The role of alpha 1- and alpha 2-adrenoceptors in the coronary vasoconstrictor responses to neuronally released and exogenous noradrenaline in the dog

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.

Antidepressant-like action of intracerebral 6-fluoronorepinephrine, a selective full α-adrenoceptor agonist

The present study examined the ability of 6-fluoronorepinephrine (6FNE), a full selective α-adrenoceptor agonist, to produce antidepressant-like effects in mice. The drug, administered in the 4th ventricle, produced marked anti-immobility effects at mid-dose range in the acute forced swim, tail suspension and repeated open-space forced swim tests with minimal effect on open-field motor activity and also reversed anhedonia following lipopolysaccharide administration. Its antidepressant effects were equal to or greater than that of an established systemic antidepressant, desmethylimipramine, given subacutely. Experiments with α-adrenoceptor antagonists indicated that the drug acts primarily via the α2-receptor in contrast to endogenous catecholamines which appear to control depressive behaviour primarily via the α1-receptor. Antidepressant activity declined at higher doses signifying a possible pro-depressant effect of one of the α-adrenoceptor subtypes. Compared to the selective α2-agonist, dexmedetomidine, 6FNE showed equivalent antidepressant action in the tail suspension test but appeared to have a greater efficacy or speed of action in the repeated open-space forced swim test which produces a more sustained depression. Studies of regional brain Fos expression induced during the antidepressant tests showed that 6FNE tended to inhibit neural activity in two stress-responsive regions (locus coeruleus and paraventricular hypothalamus) but to enhance activity in two areas involved in motivated behaviour (nucleus accumbens shell and lateral septal nucleus) producing a neural pattern consistent with antidepressant action. It is concluded that 6FNE elicits a rapid and effective antidepressant and anti-stress response that may compare favourably with available antidepressants.

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.

Enhancement of noradrenergic constriction of large coronary arteries by inhibition of nitric oxide synthesis in anaesthetized dogs

1. Coronary vascular responses to bilateral carotid occlusion (BCO) and the intravenous infusion of tyramine (Tyr, 20 micrograms kg-1 min-1) and noradrenaline (NA, 0.5 microgram kg-1 min-1) were examined after bilateral vagotomy and antagonism of beta-adrenoceptors. BCO, Tyr and NA decreased large coronary artery diameter and increased mean coronary resistance and systemic arterial pressure without affecting heart rate. 2. Inhibition of nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NNA, 5 and 15 mg kg-1) significantly increased mean arterial pressure and decreased heart rate and large coronary artery diameter. Mean coronary resistance was unaffected by either dose of L-NNA. L-NNA significantly reduced depressor and coronary vasodilator responses to the endothelium-dependent vasodilator acetylcholine (ACh, 10 micrograms kg-1, i.v.). Systemic and coronary vasodilator responses to sodium nitroprusside (SNP, 5 micrograms kg-1) were unaffected by L-NNA with the exception that the dilatation of the large coronary artery was significantly enhanced by the higher dose. 3. L-NNA significantly enhanced constriction of the large coronary arteries caused by BCO, Tyr and NA but did not affect the increases in mean coronary resistance or systemic arterial pressure. 4. Inhibition of NO synthesis enhances adrenergic constriction of large coronary arteries caused by both neuronally released and exogenous noradrenaline. In contrast, L-NNA did not affect adrenergic constriction of coronary or systemic resistance vessels. Endothelium-derived NO may play an important role in the modulation of noradrenergic vasoconstriction in coronary conductance arteries.

