Current guidelines for assessing the risk of experiencing a hospitalized cardiovascular (CV) event discourage stress testing of asymptomatic individuals; however, these recommendations are based on evidence gathered primarily from those aged < 60 years, and do not address the possibility of unrecognized “silent myocardial ischemia” in middle aged and older adults.

Myocardial injury because of oxidative stress manifesting through reductions in left ventricular ejection fraction (LVEF) may occur after the administration of anthracycline-based chemotherapy (A-bC). We hypothesized that bilirubin, an effective endogenous antioxidant, may attenuate the reduction in LVEF that sometimes occurs after receipt of A-bC. We identified 751 consecutively treated patients with cancer who underwent a pre-A-bC LVEF measurement, exhibited a serum total bilirubin level <2 mg/dl, and then received a post-A-bC LVEF assessment because of symptomatology associated with heart failure. Analysis of variance, Tukey's Studentized range test, and chi-square tests were used to evaluate an association between bilirubin and LVEF changes. The LVEF decreased by 10.7 ± 13.7%, 8.9 ± 11.8%, and 7.7 ± 11.5% in group 1 (bilirubin at baseline ≤0.5 mg/dl), group 2 (bilirubin 0.6 to 0.8 mg/dl), and group 3 (bilirubin 0.9 to 1.9 mg/dl), respectively. More group 1 patients experienced >15% decrease in LVEF compared with those in group 3 (p = 0.039). After adjusting for age, coronary artery disease/myocardial infarction, diabetes mellitus, hematocrit, and the use of cardioactive medications, higher precancer treatment bilirubin levels and lesser total anthracycline doses were associated with LVEF preservation (p = 0.047 and 0.011, respectively). In patients treated with anthracyclines who subsequently develop symptoms associated with heart failure, pre-anthracycline treatment serum bilirubin levels inversely correlate with subsequent deterioration in post-cancer treatment LVEF. In conclusion, these results suggest that increased levels of circulating serum total bilirubin, an intrinsic antioxidant, may facilitate preservation of LVEF in patients receiving A-bC for cancer.

Introduction: Resting measures of pulse wave velocity have been shown to forecast future major adverse cardiovascular events (MACE), but the utility of stress induced changes in PWV to forecast events is unknown. Methods: A prospective cohort study was implemented in which 520 consecutive participants aged 55-85 years at high risk for MACE were recruited. A 1.5T MRI scanner was used to obtain PWV at baseline, low dose dobutamine (10 mcg/kg/min) and peak stress (dobutamine up to 40 mcg/kg/min and atropine up to 1.5mg administered to achieve 80% of the maximum predicted heart rate response for age). Aortic arch PWV was assessed using the velocity waveform transit time technique. Participants were followed-up at 3 month intervals by personnel who were blinded to the stress test results. Participants were stratified by gender and presence or absence of MACE. Two sample t-test was used for continuous variables while Chi-square test was used for categorical variables. Cox proportional hazards model was used to ...

Introduction: Reductions in left ventricular ejection fraction (LVEF) may occur after the administration of anthracycline-based chemotherapeutic regimens. Oxidative stress at the myocellular level has been implicated in these reductions, and as a consequence, we hypothesized that bilirubin, an effective endogenous anti-oxidant, would ameliorate some of the reductions in LVEF associated with anthracyline administration. Methods: From 1/1/2002 to 12/31/2012, we identified 751 consecutive individuals who were treated with anthracyclines at Wake Forest Baptist Medical Center, received serial LVEF measures, and exhibited basal serum bilirubin levels < 2mg/dl prior to their treatment for cancer. The correlation between pre-chemotherapy bilirubin levels and serial pre- to post-chemotherapy changes in LVEF was analyzed using linear regression models. For dose response analysis, the participants were divided into 3 groups based on their bilirubin levels. ANOVA was used to test for the difference in the mean LVEF change across groups. Tukey’s Studentized Range test was used in pairwise comparisons. Chi-square test was used for categorical variables. Results: There were 65 (35%), 86 (30%) and 68 (24%) participants whose LVEF decreased by more than 15% in Group 1 (bilirubin ≤0.5mg/dl), Group2 (bilirubin 0.6 - 0.8mg/dl) and Group 3 (bilirubin 0.9 - 1.9mg/dl) respectively (Table 1). On pairwise comparison, there was a significant decrease in LVEF between Group 1 and Group 3 (2.9, 95% CI: 0.15 - 5.7). After adjusting for age, BMI, race, CAD/MI, diabetes, hematocrit and medications; increased bilirubin and reduced BMI were associated with LVEF preservation (p = 0.028 and 0.033 respectively). Conclusions: In patients treated with anthracyclines, bilirubin level was negatively correlated with reduction in LVEF. These results have potential therapeutic implications for preserving left ventricular function in patients treated with anthracyclines.

Angiotensin II (AngII) causes hypertension (HTN) and promotes renal injury while simultaneously inducing reno-protective enzymes like heme oxygenase-1 (HO-1). We examined the modulatory role of HO on sub-pressor angiotensin II (SP-AngII) induced renal inflammation and injury. We first tested whether the SP-AngII-induced renal dysfunction, inflammation and injury are exacerbated by either preventing (chronic HO-1 inhibition) or reversing (late HO-1 inhibition) SP-AngII-induced HO (using tin protoporphyrin; SnPP). We next examined whether additional chronic or late induction of SP-AngII-induced HO (using cobalt protoporphyrin; CoPP), prevents or ameliorates renal damage. We found that neither chronic nor late SnPP altered blood pressure. Chronic SnPP worsened SP-AngII-induced renal dysfunction, inflammation, injury and fibrosis, whereas late SnPP worsened renal dysfunction but not inflammation. Chronic CoPP prevented HTN, renal dysfunction, inflammation and fibrosis, but surprisingly, not the NGAL levels (renal injury marker). Late CoPP did not significantly alter SP-AngII-induced HTN, renal inflammation or injury, but improved renal function. Thus, we conclude (a) endogenous HO may be an essential determining factor in SP-AngII induced renal inflammation, injury and fibrosis, (b) part of HO's renoprotection may be independent of blood pressure changes; and (c) further induction of HO-1 protects against renal injury, suggesting a possible therapeutic target.

