It remains unclear if the human coronary vasculature is inherently sensitive to changes in arterial Po2 and Pco2 or if coronary vascular responses are the result of concomitant increases in myocardial O2 consumption/demand ([Formula: see text]). We hypothesized that the coronary vascular response to Po2 and Pco2 would be attenuated in healthy men when [Formula: see text] was attenuated with β1-adrenergic receptor blockade. Healthy men (age: 25 ± 1 yr, n = 11) received intravenous esmolol (β1-adrenergic receptor antagonist) or volume-matched saline in a double-blind, randomized crossover study and were exposed to poikilocapnic hypoxia, isocapnic hypoxia, and hypercapnic hypoxia. Measurements made at baseline and after 5 min of steady state at each gas manipulation included left anterior descending coronary blood velocity (LADV; Doppler echocardiography), heart rate, and arterial blood pressure. LADV values at the end of each hypoxic condition were compared between esmolol and placebo. The rate-pressure product (RPP) and left ventricular mechanical energy (MELV) were calculated as indexes of [Formula: see text]. All gas manipulations augmented RPP, MELV, and LADV, but only RPP and MELV were attenuated (4-18%) after β1-adrenergic receptor blockade ( P < 0.05). Despite attenuated RPP and MELV responses, β1-adrenergic receptor blockade did not attenuate the mean LADV vasodilatory response compared with placebo during poikilocapnic hypoxia (29.4 ± 2.2 vs. 27.3 ± 1.6 cm/s) and isocapnic hypoxia (29.5 ± 1.5 vs. 30.3 ± 2.2 cm/s). Hypercapnic hypoxia elicited a feedforward coronary dilation that was blocked by β1-adrenergic receptor blockade. These results indicate a direct influence of arterial Po2 on coronary vascular regulation that is independent of [Formula: see text]. NEW & NOTEWORTHY In humans, arterial hypoxemia led to an increase in epicardial coronary artery blood velocity. β1-Adrenergic receptor blockade did not diminish the hypoxemic coronary response despite reduced myocardial O2 demand. These data indicate hypoxemia can regulate coronary blood flow independent of myocardial O2 consumption. A plateau in the mean left anterior descending coronary artery blood velocity-rate-pressure product relationship suggested β1-adrenergic receptor-mediated, feedforward epicardial coronary artery dilation. In addition, we observed a synergistic effect of Po2 and Pco2 during hypercapnic hypoxia.

A control system model was developed to analyze data on in vivo coronary blood flow regulation and to probe how different mechanisms work together to control coronary flow from rest to exercise, and under a variety of experimental conditions, including cardiac pacing and with changes in coronary arterial pressure (autoregulation). In the model coronary flow is determined by the combined action of a feedback pathway signal that is determined by the level of plasma ATP in coronary venous blood, an adrenergic open-loop (feed-forward) signal that increases with exercise, and a contribution of pressure-mediated myogenic control. The model was identified based on data from exercise experiments where myocardial oxygen extraction, coronary flow, cardiac interstitial norepinephrine concentration, and arterial and coronary venous plasma ATP concentrations were measured during control and during adrenergic and purinergic receptor blockade conditions. The identified model was used to quantify the relative contributions of open-loop and feedback pathways and to illustrate the degree of redundancy in the control of coronary flow. The results indicate that the adrenergic open-loop control component is responsible for most of the increase in coronary blood flow that occurs during high levels of exercise. However, the adenine nucleotide-mediated metabolic feedback control component is essential. The model was evaluated by predicting coronary flow in cardiac pacing and autoregulation experiments with reasonable fits to the data. The analysis shows that a model in which coronary venous plasma adenine nucleotides are a signal in local metabolic feedback control of coronary flow is consistent with the available data.

