Subtypes of beta-adrenergic receptors in bovine coronary arteries

Remote ischemic preconditioning triggers changes in autonomic nervous system activity: implications for cardioprotection

Cardioprotective efficacy of remote ischemic preconditioning (RIPC) remains controversial. Experimental studies investigating RIPC have largely monitored cardiovascular changes during index ischemia and reperfusion with little work investigating changes during RIPC application. This work aims to identify cardiovascular changes associated with autonomic nervous system (ANS) activity during RIPC and prior to index ischemia. RIPC was induced in anesthetized male C57/Bl6 mice by four cycles of 5 min of hindlimb ischemia using inflated cuff (200 mmHg) followed by 5 min reperfusion. Electrocardiography (ECG) and microcirculatory blood flow in both hindlimbs were recorded throughout RIPC protocol. Heart rate variability (HRV) analysis was performed using ECG data. Hearts extracted at the end of RIPC protocol were used either for measurement of myocardial metabolites using high‐performance liquid chromatography or for Langendorff perfusion to monitor function and injury during 30 min index ischemia and 2 h reperfusion. Isolated‐perfused hearts from RIPC animals had significantly less infarct size after index ischemia and reperfusion (34 ± 5% vs. 59 ± 7%; mean ± SE P < 0.05). RIPC protocol was associated with increased heart rate measured both in ex vivo and in vivo. Frequency ratio of HRV spectra was altered in RIPC compared to control. RIPC was associated with a standard hyperemic response in the cuffed‐limb but there was a sustained reduction in blood flow in the uncuffed contralateral limb. RIPC hearts (prior to index ischemia) had significantly lower phosphorylation potential and energy charge compared to the control group. In conclusion, RIPC is associated with changes in ANS activity (heart rate, blood flow, HRV) and mild myocardial ischemic stress that would contribute to cardioprotection.

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.

Vascular nitric oxide: Beyond eNOS

As the first discovered gaseous signaling molecule, nitric oxide (NO) affects a number of cellular processes, including those involving vascular cells. This brief review summarizes the contribution of NO to the regulation of vascular tone and its sources in the blood vessel wall. NO regulates the degree of contraction of vascular smooth muscle cells mainly by stimulating soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), although cGMP-independent signaling [S-nitrosylation of target proteins, activation of sarco/endoplasmic reticulum calcium ATPase (SERCA) or production of cyclic inosine monophosphate (cIMP)] also can be involved. In the blood vessel wall, NO is produced mainly from l-arginine by the enzyme endothelial nitric oxide synthase (eNOS) but it can also be released non-enzymatically from S-nitrosothiols or from nitrate/nitrite. Dysfunction in the production and/or the bioavailability of NO characterizes endothelial dysfunction, which is associated with cardiovascular diseases such as hypertension and atherosclerosis.

Both beta1- and beta2-adrenoceptors contribute to feedforward coronary resistance vessel dilation during exercise

During exercise, beta-feedforward coronary vasodilation has been shown to contribute to the matching of myocardial oxygen supply with the demand of the myocardium. Since both beta(1)- and beta(2)-adrenoceptors are present in the coronary microvasculature, we investigated the relative contribution of these subtypes to beta-feedforward coronary vasodilation during exercise as well as to infusion of the beta(1)-agonist norepinephrine and the beta(1)- and beta(2)-agonist isoproterenol. Chronically instrumented swine were studied at rest and during graded treadmill exercise (1-5 km/h) under control conditions and after beta(1)-blockade with metoprolol (0.5 mg/kg iv) and beta(1)/beta(2)-blockade with propranolol (0.5 mg/kg iv). The selectivity and degree of beta-blockade of metoprolol and propranolol were confirmed using isoproterenol infusion (0.05-0.4 microg. kg(-1).min(-1)) under resting conditions. Isoproterenol-induced coronary vasodilation was mediated through the beta(2)-adrenoceptor, whereas norepinephrine-induced coronary vasodilation was principally mediated through the beta(1)-adrenoceptor. Exercise resulted in a significant increase in left ventricular norepinephrine release and epinephrine uptake. beta(1)-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 Po(2). Consequently, metoprolol caused a clockwise rotation of the relationship between myocardial oxygen consumption and coronary venous Po(2). Additional beta(2)-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 Po(2). In conclusion, both beta(1)- and beta(2)-adrenoceptors contribute to the beta-feedforward coronary resistance vessel dilation during exercise.

