During exercise, β-feedforward coronary vasodilation has been shown to contribute to the matching of myocardial oxygen supply with the demand of the myocardium. Since both β1- and β2-adrenoceptors are present in the coronary microvasculature, we investigated the relative contribution of these subtypes to β-feedforward coronary vasodilation during exercise as well as to infusion of the β1-agonist norepinephrine and the β1- and β2-agonist isoproterenol. Chronically instrumented swine were studied at rest and during graded treadmill exercise (1–5 km/h) under control conditions and after β1-blockade with metoprolol (0.5 mg/kg iv) and β1/β2-blockade with propranolol (0.5 mg/kg iv). The selectivity and degree of β-blockade of metoprolol and propranolol were confirmed using isoproterenol infusion (0.05–0.4 μg· kg−1·min−1) under resting conditions. Isoproterenol-induced coronary vasodilation was mediated through the β2-adrenoceptor, whereas norepinephrine-induced coronary vasodilation was principally mediated through the β1-adrenoceptor. Exercise resulted in a significant increase in left ventricular norepinephrine release and epinephrine uptake. β1-Adrenoceptor blockade with metoprolol had very little effect under resting conditions. However, during exercise, metoprolol attenuated the increase in myocardial oxygen supply in excess of the reduction in myocardial oxygen demand, as evidenced by a progressive decrease in coronary venous Po2. Consequently, metoprolol caused a clockwise rotation of the relationship between myocardial oxygen consumption and coronary venous Po2. Additional β2-adrenoceptor blockade with propranolol further inhibited myocardial oxygen supply during exercise, resulting in a further clockwise rotation of the relationship between myocardial oxygen consumption and coronary venous Po2. In conclusion, both β1- and β2-adrenoceptors contribute to the β-feedforward coronary resistance vessel dilation during exercise.
- coronary circulation
in the normal heart, coronary blood flow is tightly regulated in response to changing myocardial metabolic needs to maintain a consistently high level of myocardial O2 extraction (10, 16, 44). The close coupling of coronary blood flow and myocardial O2 demand has been proposed to depend primarily on messengers released from the myocardium and endothelium but is also modulated by the autonomic nervous system (10, 11, 44, 45). Thus an increase in sympathetic activity, as occurs during exercise, not only results in β-adrenoceptor-mediated increases in heart rate and contractility but simultaneously produces feedforward β-adrenoceptor-mediated coronary vasodilation (3, 12, 13, 19, 26, 27). This feedforward β-adrenoceptor vasodilation may account for as much as 25% of the coronary vasodilation observed during exercise (20).
The contribution of β1- versus β2-adrenoceptor subtypes to feedforward vasodilation during exercise remains incompletely understood. There is general agreement that whereas β1-adrenoceptors are the predominant receptor subtype in large conduit coronary arteries (1, 35, 36, 38, 40, 42, 47⇓–49, 52), β2-adrenoceptors are the predominant receptor subtype in small coronary arteries (41) and particularly coronary arterioles (25, 34). Indeed, two early studies suggest that β2-adrenergic receptors contribute to the exercise-induced increase in coronary blood flow in the dog (9, 31). Unfortunately, in these studies, myocardial O2 consumption (MV̇O2) was not measured, so it could not be excluded that the small reductions in coronary flow velocities observed during exercise in the presence of selective β2-adrenergic receptor blockade were, at least in part, due to a decrease in myocardial O2 demand (5). Given the influence of the β1-adrenoceptors on heart rate and contractility, the simultaneous measurement of MV̇O2 and myocardial O2 supply is essential to allow an assessment of the possible role of β1-adrenoceptors, which have been shown to be present in coronary arterial microvessels > 55 μm in diameter (34), in the exercise-induced feedforward coronary resistance vessel dilation. The latter is likely in view of the observations that both β1- and β2-adrenoceptors contributed to the coronary vasodilation produced by intravenous infusion of exogenous norepinephrine (33, 43).
In light of these considerations, the aim of the present study was to investigate the contribution of β1- and β2-adrenoceptors to exercise-induced β-adrenergic feedforward coronary resistance vessel dilation. For this purpose, we exercised chronically instrumented swine on a motor-driven treadmill and measured coronary blood flow and MV̇O2. Swine were studied at rest and during a staged exercise protocol under control conditions and in the presence of selective β1- and nonselective β1/β2-adrenoceptor blockade.