The Journal of PhysiologyVolume 591, Issue 23 p. 5795-5797 CROSSTALKFree Access CrossTalk opposing view: Guyton's venous return curves should not be taught Daniel A. Beard, Daniel A. Beard Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USASearch for more papers by this authorEric O. Feigl, Eric O. Feigl Department of Physiology and Biophysics, University of Washington, Seattle, WA, USASearch for more papers by this author Daniel A. Beard, Daniel A. Beard Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USASearch for more papers by this authorEric O. Feigl, Eric O. Feigl Department of Physiology and Biophysics, University of Washington, Seattle, WA, USASearch for more papers by this author First published: 29 November 2013 https://doi.org/10.1113/jphysiol.2013.260034Citations: 16 Email: beardda@gmail.com AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat [ Daniel Beard is a Professor in the Department of Molecular and Integrative Physiology and School of Medicine Collegiate Professor at the University of Michigan. He received a B.S. in Biomedical Engineering from Boston University in 1993 and an M.S. in Applied Mathematics and Ph.D. in Bioengineering from the University of Washington in 1997. Dr Beard is the Director of the Virtual Physiological Rat (VPR) project, an NIH National Center for Systems Biology, working to analyse, interpret, simulate, and ultimately predict physiological function in health and disease. Eric Feigl MD graduated from the University of Minnesota and had postdoctoral training in Sweden and NIH. He is a Professor of Physiology in the Medical School of the University of Washington and his honours include The Katz Prize from the American Heart Association, the Outstanding Research Award of the International Society for Heart Research, the Wiggers Award and the Berne Award of the American Physiological Society. Dr Feigl has taught cardiovascular medicine to medical students for several decades without using the confusing Guyton model.] The thesis of this essay is that Guyton's model of the systemic circulation has been so misinterpreted by Guyton and others that it generates more confusion than insight. Because of the capacitance of arteries and veins, blood pressure will attain a finite value if blood flow is quickly stopped without a change in vascular tone. This static pressure is called the 'mean systemic pressure'PMS. Guyton devised an experimental preparation in open chest dogs where an external artificial pump drew blood from the right atrium and delivered it to the pulmonary artery. The output flow rate of the pump was manually adjusted by raising and lowering a collapsible tube (Starling resistor) at the inlet (right atrial) side of the pump. In this way the steady state cardiac output flow of the left ventricle was set as the independent experimental variable. In other words, the artificial pump opened the circuit of the circulation and haemodynamically isolated the right atrium from the heart so that the systemic circulation could be studied in quasi-isolation. As will be explained, the confusion over Guyton's results and associated model can be traced to the mistaken interpretation of the measured right atrial pressure as an independent causal variable governing the behaviour of the system. Using the preparation with a Starling resistor, Guyton made steady state measurements of right atrial pressure at different pump output levels, including mean systemic pressure when pump output was zero (Guyton, 1955, 1959). An example of Guyton's original data is shown in Fig. 1A. Remember that these are steady state measurements where venous return equals cardiac output by definition. Figure 1Open in figure viewerPowerPoint A, data from Guyton et al. (1959) are plotted showing the steady state relation between flow (F= cardiac output = venous return) and right atrial pressure (PRA) measured when flow was altered by limiting the inflow to an artificial pump with a collapsible tube. B, the data from A are replotted with flow plotted on the abscissa correctly indicating that cardiac output is the independent variable. The intercept on the ordinate is the right atrial pressure (equal to the mean systemic pressure) when flow (cardiac output) is zero. C, the calculated arterial pressure according to Guyton's model, with constant systemic vascular resistance, when flow is varied over the range defined in A and B. See Beard & Feigl (2011) for details. Figure reproduced, with permission, from the American Journal of Physiology Heart – Heart and Circulatory Physiology. Guyton's first misinterpretation was to assume that the independent (causal) variable in his experiment was the right atrial pressure because it was determined by manually adjusting the elevation of the Starling resistor. However, adjusting the height of the collapsible tube was just a means of changing the pump output, which was the true independent variable. This point is verified by the observations of Grodins et al. (1960) and Levy (1979), who used the same external pump right heart bypass preparation without a Starling resistor so the pump just connected the right atrium to the pulmonary artery. Grodins et al. and Levy obtained very similar results to Guyton when the pump rate was manually dialled up or down, thus eliminating confusion about the independent variable; clearly it is the pump (cardiac) output. Guyton's second major misconception was the assertion that the driving pressure for steady state cardiac output ('venous return') is mean systemic pressure minus right atrial pressure (PMS–PRA). This interpretation has no physical or physiological basis. The static mean systemic pressure only exists when cardiac output flow is zero. When there is flow, the mean systemic pressure exists only as an abstraction of a mathematical model. It is possible to define a resistance, RVR, relating the flow (F) and the mean systemic pressure minus right atrial pressure. This is Guyton's famous venous return equation. (1) R VR is an equivalent resistance (Guyton's 'resistance to venous return') that is simply a consequence of this equation, and does not represent an actual physical structure or mechanism. It is misleading to think of PMS as a driving force for flow through a resistor. PMS is solely a function of the magnitude of blood volume