Carbon monoxide (CO) is an endogenously produced gas resulting from the degradation of heme by heme oxygense or from fatty acid oxidation. Heme oxygenase (HO) enzymes are constitutively expressed in the kidney (HO-2) and HO-1 is induced in the kidney in response to several physiological and pathological stimuli. While the beneficial actions of HO in the kidney have been recognized for some time, the important role of CO in mediating these effects has not been fully examined. Recent studies using CO inhalation therapy and carbon monoxide releasing molecules (CORMs) have demonstrated that increases in CO alone can be beneficial to the kidney in several forms of acute renal injury by limiting oxidative injury, decreasing cell apoptosis, and promoting cell survival pathways. Renal CO is also emerging as a major regulator of renal vascular and tubular function acting to protect the renal vasculature against excessive vasoconstriction and to promote natriuresis by limiting sodium reabsorption in tubule cells. Within this review, recent studies on the physiological actions of CO in the kidney will be explored as well as the potential therapeutic avenues that are being developed targeting CO in the kidney which may be beneficial in diseases such as acute renal failure and hypertension.

Obsessive-compulsive disorder (OCD) in children and adolescents is a chronic disease characterized by a poor prognosis. Often it is detected months and even years after its onset. Comorbid disorders hinder specialized care of children and adolescents with this condition. This article includes recommendations for evaluation and a review of treatment of pediatric patients with OCD. Evaluation should include a diagnostic interview and assessment of symptoms and functional impairment using scales (Yale Brown and OCD impairment). Treatment should be multimodal, including medication and psychosocial interventions. Pharmacological treatment must consider comorbidity and is based on the use of antidepressants (clomipramine, sertraline, fluvoxamine, and fluoxetine) that are safe and effective over the long term, and other medications are explored such as rulizole. Psychosocial treatment includes psychoeducation and psychotherapy. Cognitive behavioral therapy (CBT) has shown efficacy in controlled clinical trials, and is considered the first therapeutic option by many experts and clinical guidelines. The objective of CBT is to enable the patient to control their thoughts and restore functionality. Factors affecting prognosis for response to treatment include family history of the illness, limited introspection, cognitive deficits, functional impairment, longer duration of the illness, and family accommodation to the patient’s symptoms. This article proposes an algorithm that recommends beginning treatment with sertraline (12 weeks). If a response is not achieved, the algorithm proposes that another medication be used for 8 weeks. If this fails to produce an improvement, we recommend a CBT program. Once a response is achieved, the pharmacological treatment is extended for a period of one year.

Heme oxygenase-1 induction (HO-1) elicits chronic weight loss in several rodent models of obesity. Despite these findings, the mechanism by which HO-1 induction reduces body weight is unclear. Chronic HO-1 induction does not alter food intake, suggesting other mechanisms such as increases in metabolism and activity may be responsible for the observed reduction of body weight. In this study, we investigated the mechanism of weight loss elicited by chronic HO-1 induction in a model of genetic obesity due to melanocortin-4 receptor (MC4R) deficiency. Experiments were performed on loxTB MC4R-deficient mice as well as lean controls. Mice were administered cobalt protoporphyrin (CoPP, 5 mg kg−1), an inducer of HO-1, once weekly, from 4 to 23 weeks of age. Body weights were measured weekly and fasted blood glucose and insulin, as well as food intake were determined at 18 weeks of age. Oxygen consumption (VO2), CO2 production (VCO2), activity and body heat production were measured at 20 weeks of age. Chronic CoPP treatment resulted in a significant decrease in body weight from 5 weeks on in loxTB mice. Chronic CoPP treatment resulted in a significant decrease in fasted blood glucose levels, plasma insulin and a significant increase in plasma adiponectin levels in MC4R-deficient mice. Chronic CoPP treatment increased VO2 (47±4 vs 38±3 ml kg−1 per min, P<0.05) and VCO2 (44±7 vs 34±4 ml kg−1 per min, P<0.05) in treated vs non-treated, MC4R-deficient mice (n=4). Heat production (10%) and activity (18%) were also significantly (P<0.05) increased in CoPP-treated MC4R-deficient mice. Our results suggest that chronic HO-1 induction with CoPP induction elicits weight loss by increasing metabolism and activity by an MC4R-independent pathway.

Recent studies have indentified imidazole-dioxolane based compounds as novel heme oxyenase (HO) inhibitors. While these compounds have been demonstrated to be specific HO inhibitors in vitro, they have yet to be used to inhibit renal HO activity in vivo. The goal of this study was to determine the effectiveness of the imidazole-dioxolane HO-1 inhibitor, QC-13, in the inhibition of renal HO activity in vivo. HO-1 was induced in mice by treatment with cobalt protoporphyrin (CoPP). After 5 days, QC-13 was delivered either by continuous intrarenal medullay interstitial infusion (IRMI) into one kidney at several concentrations for 72h or by two intraperitoneal injections over a 48-h period. IRMI infusion of QC-13 at a concentration of 25μM resulted in a significant decrease in medullary but not cortical HO activity as compared to CoPP treated kidneys. IRMI infusion of QC-13 at a lower concentration (2.5μM) had no effect on either medullary or cortical HO activity in CoPP treated mice. In contrast, administration of QC-13 at a higher concentration (250μM) resulted in a significant decrease in both medullary and cortical HO activity in CoPP treated mice. Systemic administration of QC-13 resulted in significant decrease both renal cortical and medullary HO activity in CoPP treated mice. In contrast to classical porphyrin based HO inhibitors, IRMI infusion of QC-13 did not induce HO-1 protein levels as determined by Western blot analysis of medullary protein samples. Our results demonstrated that imidazole-dioxolane inhibitors are renal HO inhibitors in vivo and can inhibit HO activity independent of HO-1 induction. These inhibitors may be useful tools to elucidate the role of renal HO-1 in numerous physiologic and pathophysiologic conditions.

We have previously demonstrated that moderate hyperbilirubinemia decreases blood pressure in ANG II-dependent hypertension through mechanisms that decrease oxidative stress and increase nitric oxide levels. Since decreases in renal hemodynamics play an important role in mediating the hypertensive actions of ANG II, the goal of the present study was to examine the effect of moderate hyperbilirubinemia on glomerular filtration rate (GFR) and renal blood flow (RBF) in a mouse model of ANG II hypertension. Mice were made moderately hyperbilirubinemic by two methods: indinavir or specific morpholino antisense oligonucleotides against UGT1A1, which is the enzyme responsible for the conjugation of bilirubin in the liver. GFR and RBF were measured in mice after implantation of an osmotic minipump delivering ANG II at a rate of 1 μg·kg(-1)·min(-1). GFR was measured by continuous infusion of I(125)-labeled iothalamate on days 5 and 6 of ANG II infusion in conscious mice. RBF was measured on day 7 of ANG II infusion in anesthetized mice. Blood levels of unconjugated bilirubin were significantly increased in mice treated with indinavir or anti-UGT1A1 (P = 0.002). ANG II decreased GFR by 33% of control (n = 9, P = 0.004), and this was normalized by moderate hyperbilirubinemia (n = 6). Next, we examined the effect of moderate hyperbilirubinemia on RBF in ANG II-infused mice. ANG II infusion significantly decreased RBF by 22% (P = 0.037) of control, and this decrease was normalized by moderate hyperbilirubinemia (n = 6). These results indicate that improvement of renal hemodynamics may be one mechanism by which moderate hyperbilirubinemia lowers blood pressure in this model.