Effects of endothelin-1 on the relaxation of rat coronary arteries

To analyze the effects of endothelin-1 on the b-adrenergic response of the coronary circulation, 2-mm-long segments of coronary arteries from rats were prepared for isometric tension recording in organ baths. The relaxation to isoproterenol (3 x 10(-8) M), field electrical stimulation (4 Hz, 0.1-millisecond duration, 10 seconds), acetylcholine (3 x 10(-8) M), and sodium nitroprusside (10(-9) M) was recorded in arteries precontracted with U46619 (10(-7) to 5 x 10(-7) M) before and after treatment with endothelin-1 (3 3 10210 and 1029 M). The relaxation to isoproterenol was increased by treatment with endothelin-1 and with the endothelin ET(B) antagonist BQ788 (10(-6) M) but not with the endothelin ET(A) antagonist BQ123 (10(-6) M) or with the blocker of protein kinase C chelerythrine (10(-5) M). In the presence of BQ788, BQ123, or chelerythrine, endothelin-1 did not modify the relaxation to isoproterenol. Treatment with endothelin-1 did not modify the relaxation to electrical stimulation, acetylcholine, or sodium nitroprusside. These results suggest that endothelin-1 may potentiate coronary beta-adrenergic vasodilatation, at least in part due to stimulation of endothelin ET(A) receptors and activation of protein kinase C.

The rush to adrenaline: drugs in sport acting on the beta-adrenergic system

Athletes attempt to improve performance with drugs that act on the beta-adrenergic system directly or indirectly. Of three beta-adrenoceptor (AR) subtypes, the beta(2)-AR is the main target in sport; they have bronchodilator and anabolic actions and enhance anti-inflammatory actions of corticosteroids. Although demonstrable in animal experiments and humans, there is little evidence that these properties can significantly improve performance in trained athletes. Their actions may also be compromised by receptor desensitization and by common, naturally occurring receptor mutations (polymorphisms) that can influence receptor signalling and desensitization properties in individuals. Indirectly acting agents affect release and reuptake of noradrenaline and adrenaline, thereby influencing all AR subtypes including the three beta-ARs. These agents can have potent psychostimulant effects that provide an illusion of better performance that does not usually translate into improvement in practice. Amphetamines and cocaine also have considerable potential for cardiac damage. beta-AR antagonists (beta-blockers) are used in sports that require steadiness and accuracy, such as archery and shooting, where their ability to reduce heart rate and muscle tremor may improve performance. They have a deleterious effect in endurance sports because they reduce physical performance and maximum exercise load. Recent studies have identified that many beta-AR antagonists not only block the actions of agonists but also activate other (mitogen-activated PK) signalling pathways influencing cell growth and fate. The concept that many compounds previously regarded as 'blockers' may express their own spectrum of pharmacological properties has potentially far-reaching consequences for the use of drugs both therapeutically and illicitly.

Treatment of vascular smooth muscle cells with estradiol and beta-adrenergic agonists has an additive effect on cAMP levels, but no additive effect on inhibition of collagen synthesis

Several studies have suggested that increased cell levels of cAMP result in decreased rates of collagen synthesis. Oestrogen treatment of vascular smooth muscle cells (VSMCs) has been shown to cause increased levels of cAMP and decreased rates of collagen synthesis. Beta-adrenergic agonists are also known to increase cellular levels of cAMP in VSMCs, although the effect of beta-adrenergic agonists on collagen synthetic rates in VSMCs is unknown. Since beta-agonists and oestrogens are commonly used clinical agents these studies were conducted to determine the potential of these agents to have an additive effect on cell cAMP levels and inhibition of collagen synthetic rates. When VSMCs were treated with both oestrogen and isoproterenol there was an additive effect on cellular cAMP levels although the observed decrease in collagen synthetic rates was the same as observed in cells treated with just oestrogen. Treatment of VSMCs with propranolol inhibited isoproterenol-induced changes in cAMP but had no effect on either oestrogen-induced increases in cAMP levels or inhibition of collagen synthesis. The cellular location of cAMP following beta-adrenergic agonist treatment was different from the distribution of cAMP in control or oestrogen-treated VSMCs. This difference in cellular distribution of cAMP may partially explain the absence of collagen synthesis inhibition following beta-adrenergic agonist treatment of VSMCs.