and systemic vascular capacitance. In steady state, PMS is not a function of flow, and flow is not a function of PMS. However, right atrial pressure is a function of flow in the Guyton model as emphasized by Beard & Feigl (2011). Another way of stating it is: the mean systemic pressure (only manifest when flow is zero) is the transmural pressure distending the vasculature, but this pressure cannot generate flow along the vascular tree because pressure is equal everywhere in the systemic circulation when flow is zero. A third misinterpretation, not made by Guyton but by those who followed him using his model, is that vascular capacitance provides a driving force for venous return. A passive elastic element can store pressure energy and smooth pulsations from beat to beat but cannot generate pressure to overcome resistive losses during steady state (see Discussion in Beard & Feigl (2011)). These major misconceptions are promoted by how the relationship between flow and right atrial pressure is presented in textbooks. Guyton's model, (usually in graphic form) has been used to explain the relation between cardiac output (venous return) and right atrial pressure as shown in Fig. 1A. This is misleading because right atrial pressure is graphed on the x-axis abscissa implying that right atrial pressure is the independent, causative variable. In Guyton's model right atrial pressure is determined by the flow through the systemic circulation. Therefore the graph shown in Fig. 1B where cardiac output is the independent variable plotted on the x-axis is less misleading. The flow determines the right atrial pressure. The right atrial pressure does not represent a 'back pressure' limiting venous return. Why does the Guyton model predict a decrease in right atrial pressure as cardiac output increases? This is because the model assumes a constant total systemic vascular resistance, and thus predicts that the arterial pressure increases as cardiac output increases as shown in Fig. 1C. An increase in arterial pressure results in increased filling of the arterial capacitance in the model. Since systemic blood volume is constant in the model definition, volume is shifted from the venous capacitance to the arterial capacitance in the model. The decreased filling of the venous capacitance is reflected in the diminished venous pressure and right atrial pressure as cardiac output is augmented. However, what actually happens in the circulation is more complicated than indicated by the simple constant-coefficient lumped Guyton model. What really occurs during physiological adjustments is that vascular resistance changes (reflexes, local metabolic control, etc.) and the distribution of cardiac output changes to the various organs in parallel (skeletal muscle, skin, kidney, gut, etc.), each with its own separate resistance and capacitance. The three misinterpretations described above have potential clinical consequences. For example, in a condition when right atrial pressure is low a clinician might withhold giving needed fluids under the misguided thought that increasing right atrial pressure will decrease the gradient for 'venous return' (PMS–PRA) and cardiac output. Furthermore, the simple series Guyton model with only single arterial and venous compartments and a fixed total vascular resistance is not sufficient to describe what happens in exercise, fever, postural changes, shock, heart failure, etc. Numerous critical analyses, discussions, and letters to the editor have exposed the misinterpretations of Guyton's venous return curves: Grodins et al. (1960), Levy (1979), Brengelmann (2003), Brengelmann (2006), Beard & Feigl (2011). Nevertheless, confused interpretations of the Guyton model continue to be presented in textbooks that bewilder students and mislead clinicians. The Guyton model and, critically, Guyton's misinterpretation of the model generates more confusion than clarity and should be discarded. Sometimes it is possible to get a correct answer for the wrong reasons. However, the hazard is that one is likely to get other answers that are incorrect if one starts with a faulty premise. Guyton's idea, that venous return (equal to cardiac output) is determined by the pressure difference between mean systemic pressure and right atrial pressure (PMS–PRA) is physically and physiologically wrong. This idea should not be taught because it results in confused thinking about the cardiovascular system. Call for comments Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief comment. Comments may be posted up to 6 weeks after publication of the article, at which point the discussion will close and authors will be invited to submit a 'final word'. To submit a comment, go to http://jp.physoc.org/letters/submit/jphysiol;591/23/5795 Supporting Information Disclaimer: Supplementary materials have been peer-reviewed but not copyedited. Filename Description tjp5914-sup-0001-Comments.pdf61 KB Comments. tjp5914-sup-0002-Andrew.pdf49.1 KB Last words by Andrew. tjp5914-sup-0003-BeardandFeigl.pdf49.2 KB Last words by Beard and Feigl. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. References Beard DA & Feigl EO (2011). Understanding Guyton's venous return curves. Am J Physiol Heart Circ Physiol 301, H629– H633. Brengelmann GL (2003). A critical analysis of the view that right atrial pressure determines venous return. J Appl Physiol 94, 849– 859. Brengelmann GL (2006). Counterpoint: the classical Guyton view that mean systemic pressure, right atrial pressure, and venous resistance govern venous return is not correct. J Appl Physiol 101, 1525– 1526 (Discussion pp. 1526–1527). Grodins FS, Stuart WH & Veenstra RL (1960). Performance characteristics of the right heart bypass preparation. Am J Physiol 198, 552– 560. Guyton AC (1955). Determination of cardiac output by equating venous return curves with cardiac response curves. Physiol Rev 35, 123– 129. Guyton AC, Abernathy B, Langston JB, Kaufmann BN & Fairchild HM (1959). Relative importance of venous and arterial resistances in controlling venous return and cardiac output. Am J Physiol 196, 1008– 1014. Levy MN (1979). The cardiac and vascular factors that determine systemic blood flow. Circ Res 44, 739– 747. Citing Literature Volume591, Issue23December 2013Pages 5795-5797 FiguresReferencesRelatedInformation