Studies were performed in accordance with the American Physiological Society's “Guiding Principles in the Care and Use of Laboratory Animals” and with prior approval of the Animal Care Committee of the Erasmus Medical Center. Fifteen 2 to 3-mo-old Yorkshire × Landrace pigs (22 ± 1 kg at the time of surgery) of either sex (males and females) entered the study.
Swine were sedated with an intramuscular injection of Zoletil (2.5 mg/kg tiletamine + 2.5 mg/kg zolazepam) and Rompun (2.25 mg/kg xylazine), intubated, and ventilated with a mixture of O2 and N2 (1:2) to which 2 to 3% (vol/vol) isoflurane was added to maintain anesthesia. Under sterile conditions, the chest was opened via the fourth left intercostal space, and fluid-filled polyvinylchloride catheters were inserted into the aortic arch, pulmonary artery, and left atrium for blood pressure measurement, blood sampling, and infusion of drugs (12, 32). A microtipped pressure transducer (P4.5, Konigsberg Instruments) was inserted into the left ventricle via the apex to measure left ventricular (LV) pressure and its first derivative (dP/dt) as an index of contractility. Transit-time flow probes (Transonic Systems) were positioned around the ascending aorta for measurement of cardiac output and around the left anterior descending coronary artery to measure the coronary blood flow. Small angiocatheters were inserted into the anterior interventricular vein for coronary venous blood sampling. Finally, in a subset of five animals, a small angiocatheter was also inserted into the left anterior descending coronary artery for intracoronary administration of norepinephrine. Catheters were tunneled to the back, and animals were allowed to recover, receiving analgesia (0.3 mg buprenorphine im) for 2 days and antibiotic prophylaxis (25 mg/kg amoxicillin and 5 mg/kg gentamicin iv) for 5 days.
Studies were performed 1–3 wk after surgery with animals resting or exercising on a motor-driven treadmill up to 80–90% of maximal heart rate. The different protocols were performed on different days and in random order.
Protocol I: β1-adrenoceptor selectivity of metoprolol.
The β1-adrenoceptor blocking properties of metoprolol and β1/β2-adrenoceptor blocking properties of propranolol were tested in five swine. With swine resting quietly, the mixed β1/β2-adrenoceptor agonist isoproterenol (Sigma-Aldrich) was infused intravenously in consecutive doses of 0.05, 0.1, and 0.2 μg·kg−1·min−1; each dosage was infused for 5 min. During the last minute of each dose, hemodynamic variables (consisting of heart rate, aortic blood pressure, LV pressure and dP/dt, cardiac output, and coronary blood flow) were measured and blood samples were taken. After 15 min of washout when hemodynamic variables had returned to baseline values, the β1-receptor antagonist metoprolol (0.5 mg/kg iv; CIBA-Geigy BV) was administered and, 5 min later, infusions of isoproterenol (0.05–0.4 μg·kg−1·min−1) were repeated. Ninety minutes later, isoproterenol infusions (0.1–0.4 μg·kg−1·min−1) were again repeated in the presence of the β1/β2-adrenoceptor antagonist propranolol (0.5 mg/kg iv; Sigma-Aldrich).
Protocol II: role of β1- and β2-adrenoceptors to exercise-induced coronary vasodilation.
On a different day, with swine (n = 9) resting quietly on the treadmill, hemodynamic measurements were obtained and blood samples collected. Hemodynamic measurements were then repeated, and rectal temperature was measured with animals standing on the treadmill. Subsequently, a five-stage (1–5 km/h) treadmill exercise protocol was started; each exercise stage lasted ∼3 min in duration. Hemodynamic variables were continuously recorded and blood samples collected during the last 30 s of each stage when hemodynamics had reached a steady state. After the completion of the exercise protocol, the animals were allowed to rest on the treadmill for 90 min, resulting in a complete return of hemodynamic variables to baseline values. Subsequently, animals received an intravenous 10-min infusion of the β1-adrenoceptor antagonist metoprolol (0.5 mg/kg), and the exercise protocol was repeated. After another 90 min of rest, the β1/β2-adrenoceptor antagonist propranolol (0.5 mg/kg) was administered intravenously over 10 min, and the exercise protocol was repeated.
Protocol III: contribution of β1- and β2-adrenoceptors to norepinephrine-induced coronary vasodilation.