Impact of alpha-adrenergic coronary vasoconstriction on the transmural myocardial blood flow distribution during humoral and neuronal adrenergic activation

Increased heart rate and left ventricular pressure during humoral and neuronal adrenergic activation act to restrict blood flow preferentially in the subendocardium. The hypothesis was advanced that alpha-adrenergic coronary vasoconstriction preferentially in the subepicardium may counterbalance the enhanced extravascular compression in the subendocardium and serve to maintain blood flow transmurally uniform. In 40 anesthetized dogs, regional myocardial blood flow was determined with colored microspheres; wall function, with sonomicrometry. Humoral adrenergic activation (HAA) was induced by a combination of intravenous atropine, intravenous norepinephrine, and atrial pacing during baseline coronary vasomotor tone (group 1, n = 6) and in the presence of maximal coronary vasodilation with intravenous dipyridamole (group 2, n = 6). In an additional group, HAA was induced by intravenous norepinephrine in the presence of dipyridamole but without atropine and atrial pacing in order to increase end-diastolic left ventricular pressure (group 3, n = 6). Measurements were performed at rest, during HAA, and during ongoing HAA with the intracoronary infusion of the alpha-antagonist phentolamine (Phen). At unchanged mean aortic pressure, Phen improved blood flow particularly to the inner layers as follows: from 1.42 +/- 0.40 (mean +/- SD) to 1.90 +/- 0.40 mL/(min.g) (group 1, P < .05), from 4.99 +/- 2.31 to 5.53 +/- 2.56 mL/(min.g) (group 2, P < .05), and from 6.01 +/- 1.41 to 6.29 +/- 1.27 mL/(min.g) (group 3, P < .05), associated with a decrease in outer layer blood flow in groups 2 and 3. In 16 additional dogs, beta-adrenoceptors were blocked by propranolol and muscarinic receptors by atropine. Neuronal adrenergic activation (NAA) was induced by cardiac sympathetic nerve stimulation (CSNS) during baseline coronary vasomotor tone (group 4, n = 8) and in the presence of maximal vasodilation (group 5, n = 8). Measurements were performed at rest, during a first CSNS, and 20 minutes later during a second CSNS+Phen. The reproducibility of two consecutive episodes of CSNS 20 minutes apart was demonstrated in a separate set of experiments (n = 6). At matched mean aortic pressures, Phen improved blood flow to all myocardial layers in group 4, whereas in group 5, Phen induced a redistribution of myocardial blood flow toward subepicardial layers [from 4.44 +/- 0.96 to 4.81 +/- 0.83 mL/(min.g), P < .05] at the expense of inner layers. With the addition of Phen, there was no change in regional wall function in any group of dogs studied. Thus, during HAA, alpha-adrenergic coronary vasoconstriction does not exert a beneficial effect on transmural blood flow distribution. During NAA, a beneficial effect of alpha-adrenergic coronary vasoconstriction becomes apparent only under conditions of maximal coronary vasodilation.

Causative role of coronary microvessels for the development and progression of chronic myocardial lesions in spontaneously hypertensive rats (SHR)

The pathomechanisms responsible for the development and progression of myocardial alterations in hypertensive heart disease are largely unknown. Using newly developed preparation and measuring procedures in 78 SHR and 82 controls aged 3-78 weeks, topological relations were detected between focal morphological appearances of chronic myocardial ischemia (fml.) and pathological microvessel (mv.) reactions characterized by morphometric signs of chronic contractions. The smallest ramifications are of particular pathogenic importance. A generalized peak of pathological mv. reactions between the 16th and 24th weeks is responsible for the development of first fml. The further progression of the area density of fml. from 1.26 +/- 0.85% (24th week) to 31.82 +/- 8.60% (78th week) is attributable to the further increase in pathological mv. reactions caused by organ-specific influences. The histological and morphometric findings suggest that the pathological mv. reactions are aggravated by their own effects at the local level.