Letters to the EditorLetter to the editor: “A return to the venous return controversy: a visual aid for combatants”James R. MunisJames R. MunisDepartments of Anesthesiology, Physiology and Biomedical Engineering, Mayo Clinic, Rochester, MinnesotaPublished Online:01 Feb 2013https://doi.org/10.1152/ajpheart.00762.2012MoreSectionsPDF (174 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat to the editor: I have followed with interest the controversy surrounding the determinants of venous return, specifically the latest contribution by Beard and Feigl (1) published in the American Journal of Physiology-Heart and Circulatory Physiology (1–5). Since this debate has continued for several years without apparent resolution, I would like to offer the following suggestions for a common ground visual aid that may be of use in clarifying some of the points of difference. As a starting point, since the circulation is a complete closed loop, it would be helpful for any physical or conceptual model of the systemic circulation to reflect this anatomic reality. Such a visual aid and conceptual model might help to avoid errors in interpretation that can arise from analysis of only one segment of the circulation in isolation from the dynamics of a closed loop.In addition, it would be helpful to remember how mean systemic pressure (PMS), the supposed upstream pressure driving venous return in the classic Guyton view, was defined in the first place. In 1897, Ernest Starling pointed out that, “Somewhere in the circulation there must be a point where the pressure is neither raised nor lowered and where, therefore, the pressure is independent of cardiac activity” (7). By definition, neither the pressure nor, therefore, the vascular wall tension changes as a function of cardiac activity at that point. Consequently, in the simple model depicted below (Fig. 1), neither volume nor pressure change at the point of PMS, which occurs in the peripheral venous compartment.Fig. 1.A simplified 3 compartment model of the systemic circulation. AC, arterial compartment with low capacitance or compliance (CA); PA, arterial pressure; PVC, peripheral venous compartment with a greater capacitance or compliance (CV); CVC, central venous compartment with a compliance that can accommodate changing central blood volumes. The AC and PVC could just as well be drawn as closed, elastic elements with differential compliances. PMS, mean systemic (circulatory arrest) pressure; PRA, right atrial pressure; VR, venous return; Rv, resistance to venous return (a more useful mathematical than anatomical concept, not depicted in the figure but included in the standard equation for venous return, Eq. 2); PATM, atmospheric pressure; V, blood volume; VO, unstressed blood volume; SVR, systemic vascular resistance; CO, cardiac output; Ra, right atrium.Download figureDownload PowerPointIt may be helpful in establishing new parameters for this debate if those who interpret right atrial pressure (PRA) as a “back pressure” impeding venous return and who posit the gradient between PMS and PRA as a driving force that determines venous return would comment on the following two observations:First, by definition, only one of the two boundaries (PRA) of that pressure gradient changes with cardiac activity and vascular flow. PMS does not change in an isovolemic situation. PRA cannot decrease until the heart moves blood from the central venous compartment (CVC) to the arterial compartment (AC). This is consistent with Levy's observation in a right heart bypass model that central venous pressure falls as a result of an increase in the right heart pump flow setting and is not the cause of an increased flow rate (4). Venous return is only enabled by the action of the heart, not by the static elastic properties of the venous compartment. The former correlates with flow; the latter does not, since it remains unchanged with or without flow. To use an even more visual analogy, if it were possible to facilitate steady-state flow with an isovolemic and isometric elastic element, then oil pipelines could achieve increased flow rates by the simple inclusion of a static elastic element under stretch somewhere upstream in the pipeline. A “restorative” function of the heart or another pump operating on the upstream volume source (to use Magder's terminology, Ref. 5) does not explain away that analogy. An isometric elastic element involves no energy exchange and can move nothing while remaining isometric. Similarly, the locus of PMS in the veins remains isometric during steady-state flow and is constrained from moving blood.Second, since our observational study in 2001 (6), there have been 25 other studies confirming that the gradient between peripheral venous pressure (close to, if not identical with, PMS) and PRA are essentially unchanged within the same patient under widely varying clinical conditions and flow states. The model depicted here accommodates those observations, but a model where the PMS-PRA gradient changes and determines venous return does not. The model below predicts, in an isovolemic state, a fall in PRA with an increase in cardiac activity but an essentially unchanged peripheral venous compartment volume and peripheral venous pressure, corresponding to an unchanged PMS. During the transition between circulatory arrest and circulatory function, the heart moves blood from the CVC to the AC. By definition, at circulatory arrest, PMS = PRA. As blood is evacuated from the CVC by the heart, PRA falls below PMS and a gradient develops. In fact, PRA is reasonably considered as a local distortion of PMS, caused by the activity of the heart. According to the large body of clinical data available, where PRA (equivalent to central venous pressure, or CVP) and PMS (peripheral venous pressure) have been simultaneously measured, there appears to be a threshold effect where increasing cardiac activity is coupled to an increase in cardiac output and venous return but where neither PMS nor PRA changes unless blood volume changes. In accordance with clinical observations, the visual aid presented here predicts that both PMS and PRA will respond to volume changes while also accommodating such a threshold effect in the PMS-PRA difference.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the author.AUTHOR CONTRIBUTIONSJ.R.M. prepared figure; drafted, edited, and revised manuscript; and approved final version of manuscript.REFERENCES1. Beard DA , Feigl EO. Understanding Guyton's venous return curves. Am J Physiol Heart Circ Physiol 301: H629–H633, 2011.Link | ISI | Google Scholar2. Brengelmann GL. A critical analysis of the view that right atrial pressure determines venous return. J Appl Physiol 94: 849–859, 2003.Link | ISI | Google Scholar3. Guyton AC. Determination of cardiac output by equating venous return curves with cardiac response curves. Physiol Rev 35: 123–129, 1955.Link | ISI | Google Scholar4. Levy MN. The cardiac and vascular factors that determine systemic blood flow. Circ Res 44: 739–747, 1979.Crossref | PubMed | ISI | Google Scholar5. Magder S , Brengelmann GL. Point:Counterpoint: The classical Guyton view that mean systemic pressure, right atrial pressure, and venous resistance govern venous return is/is not correct. J Appl Physiol 101: 1523–1527, 2006.Link | ISI | Google Scholar6. Munis JR , Bhatia S , Lozada LJ. Peripheral venous pressure as a hemodynamic variable in neurosurgical patients. Anesth Analg 92: 172–179, 2001.Crossref | PubMed | ISI | Google Scholar7. Starling EH. The Arris and Gale Lectures on some points in the pathology of heart disease; Lecture II. The effects of heart failure on the circulation. Lancet 149: 652–655, 1897.Crossref | Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: J. R. Munis, Depts. of Anesthesiology, Physiology and Biomedical Engineering, Mayo Clinic Rochester, 200 First St. SW, Rochester, MN, 55905 (e-mail: munis.[email protected]edu). Download PDF Previous Back to Top Next FiguresReferencesRelatedInformationRelated ArticlesReply to “Letter to the editor: A return to the venous return controversy: a visual aid for combatants'” 01 Feb 2013American Journal of Physiology-Heart and Circulatory Physiology More from this issue > Volume 304Issue 3February 2013Pages H487-H488 Copyright & PermissionsCopyright © 2013 the American Physiological Societyhttps://doi.org/10.1152/ajpheart.00762.2012PubMed23377150History Published online 1 February 2013 Published in print 1 February 2013 Metrics

