On the influence of hydrogen ion concentration and of anoxaemia upon the heart volume

Perspectives on the History of Coronary Physiology: Discovery of Major Principles and Their Clinical Correlates

Coronary circulation plays an essential role in delivering oxygen and metabolic substrates to satisfy the considerable energy demand of the heart. This article reviews the history that led to the current understanding of coronary physiology, beginning with William Harvey's revolutionary discovery of systemic blood circulation in the 17th century, and extending through the 20th century when the major mechanisms regulating coronary blood flow (CBF) were elucidated: extravascular compressive forces, metabolic control, pressure-flow autoregulation, and neural pathways. Pivotal research studies providing evidence for each of these mechanisms are described, along with their clinical correlates. The authors describe the major role played by researchers in the 19th century, who formulated basic principles of hemodynamics, such as Poiseuille's law, which provided the conceptual foundation for experimental studies of CBF regulation. Targeted research studies in coronary physiology began in earnest around the turn of the 20th century. Despite reliance on crude experimental techniques, the pioneers in coronary physiology made groundbreaking discoveries upon which our current knowledge is predicated. Further advances in coronary physiology were facilitated by technological developments, including methods to measure phasic CBF and its regional distribution, and by biochemical discoveries, including endothelial vasoactive molecules and adrenergic receptor subtypes. The authors recognize the invaluable contribution made by basic scientists toward the understanding of CBF regulation, and the enormous impact that this fundamental information has had on improving clinical diagnosis, decision-making, and patient care.

Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Protein disulfide isomerase may facilitate the efflux of nitrite derived S-nitrosothiols from red blood cells

Protein disulfide isomerase (PDI) is an abundant protein primarily found in the endoplasmic reticulum and also secreted into the blood by a variety of vascular cells. The evidence obtained here, suggests that PDI could directly participate in the efflux of NO⁺ from red blood cells (RBC). PDI was detected both in RBC membranes and in the cytosol. PDI was S-nitrosylated when RBCs were exposed to nitrite under ∼50% oxygen saturation but not under ∼100% oxygen saturation. Furthermore, it was observed that hemoglobin (Hb) could promote PDI S-nitrosylation in the presence of ∼600 nM nitrite. In addition, three lines of evidence were obtained for PDI–Hb interactions: (1) Hb co-immunoprecipitated with PDI; (2) Hb quenched the intrinsic PDI fluorescence in a saturable manner; and (3) Hb–Fe(II)–NO absorption spectrum decreased in a [PDI]-dependent manner. Finally, PDI was detected on the surface RBC under ∼100% oxygen saturation and released as soluble under ∼50% oxygen saturation. The soluble PDI detected under ∼50% oxygen saturation was S-nitrosylated. Based on these data it is proposed that PDI is taken up by RBC and forms a complex with Hb. Hb–Fe(II)–NO that is formed from nitrite reduction under ∼50% O₂, then transfers NO⁺ to either Hb–Cys β93 or directly to PDI resulting in S-nitroso-PDI which transverses the RBC membrane and attaches to the RBC surface. When RBCs enter tissues the S-nitroso-PDI is released from the RBC-surface into the blood where its NO⁺ is transferred into the endothelium thereby inducing vasodilation, suggesting local oxygen-dependent dynamic interplays between nitrite, NO and S-nitrosylation.

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Red blood cells (RBC) contain protein disulfide isomerase (PDI) that can associate with hemoglobin.

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Formation of S-nitroso-PDI is an oxygen- and Hb-dependent process.

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S-nitroso-PDI associates with RBC surface in an oxygen dependent manner that facilitates its release under hypoxia.