Kinin receptors mediating the effects of bradykinin on the coronary circulation in anaesthetized greyhounds

Bradykinin (BK, 0.05 micrograms/kg) or glyceryl trinitrite (5 micrograms/kg) injected into the left circumflex coronary artery of anaesthetized, open-chest greyhounds, caused pronounced increases in large coronary artery diameter (CD) and coronary blood flow (CBF), whereas des-Arg9-BK (0.05-0.3 micrograms/kg), a selective bradykinin B1 agonist, dose dependently elevated CBF but had little effect on CD. BK-induced increases in CD and CBF were not affected by the intracoronary infusion of a selective B1 receptor antagonist, des-Arg9-[Leu8]BK (40 micrograms/min), but were significantly reduced by the infusion of a selective B2 receptor antagonist, D-Arg0-[Hyp3,Thi5,8,D-Phe7] BK (10-12 micrograms/min). The antagonism was reversible and specific for BK since responses to glyceryl trinitrate were not affected. Bilateral vagotomy (n = 3) or autonomic blockade with atropine (0.1 mg/kg i.v.) and propranolol (1 mg/kg i.v.) (n = 5) resulted in significant attenuation of BK-induced increases in CBF but not that of CD. It is concluded that BK is a potent dilator of both conductance and resistance coronary vessels in anaesthetized greyhounds. The dilatation of conductance vessels appears to involve a selective interaction with B2 receptors, while BK-induced increase in CBF may be mediated by both B1 and B2 receptors and involve participation of neuroreflex mechanisms.

Alpha-adrenoceptor subtypes in the coronary circulation

The pathophysiological role of sympathetic coronary innervation in myocardial ischemia is not clear, probably due to the complexities of adrenergic vascular control. In the canine coronary bed in vivo under beta-adrenergic blockade, alpha 1- as well as alpha 2-adrenoceptor-mediated constrictions can be elicited with predominance of the former in the epicardial conductance arteries, and of the latter in coronary resistance vessels. However, this distribution of functional responsiveness cannot indicate distribution of receptor density and cannot remain unchanged under differing conditions. First, each of these two classes of alpha-adrenoceptors consists of a mixture of different, interacting subtypes; second, the smooth muscular responsiveness to these two classes of alpha-adrenoceptors is differently modulated by contractile preactivation, by beta 2-blockade, and by the influence of sympathetic cotransmitters; third, alpha-adrenoceptors on endothelial cells and on sympathetic nerve endings can substantially modulate sympathetic coronary constriction. Thus, this neurogenic coronary control possesses a great functional plasticity, which is not yet fully evaluated with the presently available pharmacological tools.

Contribution of postsynaptic alpha 2-adrenoceptors to reflex sympathetic constriction of stenotic coronary vessels

Increases in the activity of efferent cardiac sympathetic nerves by 35 +/- 9% were induced by 60 s bilateral occlusion of the common carotid arteries (BCO) in anesthetized dogs. Under control conditions the reflex rise in sympathetic nerve activity enhanced left ventricular pressure (115 +/- 4 mm Hg) by 47% and regional myocardial oxygen consumption (9.7 +/- 1.1 ml/min.100 g) by 56%. Simultaneously, end-diastolic circumflex coronary resistance (0.99 +/- 0.11 mm Hg.min.100 g/ml) decreased by 16%. After exhaustion of coronary dilator reserve by production of a severe coronary stenosis, BCO enhanced left ventricular pressure (107 +/- 4 mm Hg) by 49%, oxygen consumption of the poststenotic area (7.6 +/- 0.8 ml/min.100 g) increased by 21%, and circumflex coronary resistance (0.54 +/- 0.05 mm Hg.min.100 g/ml) also increased by 19%. The reflex increase in coronary resistance during BCO was abolished after infusion of the alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.). Administration of rauwolscine, however, did not prevent the reflex increase of left ventricular pressure and regional myocardial oxygen consumption. Comparable increases in poststenotic coronary resistance during BCO were found in dogs which either received propranolol (2 mg/kg i.v.) or in which the reflex rise in mean aortic pressure was limited to 13 +/- 3 mm Hg. In both experimental groups, rauwolscine also effectively prevented the BCO-induced rise in coronary resistance. In contrast, the reflex increase of total peripheral resistance was not significantly reduced by rauwolscine, but was blunted after additional administration of the selective alpha 1-adrenoceptor antagonist prazosin (1.2 mg/kg i.v.). We conclude that: 1) Poststenotic coronary vasoconstriction occurs during shortlasting increases in efferent cardiac sympathetic discharge within the physiological range. 2) This increase in poststenotic coronary resistance is significantly reduced after administration of the alpha 2-adrenoceptor antagonist rauwolscine. 3) In contrast to poststenotic coronary resistance, functionally innervated alpha 2-adrenoceptors are of minimal importance for the reflex increase in total peripheral resistance.