In five additional swine, we studied the involvement of β1-adrenoceptors in the vasodilation produced by norepinephrine. Since we observed in a pilot experiment that the intravenous infusion of norepinephrine resulted in a marked systemic pressor response accompanied by a reflex-bradycardia (with no change in MV̇O2), we elected to infuse norepinephrine selectively into the left anterior descending coronary artery at doses of 1 and 10 ng/min, lasting 5 min each. Changes in coronary and systemic hemodynamics were measured, and arterial and coronary venous blood samples were taken. After 15 min of rest, when hemodynamic variables had returned to steady state values, metoprolol (0.5 mg/kg iv) was administered and intracoronary infusions of norepinephrine (1, 10, and 20 ng/min) were repeated.
Data Acquisition and Analysis
Digital recording and off-line analysis of hemodynamic and blood gas data have been described in detail elsewhere (12, 32). In short, myocardial O2 delivery was computed as the product of left anterior descending coronary artery blood flow and arterial blood O2 content. MV̇O2 in the region of myocardium perfused by the left anterior descending coronary artery was calculated as the product of coronary blood flow and the difference in O2 content [determined as the sum of dissolved O2 (Po2), measured by ABL600 (Radiometer) and Hb-bound O2 (So2), measured by OSM3 Hemoximeter (Radiometer)] between arterial and coronary venous blood. Myocardial O2 extraction was computed as the ratio of MV̇O2 and myocardial O2 delivery. Coronary vascular conductance was calculated as the ratio between coronary blood flow and blood pressure. Systemic vascular conductance was calculated as the ratio of cardiac output and blood pressure.
Catecholamine levels were determined in arterial and coronary venous blood of four swine as previously described (30). Myocardial norepinephrine release and epinephrine uptake were determined as the product of coronary blood flow and arteriovenous-concentration differences.
Statistical analysis of hemodynamic data was performed using a two-way (drug treatment and exercise) analysis of variance for repeated measures. When significant effects were detected, post hoc testing for the effects of drug treatment and exercise was performed using Scheffé's test. To test for the effects of drug treatment on the relationship between MV̇O2 and coronary venous O2 tension (PcvO2) and saturation (ScvO2), regression analysis was performed with the animal as a dummy variable and with drug treatment (parallel shift), MV̇O2, and their interaction (rotation of the relation) as independent variables. Statistical significance was accepted when P ≤ 0.05. Data are presented as means ± SE.
β1-Adrenoceptor Selectivity of Metoprolol
The mixed β-adrenoceptor agonist isoproterenol resulted in dose-dependent increases in heart rate, maximum LV dP/dt (dP/dtmax), and systemic vascular conductance and produced a dose-dependent decrease in blood pressure (Fig. 1). Metoprolol resulted in a rightward shift of the dose-response curve of heart rate and LV dP/dtmax but had no effect on the isoproterenol-induced increases in systemic vascular conductance and the decrease in mean aortic pressure (Fig. 1). These findings indicate that metoprolol in a dose of 0.5 mg/kg blocks the β1-adrenoceptor but is devoid of β2-adrenoceptor blocking properties. In contrast, propranolol completely abolished the systemic vasodilation by isoproterenol and caused a further shift in the dose-response curves of heart rate and LV dP/dtmax (Fig. 1), consistent with the blockade of both β1- and β2-adrenoceptors.
Role of β1- and β2-Adrenoceptors to Exercise-Induced Coronary Vasodilation
Systemic hemodynamic responses to exercise.
Exercise produced increases in heart rate, LV systolic pressure, dP/dtmax, left atrial pressure, and cardiac output (Table 1). The increase in cardiac output was paralleled by a similar increase in systemic vascular conductance so that mean aortic pressure remained unchanged.
Under resting conditions, metoprolol and propranolol resulted in similar decreases in heart rate (∼10%) and LV dP/dtmax (∼20%) but had no significant effect on LV peak systolic, left atrial, and mean aortic blood pressures; cardiac output; or systemic vascular conductance (Table 1). However, both metoprolol and propranolol markedly blunted the exercise-induced increases in heart rate, LV dP/dtmax, and, to a lesser extent, cardiac output (Table 1). Although the effects of propranolol on heart rate and LV dP/dtmax were slightly greater than those of metoprolol, the relationship between heart rate and LV dP/dtmax was similarly altered by metoprolol and propranolol, suggesting that the degree of β1-adrenoreceptor blockade was similar with the two β-blockers (Fig. 2). Conversely, the systemic vasodilation that occurred during exercise was slightly blunted by propranolol, but not metoprolol, indicating that exercise results in β2-adrenoceptor-mediated systemic vasodilation.
