Influence of myocardial oxygen demand on the coronary vascular response to arterial blood gas changes in humans.

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 ß1-adrenergic receptor blockade. Healthy men (age: 25 ± 1 yr, n = 11) received intravenous esmolol (ß1-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 ß1-adrenergic receptor blockade ( P < 0.05). Despite attenuated RPP and MELV responses, ß1-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 ß1-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. ß1-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 ß1-adrenergic receptor-mediated, feedforward epicardial coronary artery dilation. In addition, we observed a synergistic effect of Po2 and Pco2 during hypercapnic hypoxia.

Control of coronary blood flow during exercise.

During exercise, coronary blood flow increases to match the augmented myocardial oxygen demand because of tachycardia. Coronary vasodilation during exercise is via a combination of feedforward and feedback control mechanisms. Feedforward control is mediated by sympathetic ß-adrenoceptor vasodilation. Feedback vasodilator control is via a novel hypothesis where adenine nucleotides released from red blood cells act on endothelial purinergic receptors.

Adenine nucleotide control of coronary blood flow during exercise.

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.

Fibroblast growth factor-2 regulates myocardial infarct repair: effects on cell proliferation, scar contraction, and ventricular function.

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.

Human plasma ATP concentration.

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.

Myocardial oxygenation and adenosine release in isolated guinea pig hearts during changes in contractility.

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.

Plasma ATP during exercise: possible role in regulation of coronary blood flow.

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.

Matching coronary blood flow to myocardial oxygen consumption.

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.

Nucleotide coronary vasodilation in guinea pig hearts.

The role of P1 receptors and P2Y1 receptors in coronary vasodilator responses to adenine nucleotides was examined in the isolated guinea pig heart. Bolus arterial injections of nucleotides were made in hearts perfused at constant pressure. Peak increase in flow was measured before and after addition of purinoceptor antagonists. Both the P1 receptor antagonist 8-(p-sulfophenyl)theophylline and adenosine deaminase inhibited adenosine vasodilation. AMP-induced vasodilation was inhibited by P1 receptor blockade but not by adenosine deaminase or by the selective P2Y1 antagonist N6-methyl-2'-deoxyadenosine 3',5'-bisphosphate (MRS 2179). ADP-induced vasodilation was moderately inhibited by P1 receptor blockade and greatly inhibited by combined P1 and P2Y1 blockade. ATP-induced vasodilation was antagonized by P1 blockade but not by adenosine deaminase. Addition of P2Y1 blockade to P1 blockade shifted the ATP dose-response curve further rightward. It is concluded that in this preparation ATP-induced vasodilation results primarily from AMP stimulation of P1 receptors, with a smaller component from ATP or ADP acting on P2Y1 receptors. ADP-induced vasodilation is largely due to P2Y1 receptors, with a smaller contribution by AMP or adenosine acting via P1 receptors. AMP responses are mediated solely by P1 receptors. Adenosine contributes very little to vasodilation resulting from bolus intracoronary injections of ATP, ADP, or AMP.

Measurement of adenine nucleotides in plasma.

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.

Control of coronary blood flow during exercise.

Under normal physiological conditions, coronary blood flow is closely matched with the rate of myocardial oxygen consumption. This matching of flow and metabolism is physiologically important due to the limited oxygen extraction reserve of the heart. Thus, when myocardial oxygen consumption is increased, as during exercise, coronary vasodilation and increased oxygen delivery are critical to preventing myocardial underperfusion and ischemia. Exercise coronary vasodilation is thought to be mediated primarily by the production of local metabolic vasodilators released from cardiomyocytes secondary to an increase in myocardial oxygen consumption. However, despite various investigations into this mechanism, the mediator(s) of metabolic coronary vasodilation remain unknown. As will be seen in this review, the adenosine, K(+)(ATP) channel and nitric oxide hypotheses have been found to be inadequate, either alone or in combination as multiple redundant compensatory mechanisms. Prostaglandins and potassium are also not important in steady-state coronary flow regulation. Other factors such as ATP and endothelium-derived hyperpolarizing factors have been proposed as potential local metabolic factors, but have not been examined during exercise coronary vasodilation. In contrast, norepinephrine released from sympathetic nerve endings mediates a feed-forward betaadrenoceptor coronary vasodilation that accounts for approximately 25% of coronary vasodilation observed during exercise. There is also a feed-forward alpha-adrenoceptor-mediated vasoconstriction that helps maintain blood flow to the vulnerable subendocardium when heart rate, myocardial contractility, and oxygen consumption are elevated during exercise. Control of coronary blood flow during pathophysiological conditions such as hypertension, diabetes mellitus, and heart failure is also addressed.