Dr. Andrew's proposal (Andrew, 2013) to rename Guyton's venous return curve the ‘venous pressure curve’ seems unlikely to overcome the decades of misinterpretation and confusion that the Guyton model of the systemic circulation has engendered.

Based on observations that as cardiac output (as determined by an artificial pump) was experimentally increased the right atrial pressure decreased, Arthur Guyton and coworkers proposed an interpretation that right atrial pressure represents a back pressure restricting venous return (equal to cardiac output in steady state). The idea that right atrial pressure is a back pressure limiting cardiac output and the associated idea that “venous recoil” does work to produce flow have confused physiologists and clinicians for decades because Guyton's interpretation interchanges independent and dependent variables. Here Guyton's model and data are reanalyzed to clarify the role of arterial and right atrial pressures and cardiac output and to clearly delineate that cardiac output is the independent (causal) variable in the experiments. Guyton's original mathematical model is used with his data to show that a simultaneous increase in arterial pressure and decrease in right atrial pressure with increasing cardiac output is due to a blood volume shift into the systemic arterial circulation from the systemic venous circulation. This is because Guyton's model assumes a constant blood volume in the systemic circulation. The increase in right atrial pressure observed when cardiac output decreases in a closed circulation with constant resistance and capacitance is due to the redistribution of blood volume and not because right atrial pressure limits venous return. Because Guyton's venous return curves have generated much confusion and little clarity, we suggest that the concept and previous interpretations of venous return be removed from educational materials.