Enzymatic function of hemoglobin as a nitrite reductase that produces NO under allosteric control

Hypoxic vasodilation is a fundamental, highly conserved physiological response that requires oxygen and/or pH sensing coupled to vasodilation. While this process was first characterized more than 80 years ago, the precise identity and mechanism of the oxygen sensor and mediators of vasodilation remain uncertain. In support of a possible role for hemoglobin (Hb) as a sensor and effector of hypoxic vasodilation, here we show biochemical evidence that Hb exhibits enzymatic behavior as a nitrite reductase, with maximal NO generation rates occurring near the oxy-to-deoxy (R-to-T) allosteric structural transition of the protein. The observed rate of nitrite reduction by Hb deviates from second-order kinetics, and sigmoidal reaction progress is determined by a balance between 2 opposing chemistries of the heme in the R (oxygenated conformation) and T (deoxygenated conformation) allosteric quaternary structures of the Hb tetramer--the greater reductive potential of deoxyheme in the R state tetramer and the number of unligated deoxyheme sites necessary for nitrite binding, which are more plentiful in the T state tetramer. These opposing chemistries result in a maximal nitrite reduction rate when Hb is 40-60% saturated with oxygen (near the Hb P50), an apparent ideal set point for hypoxia-responsive NO generation. These data suggest that the oxygen sensor for hypoxic vasodilation is determined by Hb oxygen saturation and quaternary structure and that the nitrite reductase activity of Hb generates NO gas under allosteric and pH control.

Effects of hypoxia and hypercapnia on the force-velocity relation of rabbit myocardium

The separate effects of hypoxia and hypercapnia on the force-velocity relation of rabbit myocardium were compared in 10 papillary or trabecular muscles superfused using control (95% O2-5% CO2), hypoxic (18% O2), and hypercapnic (20% CO2) physiological salt solutions. This level of hypoxia did not irreversibly damage the muscles and reduced peak isometric force by 53 +/- 11%. The level of hypercapnia was chosen to match the force depression (50 +/- 12%) produced by hypoxia. Multiple force-velocity points were measured by applying critically damped isotonic force steps at 90% of the time to peak isometric force and at the time to 50% peak isometric force. These points defined the force-velocity relation and maximum velocity of shortening, the extrapolated isometric force, and the maximum power of nonpotentiated and postextrasytolic potentiated contractions. Hypoxia and hypercapnia reduced maximum force and maximum power nearly equally. Maximum velocity of shortening decreased more during hypoxia (21 +/- 12%) than during hypercapnia (12 +/- 9%) (p less than 0.01). Postextrasystolic potentiation completely reversed the reduction of maximum velocity of shortening during hypercapnia but not during hypoxia. A 6% internal load could account for the reduction in maximum velocity of shortening during hypercapnia and all but 9% of the reduction in maximum velocity of shortening during hypoxia. The relative time course of the force-velocity relation was not altered by either hypoxia or hypercapnia. We conclude that hypercapnia reduces the effect of activation because increased activation (by postextrasystolic potentiation) restored the force-velocity relation and maximum velocity of shortening to control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Progressive hypoxemia limits left ventricular oxygen consumption and contractility

To study the cardiac effects of progressive hypoxemia, we measured the left ventricular end-systolic pressure-volume relation (ESPVR), myocardial oxygen consumption (MVO2), and myocardial oxygen delivery (MQO2) in eight thoracotomized dogs anesthetized with fentanyl and droperidol. We specifically looked for evidence of oxygen supply limitation of MVO2 and depressed contractility (altered ESPVR) during stepwise decreases in inspired oxygen fraction. We hypothesized that the reported relation between MVO2 and left ventricular pressure-volume area (PVA) may hold when inadequate MQO2 determines MVO2, which then may limit PVA, manifested partly as a change in the ESPVR. Initially, as arterial oxygen saturation was decreased from 95 +/- 3% to 64 +/- 14%, coronary blood flow increased so that MQO2 was maintained with no change in myocardial extraction ratio (ERm = MVO2/MQO2). During this first phase, lactate utilization, PVA, and ESPVR did not change. When oxygen saturation was further reduced, coronary blood flow rose no higher and ERm increased, but not enough to maintain MVO2. Lactate consumption decreased and ST segments rose, signaling a change from aerobic metabolism. MVO2 decrease was associated with a fall in PVA, which was due to a fall in blood pressure and a significant depression of the ESPVR. Specifically, the volume intercept of the ESPVR increased in all dogs (6.5-20.1 ml, p less than 0.0001), accounting for two thirds of the increase in end-systolic volume. The slope of the ESPVR decreased during hypoxia (13.3-6.1 mm Hg/ml, p less than 0.02), accounting for only one third of the observed increase in end-systolic volume. We believe that the evidence of anaerobic metabolism, the decrease in PVA, and the depression of the ESPVR demonstrates onset of oxygen supply limitation of MVO2. Our data are consistent with the hypothesis that limited MVO2 may limit PVA. The hypoxic volume intercept alteration of the ESPVR is different from changes in the slope of ESPVR seen with other interventions. This may be analogous to recent observations in isolated muscle that show hypoxic depression in contractility to be different from other interventions.