Myocardial O2 balance and coronary vasodilator responses to exercise.
The exercise-induced increases in heart rate and the product of heart rate and LV systolic pressure resulted in equivalent increases of MV̇O2 (Fig. 3, and Table 2). During control exercise, the increased MV̇O2 was met by a commensurate increase in myocardial O2 delivery (Table 2), allowing myocardial O2 extraction to remain constant, thereby leaving PcvO2 and ScvO2 unchanged from their resting values (Fig. 4). Plasma norepinephrine concentrations were higher in coronary venous compared with arterial blood, indicating the release of norepinephrine from the sympathetic nerve terminals in the heart. The release of norepinephrine increased with increasing exercise intensity (Fig. 5). In contrast, plasma concentrations of epinephrine were slightly higher in arterial compared with coronary venous blood, indicating myocardial uptake of epinephrine. The uptake of epinephrine also increased slightly with increasing exercise intensity (Fig. 5).
Metoprolol and propranolol had no effect on MV̇O2, myocardial O2 delivery, myocardial O2 extraction, or ScvO2 under resting conditions but resulted in a slight increase in PcvO2 (Fig. 4). The increase in PcvO2 in conjunction with an unchanged ScvO2 is consistent with a rightward shift of the O2 dissociation curve, which was likely the result of a small decrease in pH (Table 2) and a small increase in body temperature (from 39.1 ± 0.1°C during control to 39.6 ± 0.3°C after metoprolol and 39.5 ± 0.2°C after propranolol, both P < 0.05 vs. control), possibly as the result of reduced skin perfusion because of β1-blockade (15). Although metoprolol and propranolol blunted the increases in heart rate and LV systolic pressure during exercise, they did not influence the relationship between heart rate and MV̇O2, whereas they had a negligible effect on the relationship between the rate-pressure product and MV̇O2 (Fig. 3). These data suggest that the administration of metoprolol and propranolol did not change the efficiency of contraction. The reduced exercise-induced increase in heart rate in the presence of metoprolol also blunted the exercise-induced increase in MV̇O2 and myocardial O2 delivery. However, the attenuation of myocardial O2 delivery slightly outweighed the decrease in MV̇O2, thereby forcing the myocardium to increase its O2 extraction and resulting in a progressive decrease in PcvO2 and ScvO2 with increasing exercise intensity (Fig. 4). These observations are consistent with a significant contribution of β1-adrenoceptors to the β-adrenergic feedforward coronary resistance vessel dilation that occurs during exercise. Propranolol caused a further rotation of the relationships between myocardial O2 demand and PcvO2 and ScvO2 compared with metoprolol (Fig. 4, and Table 3). These findings indicate that β2-adrenoceptors also contribute to β-adrenergic feedforward coronary resistance vessel dilation during exercise.
Myocardial O2 Balance and Coronary Vasodilator Responses to Isoproterenol and Norepinephrine
Isoproterenol resulted in dose-dependent coronary vasodilation as reflected by the increase in coronary blood flow (Fig. 6). This coronary vasodilation was in part due to the increase in MV̇O2 but also reflected direct coronary β-adrenergic vasodilation, as the resultant increase in myocardial O2 delivery exceeded the increase in MV̇O2, leading to increases in PcvO2 and ScvO2 (Fig. 6). Metoprolol did not impair isoproterenol-induced coronary vasodilation (Fig. 6), indicating that the isoproterenol-mediated coronary vasodilation is principally β2-adrenoceptor mediated. Indeed, propranolol abolished the isoproterenol-induced increases in MV̇O2 and hence coronary blood flow, as well as the increases in PcvO2 and ScvO2 (data not shown).