Denopamine, a selective beta1-receptor agonist and a new coronary vasodilator

Up until now, it has been suggested that nitrate and/or calcium channel blockers were effective against variant angina pectoris. On the other hand, it is known that about 20% of variant angina pectoris was refractory to both nitrate and calcium channel blockers. In Japan, it has been reported that denopamine, which is an oral beta1-adrenoceptor selective agonist developed by the Japanese pharmaceutical industry (Tanabe Seiyaku), is effective in those refractory cases. To date, in Japan nine cases have been recognized of patients with vasospastic angina pectoris whose symptoms were relieved by taking denopamine, including one case in which the author has had personal experience. Eight of these nine cases were refractory, and were not relieved by combined therapy using both nitrate and a calcium channel blocker. It was also documented that denopamine was effective in cases where attacks were not relieved by prazosin or magnesium, which have been documented as effective in other refractory cases. In a study of canine coronary arteries, localization of beta-adrenoceptor subtypes was documented, with the beta1-adrenoceptor predominantly found in the conduit coronary artery. In recent years it has been emphasized that the principal role of sympathetic nerves was not associated with the constrictive action of alpha-adrenoceptors, but with the coronary dilative action of beta-adrenoceptors. It would therefore be worthwhile to determine whether denopamine is able to relieve vasospastic angina pectoris in many more cases.

Differential distribution of beta-adrenergic receptor subtypes in blood vessels of knockout mice lacking beta(1)- or beta(2)-adrenergic receptors

beta-Adrenergic receptors (beta-AR) are essential regulators of cardiovascular homeostasis. In addition to their prominent function in the heart, beta-AR are located on vascular smooth muscle cells, where they mediate vasodilating effects of endogenous catecholamines. In this study, we have investigated in an isometric myograph different types of blood vessels from mice lacking beta(1)- and/or beta(2)-adrenergic receptor subtypes (beta(1)-KO, beta(2)-KO, beta(1)beta(2)-KO). In wild-type mice, isoproterenol induced relaxation of segments from thoracic aorta, carotid, femoral and pulmonary arteries, and portal vein. The relaxant effect of beta-receptor stimulation was absent in femoral and pulmonary arteries from beta(1)-KO mice. In aortic and carotid arteries and in portal veins, the vasodilating effect of isoproterenol was reduced in mice lacking beta(1)- or beta(2)-receptors. However, in these vessels the vasodilating effect was only abolished in double KO mice lacking both beta(1)- and beta(2)-receptors. Vessel relaxation induced by forskolin did not differ between wild-type and KO mice. Similar contributions of beta(1)- and beta(2)-receptors to isoproterenol-induced vasorelaxation were found when vessels from KO mice were compared with wild-type arteries in the presence of subtype-selective beta-receptor antagonists. These studies demonstrate that beta(1)-adrenergic receptors play a dominant role in the murine vascular system to mediate vasodilation. Surprisingly, beta(2)-receptors contribute to adrenergic vasodilation only in a few major blood vessels, suggesting that differential distribution of beta-adrenergic receptor subtypes may play an important role in redirection of tissue perfusion.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Neural regulation of coronary vascular resistance: role of nitric oxide in reflex cholinergic coronary vasodilation in normal and pathophysiologic states

A number of reflexes participate in the control of coronary vascular resistance through activation of the sympathetic or parasympathetic nervous system. Classically, activation of vagal efferent fibers to the heart results in vasodilation due to the release of acetylcholine and activation of muscarinic receptors. Recently, we have found that activation of a number of reflexes in conscious dogs, the Bezold-Jarisch reflex and the carotid chemoreflex in particular, results in cholinergic coronary vasodilation which is blocked by an inhibitor of nitric oxide synthesis, nitro-L-arginine. After the development of pacing-induced heart failure, the cholinergic dilation subsequent to activation of the Bezold-Jarisch or carotid chemoreflex is essentially abolished, since coronary blood vessels no longer produce nitric oxide. In contrast, after brief exercise training, there is a potentiation of Bezold-Jarisch reflex-induced coronary vasodilation since exercise upregulates nitric oxide production by coronary blood vessels. Since the Bezold-Jarisch reflex may be important as a compensatory mechanism during acute myocardial infarction, and the carotid chemoreflex is the acute mechanisms responsible for ameliorating systemic hypoxemia, the role of nitric oxide in reflex cholinergic coronary vasodilation may be essential in the compensatory vascular adjustments evoked by these and other reflexes.

