Moderate increases in extracellular K+ produce vasodilation in fourth order cremasteric arterioles in the anesthetized rat. We studied the contribution of different subtypes of K+ channels to this response. Cremaster muscle arteriolar diameters were observed during superfusion with buffer containing 5–30 mM K+ in the absence (control) and presence of barium (Ba2+, 50 μM), glibenclamide (GLIB, 1 μM), or iberiotoxin (IBTX, 100 nM) to block inward-rectifier, ATP-sensitive, or Ca2+-activated K+ channels, respectively. Under control conditions, vessels dilated in response to 10–25 mM K+ and constricted at higher concentrations. At 5 mM K+, vessel diameters were significantly decreased by GLIB and Ba2+, but not IBTX, suggesting that basal diameter was regulated by inward-rectifier and ATP-sensitive K+ channels. In contrast, Ba2+, but not GLIB or IBTX, prevented K+-induced dilation. The data indicate that the inward-rectifier K+ channel (blocked by low concentrations of Ba2+, but not GLIB or IBTX) was most likely responsible for the K+-induced arteriolar dilation.

Background This study examined the influences of isoflurane versus halothane anesthesia on basal and agonist-stimulated nitric oxide in the cerebellum of intact rats. Nitric oxide was measured using the hemoglobin-trapping method in an in vivo microdialysis technique. This method uses the stoichiometric reaction of nitric oxide with oxyhemoglobin to produce methemoglobin and nitrate; the change in methemoglobin concentration is measured spectrophotometrically to estimate nitric oxide concentration. Methods Male Wistar rats were anesthetized with isoflurane (1.4%) or halothane (1.2%), mechanically ventilated and paralyzed (intravenous pancuronium, 1 mg/kg). Microdialysis probes were implanted into the cerebellum. Bovine oxyhemoglobin dissolved in artificial cerebrospinal fluid was pumped through the probe (2 microl/min) and assayed at 15-min intervals. The glutamatergic agonist, kainic acid (KA, 5 mg/kg, intraarterially), was used to stimulate nitric oxide production. NG-nitro L-arginine methyl ester (L-NAME, 40 mg/kg, intravenously) was used to inhibit nitric oxide synthase. Results Unstimulated cerebellar nitric oxide concentrations were stable and greater during anesthesia with isoflurane (532+/-31 nM; mean +/- SEM) than with halothane (303+/-23 nM). L-NAME pretreatment reduced nitric oxide concentrations during isoflurane, but not halothane, anesthesia. Infusion of KA increased nitric oxide in both groups; however, the increase in nitric oxide was significantly greater during isoflurane anesthesia. Pretreatment with L-NAME inhibited the response to KA in both groups. Conclusions Nitric oxide production in the cerebellum, monitored by microdialysis, was greater during isoflurane anesthesia than during halothane anesthesia. Increased nitric oxide production during isoflurane anesthesia would be expected to impact central neuronal function and cerebral blood flow and vascular resistance.

The contributions of the vasodilators nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) were investigated in the rat cremaster muscle microcirculation during halothane, isoflurane, or ketamine anesthesia. After inhibition of prostaglandin synthesis with indomethacin, changes in diameter of fourth-order arterioles to acetylcholine (ACh) or bradykinin (BK) were studied in the presence or absence of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthase, and/or 20 mM K+, an inhibitor of EDHF action. L-NMMA inhibited ACh- and BK-induced vasodilation during isoflurane but not halothane or ketamine anesthesia. Superfusion of the muscle with buffer containing 20 mM K+ dilated arterioles. EDHF was responsible for most of the NO-independent response to ACh, because 20 mM K+ unmasked ACh-stimulated, NO-dependent relaxation during halothane or ketamine anesthesia. However, 20 mM K+ did not inhibit BK-induced vasodilation during halothane or ketamine anesthesia. Our data suggest that anesthetics can alter the balance between NO and EDHF vasodilation in the microcirculation and that NO-dependent mechanisms are enhanced and EDHF action inhibited during isoflurane anesthesia.