Selective intracoronary infusion of norepinephrine in doses of 1 and 10 ng/min had minimal systemic hemodynamic effects, as neither mean aortic pressure (83 ± 2 mmHg at baseline and 83 ± 4 mmHg during norepinephrine) nor heart rate (102 ± 6 beats/min at baseline and 109 ± 9 beats/min during norepinephrine) were significantly altered. However, intracoronary infusion of norepinephrine did result in an increase in global LV dP/dt from 2,260 ± 120 to 3,150 ± 180 mmHg/s (P < 0.05) that was accompanied by increases in coronary blood flow (Fig. 6). The increase in myocardial O2 supply slightly exceeded the increase in myocardial O2 demand as evidenced by a small increase in PcvO2 and ScvO2 (Fig. 6). The administration of metoprolol not only attenuated the norepinephrine-induced increase in MV̇O2 but also blunted the vasodilator effects of intracoronary norepinephrine, suggesting that these effects were principally mediated through the β1-adrenoceptor.
The present study assessed the contribution of β1- and β2-adrenergic receptor subtypes to the feedforward coronary resistance vessel dilation that occurs during exercise. The main findings are as follows. First, selective β1-adrenergic receptor blockade caused a clockwise rotation of the relationship between MV̇O2 and PcvO2, indicating that β1-adrenoceptors contribute to β-adrenergic feedforward vasodilation of the coronary resistance vessels during exercise. Second, combined β1/β2-adrenoceptor blockade caused a further downward rotation of the relationship between MV̇O2 and PcvO2, indicating that β2-adrenoceptors also contribute to the β-adrenergic feedforward vasodilation. Third, isoproterenol-induced coronary vasodilation was not affected by β1-adrenoceptor blockade but was abolished by the combined β1/β2-adrenoceptor blockade, indicating that isoproterenol-induced vasodilation was principally mediated by β2-adrenoceptors. Finally, norepinephrine-induced coronary vasodilation was attenuated by β1-adrenoceptor blockade, indicating that norepinephrine exerts its effects on the coronary vasculature principally through β1-adrenoceptors. The implications of these findings will be discussed below.
In the present study we assessed the role of β1-adrenoceptors in coronary vasodilation in response to treadmill exercise by using the selective β1-adrenoceptor antagonist metoprolol. However, to examine the role of β2-adrenoceptors, selective β2-adrenoceptor blockade is not suitable since selective β2-adrenoceptor blockade not only targets postsynaptic β2-receptors but can also blunt the presynaptic β2-receptor-mediated facilitation of norepinephrine release, resulting in lower norepinephrine levels (5, 46). In view of the lack of significant α-adrenergic resistance vessel control in swine (12, 39), a reduction in norepinephrine would lead to a reduced postsynaptic β1-adrenoceptor-mediated coronary vasodilation. To avoid this confounding influence, we studied the effect of β2-adrenoceptor blockade in the presence of β1-adrenoceptor blockade.
The interpretation of our results critically depends on the selectivity of metopropol as a β1-adrenoceptor antagonist. There is general agreement that β-adrenergic vasodilation of the resistance vessels within skeletal muscles and subcutaneous and cutaneous tissue is principally mediated via β2-adrenoceptors (2, 4, 8, 17, 18). Importantly, the isoproterenol- and exercise-induced systemic vasodilation, which were abolished and blunted, respectively, by propranolol, were not affected by metoprolol, indicating that metoprolol, in the dose employed, resulted in selective β1-adrenoceptor blockade. Moreover, the exercise-induced positive chronotropic and particularly inotropic effects were significantly attenuated by both metoprolol and propranolol, indicating significant β1-adrenergic receptor blockade by both β-blocking agents. Furthermore, the effects of metoprolol and propranolol on the relationship between heart rate and LV dP/dtmax were virtually identical (Fig. 2), suggesting a similar level of β1-adrenoceptor blockade. Conversely, the effect of propranolol on isoproterenol-induced increases in heart rate and LV dP/dtmax was significantly greater than the effect of metoprolol. These findings could be interpreted to suggest that propranolol produced a significantly greater degree of β1-adrenoceptor blockade compared with metoprolol. However, the unperturbed systemic vasodilation and drop in aortic blood pressure in response to isoproterenol (in the presence of metoprolol) likely resulted in baroreflex-mediated inactivation of the parasympathetic nervous system and activation of the sympathetic nervous system. Consequently, heart rate may have increased because of parasympathetic withdrawal and isoproterenol- and/or norepinephrine- and epinephrine-mediated activation of atrial β2-adrenergic receptors, which may constitute up to 30% of the atrial β-adrenoceptors in the mammalian heart (5, 51). Similarly, LV dP/dtmax may have increased because of ventricular β2-adrenoceptor activation, constituting up to 20% of the ventricular β-adrenoceptors (5, 51), in conjunction with the positive inotropic effect of the increase in heart rate (treppe or staircase effect) (28). In contrast, propranolol abolished the β2-adrenoceptor-mediated chronotropic and inotropic effects of isoproterenol as well as the β2-mediated systemic vasodilation, preventing both direct and indirect effects of isoproterenol on heart rate and contractility.