Neuronal control of coronary blood flow

Controversies on acetylcholine-induced increases or decreases in coronary blood flow arise from obvious species differences, the role of endothelium in mediating vascular smooth muscle responses, and the marked negative chronotropic and inotropic effects of acetylcholine. In man, there appears to be a predominant dilation of intact epicardial coronary arteries and a constriction of artherosclerotic segments. However, at present there is no evidence for a vagal initiation of myocardial ischemia. Coronary vascular beta-adrenergic receptors mediate dilation, but appear to be functionally insignificant during sympathetic activation. The beta-adrenergic mechanism contributing to myocardial ischemia are indirect, mediated by a tachycardia-related redistribution of blood flow away from the ischemic myocardium. alpha-Adrenergic receptors mediating epicardial coronary artery constriction in experimental studies appear not to be responsible for the initiation of ischemia in patients with angina at rest. However, alpha-adrenergic constriction of coronary resistance vessels resulting in the precipitation of post-stenotic myocardial ischemia was demonstrated in experimental studies and recently confirmed in patients with effort angina. Non-adrenergic, non-cholinergic neurotransmitters exist; however, their role in regulating coronary blood flow remains entirely unclear.

Effects of oral pretreatment with metoprolol on left ventricular wall motion, infarct size, hemodynamics, and regional myocardial blood flow in anesthetized dogs during thrombotic coronary artery occlusion and reperfusion

OBJECTIVES

To study the effects of oral pretreatment with metoprolol over 3 days on hemodynamics, left ventricular function, regional myocardial blood flow, and infarct size in an anesthetized dog model of thrombotic occlusion of the anterior descending coronary artery treated with thrombolysis.

METHODS

Ten dogs received 200 mg metoprolol (Selozok) orally and 8 dogs received placebo for 3 days twice daily and 1 hour before the experiment. Under general anesthesia, thrombotic occlusion was provoked by the copper-coil technique. Intracardiac pressures and their derivatives, cardiac output (thermodilution method), regional coronary blood flow (microspheres), global and regional left ventricular function (ventriculography), and infarct size (triphenyltetrazolium staining) were measured. Measurements were performed during control, after 60 minutes of occlusion, and after 30 and 90 minutes of reperfusion. Thrombolysis was performed in all dogs 60 minutes after occlusion by intravenous infusion of 10 micrograms/kg/min of rt-PA for 30 minutes.