BACKGROUND:Halothane and isoflurane alter signal transduction and function in several cell types. Vascular responses to these anesthetics may be attributable to agent-specific effects on vasoactive mediator production. This study investigated the effects of halothane and isoflurane on basal and agonist-stimulated prostacyclin production by endothelial cells. METHODS:Prostacyclin production by cultured bovine aortic endothelial cells was monitored by radioimmunoassay of 6-keto-prostaglandin F1 alpha, the stable breakdown product of prostacyclin. RESULTS:Neither halothane nor isoflurane (0.3-1 mM), altered prostacyclin production. Bradykinin (1 microM), adenosine triphosphate (ATP) (10 microM), and melittin (1 microgram.ml-1) stimulated prostacyclin production. Isoflurane had no effect on responses to bradykinin, ATP, or melittin. Halothane inhibited the response to bradykinin but not the response to ATP or melittin. Pretreatment with pertussis toxin (100 ng.ml-1), to inhibit the function of the guanosine triphosphate-binding protein G alpha i, did not alter the response to bradykinin in the presence or absence of halothane. Pretreatment with phorbol 12-myristate 13-acetate (100 nM), to stimulate protein kinase C activity, did not alter bradykinin-stimulated prostacyclin production and prevented the inhibition of the response to bradykinin by halothane. CONCLUSIONS:Isoflurane had no effect on the increase in prostacyclin production stimulated by bradykinin. Halothane inhibited the bradykinin-stimulated prostacyclin production but not that stimulated by ATP or melittin. These results suggest that the halothane-mediated inhibition of bradykinin-stimulated prostacyclin production does not involve a pertussis toxin-sensitive G-protein and may result from an interaction of halothane at some other step in the signal transduction pathway, including the inhibition of protein kinase C.

The systemic and regional hemodynamic effects of inhibition of endothelium-derived relaxing factor/ nitric oxide (EDRF/NO) were studied in awake, indo-methacin-treated rats. The radiolabeled microsphere method was used to determine the cardiac output, systemic vascular resistance (SVR), and regional blood flows and regional vascular resistances in 12 tissues before and after infusion of the EDRF/NO synthesis inhibitor, NG-monomethyl-L-arginine (NMMA, 100 mg/kg), and after reversal of NMMA by infusion of L-arginine (300 mg/kg). NMMA infusion resulted in increases in the blood pressure and SVR. After NMMA, blood flows were decreased to the cerebrum, heart, kidney, spleen, gastrointestinal tract, skin, ear, and white fat, whereas flow in the hepatic artery was increased. Vascular resistances were increased in every tissue studied except the hepatic artery, in which the resistance decreased after NMMA. L-arginine restored the vascular resistance to control values in 8 of the 12 tissues. The magnitude of the increase in the regional resistance was not uniform among the organs studied, and ranged from a maximum of 253% in brown fat to 22% in heart. These results indicate that EDRF/NO is an important mediator of regional hemodynamic control in numerous tissues of the intact rat. The marked heterogeneity in the magnitude of basal EDRF/ NO-dependent tone suggests that the mechanisms mediating this cardiovascular control system are regulated locally.

Halothane and isoflurane have different effects on the peripheral vasculature. Halothane decreases blood pressure primarily by decreasing cardiac contractility, whereas isoflurane acts primarily as a peripheral vasodilator. These peripheral vascular actions may result from different effects of the anesthetics on endothelial cell function and the release of endothelium-derived vasoactive mediators. The ability of these agents at clinically relevant concentrations to alter agonist-induced calcium mobilization in single cultured bovine aortic endothelial cells was tested using the fluorescent indicator fura-2. Neither halothane (0.3, 0.5, and 2 mM) or isoflurane (0.5 and 2 mM) altered basal calcium ([Ca]i = 49 +/- 5 nM); however, the calcium transient normally elicited by 10 nM bradykinin (peak [Ca]i = 307 +/- 22 nM) was inhibited significantly by halothane but not isoflurane. Neither anesthetic altered the calcium response to ATP (10 microM). These findings suggest that anesthetics may have specific effects on receptor-mediated endothelial cell functions that could influence hemodynamics.

The effects of the endothelium-derived relaxing factor (EDRF) inhibitors NG-monomethyl-L-arginine (L-NMMA) and methylene blue (MB) on resting hemodynamics and responses to vasodilators were studied in the intact rat anesthetized with pentobarbital sodium. L-NMMA infusions (100 mg/kg) significantly increased mean blood pressure by 48%; this effect was rapidly reversed by L-arginine (300 mg/kg). MB (50 mg/kg) decreased mean blood pressure by 24%. Both MB and L-NMMA significantly attenuated the vasodepressor responses to acetylcholine, ATP, and adenosine. By use of radiolabeled microspheres, it was determined that the blood pressure increase after L-NMMA was due to a marked increase in systemic vascular resistance (SVR; from 1.3 +/- 0.1 to 3.1 +/- 0.3 mmHg.ml-1.min-1) and decreased cardiac output. L-NMMA increased vascular resistance in brain, cerebellum, skin, skeletal muscle, ear, white and brown fat, kidney, spleen, hepatic artery, and gastrointestinal tract. Flow decreased in the skin, kidneys, ear, white and brown fat, gastrointestinal tract, portal venous circulation, and liver in response to L-NMMA. In contrast, MB decreased heart rate, blood pressure, and SVR significantly. MB increased blood flow and decreased vascular resistance in several organs, including the brain, and skeletal muscle. These results indicate that both MB and L-NMMA can inhibit agonist-induced EDRF-mediated vasodepressor responses. However, inhibition of agonist-induced responses did not predict the general and regional hemodynamic responses to L-NMMA or MB infusion.