The adenine nucleotide hypothesis postulates that the ATP released from red blood cells is broken down to ADP and AMP in coronary capillaries and that ATP, ADP, and AMP act on purinergic receptors on the surface of capillary endothelial cells. Purinergic receptor activation initiates a retrograde conducted vasodilator signal to the upstream arteriole that controls coronary blood flow in a negative feedback manner. A previous study (M. Farias 3rd, M. W. Gorman, M. V. Savage, and E. O. Feigl, Am J Physiol Heart Circ Physiol 288: H1586-H1590, 2005) demonstrated that coronary venous plasma ATP concentration increased during exercise and correlated with coronary blood flow. The present experiments test the adenine nucleotide hypothesis by examining the balance between oxygen delivery (via coronary blood flow) and myocardial oxygen consumption during exercise before and after purinergic receptor blockade. Dogs (n = 7) were chronically instrumented with catheters in the aorta and coronary sinus and a flow transducer around the circumflex coronary artery. During control treadmill exercise, myocardial oxygen consumption increased and the balance between oxygen delivery and myocardial oxygen consumption fell as indicated by a declining coronary venous oxygen tension. Blockade of P1 and P2Y(1) purinergic receptors combined with inhibition of nitric oxide synthesis significantly decreased the balance between oxygen delivery and myocardial oxygen consumption compared with control. The results support the hypothesis that ATP and its breakdown products ADP and AMP are part of a negative feedback control mechanism that matches coronary blood flow to myocardial oxygen consumption at rest and during exercise.

A method to simultaneously measure oxygenation in vascular, intracellular, and mitochondrial spaces from optical spectra acquired from muscle has been developed. In order to validate the method, optical spectra in the visible and near-infrared regions (600–850 nm) were acquired from solutions of myoglobin, hemoglobin, and cytochrome oxidase that included Intralipid as a light scatterer. Spectra were also acquired from the rabbit forelimb. Three partial least squares (PLS) analyses were performed on second-derivative spectra, each separately calibrated to myoglobin oxygen saturation, hemoglobin oxygen saturation, or cytochrome aa3 oxidation. The three variables were measured from in vitro and in vivo spectra that contained all three chromophores. In the in vitro studies, measured values of myoglobin saturation, hemoglobin saturation, and cytochrome aa3 oxidation had standard errors of 5.9%, 7.4%, and 12.2%, respectively, with little cross-talk between the in vitro measurements. In the progression from normal oxygenation to ischemia in the rabbit forelimb, hemoglobin desaturated first, followed by myoglobin, while cytochrome aa3 reduction occurred last. The ability to simultaneously measure oxygenations in the vascular, intracellular, and mitochondrial compartments will be valuable in physiological studies of muscle metabolism and in clinical studies when oxygen supply or utilization are compromised.