RESULTS

During control cardiac output was lower, total peripheral resistance higher, and Tau and the left ventricular isovolumic relaxation time greater in the metoprolol group. During occlusion and after reperfusion, there were no significant hemodynamic differences between both groups. Blood flow to the area at risk and circumflex territory during occlusion were, respectively, 12.8 +/- 5.80 ml/100 g/min versus 9.65 +/- 8.35 ml/100 g/min (p > 0.05) and 42.58 +/- 7.86 ml/100 g/min versus 61.52 +/- 20.43 ml/100 g/min (p = 0.01) in the metoprolol- and placebo-treated dogs. The ratios of flow area at risk/circumflex territories in the epicardial, midmyocardial, and endocardial layers were, respectively, 0.44 +/- 0.20, 0.19 +/- 0.09, and 0.20 +/- 0.13 in the metoprolol- versus 0.24 +/- 0.16, 0.08 +/- 0.06, and 0.06 +/- 0.07 (p > or = 0.04) in the placebo-treated dogs. The ratio of flow endocardium/epicardium was higher (p > or = 0.02) in the active treatment group during the control period, both in the area at risk and circumflex territory; this was also the case in the circumflex territory at the end of the experiment (p = 0.003). Thirty minutes after occlusion, blood flow to the three layers of the area at risk rose to 2-3 times control values in both groups; a significant increase above control values also occurred in the circumflex territory. After 90 minutes reperfusion, blood flow to both territories was similar in both groups but was comparable to the control; however, in necrotic tissue of the subendocardial layer of both groups, flow fell below control values (p < 0.05). End-systolic volume rose from 21.2 +/- 7.4 ml to 36.1 +/- 11.5 ml (p < 0.05), end-diastolic volume remained constant (46.0 +/- 13.8 vs. 47.9 +/- 12.1 ml; p > 0.05), and ejection fraction fell from 53.9 +/- 8.3% to 25.8 +/- 10.2% (p < 0.05) at the end of the experiment in the metoprolol group. Respective figures for the placebo group were 19.4 +/- 7.9 versus 27.9 +/- 10.9 (p < 0.05), 38.5 +/- 13.0 versus 42.1 +/- 11.0 (p > 0.05), and 50.6 +/- 5.7 versus 35.5 +/- 11.7 (p < 0.05). Fractional shortening of the chords analyzed was similar in both groups during the control period; it fell significantly at the end of the experiment in three chords of the metoprolol group and in five chords of the placebo group. The apical chord in the placebo, but not in the metoprolol, dogs was dyskinetic: fractional shortening was -0.86 +/- 9.7 versus 7.5 +/- 13.5% (p > 0.05). The area at risk was 41.6 +/- 10.6 cm2 in metoprolol- and 40.5 +/- 7.2 cm2 in placebo-treated dogs (p > 0.05); the infarct size, expressed as a percentage of the area at risk, was 29.0 +/- 22.5% and 45.3 +/- 23.6% (p = 0.02), respectively.

CONCLUSIONS

Oral pretreatment with metoprolol limited infarct size and improved regional left ventricular function, probably due to its negative chronotropic and inotropic effects, and also due to an enhancement of collateral flow fr

Alpha- and beta-adrenergic control of large coronary arteries in conscious calves

Large and small coronary arteries are subject to control by alpha- and beta-adrenergic mechanisms. However, controversy exists as to the distribution and physiological effects of alpha- and beta-adrenergic receptor subtypes in large coronary arteries. Studies in our laboratory have addressed these questions in conscious calves, chronically instrumented to measure large coronary artery diameter and coronary blood flow. Additionally, adrenergic receptor subtype distribution was determined using ligand binding assays in membrane preparations isolated from large coronary arteries of calves. Physiological results demonstrate, in contrast to the results of most previous studies, that both alpha 1- and alpha 2-adrenergic receptors elicit constriction of the large coronary artery. Studies with ganglionic blockade indicate that the constriction was unaltered by autonomic reflexes or presynaptic release of neurotransmitters. Selective beta-adrenergic receptor activation demonstrated that both beta 1- and beta 2-adrenergic receptors elicit dilation of large coronary arteries, and that the vasodilation was direct, i.e., it was not mediated by increases in coronary blood flow. Biochemical characterization of adrenergic subtype density indicated the presence of both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes. Thus, both biochemical and physiological data support the concept that large coronary arteries are regulated by both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes.

Endothelium and blood flow mediated vasomotion in the conscious dog

Numerous in vitro studies have demonstrated the important role of the vascular endothelium on the vasoactivity of vascular smooth muscle. Experimentation, particularly in conscious animals, is required to study the integrated role of endothelium in the regulation of vascular tone. This article reviews some of the evidence demonstrating endothelium mediated vasodilation and inhibition of vasoconstriction by the endothelium in the chronically instrumented conscious animal. Furthermore, a role for endothelial cells has been shown in the mechanism of blood flow-mediated vasodilation. Finally, the endothelium, through elaboration of constricting factors, e.g., endothelin, can also induce potent vasoconstriction. In the conscious animal endothelin elicits markedly differing degrees of vasoconstriction among the various regional vascular beds.