Minimal alpha 1- and alpha 2-adrenoceptor-mediated coronary vasoconstriction in the anaesthetized swine

alpha-Adrenoceptor-mediated coronary vasoconstriction contributes to the initiation and aggravation of experimental and clinical myocardial ischaemia. However, the extent of alpha 1- and alpha 2-adrenoceptor-mediated constriction has not been characterized in the porcine coronary circulation despite the frequent use of this experimental model. Fifteen swine were anaesthetized with either alpha-chloralose, enflurane or isoflurane to determine the amount of alpha-adrenoceptor-mediated coronary constriction elicited by either the selective alpha 1-adrenoceptor agonist methoxamine or the selective alpha 2-adrenoceptor agonist azepexole. The left anterior descending coronary artery was cannulated and perfused by an external pump delivering constant blood flow from the carotid artery. Following bilateral cervical vagotomy and beta-adrenoceptor blockade with propranolol (2 mg kg-1), graded dosages of either one of the alpha-adrenoceptor agonists (9-45 micrograms kg-1 min-1) were infused into the coronary perfusion line while coronary arterial pressure (CAP) was measured through a distal side arm of the cannula to detect changes in coronary vascular resistance. Infusion of the alpha-adrenoceptor agonists was terminated when systemic arterial pressure increased. Sonomicrometers were used to measure anterior left ventricular wall thickening for the assessment of regional contractile function. During methoxamine infusion, no increase in vascular resistance was observed during alpha-chloralose, enflurane or isoflurane anaesthesia, whereas the infusion of azepexole increased CAP from 103 +/- 31 mmHg to 120 +/- 35 mmHg (alpha-chloralose), from 101 +/- 16 mmHg to 122 +/- 11 mmHg (enflurane) and from 84 +/- 20 mmHg to 94 +/- 19 mmHg (isoflurane), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Reflex epicardial coronary vasoconstriction elicited by nicotine in anaesthetized dogs

The effects of arterial chemoreceptor activation by nicotine on coronary artery diameter was studied in anaesthetized, artificially ventilated dogs. Left circumflex coronary artery diameter, coronary blood flow, calculated mean coronary resistance, systemic arterial blood pressure and heart rate were measured. In control dogs (n = 10) the injection of nicotine (100 micrograms) into the carotid artery evoked an increase of arterial pressure (+22 +/- 9 mm Hg) and a decrease in heart rate (-36 +/- 13 beats/min), and tended to increase coronary blood flow (+7 +/- 4 ml/min). Intracarotid nicotine had no effect on large coronary artery diameter (+0.02 +/- 0.03 mm) or total coronary resistance (+0.04 +/- 0.09 mm Hg min/ml) under these conditions. When heart rate was controlled by (1) beta-adrenoceptor blockade (propranolol, 1 mg/kg i.v.) plus pacing of the right ventricle (n = 4) or (2) beta-adrenoceptor blockade plus bilateral vagotomy (n = 7), the chemoreflex-induced constriction of the large coronary artery (-0.07 +/- 0.02 mm and -0.12 +/- 0.03 mm, respectively; p less than 0.05). In contrast, there was no chemoreflex-induced change in total coronary resistance after beta-adrenoceptor blockade plus pacing (+0.01 +/- 0.09 mm Hg min/ml, but after beta-adrenoceptor blockade plus vagotomy coronary resistance was increased (+0.75 +/- 0.31 mm Hg min/ml; p less than 0.05). The constriction of both large and small coronary arteries was abolished by phentolamine (0.5 mg/kg i.v.). These results suggest that carotid body chemoreceptor stimulation by nicotine can produce reflex alpha-adrenoceptor-mediated constriction of both large and small coronary arteries, and that the constriction of the small vessels is balanced by vagally-mediated dilatation.