The investigation of the functional role of β-adrenoceptor subtypes in sympathetic feedforward coronary vasodilation during exercise is complicated by the direct myocardial effects of not only β1- but also β2-adrenoceptor activation (5, 51). Thus the use of β-receptor antagonists will affect not only coronary vascular but also myocardial β-adrenoceptors, thereby reducing heart rate and contractility and hence myocardial metabolic demand and indirectly reducing coronary blood flow. The matching of coronary resistance vessel tone to myocardial O2 demand is best studied by examining the relationship between coronary venous O2 levels and MV̇O2 (10, 26, 44). An increase in resistance vessel tone decreases myocardial O2 supply at a given level of MV̇O2, resulting in an increase in myocardial O2 extraction and thus a decrease in coronary venous O2 levels. The coronary venous O2 level thus represents an index of myocardial tissue oxygenation, i.e., the balance between O2 delivery and O2 consumption, that is ultimately determined by coronary resistance vessel tone (10, 26, 44). ScvO2 and PcvO2 represent different aspects of O2 sensing in the tissue. PcvO2 is a reflection of tissue Po2 and as such gives direct information about the oxygenation status of the myocardium and the smooth muscle cells in the coronary resistance vessels, which likely plays a role in the release of mediators of vasodilation from cardiomyocytes and/or the activity of K+ channels in vascular smooth muscle cells (44, 45). More recently, erythrocytes have been shown to act as O2 sensors, as they can release ATP and/or nitric oxide in response to a decrease in Hb O2 saturation (14). These observations suggest that both ScvO2 and PcvO2 may represent important “target” variables in the control of coronary resistance vessel tone. Consequently, in the present study we presented both PcvO2 and ScvO2 data in Figs. 4 and 6.
Role of β1- and β2-Adrenoceptors in Coronary Resistance Vessel Dilation Produced by Pharmacological Agents and Exercise
β1-Adrenoceptors are the predominant receptor subtype in large conduit coronary arteries of various species, including humans (1, 42), monkeys (42), cows (38, 47, 52), dogs (35, 36, 42, 48, 49), and pigs (40). However, several studies have demonstrated the presence of significant numbers of β2-receptors as well on large coronary arteries (1, 40, 47, 52). These radioligand binding studies are corroborated by observations in isolated large coronary arteries that both β1- and β2-receptors contribute to the vasodilation induced by isoproterenol (35, 36, 38, 42) and by in vivo observations showing that, under constant coronary inflow conditions, coronary conductance vessel dilation by isoproterenol or epinephrine involves both β1- and β2-adrenoceptors (47, 49, 52).
There is general agreement that β2-adrenoceptors are the predominant receptor subtype in the coronary microcirculation. Thus radioligand binding studies in transmural sections of the dog heart demonstrated that arterioles 16–55 μm in diameter contained predominantly (∼90%) β2-adrenoceptors (34), which is consistent with almost exclusive β2-adrenoceptor mRNA and protein expression data in subendocardial and subepicardial coronary arterioles (∼75 μm; range, 50–100 μm) isolated from the porcine left ventricle (25). Also, in small coronary arteries and arterioles (average diameter, ∼72 μm; and range, 43–209 μm) isolated from the left ventricle of explanted failing human hearts, the principal adrenergic receptor appears to be the β2-adrenoceptors subtype (41). However, in the latter study, it cannot be excluded that β1-receptors may have been downregulated because of long-lasting cardiac failure. Finally, there is some evidence to suggest that in small resistance arteries (100–400 μm in diameter) there is a more equal distribution between β1- and β2-receptor subtype (34). These small arteries may contain as much as 25–40% of the total coronary vascular resistance (6, 7) so that both β1- and β2-adrenoceptors can contribute to coronary blood flow regulation.