Mitochondrial dysfunction is involved in pathopysiology of ischemia-reperfusion-induced acute kidney injury (AKI). The p66shc adaptor protein is a newly recognized mediator of mitochondrial dysfunction, which might play a role in AKI-induced renal tubular injury. Oxidative stress-mediated Serine36 phosphorylation of p66shc facilitates its transportation to the mitochondria where it oxidizes cytochrome c and generates excessive amount of reactive oxygen species (ROS). The consequence is mitochondrial depolarization and injury. Earlier we determined that p66shc plays an essential role in injury of cultured mouse renal proximal tubule cells during oxidative stress. Here, we studied the role of p66shc in ROS generation and consequent mitochondrial dysfunction during oxidative injury in renal proximal tubule cells. We employed p66shc knockdown renal proximal tubule cells and cells that overexpress wild-type, Serine phosphorylation (S36A), or cytochrome c-binding (W134F) mutants of p66shc. Inhibition of the mitochondrial electron transport chain or the mitochondrial permeability transition revealed that hydrogen peroxide-induced injury is mitochondrial ROS and consequent mitochondrial depolarization dependent. We also found that through Ser36 phosphorylation and mitochondria/cytochrome c binding, p66shc mediates those effects. We propose a similar mechanism in vivo as we demonstrated mitochondrial binding of p66shc as well as its association with cytochrome c in the postischemic kidneys of mice. Thus, manipulating p66shc might offer a new therapeutic modality to ameliorate renal ischemic injury.

Induction of heme oxygenase-1 (HO-1) in the renal medulla increases carbon monoxide and bilirubin production and decreases ANG II-mediated superoxide production. The goal of this study was to determine the importance of increases in bilirubin to the antioxidant effects of HO-1 induction in cultured mouse thick ascending loop of Henle (TALH) and inner medullary collecting duct (IMCD3) cells. Bilirubin levels were decreased by using small interfering RNAs (siRNAs) targeted to biliverdin reductase (BVR), which is the cellular enzyme responsible for the conversion of biliverdin to bilirubin. Treatment of cultured TALH or IMCD-3 cells with BVR siRNA (50 or 100 nM) resulted in an 80% decrease in the level of BVR protein and decreased cellular bilirubin levels from 46 +/- 5 to 23 +/- 4 nM (n = 4). We then determined the effects of inhibition of BVR on ANG II-mediated superoxide production. Superoxide production induced by ANG II (10(-9) M) significantly increased in both TALH and IMCD-3 cells. Treatment of TALH cells with BVR siRNA resulted in a significant increase in ouabain-sensitive rubidium uptake from 95 +/- 6 to 122 +/- 5% control (n = 4, P < 0.05). Lastly, inhibition of BVR with siRNA did not prevent the decrease in superoxide levels observed in cells pretreated with the HO-1 inducer, hemin. We conclude that decreased levels of cellular bilirubin increase ANG II-mediated superoxide production and sodium transport; however, increases in bilirubin are not necessary for HO-1 induction to attenuate ANG II-mediated superoxide production.

Heme oxygenase (HO) is the enzyme responsible for the breakdown of heme-generating carbon monoxide (CO) and biliverdin in this process. HO-2 is the constitutively expressed isoform in most tissues, such as the kidney and vasculature. CO generated by HO is believed to be an important vasodilator in the renal circulation along with another gas, nitric oxide (NO). To determine the importance of HO-2 in the regulation of blood pressure and renal blood flow (RBF), we treated HO-2 knockout (KO) mice chronically with either ANG II or N(G)-nitroarginine methyl ester (l-NAME). Basal blood pressures were not different between wild-type (WT), heterozygous (HET), or KO mice and averaged 113 +/- 3 vs. 115 +/- 2 vs. 116 +/- 2 mmHg. Similar increases in blood pressure to chronic ANG II as well as l-NAME treatment were observed in all groups with blood pressures increasing an average of 30 mmHg in response to ANG II and 15 mmHg in response to l-NAME. Basal RBFs were not different between the groups averaging 6.0 +/- 0.5 (n = 6) vs. 4.8 +/- 0.6 (n = 10) vs. 5.8 +/- 0.7 (n = 6) ml*min(-1)*g(-1) kidney weight in WT, HET, and KO mice. HO-2 KO and HET mice exhibited an attenuated decrease in RBF in response to acute administration of ANG II, while no differences were observed with l-NAME. Our data indicate that blood pressure and RBF responses to increased ANG II or inhibition of nitric oxide are not significantly enhanced in HO-2 KO mice.

Population studies indicate that moderate hyperbilirubinemia is associated with reduced incidence of cardiovascular diseases, including hypertension. Despite this correlative evidence, no studies have directly tested the hypothesis that moderate increases in plasma bilirubin levels can attenuate the development of hypertension. This hypothesis was tested by treating mice with Indinavir, a drug that competes with bilirubin for metabolism by UDP-glucuronosyltransferase 1A1 (UGT1A1). Treatment of mice with Indinavir (500 mg x kg(-1) x day(-1), gavage) resulted in a twofold increase in plasma unconjugated bilirubin levels. Next, we determined the effect of Indinavir-induced changes in plasma bilirubin on the development of ANG II-dependent hypertension. Moderate hyperbilirubinemia was induced 3 days before the implantation of an osmotic minipump that delivered ANG II at a rate of 1 microg x kg(-1) x min(-1). ANG II infusion increased mean arterial pressure (MAP) by 20 mmHg in control mice but by only 6 mmHg in mice treated with Indinavir (n = 6). Similar to Indinavir treatment, direct infusion of bilirubin (37.2 mg x kg(-1) x day(-1) i.v.) resulted in a twofold increase in plasma bilirubin levels and also attenuated the development of ANG II-dependent hypertension. Moderate hyperbilirubinemia resulted in an increase in plasma nitrate/nitrite levels, which averaged 36 +/- 2 vs. 50 +/- 7 microM in ANG II vehicle vs. Indinavir-treated mice (n = 5). Moderate hyperbilirubinemia resulted in attenuation of vascular oxidative stress as determined by dihydroethidium staining of aortic segments. These results indicate that moderate hyperbilirubinemia prevents ANG II-dependent hypertension by a mechanism that may involve decreases in vascular oxidative stress.