Exercise-induced fall in coupling of human beta 2-adrenergic receptors

The purpose of this study was to quantitatively examine to what extent acute exertion diminishes the activity of beta-adrenergic receptors through diminishing coupling. Normal neutrophil membrane preparations containing beta 2-adrenergic receptors were obtained from healthy human volunteers, then exposed to autologous venous plasma obtained before and after acute forearm exertion. There was an acute, dramatic diminution in coupling (P less than .003) after only a few minutes of exercise. Preexertion plasma had no significant effect upon coupling. Substantial diminution in beta-adrenergic receptor sensitivity is thus acutely demonstrable in this model which serves as a model for both metabolic acidosis and acute exertion.

Coupling of human beta 2-adrenergic receptors: relationship to redox potential

The purpose of this study was to examine the quantitative relationship between the redox potential of a redox couplet and the alterations it induces in coupling of receptor occupation with enzyme activation. Normal neutrophil membrane preparations containing beta 2-adrenergic receptors were exposed to equimolar mixtures of the following redox couplets: ferrocyanide-ferricyanide, hemoglobin-methemoglobin, ascorbate-dehydroascorbate, lactate-pyruvate, glutathione ox-red, beta-hydroxybutyrate-acetoacetate, and NAD-NADH. There was a linear relationship between the redox potential of the couplets and the degree of change in coupling (p less than 0.001). The apparent redox potential of the high affinity complex was +0.30 +/- 0.093 V. The effect of lactate to uncouple beta-adrenergic receptors was partially blocked by preexposure to isoproterenol. Thus, high affinity state formation is regulated by redox couplets in a manner dependent on their redox potential.

Effects of endogenous redox-active compounds on coupling of human beta 2-adrenergic receptors

Altered redox states such as metabolic acidosis may impair beta-adrenergic receptor responsiveness. Beta-adrenergic receptor function requires formation of a high affinity, "coupled" state of the receptor. The degree of coupling is reflected in the ratio of dissociation constants, KL/KH, for the low and high affinity states of the receptor. It has previously been demonstrated that 16 mM lactate and pH 7.1 induce independent defects in beta-adrenergic receptor function. The purpose of this study was to examine further how endogenous redox agents might alter high affinity state formation. Normal neutrophil membrane preparations containing beta-adrenergic receptors were exposed to several concentrations of three redox couplets native to plasma: lactate (L)-pyruvate (P), beta-hydroxybutyrate (BOHB)-acetoacetate (AcAc), and glutathione (GSH-GSSG). BOHB, AcAc, and P had no isolated effect on high affinity state formation while 10 mM lactate diminished KL/KH by 30% (p less than 0.001). Dropping the pH from 7.4 to 7.1 resulted in a 50% to 70% reduction in KL/KH (p less than 0.001), independent of metabolite present. GSH or GSSG exposure resulted in a concentration-dependent fall in KL/KH value. Thus, high affinity state formation is regulated by redox couplets and pH independently. The reduced responsiveness of beta-adrenergic receptors observed in such states as metabolic acidosis could result from direct effects of redox couplets in addition to those of low pH.

Local vasoactivity of oxygen and carbon dioxide in the right coronary circulation of the dog and pig