Regional perfusion in hearts with acute coronary artery occlusion and subsequent beta 2- and alpha 1-adrenergic blockade

Blockade of cardiac adrenoceptor subtypes, coronary or myocardial, might elicit compensatory interaction from remaining unblocked subtypes. An attempt to explore this interplay was made by studying regional myocardial blood flow alterations associated with beta 2-adrenergic blockade followed by alpha 1-adrenergic blockade in anaesthetized cats with acute coronary occlusion. In order to maintain constant needs for perfusion, atrial pacing was established and the aortic blood pressure was kept constant. In myocardium remote from the ischaemic region, beta 2-adrenergic blockade produced higher endocardial blood flow whereas no flow changes were observed close to the ischaemic region. With subsequent alpha 1-adrenergic blockade, blood flow increased endocardially in non-ischaemic regions, but remained unchanged in epicardial tissue. Control experiments without coronary ligation revealed no increase in left ventricular oxygen consumption during the experiments and support the theory that the observed blood flow increase in the coronary ligation group, following drug interventions, was not caused by increased cardiac work. This study indicates that combined beta 2- and alpha 1-adrenergic blockade alters the balance between receptor subtypes. Unopposed beta 1-mediated vasodilation is the most likely candidate to explain why endocardial flow was increased.

Coronary vasomotion in response to sympathetic stimulation in humans: importance of the functional integrity of the endothelium

The coronary vasomotor response to the cold pressor test was studied with use of quantitative coronary angiography in 32 patients without evidence of coronary artery disease and 55 patients with such disease; in a subset of 22 patients (9 with normal coronary arteries and 13 with coronary artery disease), the effects of the cold pressor test were compared with the effects of the endothelium-dependent vasodilator acetylcholine with simultaneous intracoronary Doppler flow velocity measurements to assess the influence of endothelial dysfunction. The cold pressor test induced vasodilation of 8.9 +/- 5.7% in all 77 analyzed vessel segments of the group with normal arteries (p less than 0.01). In contrast, in patients with coronary artery disease, the 52 analyzed stenotic segments were constricted by -12.1 +/- 9.5% (p less than 0.01), the 57 analyzed vessel segments with luminal irregularities were constricted by -8.9 +/- 5.2% (p less 0.01) and 40 (85%) of 47 angiographically normal segments also were constricted by -7.0 +/- 4.9% (p less than 0.05). Preserved vasodilating capability was demonstrated by intracoronary nitroglycerin in all analyzed segments. In nine patients with normal coronary arteries, the analyzed vessel segments were dilated in response to both the cold pressor test and intracoronary acetylcholine by 10.9 +/- 5.4% and 13.4 +/- 4.7%, respectively. In contrast, in all 13 patients with coronary artery disease, vasoconstriction of identical vessel segments by -9.1 +/- 3.7% and -23 +/- 10.4%, respectively, was observed after both the cold pressor test and intracoronary acetylcholine. Intracoronary propranolol did not significantly affect either the vasodilative response in 11 normal coronary arteries (11.3 +/- 4.4% before and 8.6 +/- 4.3% after beta-blockade) or the vasoconstrictor response in 8 atherosclerotic coronary arteries (-11.4 +/- 4.6% before and -14.6 +/- 5.3% after beta-blockade). The dilation of normal and the constriction of atherosclerotic coronary arteries with cold pressor testing exactly mirror the response to the endothelium-dependent dilator acetylcholine. Endothelial dysfunction in coronary atherosclerosis resulted in a loss of normal dilator function and permitted vasoconstrictor responses to sympathetic stimulation. Thus, coronary vasomotion of large epicardial arteries in response to sympathetic stimulation by the cold pressor test in humans is intimately related to the integrity of endothelial function.

Pharmacology of acute effort angina

From the pharmacologic point of view, each of the major types of antianginal agents--calcium antagonists, beta-blockers, and nitrates--seem to act at least in part by an improvement of the myocardial blood supply. The recently elucidated mechanism of action of nitrates, acting on a common pathway with the endothelium-derived relaxation factor (EDRF), suggests an important role for guanylate cyclase and cyclic GMP in maintaining coronary artery patency in patients with coronary atheroma. The efficacy of calcium antagonists, even in effort-induced angina, is in accord with a current hypothesis that physical exercise in the presence of coronary stenosis can cause relative coronary vasoconstriction, or at the least, failure of full dilation. Therefore, calcium antagonists all act, at least in part, on the "supply" side of the supply-demand equation. Beta-adrenergic blockers appear to have as their major mode of action a reduction of heart rate, which not only reduces the oxygen demand but, through an anti-ischemic effect, also appears to improve the endocardial blood supply (in relation to the heart rate). Thus beta-blockade indirectly enhances the supply side of the equation. The intriguing situation arises whereby all three major types of antianginal compounds may also act by a common mechanism of anginal relief, namely, improvement in the coronary blood supply, in addition to the diverse mechanisms specific to each type of compound. That conclusion does not mean the the "demand" side of the equation can be ignored. Rather, the critical importance of a reduced myocardial blood supply in the production of anginal syndromes is highlighted.