BACKGROUNDInduction of heme oxygenase-1 (HO-1) attenuates the development of angiotensin II (Ang II)-dependent hypertension in mice. However, the mechanism by which HO-1 lowers blood pressure in this model is not clear. This study was designed to determine whether induction of HO-1 results in an improvement in vascular relaxation in Ang II hypertensive mice.METHODSMice were treated with either of the vehicles (control), the HO-1 inducer cobalt protoporphyrin (COPP; 50 mg/kg), Ang II (1 mu g/kg/min, 14 days), or Ang II + CoPP. COPP was administered as a single bolus dose 2 days prior to subcutaneous implantation of the osmotic minipump containing Ang II.Vascular relaxation was examined in isolated carotid arteries precontracted with the thromboxane mimetic U46619 (0.4 mu g/ml).RESULTSEndothelial dependent relaxation to acetylcholine (ACh; 1 mu mol/l) was significantly impaired in Ang II-treated mice compared to control mice (56 +/- 3% vs. 40 +/- 4%, P < 0.05, n >= 6). Similarly, endothelial independent relaxation to sodium nitroprusside (SNP; 1 mu mol/l) was significantly impaired in Ang II mice (56 6% vs. 28 6%, P < 0.05, n ! 6). Relaxation in response to the carbon monoxide donor, CORM-A1 (100 mu mol/l), was attenuated after Ang II treatment (75 +/- 7% vs. 59 +/- 7%, P < 0.05, n >= 6). COPP treatment induced HO-1 but not HO-2 protein in the aorta, as measured by western blot analysis. COPP treatment had no effect on vascular responses in control mice and did not improve ACh (26 +/- 5%, n = 15), SNP (23 +/- 4%, n = 15), or CORM-A1 (46 7%, n = 10) dependent relaxation in Ang II treated mice.CONCLUSIONS These results suggest that induction of HO-1 lowers Ang II-dependent hypertension through a mechanism independent of improved vascular relaxation.

Heme oxygenase (HO)-1 induction can attenuate the development of angiotensin II (ANG II)-dependent hypertension. However, the mechanism by which HO-1 lowers blood pressure is not clear. The goal of this study was to test the hypothesis that induction of HO-1 can reduce the ANG II-mediated increase in superoxide production in cultured thick ascending loop of Henle (TALH) cells. Studies were performed on an immortalized cell line of mouse TALH (mTALH) cells. HO-1 was induced in cultured mTALH cells by treatment with cobalt protoporphyrin (CoPP, 10 microM) or hemin (50 microM) or by transfection with a plasmid containing the human HO-1 isoform. Treatment of mTALH cells with 10(-9) M ANG II increased dihydroethidium (DHE) fluorescence (an index of superoxide levels) from 35.5+/-5 to 136+/-18 relative fluorescence units (RFU)/microm2. Induction of HO-1 via CoPP, hemin, or overexpression of the human HO-1 isoform significantly reduced ANG II-induced DHE fluorescence to 64+/-5, 64+/-8, and 41+/-4 RFU/microm2, respectively. To determine which metabolite of HO-1 is responsible for reducing ANG II-mediated increases in superoxide production in mTALH cells, cells were preincubated with bilirubin or carbon monoxide (CO)-releasing molecule (CORM)-A1 (each at 100 microM) before exposure to ANG II. DHE fluorescence averaged 80+/-7 RFU/microm2 after incubation with ANG II and was significantly decreased to 55+/-7 and 53+/-4 RFU/microm2 after pretreatment with bilirubin and CORM-A1. These results demonstrate that induction of HO-1 in mTALH cells reduces the levels of ANG II-mediated superoxide production through the production of both bilirubin and CO.