Fibroblast growth factor-2 (FGF2, bFGF) has been proposed to regulate wound healing and angiogenesis, but skin wound healing in FGF2-knockout (FGF2-KO) animals is only slightly delayed. To determine the role of FGF2 in myocardial infarct repair, we studied the evolution of left ventricular geometry, cell proliferation, matrix content, and cardiac function in mice lacking or overexpressing (FGF2-Tg) FGF2. Despite having no effect on initial infarct size, deletion of FGF2 resulted in reduced fibroblast proliferation and interstitial collagen deposition, decreased endothelial proliferation and vascular density, and decreased cardiomyocyte hypertrophy. Furthermore, FGF2-KO mice demonstrated a complete absence of scar contraction, resulting in increased final infarct size and marked increases in chamber size and infarct expansion. These deficits ultimately impaired left ventricular dP/dt compared with wild-type infarcted mice. Conversely, overexpression of FGF2 increased fibroblast proliferation and collagen deposition, accelerated endothelial proliferation, and enhanced cardiomyocyte hypertrophy after infarction. These changes curbed infarct expansion and preserved left ventricular function. Thus, FGF2 is an important regulator of cell proliferation, angiogenesis, collagen synthesis, myocyte hypertrophy, scar contraction, and, ultimately, left ventricular contractile function during infarct repair. FGF2 may be more important in healing of infarcts compared with skin wounds because of the mechanical stress under which infarcts heal.

BACKGROUND:Human plasma ATP concentration is reported in many studies as roughly 1000 nmol/L. The present study tested the hypothesis that the measured plasma ATP concentration is lower if ATP release from formed blood elements is inhibited during blood sample processing. A second hypothesis was that pretreatment with aspirin to inhibit platelets would reduce the measured plasma concentration of ATP.METHODS:Blood was sampled from the antecubital vein in 20 healthy individuals 30 and 60 min after ingestion of aspirin (325 mg) or placebo. Aliquots of each blood sample were added to the usual EDTA/saline solution to inhibit ATP catabolism, or to a new stabilizing solution designed to both stop ATP catabolism and inhibit ATP release from blood elements. The stabilizing solution contained NaCl, EDTA, tricine buffer, KCl, nitrobenzylthioinosine, forskolin, and isobutylmethylxanthine. Plasma ATP was measured with the luciferin-luciferase assay with standard additions in each sample to determine ATP content. Hemoglobin concentration was used as an index of sample hemolysis, and the plasma ATP concentration was corrected for the hemolysis component.RESULTS:Aspirin pretreatment had no effect on plasma ATP concentrations. However, use of the stabilizing solution resulted in mean (SD) ATP concentrations 8-fold lower than the use of EDTA alone [28 (16) vs 236 (201) nmol/L; P <0.001].CONCLUSION:When precautions are taken to inhibit ATP release from blood elements during sample preparation, human venous plasma ATP concentration is much lower than previously reported.

This essay looks at the historical significance of an APS classic paper that is freely available online: Berne RM. Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow. Am J Physiol 204: 317–322, 1963 ( http://ajplegacy.physiology.org/cgi/reprint/204/2/317 ).

Previous work from this laboratory using near-infrared optical spectroscopy of myoglobin has shown that approximately 20% of the myocardium is hypoxic in buffer-perfused hearts that are perfused with fully oxygenated buffer at 37 degrees C. The present study was undertaken to determine cardiac myoglobin saturation in buffer-perfused hearts when cardiac contractility was increased with epinephrine and decreased during cardiac arrest with KCl. Infusion of epinephrine to achieve a doubling of contractility, as measured by left ventricular maximum pressure change over time (dP/dt), resulted in a decrease in mean myoglobin saturation from 79% at baseline to 65% and a decrease in coronary venous oxygen tension from 155 mmHg at baseline to 85 mmHg. Cardiac arrest with KCl increased mean myoglobin saturation to 100% and coronary venous oxygen tension to 390 mmHg. A previously developed computer model of oxygen transport in the myocardium was used to calculate the probability distribution of intracellular oxygen tension and the hypoxic fraction of the myocardium with an oxygen tension below 0.5 mmHg. The hypoxic fraction of the myocardium was approximately 15% at baseline, increased to approximately 30% during epinephrine infusion, and fell to approximately 0% during cardiac arrest. The coronary venous adenosine concentration changed in parallel with the hypoxic fraction of the myocardium during epinephrine and KCl. It is concluded that catecholamine stimulation of buffer-perfused hearts increases hypoxia in the myocardium and that the increase in venous adenosine concentration is a reflection of the larger hypoxic fraction of myocardium that is releasing adenosine.