1. Eight mongrel dogs were anaesthetized with sodium thiamylal and chloralose-urethane, ventilated, vagotomized and heparinized. Five Poland-China pigs were anaesthetized with sodium thiamylal and nitrous oxide, ventilated, vagotomized and heparinized. 2. Extracorporeal perfusion of the right coronary artery at constant pressure (100 mmHg) was instituted. A lung from a donor animal was interposed in the coronary perfusion circuit to effect changes in CO2 and O2 tensions in the coronary arterial blood while systemic blood gases were maintained at normal levels. 3. Local hypoxia (PO2 range 17-22 mmHg) produced a 25-75% decrease in coronary vascular resistance (P less than 0.05) and a 0-24% (not significant) decrease in right ventricular dP/dt. 4. Local changes in PCO2 over the range 8-105 mmHg were associated with a 17-58% decrease in coronary vascular resistance (P less than 0.05), a 19-24% decrease in right ventricular dP/dt (P less than 0.05) with no change in right ventricular end-diastolic pressure, and a 1-18% (not significant) decrease in heart rate. 5. These studies suggest that local decreases in O2 or increases in CO2 tensions produce decreases in right coronary vascular resistance that are in the opposite direction to those that would be expected from the observed changes in heart rate and contractility (two primary determinants of myocardial oxygen consumption). 6. These data support the hypothesis that CO2 and O2 are locally vasoactive in the coronary circulation.

Chronotropic response of isolated atria to acid base alterations

The effect of acid-base alterations on spontaneous rate was analysed using isolated atria exposed to cumulative degrees of acidosis produced either by adding HCl or by increasing PCO2 in the incubation medium. Frequncy vs. pH curves were made to assess chronotropic response to acid-base changes. Heart rate was increased in alkalosis and decreased when the pH of the medium was lowered. Both "respiratory" and "metabolic" alterations affected the contraction rate to the same extent. Decreasing pH from normal values seemed to decrease heart rate more than the enhancement produced by the same change in pH towards the alkalotic side. When frequency was plotted as a function of hydrogen ion activity (aH+) a more linear relationship was obtained, either with pure "metabolic" or with "respiratory" acid-base alterations. Increasing (aH+) from normal values seemed to decrease heart rate to the same extent (respiratory alterations) or even less (metabolic alterations) than the enhancement produced by the same change in (aH+) towards the alkalotic side. Neither the increase in rate produced by alkalosis nor the decrease induced by acidosis were prevented by blocking the neurotransmitters by atropine or propranolol.

Influences of hypercapnia on cardiac function in the newborn lamb

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Hemodynamic studies on cholera. Effects of hypovolemia and acidosis

The hemodynamic effects of hypovolemia and acidosis were studied in 23 patients with cholera. Studies were made before and during fluid replacement and administration of alkali.

The major hemodynamic abnormalities encountered before rehydration can be ascribed to a reduction in circulating blood volume. Hypovolemia was associated with a reduction in cardiac output, blood pressures, and central blood volume. Restoration of blood volume returned these variables toward normal.

The chief effect of acidosis appeared to be a redistribution of blood from the peripheral to the central circulation; consequently, central blood volume, lesser circulation pressures, and cardiac output were relatively well maintained despite hypovolemia. Fluid administration without correction of acidosis favored a disproportionate increase in central blood volume, while reduction in hydrogen ion concentration attending fluid replacement resulted in a more even distribution of the circulating blood volume and reduced the possibility of engorgement of the pulmonary bed.

It is postulated that this redistribution of blood stems from peripheral venoconstriction and a reduction in the capacity of venous reservoirs induced by acidosis.

The individual effects of CO2, bicarbonate and pH on the electrical and mechanical activity of isolated rabbit auricles

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The Cardiovascular Effect of Induced Controlled Anoxemia

Cardiovascular responses were studied by means of the ballistocardiograph and electrocardiograph in 16 normal subjects and 11 patients with suspected coronary artery disease before, during and after the induction of oximetrically controlled anoxemia. Progressive and significant increases occurred in cardiac output, pulse rate, left ventricular work, maximum cardiac force and pulse pressure during anoxemia in all of the normal subjects and in the 5 patients whose ballistocardiograms allowed the calculation of these variables. The increase in cardiac output resulted almost entirely from an acceleration of cardiac rate. In the normal subjects control ballistocardiograms were normal in form and remained normal throughout anoxemia. Of the patients, 1 with normal, 2 with borderline and 1 with abnormal ballistocardiograms became abnormal or more abnormal. Two of the patients had a positive electrocardiographic test for coronary insufficiency while the test was negative in all of the normal subjects.

The linear relationship between left ventricular work and arterial oxygen saturation suggests that when cardiovascular function is tested by means of anoxemic stress the level of arterial desaturation should be controllable. This is made possible by the use of a variable oxygen-nitrogen mixture and an oximeter.