HomeHypertensionVol. 51, No. 3Role of Carbon Monoxide in Blood Pressure Regulation Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBRole of Carbon Monoxide in Blood Pressure Regulation David E. Stec, Heather A. Drummond and Trinity Vera David E. StecDavid E. Stec From the Department of Physiology and Biophysics, Center for Excellence in Cardiovascular-Renal Research, University of Mississippi Medical Center, Jackson. , Heather A. DrummondHeather A. Drummond From the Department of Physiology and Biophysics, Center for Excellence in Cardiovascular-Renal Research, University of Mississippi Medical Center, Jackson. and Trinity VeraTrinity Vera From the Department of Physiology and Biophysics, Center for Excellence in Cardiovascular-Renal Research, University of Mississippi Medical Center, Jackson. Originally published22 Jan 2008https://doi.org/10.1161/HYPERTENSIONAHA.107.097154Hypertension. 2008;51:597–604Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: January 22, 2008: Previous Version 1 Carbon monoxide (CO) is an odorless, colorless, tasteless gas that is generated in the environment as the result of combustion from stoves and engines among other sources. Approximately 500 people per year in the United States are victims of nonfire-associated CO poisoning according to the Centers for Disease Control and Prevention. CO poisoning is often fatal because of its interaction with hemoglobin, which renders it incapable of carrying oxygen-causing organs to become severely hypoxic. CO inhalation is believed to be fatally toxic at concentrations as little as 800 parts per million (ppm) or 0.08% in the air. Despite the lethal nature of this gas, several recent studies suggest that CO inhalation at low doses (≤250 ppm), as well as increases in CO levels using CO releasing molecules (CORMs), offers protection against ischemic injury in the heart, liver, and kidney.1–4CO is endogenously produced in the body as a result of the metabolism of heme by heme oxygenase (HO), as well as from lipid peroxidation.5,6 The catabolism of heme by HO also produces an equimolar amount of biliverdin, which is rapidly converted in the cell to bilirubin by the enzyme biliverdin reductase.7 There are 2 major isoforms of HO responsible for CO production. HO-1 is expressed at very low levels under normal conditions but is highly induced by several stimuli, including heavy metals, ultraviolet light, endotoxin, shear stress, hypoxia, and oxidants.8 HO-2 is the constitutively expressed form of the enzyme with the highest levels observed in the brain and testes.9 Experimental evidence has demonstrated that systemic induction of HO has several beneficial actions on the cardiovascular system, including lowering of blood pressure, protection against myocardial infarction, and prevention of atherosclerosis.10–12 Although the cardiovascular actions of HO induction have been established, the role of CO in mediating these responses is not clear. The purpose of this review is to outline the potential antihypertensive actions of CO and highlight areas that may pose new opportunities for the development of novel therapeutic targets for the treatment of hypertension.Altering CO Levels In Vivo: Tools of the TradeThere are 3 main approaches that have been used to alter tissue levels of CO in vivo, which need to be briefly discussed. These include inhibition/induction of HO, CO inhalation, and CORMs. Each of these approaches has its own advantages and limitations depending on the specific experimental settings in which tissue levels of CO are to be altered. HO induction/inhibition has been widely used because of the fact that most of the endogenous CO produced in vivo is derived from HO. Given the routine ability to alter HO in a tissue- and temporal-specific fashion either pharmacologically or genetically, this is an attractive option for examining CO in vivo. However, there are several limitations in altering HO levels to examine the role of CO in vivo. First among these is the specificity of the majority of the metalloporphyrin HO inhibitors, which have nonspecific effects apart from HO inhibition.13,14 Secondly, inhibition/induction of HO also alters the levels of bilirubin and free iron, both of which also can have effects on cell and tissue function apart from changes in CO.15–17CO inhalation therapy is a highly effective means of increasing tissue levels of CO. CO inhalation has been proven to protect against ischemia-reperfusion injury and improve survival of allograft transplants.2,18 One limitation with CO inhalation is the high levels of blood carboxyhemoglobin (COHb) that are achieved at CO levels (200 to 500 ppm) required to ensure significant increases in tissue levels of CO.19,20 These effects on blood COHb levels may limit the clinical applicability of CO inhalation unless lower levels of CO can achieve similar results. In support of this idea, a recent study by Kobayashi et al21 demonstrated that low-level CO inhalation (60 ppm), which did not result in a dramatic increase in blood COHb levels, was able to significantly attenuate the development of angiotensin II (Ang II)–dependent hypertension. This study provides the first evidence that chronic low level CO inhalation may be a viable option for the treatment of hypertension.CORMs are recently described compounds capable of releasing CO at physiological pH. CORMs are available as transition metal carbonyl compounds with a fast rate of CO release (CORM-2, CORM-3, and CORM-F10) or as a sodium bornanocarbonate compound (CORM-A1) with a slower rate of CO release.22–24 CORMs are advantageous in that they exhibit physiological effects without causing significant increases in blood COHb levels.1,4,25 There are 2 notable limitations with CORMs. The first being the induction of HO-1 because of the metals in the transition metal carbonyl forms, such as CORM-3.4 This induction of HO-1 can result in a further increase in endogenous CO and bilirubin levels, which may contribute to any observed physiological actions of these compounds. Significant induction of HO-1 can be avoided by use of the nonmetal-containing CORM, CORM-A1; however, this chemical is not currently available from commercial sources and must be synthesized by individual investigators. Another limitation is the inability to infuse these compounds chronically over hours or days because of the relatively short half-life of the compounds in physiological solutions. This is a major obstacle for investigators interested in performing chronic whole-animal integrative studies with these compounds.Tissue levels of CO are consistent between the rat and the mouse averaging 1 to 5 pmol/mg of tissue under control conditions.19 Treatment with the HO inducer heme arginate, which doubles blood COHb levels, modestly increases tissue CO levels primarily in the muscle, heart, and lung by 2-fold.19 However, inhalation of 500 ppm CO, which increases COHb levels >50-fold, increases CO levels in the brain, heart, kidney, liver, and lung by >15-fold.19 Specific studies examining tissue CO levels after administration of CORMs have not yet been performed. CO inhalation at high levels results in a larger increase in CO levels in more tissues than induction of HO-1 but at the expense of greatly increase blood COHb levels, which could be a significant concern clinically.CO and the Brain: CO as a Modulator of Baroreceptor FunctionOver the last 20 years, several studies have demonstrated that CO is an important signaling molecule in the brain.26,27 CO generated in the brain is generated from the metabolism of heme by HO-2, which is the major isoform of HO expressed in the brain.28,29 Both HO-2 and HO-1 are expressed in neurons and glia, with HO-1 being induced in these cells types in response to several types of brain injuries, including stroke.30–32 CO modulates the release of neurotransmitters such as oxytocin and corticotropin (adrenocorticotropic hormone)-releasing hormone.33,34 Neural generation of CO can decrease the firing rates of neurons in several nuclei of the brain, including the locus coeruleus and the nucleus tractus solitarii (NTS).35,36The NTS is an important area involved in the integration of autonomic control of the cardiovascular system, where afferent fibers from peripheral cardiovascular baroreceptors and chemoreceptors make their first central synapses.37 It is in this region of the brain where alterations in HO and CO decrease baroreceptor sensitivity. Johnson et al36 were the first to demonstrate that systemic inhibition of HO with zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) was associated with an increase in mean arterial pressure and a decrease in the gain of the baroreflex. More importantly, they were also able to increase blood pressure by unilateral or bilateral administration of ZnDPBG directly into the NTS.36 The increase in blood pressure elicited by direct NTS administration of ZnDPBG was reversed by direct injection of saline saturated with CO, indicating that decreases in the levels of CO are responsible for the pressor actions of ZnDPBG administration into the NTS. These studies were significant in that they provided the first experimental evidence that CO produced by HO may play a role in central regulation of blood pressure. Additional studies by Lo et al38 also demonstrated that direct injection of ZnDBPG into the NTS decreased baroreflex sensitivity. It should again be noted that the use of metalloporphyrins as HO inhibitors is not without controversy. Several nonspecific effects, such as direct inhibition of guanylate cyclase, inhibition of NO synthase, and induction of HO-1 have been attributed to metalloporphyrins.13,14 However, of the wide variety of metalloporphyrins, ZnDPBG has been found to be among the most specific for inhibition of HO.14,39Further evidence for a role of CO