Effects of regional alpha- and beta-blockade on resting and hyperemic coronary blood flow in conscious, unstressed humans

Our purpose was to determine if there are basal adrenergic influences on the coronary circulation in humans. We studied 56 patients with denervated hearts after cardiac transplantation and 19 normally innervated patients with angiographically normal coronary arteries. Coronary blood flow velocity was measured during cardiac catheterization with a subselective 3F intracoronary Doppler catheter. Heart rate was controlled by atrial pacing. Epicardial coronary artery diameter was measured by automated analysis of digital coronary angiograms. Coronary flow reserve was assessed by intracoronary papaverine hydrochloride (12 mg) injections. Regional sympathetic blockade was produced by intracoronary injections of phentolamine (3 mg, alpha) and propranolol (2 mg, beta) or metoprolol (3 mg, beta 1). After alpha-blockade, mean arterial pressure fell significantly (p less than 0.05) in both the denervated transplant (-5.8 +/- 1.5%) (mean +/- SEM) and normally innervated patients (-12.6 +/- 3.2%). Reductions in coronary flow velocity also were observed in these groups (-8.2 +/- 2.3% and -9.2 +/- 5.8%, respectively). Calculated coronary vascular resistance was unchanged. Similar changes were seen when patients were pretreated with beta-blockade before alpha-blockade. Nonspecific beta-blockade did not affect mean arterial pressure but decreased coronary velocity (innervated, -11.6 +/- 3.9%; denervated, -9.3 +/- 2.4%) and increased coronary vascular resistance (innervated, 15.4 +/- 6.7%; denervated, 10.2 +/- 3.7%). Coronary vascular resistance did not rise in either group after selective beta 1-blockade with metoprolol. Coronary flow reserve did not change in either patient group after either alpha- or beta-blockade. Changes in epicardial coronary artery diameter were small and generally not significant. These data suggest that alpha-receptor-mediated vascular tone is negligible in both denervated transplant patients and normally innervated patients. Additionally, the increase in vascular resistance after nonselective beta-blockade is the result of direct beta 2 vascular effects. Our data further suggest that there is little adrenergically mediated epicardial artery tone (either humoral or neural) at rest and that maximal vasodilator responses are not limited by adrenergically mediated vasomotor tone.

Noradrenaline-induced constriction of large and small coronary arteries in the anaesthetized dog

1. In the anaesthetized dog external diameter of the left circumflex coronary artery and blood flow through that artery were measured to allow the effect of noradrenaline to be compared in large arteries and resistance vessels. 2. The injection of noradrenaline (0.5 micrograms kg-1) into the coronary artery, after bilateral vagotomy and antagonism of beta-adrenoreceptors, decreased large coronary artery diameter and coronary blood flow. Calculation of resistance in the large coronary artery and in the total left circumflex coronary vascular bed revealed that noradrenaline induced increases of 66% and 89% respectively. 3. Intra-coronary (i.c.) injection of the alpha 1-adrenoreceptor agonist phenylephrine (0.5-2 micrograms kg-1) decreased large coronary artery diameter and coronary blood flow. The alpha 2-adrenoreceptor agonist B-HT 920 (0.5-2 micrograms kg-1 i.c.) decreased coronary blood flow but did not significantly affect large coronary artery diameter. 4. Antagonism of alpha 1-adrenoreceptors with prazosin (10 micrograms kg-1 i.c.) abolished the noradrenaline-induced constriction of the large coronary artery but only partially attenuated the decrease in blood flow. The alpha 2-adrenoreceptor antagonist idazoxan (50 micrograms kg-1 i.c.) partially attenuated the noradrenaline-induced decrease in coronary blood flow but did not affect the large artery constriction. 5. It is concluded that noradrenaline constricts both large and small coronary arteries. Noradrenaline-induced constriction of the resistance vessels is more powerful than in the large artery. The constriction of large arteries in response to noradrenaline is mediated by alpha 1-adrenoreceptors. Postjunctional alpha 1- and alpha 2-adrenoreceptors are both involved in the constriction of the resistance vessels.