It has been proposed that alpha-adrenoceptor vasoconstriction in coronary resistance vessels results not from alpha-adrenoceptors on coronary smooth muscle but from alpha-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the alpha-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs (n = 10), the endothelin ETA/ETB receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the alpha-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and alpha-adrenoceptor blockade results indicates that alpha-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the alpha-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if alpha-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

It was previously shown that red blood cells release ATP when blood oxygen tension decreases. ATP acts on microvascular endothelial cells to produce a retrograde conducted vasodilation (presumably via gap junctions) to the upstream arteriole. These observations form the basis for an ATP hypothesis of local metabolic control of coronary blood flow due to vasodilation in microvascular units where myocardial oxygen extraction is high. Dogs (n = 10) were instrumented with catheters in the aorta and coronary sinus, and a flow transducer was placed around the circumflex coronary artery. Arterial and coronary venous plasma ATP concentrations were measured at rest and during three levels of treadmill exercise by using a luciferin-luciferase assay. During exercise, myocardial oxygen consumption increased approximately 3.2-fold, coronary blood flow increased approximately 2.7-fold, and coronary venous oxygen tension decreased from 19 to 12.9 mmHg. Coronary venous plasma ATP concentration increased significantly from 31.1 to 51.2 nM (P < 0.01) during exercise. Coronary blood flow increased linearly with coronary venous ATP concentration (P < 0.01). Coronary venous-arterial plasma ATP concentration difference increased significantly during exercise (P < 0.05). The data support the hypothesis that ATP is one of the factors controlling coronary blood flow during exercise.

The goal of this study was to identify the most important variables affecting bioluminescent ATP, ADP and AMP measurements in plasma and to develop an assay that takes these variables into account. Blood samples were drawn from conscious dogs. A 'stop solution' containing EDTA was prepared, which greatly retarded plasma ATP degradation by chelating Mg(+2) and Ca(+2) that are co-factors for many ATPases. Stop solution and blood were mixed using a two-syringe withdrawal system. Samples were centrifuged twice in order to remove red blood cells, and ATP was measured in the supernatant using the firefly luciferase assay. Sample pH was adjusted to the optimal range (7.75-7.95) and Mg(2+) (necessary for the luciferase reaction) was added back to the sample within the luminometer 2 s prior to luciferase addition. Four assay tubes were prepared for each plasma sample, containing standard additions of 0-15 pmol added ATP, in order to quantify native plasma ATP content. In separate plasma/stop solution samples ADP + ATP was measured after converting ADP to ATP via the pyruvate kinase reaction, and AMP + ADP + ATP was measured after addition of both myokinase and pyruvate kinase. Addition of forskolin and isobutylmethylxanthine (IBMX) to the stop solution to inhibit platelets resulted in lower ATP concentrations. Measurement of ATP and haemoglobin from lysed erythrocytes revealed that haemolysis exerts a strong influence on plasma ATP concentration that must be taken into account.

Myocardial mean myoglobin oxygen saturation was determined spectroscopically from isolated guinea pig hearts perfused with red blood cells during increasing hypoxia. These experiments were undertaken to compare intracellular myoglobin oxygen saturation in isolated hearts perfused with a modest concentration of red blood cells (5% hematocrit) with intracellular myoglobin saturation previously reported from traditional buffer-perfused hearts. Studies were performed at 37 degrees C with hearts paced at 240 beats/min and a constant perfusion pressure of 80 cmH2O. It was found that during perfusion with a hematocrit of 5%, baseline mean myoglobin saturation was 93% compared with 72% during buffer perfusion. Mean myoglobin saturation, ventricular function, and oxygen consumption remained fairly constant for arterial perfusate oxygen tensions above 100 mmHg and then decreased precipitously below 100 mmHg. In contrast, mean myoglobin saturation, ventricular function, and oxygen consumption began to decrease even at high oxygen tension with buffer perfusion. The present results demonstrate that perfusion with 5% red blood cells in the perfusate increases the baseline mean myoglobin saturation and better preserves cardiac function at low oxygen tension relative to buffer perfusion. These results suggest that caution should be used in extrapolating intracellular oxygen dynamics from buffer-perfused to blood-perfused hearts.