in the central regulation of blood pressure comes from studies in which hematin, an alternative form of the HO substrate hemin, was directly injected into the NTS. Direct injection of hematin into the NTS consistently lowers blood pressure and heart rate.40,41 The depressor effects of hematin injection into the NTS are similar to those observed with injection of the excitatory amino acid glutamate. Glutamate is the principle neurotransmitter of baroreceptor afferent fibers, which terminate in the NTS.42 Glaum and Miller43 were the first to provide experimental evidence linking CO and glutamate receptors in the NTS. This report was confirmed by several studies, which have reported that blockade of HO with zinc protoporphyrin IX or ZNDPBG results in a significant decrease in the depressor and bradycardic response to l-glutamate and group II and III metabotropic glutamate receptor agonists.44–46As outlined in Figure 1, increases in CO in the NTS occur after stimulation of glutamate receptors after release of glutamate from cardiovascular afferents; however, the mechanism by which increased CO affects NTS neuronal activity (ie, increases in cGMP or direct action on K+ channels) is not known. Blockade of HO reduces CO release and decreases postsynaptic neuron activity in the NTS, which leads to increases in blood pressure. Despite our increased understanding of the acute role of CO in the central regulation of blood pressure, little information is available regarding whether alterations of central HO/CO exist in experimental and genetic models of hypertension, especially those with increase sympathetic activity, such as the spontaneously hypertensive rat. Also, the potential antihypertensive actions of chronic increases in HO/CO in the NTS have not been examined in hypertensive models, so it is difficult to determine the importance of CO in this region in the long-term control of blood pressure. Advances in gene delivery techniques in the brain, as well as brain-specific transgenic overexpression approaches, which chronically increase HO/CO levels in neurons or gila, should allow for the importance of altered central CO in the regulation of blood pressure to be determined in the future. Download figureDownload PowerPointFigure 1. Schematic summary of effects of CO in the NTS. Baroreceptor afferent neurons of the NTS release glutamate in response to activity. The glutamate the binds to G1/G3 receptors on postsynaptic neurons, which leads to an increase in CO release, activation of K+ channels, and decreases in NTS postsynaptic neuron activity. Inhibition of HO in the NTS blocks CO release, increasing postsynaptic neuron activity and increasing blood pressure.CO and the Vasculature: Does the Good Outweigh the Bad?One of the first reported biological functions of CO is its ability to dilate blood vessels in several organs.47,48 The major source of CO in the vasculature is HO-2, which is expressed in both endothelial and vascular smooth muscle cells. Like NO, the ability of CO to relax blood vessels occurs through activation of soluble guanyl cyclase (sGC), increases in cGMP, and activation of high-conductance Ca2+-activated K+ channels.22,49–51 The activation of K+ channels leads to membrane hyperpolarization, which inhibits calcium entry from voltage-activated Ca2+ channels.50,52 However, there is evidence that CO may act directly or via pathways other than cGMP to cause vasodilatation in certain vascular beds.52–54 In the kidney, studies using specific CORMs have found that increases in CO produce increases in renal blood flow that are significantly but not totally reduced by blockade of sGC and completely blocked by inhibition of K+ channels.55,56CO provides an important counterbalance against vasoconstriction mediated by such agents as Ang II and 20-hydroxyeciosatetrenoic acid, especially in the renal vasculature.54,57 CO, derived from either endothelial or smooth-muscle HO, can directly inactivate P450 enzymes in the vasculature by binding to the heme moiety in the enzyme. Induction of HO-1 in the vasculature can also directly decrease the activity of P450 enzymes by reducing the availability of heme, which is required for the function of P450 enzymes. Thus, increases in vascular CO/HO can result in decreased production of vasoconstrictors such as 20-hydroxyeciosatetrenoic acid, but it can also inhibit production of vasodilators such as prostaglandin I2 and prostaglandin E2.58,59 Interestingly, CO produced from HO can also act as a second messenger for P450 enzymes. For example, incubation of rat mesenteric arterioles with 11,12-epoxyeicosatrienoic acid stimulates CO release, and vasodilatation to 11,12-epoxyeicosatrienoic acid is abolished by inhibition of HO activity.60 Because vasodilatation to 11,12-epoxyeicosatrienoic acid and CO are both inhibited by iberiotoxin,56,60 it is likely that CO mediates dilatation to 11,12-epoxyeicosatrienoic acid through activation of K+ channels.There is a very complicated relationship between vascular NO and CO (Figure 2). Both gases can activate sGC to increase cGMP levels and cause vasodilation.61,62 Previous studies have shown that low levels of CO (0.001 to 0.1 μmol/L) can stimulate NO release, whereas higher levels of CO (≥1 μmol/L) inhibit NO synthase.63 In the renal circulation, CO buffers against excessive vasoconstriction observed after blockade of NO.64,65 This suggests that a major role for CO in the renal vasculature is to provide protection against excessive vasoconstriction when the renal NO system is deficient. In contrast, vascular smooth muscle–specific overexpression of HO-1, resulting in a 3-fold increase in vascular HO activity, attenuates cGMP production in response to NO, impairs NO-mediated vasodilatation, and causes hypertension.66 Additional evidence for deleterious affects of excessive CO production in the vasculature is supported by studies on Dahl salt-sensitive rats in which responses to acetylcholine are restored upon inhibition of HO.67 In the obese Zucker rat model of the metabolic syndrome, inhibition of HO activity results in an enhancement of acetylcholine-induced vasodilatation and lowering of blood pressure.68 All of these studies clearly indicate that excessive production of CO in the vasculature leads to alterations in NO production, endothelial dysfunction, and hypertension. However, there are also beneficial effects of increased HO/CO in the vasculature. Increases in vascular HO/CO improve acetylcholine-mediated vasodilatation in diabetes.25 One potential mechanism for the improvement in vasodilatation by CO in diabetes is a decrease in the levels of oxidants with HO-1 induction or increases in CO. This hypothesis is supported by data from endothelial cells in which HO-1 or CO reduces oxidant damage and endothelial cell sloughing in streptozotocin-induced diabetes.69–71Download figureDownload PowerPointFigure 2. Vascular CO production and its relationship to NO. A, Under normal conditions CO is produced from the breakdown of intracellular heme by HO. CO then activates sGC to increase intracellular cGMP levels, which increase activity of K+ channel to cause vasodilatation. CO may also have direct actions on K+ channels to increase their activity. NO produced by endothelial cells (EC) stimulates intracellular cGMP levels in vascular smooth muscle cells (VSMC) to cause vasodilation. B, In cases in which CO production is significantly increased in VSMCs, CO can inhibit NO synthase (NOS) in ECs decreasing NO production and reducing NO-mediated increases in cGMP, decreasing VSMC relaxation, and causing vascular dysfunction.There are several potential therapeutic applications for increases in vascular CO that are somewhat limited by the multifaceted nature of the actions of CO with NO. Increases in vascular CO would most likely be beneficial in states where endogenous NO production is reduced. The kidney would likely be a good target for increases in vascular CO, especially in conditions of excessive renal vasoconstriction. However, increases in vascular CO need to be tempered as to not interfere with endogenous NO production, which can lead to impaired vascular relaxation and increases in blood pressure. Increases in vascular CO may also be beneficial to protect the vasculature against oxidant damage and to repair the endothelium in diabetes. Again, the difficulty will be in determining the proper levels of CO needed to protect the vasculature, while at the same time avoiding any negative effects on other systems, such as NO.CO and the Kidney: Beyond the VasculatureAs outlined above, CO has an important role in the regulation of vascular tone, and this is especially true in the renal vasculature. HO-2–derived CO is critical for the maintenance of renal medullary blood flow.72 Because decreases in renal medullary blood flow are observed in several forms of hypertension,73,74 preservation of renal medullary blood flow by CO is an important adaptive mechanism to promote sodium excretion by the kidney. Another mechanism by which CO regulates renal vascular resistance is by modulation of afferent arteriolar tone, as described in the previous section. Another important mechanism that regulates the afferent arteriolar diameter is tubuloglomerular feedback (TGF). One hypothesis for the mechanism of TGF is that a rise in sodium delivery to the early distal tubule results in an increase in ADP release by the macula densa cells. ADP then acts on the A1 receptor causing contraction of the afferent arteriole, a drop in glomerular pressure, and a reduction in the single-nephron GFR75 (Figure 3). The increase in sodium delivery into this segment also activates neuronal NO synthase activity in macula densa cells. The NO produced is thought to regulate the TGF sensitivity by dilating the afferent arteriole to prevent excessive