Delineation of the distribution of beta-adrenergic receptor subtypes in canine myocardium

beta 2-Receptors constitute only 10-30% of the total beta-adrenergic receptors in mammalian ventricular myocardium, but their precise tissue location cannot be determined easily by measuring physiological variables. To delineate the distribution of beta-receptor subtypes in myocytic and vascular components of the heart, we incubated transmural sections of canine left ventricle with [125Iodo]cyanopindolol and selected concentrations of the beta 1-selective antagonist betaxolol or the beta 2-selective antagonist ICI 118,551. Detailed competition binding data were best accounted for by a two-site model in which approximately 75% of total sites were beta 1- and 25% were beta 2-receptors. The relative proportions of beta-receptor subtypes in myocytic and vascular components were assessed autoradiographically by analyzing the density of binding sites in transmural sections incubated with radioligand and subtype-selective displacers. Betaxolol (10(-7) M) reduced the density of radioligand binding sites by 44% in regions composed primarily of ventricular myocytes but by less than 5% in small coronary arterioles. ICI 118,551 (10(-7) M) reduced radioligand binding-site density by 18% in myocytic regions and by 55% in small arterioles. In myocytic regions, these data indicated a subtype composition of approximately 85% beta 1- and 15% beta 2-sites. In contrast, arterioles contained almost exclusively the beta 2-subtype. The diameters of coronary vessels in which beta 2-receptors were found to be selectively increased fell within a narrow range (mean +/- SD, 35 +/- 11 microns; range, 16-55 microns). Small mural arteries and venules did not contain a significantly higher proportion of beta 2-receptors than adjacent myocytic regions.

Effect of atenolol and pindolol on the phorbol ester-induced coronary vasoconstriction in the isolated perfused heart of the rat

1. The effects of atenolol (beta 1-adrenoceptor antagonist without partial agonistic activity) and pindolol (beta 1- and beta 2-antagonist with partial agonistic activity) were studied on basal coronary vascular tone and on the phorbol ester-induced coronary vasoconstriction in the rat perfused heart. 2. The addition of the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA; 1.8 X 10(-8)-1.6 X 10(-7) M) into the perfusion fluid during perfusion of rat heart at constant flow caused a dose-dependent, sustained increase in perfusion pressure. The vasoconstrictor response in hearts of reserpine-treated rats to infusion of TPA was similar to that of non-reserpine treated hearts. 3. Infusion of a calcium channel agonist Bay K 8644 at a concentration of 4 X 10(-7) M enhanced, whereas isoprenaline (1 X 10(-5) M), dibuturyl-cyclic AMP (1.6 X 10(-4) M) and forskolin (1 X 10(-6) M), which elevate intracellular concentrations of cyclic AMP, all inhibited the coronary vasoconstriction induced by TPA. 4. Pindolol, in doses which produced comparable inhibition of isoprenaline-induced tachycardia, dose-dependently attenuated the phorbol ester-induced increase in perfusion pressure, whereas atenolol had no effect. The inhibitory action of pindolol (2 X 10(-5) M) on TPA-induced vasoconstriction was blocked by addition of 2.2 X 10(-5) M propranolol into the perfusion fluid. When infused alone, atenolol (2 X 10(-4) M) significantly increased coronary vascular tone, but pindolol had no effect. 5. The present results indicate that pindolol has coronary vasodilator properties due to stimulation of vascular beta-adrenoceptors. If stenosis dilatation of coronary artery spasm is an important component of the anti-anginal effect of beta-blocking drugs, the possession of partial agonistic property by a beta-blocking drug may be of importance in maintaining coronary flow.