Studies into the contribution of β1- and β2-adrenoceptor subtypes to regulation of coronary blood flow and coronary resistance vessel tone in the in situ canine heart have shown variable results. In an anesthetized dog model of long diastole, to exclude the confounding influence of alterations in MV̇O2, intracoronary injections of either isoproterenol (33) or norepinephrine (43) produced coronary resistance vessel dilation that was blunted by the β1-adrenoceptor antagonist practolol as well as the β2-adrenoceptor antagonist ICI-118551, indicating that both β1- and β2-adrenoceptors contributed to the isoproterenol- and norepinephrine-induced coronary resistance vessel dilation when chronotropic and inotropic effects are absent (33, 43). In contrast, intracoronary injections of isoproterenol and the β2-adrenoceptor agonist pirbuterol, but not the β1-adrenoceptor agonist prenalterol, resulted in an increase in PcvO2 in awake dogs (48). In addition, the isoproterenol-induced increase in PcvO2 was insensitive to the β1-antagonist practolol (48), suggesting that in awake dogs the coronary resistance vessel dilation in response to intravascular administration of pharmacological agents is principally β2-receptor mediated. In accordance with these findings, isoproterenol resulted in an increase in PcvO2 in the present study that was also not affected by β1-blockade but attenuated by β1/β2-blockade. In contrast, we observed that intracoronary infusion of norepinephrine resulted in a small increase in PcvO2 that was abolished by metoprolol. Although we cannot exclude that higher dosages of norepinephrine might have been able to produce coronary vasodilation via β2-adrenoceptors, our findings suggest that in awake resting swine (in contrast to awake dogs), the coronary resistance vessel dilation produced by norepinephrine is principally mediated via β1-adrenoceptors. The differential involvement of the β1- and β2-adrenoceptors in the coronary vasodilator responses to norepinephrine and isoproterenol in the present study is likely related to their respective potency for the β1- and β2-adrenoceptors. Norepinephrine is a preferential β1-agonist, whereas isoproterenol can activate both β1- and β2-adrenoceptors (46).
Exercise results in the activation of the sympathetic nervous system and thereby an increase in circulating norepinephrine and epinephrine as well as in an increase in norepinephrine release from cardiac nerve fibers (19, 20, present study). The blockade of β-adrenoceptors has been shown to blunt the exercise-induced increase in myocardial O2 supply in excess of the decrease in myocardial O2 demand in dogs (3, 9, 19, 26, 31), humans (13, 27, 50), and swine (12, present study), consistent with the concept of β-feedforward coronary vasodilation during exercise. In the present study, we observed that selective β1-adrenoceptor blockade resulted in a downward rotation of the relationship between MV̇O2 and PcvO2, reflecting a progressive blunting of exercise-induced coronary vasodilation. Additional blockade of β2-adrenoceptors resulted in a further downward rotation of this relationship between MV̇O2 and PcvO2. These findings demonstrate for the first time that both β1- and β2-adrenoceptors contribute to β-adrenergic coronary resistance vessel dilation during exercise.
Exercise-induced β-adrenergic feedforward vasodilation is principally mediated via the local release of norepinephrine from sympathetic nerve terminals, with minor contributions of circulating norepinephrine and epinephrine (10, 20, 21, 23) or neuronal release of epinephrine (29, present study). The release of norepinephrine from the sympathetic nerve terminals, which are located in the adventitial-medial border of the coronary arterial tree (16, 22), will principally stimulate the innervated β1-receptors (24), which then contribute to the exercise-induced coronary vasodilation. Indeed, our experiments with intracoronary norepinephrine infusions confirm that norepinephrine preferentially activates the coronary β1-adrenoceptors (46). The involvement of the β2-adrenoceptors in the exercise-induced coronary vasodilation is likely to be due, at least in part, to the activation of the noninnervated β2-receptors located on the endothelium (37) and in the medial layer (25) of the coronary microcirculation by the exercise-induced increase in circulating levels of epinephrine.
In conclusion, in swine coronary blood flow and myocardial O2 demand are tightly coupled to maintain a constant O2 extraction and coronary venous Po2 during exercise. The present study demonstrates that both β1- and β2-adrenoceptors contribute to the exercise-induced feedforward vasodilation in the porcine coronary microcirculation.
This study was supported by Shan Xi Province, P. R. China, (oversea scholarship to F. Gao), as well as from The Netherlands Heart Foundation Grant 2000T042 (to D. Merkus and V. J. de Beer).
No conflicts of interest are declared by the author(s).
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