vasoconstriction.76 However, in the spontaneously hypertensive rat, 2-kidney 1-clip, and Ang II–dependent hypertension models, NO half-life is markedly reduced, resulting in TGF hypersensitivity.77,78 As mentioned earlier, there are considerable similarities in the actions of CO and NO on the vasculature; however, the role of CO on TGF regulation is not clearly understood. CO at low concentrations releases NO from intracellular stores, whereas at high concentration it inhibits NO synthase activity.63 HO inhibition alone has no effect on TGF sensitivity, but inhibition of HO activity after NO synthase inhibition further increases TGF sensitivity.65 Therefore, in models of hypertension where NO bioavailability is reduced, CO production may be critical to maintain an afferent arteriolar diameter against excessive TGF-mediated constriction. Most of the CO generated in the renal vasculature is derived from HO-2; however, it is possible that HO-1 may be an important source of vascular CO, especially in hypertensive models, such as Ang II hypertension, where HO-1 is induced. CO may further regulate TGF sensitivity by inhibiting superoxide production from reduced nicotinamide-adenine dinucleotide phosphate oxidase, consequently increasing NO availability by reducing its conversion to peroxynitrate. Although the vascular functions of CO have been established in the kidney, the role of CO in tubule cells has not been thoroughly examined. Download figureDownload PowerPointFigure 3. Modification of renal function by CO. Top, black, An increase in sodium delivery to the early distal tubule causes an increase in ADP secretion from the macula densa cells. ADP acts through the A1 receptor to cause afferent arteriolar constriction. This response is modulated by NO, which causes vasodilation. CO may participate in this response by reducing oxidative stress or directly acting on sGC to increase cGMP-mediated relaxation. Bottom, blue, CO is known to open K+ channels, allowing K+ outflux, which enhances its availability to the Na+-K+-2Cl− cotransporter (NKCC), resulting in increased Na+ reabsorption. However, increased CO may also inhibit apical migration of NKCC via increased cGMP levels, which could reduce Na+ reabsorption. nNOS indicates neuronal NO synthase.There have been conflicting reports on the role of CO as compared with HO induction in the regulation of tubular function. In isolated thick-ascending loop of Henle (TALH) tubule segments, CO stimulates the apical 70-pS K+ channel.79 Apical K+ channels are essential for the recycling of K+ to maintain the activity of the Na+-K+-2Cl− cotransporter, which is the major transporter involved in Na+ reabsorption in the TALH.80,81 CO-mediated stimulation of apical K+ channel activity increases K+ availability for the Na+-K+-2Cl− cotransporter, resulting in increased Na+ reabsorption in the TALH. This observation is supported by additional data from in vivo microperfusion experiments, which demonstrate that blockade of HO activity results in a decrease in sodium and fluid reabsorption in the TALH.82 However, results from whole-animal studies in which the levels of HO are altered have not supported an antinaturetic function of CO in the kidney. Recent studies by Li et al83 have documented an increase in CO production in the renal medulla in response to increases in renal perfusion pressure. Because increases in renal perfusion pressure lead to increases in sodium and water excretion,84 the reported antinatriuretic action of CO in the TALH does not correspond with what is occurring in this nephron segment physiologically. HO induction with hemin increases sodium and water excretion in the absence of any effects on glomerular filtration rate, and this effect is blocked by previous HO inhibition.85 Lastly, chronic inhibition of HO in the renal medulla attenuates pressure-natriuresis and hypertension in response to increases in sodium intake.83 Whether salt-sensitive hypertension resulting from inhibition of HO in the medulla is the result of changes in medullary blood flow or alterations in tubular function is not known and warrants further investigation.CO signaling in renal tubular epithelial cells has not been extensively studied; however, because it uses similar signaling pathways as NO in the vasculature, is it possible that CO is able to activate similar pathways as NO in tubular epithelial cells. For example, both CO and NO increase cGMP levels in renal tubules, which activates cGMP-stimulated phosphodiesterase (phosphodiesterase II), enhancing the degradation of cAMP.86 Because cAMP levels have been linked to apical positioning of the Na+-K+-2Cl− cotransporter, it is possible that chronic increases in CO levels may decrease Na+ reabsorption via alterations in apical levels of the Na+-K+-2Cl− cotransporter. This possibility remains to be tested in vivo.Another possible pathway by which increases in CO may affect renal tubular epithelial cell function is through interactions with superoxide anion generation. Superoxide generation has been reported to stimulate sodium reabsorption through direct actions, as well as by decreasing the availability of NO in the tubule.87,88 CO has not been traditionally looked on as an antioxidant; however, recent studies have reported that CO can decrease superoxide generation via inhibition of reduced nicotinamide-adenine dinucleotide phosphate oxidase activity.89–91 Recently, we demonstrated that previous induction of HO in the kidney was associated with a decrease in Ang II–stimulated superoxide production in a model of Ang II hypertension.92 We also have additional experimental evidence that increases in CO alone can attenuate Ang II–stimulated superoxide production in both cultured TALH and mouse inner medullary collecting duct cells (unpublished observation). Collectively, these results suggest that decreases in superoxide anion production may be a potential mechanism by which chronic increases in CO can affect sodium transport, especially in cases in which renal Ang II levels are increased. However, the role of CO as a potential antioxidant in the kidney is an emerging concept, and its importance remains to be tested in vivo.PerspectivesParacelsus, often referred to as the father of toxicology, wrote, “All things are poison and nothing is without poison, only the dose permits something not to be poisonous.” This statement is especially true for CO, because levels as low as 800 ppm in the air can be fatal. However, a growing body of literature indicates that low-level CO inhalation (<100 ppm) or treatment with CO donors has several beneficial cardiovascular actions, including lowering of blood pressure,21 protection against cardiac ischemia,1,93 and protection against diabetes induced vascular injury.25 There are also substantial reports that induction of HO-1, which results in increased CO production, also lowers blood pressure in several forms of hypertension.92,94,95 To what extent do increases in CO specifically contribute to the blood pressure lowering observed with induction of HO? It has been difficult to specifically address this question in the past given the limited ability to specifically increase CO levels in various tissues either chemically or genetically. However, with the emerging use of genetically engineered animal models and further development of CORMs, it will soon be possible to explore the potential antihypertensive properties of specific increases in CO production. Translational studies examining the efficacy of CO inhalation therapy are warranted to further develop CO as a novel antihypertensive agent.This paper was sent to Ernesto L. Schiffrin, associate editor, for review by expert referees, editorial decision, and final disposition.Sources of FundingWe acknowledge the support of grants from the American Heart Association (0430094N and 0755330B), as well as the National Institutes of Health (PO1HL-5197).DisclosuresNone.FootnotesCorrespondence to David E. Stec, Department of Physiology and Biophysics, University of Mississippi Medical Center, 2500 North State St, Jackson, MS 39216-4505. E-mail [email protected] References 1 Guo Y, Stein AB, Wu WJ, Tan W, Zhu X, Li QH, Dawn B, Motterlini R, Bolli R. Administration of a CO-releasing molecule at the time of reperfusion reduces infarct size in vivo. Am J Physiol Heart Circ Physiol. 2004; 286: H1649–H1653.CrossrefMedlineGoogle Scholar2 Kaizu T, Nakao A, Tsung A, Toyokawa H, Sahai R, Geller DA, Murase N. Carbon monoxide inhalation ameliorates cold ischemia/reperfusion injury after rat liver transplantation. Surgery. 2005; 138: 229–235.CrossrefMedlineGoogle Scholar3 Neto JS, Nakao A, Toyokawa H, Nalesnik MA, Romanosky AJ, Kimizuka K, Kaizu T, Hashimoto N, Azhipa O, Stolz DB, Choi AM, Murase N. Low-dose carbon monoxide inhalation prevents development of chronic allograft nephropathy. Am J Physiol Renal Physiol. 2006; 290: F324–F334.CrossrefMedlineGoogle Scholar4 Vera T, Henegar JR, Drummond HA, Rimoldi JM, Stec DE. Protective effect of carbon monoxide-releasing compounds in ischemia-induced acute renal failure. J Am Soc Nephrol. 2005; 16: 950–958.CrossrefMedlineGoogle Scholar5 Tenhunen R, Marver HS, Schmid R. Microsomal heme oxygenase. Characterization of the enzyme. J Biol Chem. 1969; 244: 6388–6394.CrossrefMedlineGoogle Scholar6 Wolff DG, Bidlack WK. The formation of carbon monoxide during peroxidation of microsomal lipids. Biochem Biophys Res Commun. 76 A.D.; 73: 850–857.CrossrefMedlineGoogle Scholar7 Tenhunen R. The enzymatic conversion of heme to bilirubin in vivo. Ann Clin Res. 1976; 8 (suppl 17): 2–9.MedlineGoogle Scholar8 Abraham NG, Kappas A. Heme oxygenase and the cardiovascular-renal system. Free Radic Biol Med. 2005; 39: 1–25.CrossrefMedlineGoogle Scholar9 Zakhary R, Poss KD, Jaffrey SR, Ferris CD, Tonegawa S, Snyder SH. Targeted ge