Parasympathetic coronary vasoconstriction induced by nicotine in conscious calves

We studied the effects of intracoronary injection of nicotine and acetylcholine on coronary blood flow in nine conscious calves chronically instrumented to measure coronary blood flow, left ventricular (LV) and mean arterial pressure, LV dP/dt, and heart rate. Nicotine (5 micrograms/kg i.c.) elicited a biphasic response in coronary blood flow consisting of an initial vasoconstriction (phase 1; blood flow fell by 52 +/- 5.4% from a baseline of 66 +/- 7.5 ml/min) followed by vasodilation (phase 2, blood flow rose 119 +/- 12.7% above baseline). The change in coronary blood flow with nicotine was not associated with changes in LV systolic pressure, mean arterial pressure, or heart rate. The change in coronary blood flow was unaffected by combined alpha- and beta-adrenoceptor blockade with prazosin, rauwolscine, and propranolol but was abolished by either muscarinic blockade with atropine or ganglionic blockade with hexamethonium. Acetylcholine (0.5 microgram/kg i.c.), without affecting mean arterial pressure, elicited changes in coronary blood flow similar to those observed with nicotine, producing an initial phase of coronary vasoconstriction (blood flow fell by 71 +/- 4.9%) followed by vasodilation (blood flow rose by 228 +/- 20.7%). Both phases of the response to acetylcholine were abolished by muscarinic blockade but were unaffected by ganglionic blockade. When nicotine was injected into the left circumflex coronary artery, no change in blood flow was observed in the left anterior descending coronary artery, indicating the lack of involvement of global reflex pathways. These results suggest that nicotine locally stimulates parasympathetic nerves, which constrict the coronary circulation via a muscarinic mechanism.

Evidence that high affinity (3H)clonidine binding cooperates with H2-receptors in the canine coronary smooth muscle membrane

We have investigated the effects of myocardial ischemia and exogenous histamine and 4-methylhistamine on the regulation of membrane bound alpha 2- and beta-adrenoreceptors (ARs) in the canine coronary artery smooth muscle (CAS). The results indicate that exposure of CAS to ischemia and histamine is associated with the stimulation of adenylate cyclase and with a down-regulation of alpha 2-ARs which is accompanied by the sequestration of alpha 2-AR sites into light membrane particles. The increased number of beta-AR sites in CAS represents a c-AMP mediated adaptational pathway in compromised CAS.

Demonstration of beta 1-adrenoceptor mediating relaxation of porcine coronary artery by radioligand binding and pharmacological methods

beta-adrenoceptors in the porcine coronary artery were characterized by a radioligand binding assay using (-)-[3H]dihydroalprenolol (DHA) and also by measuring the relaxant response of isolated coronary artery to norepinephrine. Specific (-)-[3H]DHA binding in the porcine coronary artery was saturable, reversible and of high affinity (Kd = 1.6 nM) with a maximal number of binding sites of 63 fmol/mg protein, and it showed a pharmacological specificity as well as stereoselectivity which characterized beta-adrenoceptors. The Hofstee analysis of inhibition of (-)-[3H]DHA binding by atenolol, practolol and ICI 118551 has shown that the averaged concentration of beta 1 and beta 2-adrenoceptors in this tissue was 68% and 32% respectively. The relaxant response of isolated coronary artery to norepinephrine was competitively antagonized by (-)propranolol, (+)propranolol, atenolol, practolol and ICI 118551. The pA2 values of these adrenoceptor antagonists were significantly correlated with the Ki values for beta 1 but not beta 2-adrenoceptors determined by the (-)-[3H]DHA binding assay. Thus, the present study demonstrates that the relaxant response of porcine coronary artery to norepinephrine is predominantly mediated through the stimulation of beta 1-adrenoceptors on vascular smooth muscles.

Autonomic control of large coronary arteries and resistance vessels

The resistance coronary vessels as well as the large coronary arteries are regulated to a major extent by autonomic mechanisms. Neural and pharmacologic activation of alpha-adrenergic pathways can reduce coronary blood flow and the cross-sectional area of large coronary arteries, despite concomitant increases in coronary distending pressure. Activation of either beta 1- or beta 2-adrenergic receptors can induce potent dilation of both coronary resistance vessels and large coronary arteries, whereas blockade of these receptors can elicit profound constriction in the presence of elevated beta-adrenergic tone. The regulation of the coronary circulation by parasympathetic mechanism remains the most controversial, likely because of major species differences. Parasympathetic coronary dilation is uniformly observed in the dog, but in primates, important aspects of parasympathetic-mediated coronary vasoconstriction have also been demonstrated.