Hypoxemia during anesthesia can cause severe morbidity and mortality. To determine how the volatile anesthetics alter the normal hemodynamic compensation for hypoxemia, we investigated the effects of various anesthetics on regional blood flows during normoxemia and during normocapnic hypoxemia (FIO2 0.12 for 20 min) in rats. Using the radioactive microsphere method, organ blood flows were determined in animals anesthetized with 1 MAC of halothane, enflurane, or isoflurane and in awake animals. Brain blood flow increased significantly with hypoxemia in awake animals. However, brain blood flow decreased in all anesthetized animals that were hypoxemic. Coronary blood flow also increased significantly with hypoxemia in awake animals. In the presence of volatile anesthetics, coronary blood flow decreased, a decrease that was unchanged with hypoxemia. Thus, there was a large difference in brain and coronary blood flows between awake hypoxemic and anesthetized hypoxemic animals. Hypoxemia did not alter the magnitude of renal, gastrointestinal tract, or total hepatic blood flows in awake animals. However, all three blood flows decreased significantly in anesthetized hypoxemic animals. We conclude that volatile anesthetics modify the compensatory responses to hypoxemia that occur in awake animals, resulting in decreased blood flow to vital organs.

The authors compared the hemodynamic effects of isoflurane anesthesia in normotensive (WKY) and genetically hypertensive (SHR) rats. Eighteen male SHR and 18 WKY rats were subdivided into conscious animals and those anesthetized with isoflurane, 1.2 vol% inspired. During brief isoflurane anesthesia, cannulae were placed in the left cardiac ventricle, the femoral artery, and the femoral vein. Central and regional hemodynamics were determined with 85Sr-labeled microspheres (15 +/- 1 micron) using the reference sample technique in both conscious and anesthetized animals. Isoflurane anesthesia caused similar reductions in mean arterial blood pressure (MAP) in all rats. This was due to a significant decrease in systemic vascular resistance in WKY rats, whereas MAP declined due to a significant decrease in cardiac output in SHR rats. In the anesthetized WKY rat, the decrease in total systemic vascular resistance resulted from significant decreases in vascular resistance of the brain and nonrespiratory skeletal muscles. In the anesthetized SHR rat, both decreases (cerebellum, hepatic artery) and increases (GI tract, skin, diaphragm) in regional vascular resistances occurred, resulting in no net change in total systemic vascular resistance. In both SHR and WKY rats, isoflurane redistributed blood flow in favor of the brain at the expense of blood flow to the GI tract, diaphragm, and skin. Blood flows to the liver, GI tract, and skin were significantly less in the anesthetized SHR as compared with WKY rats. It is concluded that isoflurane influences central and regional hemodynamics differently in hypertensive, as compared with normotensive, rats.

The dose-dependent actions of bupivacaine on the microvasculature were evaluated by television microscopy in an in vivo rat cremaster muscle preparation. Animals were anesthetized with chloralose and urethane. Mean arterial pressure was measured via a carotid artery cannula; heart rate was calculated from the phasic pressure trace. The cremaster muscle was suffused with a balanced electrolyte solution that was controlled for temperature, pH, PO2, PCO2, and osmolarity to provide a physiologic environment. Internal diameters of fourth-order arterioles were measured with an electronic vernier displayed on the video monitor. Arteriolar diameters were measured every 30 s during a 10-min control period, a 10-min period of topical application of bupivacaine hydrochloride, and a 30-min recovery period. Bupivacaine 10(-1), 10(0), 10(1), and 10(2) micrograms X ml-1 produced progressive vasoconstriction to 82.7 +/- 2.9%, 75.0 +/- 5.6%, 71.0 +/- 7.0%, and 65.7 +/- 9.4% of control (P less than 0.05 for each), respectively. Bupivacaine, 10(3) and 2.5 X 10(3) micrograms X ml-1, did not alter arteriolar diameters significantly, although there was a tendency for vasodilation. In a second group of animals, arteriolar diameters were measured during intravenous bupivacaine infusion that produced stable plasma concentrations of 2.3 +/- 0.2 micrograms X ml-1. Vasoconstriction of 91.4 +/- 2.2%, of control (P less than 0.01) was observed. These results demonstrate that dose-dependent arteriolar constriction occurs even with blood bupivacaine levels that are at the upper limits of those expected to occur during regional anesthesia.