Induction of Heme Oxygenase‐1 (HO‐1) in the renal medulla increases CO and bilirubin production and decreases Angiotensin II (Ang II)‐mediated superoxide production. The goal of this study was to determine the relative importance of increases in CO and bilirubin to the antioxidant effects of HO‐1 induction in cultured thick ascending loop of Henle (TALH) cells. The effects of CO and bilirubin were separated by using small interfering RNAs (siRNAs) targeted to biliverdin reductase (BVR) which is the cellular enzyme responsible for the conversion of biliverdin to bilirubin. Treatment of cultured TALH with BVR siRNA (100 nM) resulted in a 70–90% decrease in the level of BVR protein and decreased cellular bilirubin levels from 46 ± 5 to 23 ± 4 nM (n=4). Next, we determined the effects of inhibition of BVR on Ang II‐mediated superoxide production in cultured TALH cells. Superoxide production to Ang II (10−9 M) was determined using dihydroethidium (DHE) staining followed by confocal microscopy and expressed as percent of control staining. Treatment of TALH cells with BVR siRNA (50, 100 nM) resulted in a significant increase in Ang II‐mediated superoxide production and averaged 109 ± 4, 145 ± 12, 164 ± 7%, respectively, in non‐targeting vs. 50 vs.100 nM BVR siRNA (n=4). These results demonstrate that decreased levels of cellular bilirubin increase Ang II‐mediated superoxide production in TALH cells. These studies were supported by a grant from the American Heart Association‐Southeast Affiliate.

The heme oxygenase (HO) system has received significant attention in recent years as a possible novel target for antihypertensive therapy. HO is the rate limiting enzyme in the metabolism of heme releasing bioactive molecules carbon monoxide (CO) and bilirubin each with beneficial cardiovascular actions. Induction of HO-1 has been demonstrated to lower blood pressure in several animal models of hypertension In addition to its blood pressure lowering effects, HO can also reduce target organ injury and protect against ischemic injury. Growing experimental evidence suggests that increases in either CO or bilirubin alone may also lower blood pressure and provide protection against hypertensive and ischemic end-organ damage. In this review, we will discuss the current understanding of the actions of the HO on the kidney and cardiovascular systems and how the HO system or its products may be manipulated for antihypertensive therapy.