Adrenoceptor subtypes involved in the baroreceptor reflex constriction of large coronary arteries in the anaesthetized dog

The baroreceptor reflex was invoked by bilateral occlusion of the carotid arteries in anaesthetized dogs. The effect of bilateral carotid occlusion on the diameter of the left circumflex coronary artery and late diastolic coronary resistance was examined after bilateral vagotomy and antagonism of beta-adrenoceptors and then compared to responses after selective antagonism of alpha 1- and alpha 2-adrenoceptors. In the absence of any change in heart rate, bilateral carotid occlusion decreased coronary artery diameter (-8 +/- 1%) and increased coronary resistance (59 +/- 14%). Prazosin (0.01 mg/kg i.a.), a selective antagonist of alpha 1-adrenoceptors, significantly attenuated the bilateral carotid occlusion-induced changes in coronary artery diameter (-2 +/- 2%) and late diastolic coronary resistance (20 +/- 11%). Selective antagonism of alpha 2-adrenoceptors by the intra-coronary injection of idazoxan (0.05 mg/kg) significantly reduced the bilateral carotid occlusion-induced increase in coronary resistance (14 +/- 12%) but did not affect the large artery constriction (-8 +/- 4%). When injected into the coronary circulation the alpha 1-adrenoceptor agonist phenylephrine constricted both the large artery and the resistance vessels. In contrast B-HT 920, a selective alpha 2-adrenoceptor agonist, constricted the resistance vessels but did not affect large coronary artery tone. The responses to phenylephrine and B-HT 920 were selectively antagonised by prazosin and idazoxan respectively. Reflex activation of the sympathetic nervous system results in constriction of both large coronary arteries and coronary resistance vessels when there is no change in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the carotid sinus reflex on large coronary artery diameter in anaesthetized dogs

1. The diameter of, and blood flow in, the left circumflex coronary artery was measured in anaesthetized dogs and the carotid sinus reflex was stimulated by bilateral occlusion of the carotid arteries (BCO) for 2 min. 2. The effect of BCO on coronary artery diameter and late diastolic coronary resistance was examined: (a) after bilateral vagotomy; (b) after vagotomy plus antagonism of beta-adrenoceptors with propranolol (1 mg/kg, i.v.); and (c) after vagotomy, antagonism of beta-adrenoceptors and antagonism of alpha-adrenoceptors with phentolamine (0.5 mg/kg, i.v.). 3. After vagotomy BCO increased mean arterial pressure (delta = 72 +/- 7 mmHg), heart rate (16 +/- 3 beats/min), coronary blood flow (37 +/- 11 ml/min) and coronary artery diameter (0.14 +/- 0.04 mm). Late diastolic coronary resistance initially fell (-0.26 +/- 0.13 mmHg min/ml at 30 s) but at the end of the 2 min occlusion it had returned to the baseline level (delta = 0.04 +/- 0.08 mmHg min/ml). 4. In the presence of propranolol BCO increased arterial pressure (28 +/- 12 mmHg), but did not alter heart rate (0.6 +/- 0.4 beats/min) or coronary blood flow (2 +/- 2 ml/min). There was a significant decrease in large artery diameter (-0.24 +/- 0.07 mm) and an increase in late diastolic coronary resistance (0.46 +/- 0.12 mmHg min/ml). A mechanical increase in afterload did not affect large coronary artery diameter or coronary resistance. 5. Antagonism of alpha-adrenoceptors abolished the reflex-induced constriction of the large coronary artery (delta = -0.02 +/- 0.02 mm) and the coronary resistance vessels (delta LDCR = -0.02 +/- 0.03 mmHg min ml).(ABSTRACT TRUNCATED AT 250 WORDS)