Requisite role of cardiac myocytes in coronary alpha1-adrenergic constriction

Mitochondrial DNA integrity and function are critical for endothelium-dependent vasodilation in rats with metabolic syndrome

Endothelial dysfunction in diabetes is generally attributed to oxidative stress, but this view is challenged by observations showing antioxidants do not eliminate diabetic vasculopathy. As an alternative to oxidative stress-induced dysfunction, we interrogated if impaired mitochondrial function in endothelial cells is central to endothelial dysfunction in the metabolic syndrome. We observed reduced coronary arteriolar vasodilation to the endothelium-dependent dilator, acetylcholine (Ach), in Zucker Obese Fatty rats (ZOF, 34 ± 15% [mean ± standard deviation] 10-3 M) compared to Zucker Lean rats (ZLN, 98 ± 11%). This reduction in dilation occurred concomitantly with mitochondrial DNA (mtDNA) strand lesions and reduced mitochondrial complex activities in the endothelium of ZOF versus ZLN. To demonstrate endothelial dysfunction is linked to impaired mitochondrial function, administration of a cell-permeable, mitochondria-directed endonuclease (mt-tat-EndoIII), to repair oxidatively modified DNA in ZOF, restored mitochondrial function and vasodilation to Ach (94 ± 13%). Conversely, administration of a cell-permeable, mitochondria-directed exonuclease (mt-tat-ExoIII) produced mtDNA strand breaks in ZLN, reduced mitochondrial complex activities and vasodilation to Ach in ZLN (42 ± 16%). To demonstrate that mitochondrial function is central to endothelium-dependent vasodilation, we introduced (via electroporation) liver mitochondria (from ZLN) into the endothelium of a mesenteric vessel from ZOF and restored endothelium-dependent dilation to vasoactive intestinal peptide (VIP at 10-5 M, 4 ± 3% vasodilation before mitochondrial transfer and 48 ± 36% after transfer). Finally, to demonstrate mitochondrial function is key to endothelium-dependent dilation, we administered oligomycin (mitochondrial ATP synthase inhibitor) and observed a reduction in endothelium-dependent dilation. We conclude that mitochondrial function is critical for endothelium-dependent vasodilation.

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.

Impaired coronary metabolic dilation in the metabolic syndrome is linked to mitochondrial dysfunction and mitochondrial DNA damage

Mitochondrial dysfunction in obesity and diabetes can be caused by excessive production of free radicals, which can damage mitochondrial DNA. Because mitochondrial DNA plays a key role in the production of ATP necessary for cardiac work, we hypothesized that mitochondrial dysfunction, induced by mitochondrial DNA damage, uncouples coronary blood flow from cardiac work. Myocardial blood flow (contrast echocardiography) was measured in Zucker lean (ZLN) and obese fatty (ZOF) rats during increased cardiac metabolism (product of heart rate and arterial pressure, i.v. norepinephrine). In ZLN increased metabolism augmented coronary blood flow, but in ZOF metabolic hyperemia was attenuated. Mitochondrial respiration was impaired and ROS production was greater in ZOF than ZLN. These were associated with mitochondrial DNA (mtDNA) damage in ZOF. To determine if coronary metabolic dilation, the hyperemic response induced by heightened cardiac metabolism, is linked to mitochondrial function we introduced recombinant proteins (intravenously or intraperitoneally) in ZLN and ZOF to fragment or repair mtDNA, respectively. Repair of mtDNA damage restored mitochondrial function and metabolic dilation, and reduced ROS production in ZOF; whereas induction of mtDNA damage in ZLN reduced mitochondrial function, increased ROS production, and attenuated metabolic dilation. Adequate metabolic dilation was also associated with the extracellular release of ADP, ATP, and H2O2 by cardiac myocytes; whereas myocytes from rats with impaired dilation released only H2O2. In conclusion, our results suggest that mitochondrial function plays a seminal role in connecting myocardial blood flow to metabolism, and integrity of mtDNA is central to this process.

Requisite Role of Kv1.5 Channels in Coronary Metabolic Dilation

RATIONALE

In the working heart, coronary blood flow is linked to the production of metabolites, which modulate tone of smooth muscle in a redox-dependent manner. Voltage-gated potassium channels (Kv), which play a role in controlling membrane potential in vascular smooth muscle, have certain members that are redox-sensitive.

OBJECTIVE

To determine the role of redox-sensitive Kv1.5 channels in coronary metabolic flow regulation.

METHODS AND RESULTS

In mice (wild-type [WT], Kv1.5 null [Kv1.5(-/-)], and Kv1.5(-/-) and WT with inducible, smooth muscle-specific expression of Kv1.5 channels), we measured mean arterial pressure, myocardial blood flow, myocardial tissue oxygen tension, and ejection fraction before and after inducing cardiac stress with norepinephrine. Cardiac work was estimated as the product of mean arterial pressure and heart rate. Isolated arteries were studied to establish whether genetic alterations modified vascular reactivity. Despite higher levels of cardiac work in the Kv1.5(-/-) mice (versus WT mice at baseline and all doses of norepinephrine), myocardial blood flow was lower in Kv1.5(-/-) mice than in WT mice. At high levels of cardiac work, tissue oxygen tension dropped significantly along with ejection fraction. Expression of Kv1.5 channels in smooth muscle in the null background rescued this phenotype of impaired metabolic dilation. In isolated vessels from Kv1.5(-/-) mice, relaxation to H2O2 was impaired, but responses to adenosine and acetylcholine were normal compared with those from WT mice.

CONCLUSIONS

Kv1.5 channels in vascular smooth muscle play a critical role in coupling myocardial blood flow to cardiac metabolism. Absence of these channels disassociates metabolism from flow, resulting in cardiac pump dysfunction and tissue hypoxia.

Cold-acclimation leads to differential regulation of the steelhead trout (Oncorhynchus mykiss) coronary microcirculation

The regulation of vascular resistance in fishes has largely been studied using isolated large conductance vessels, yet changes in tissue perfusion/vascular resistance are primarily mediated by the dilation/constriction of small arterioles. Thus we adapted mammalian isolated microvessel techniques for use in fish and examined how several agents affected the tone/resistance of isolated coronary arterioles (<150 μm ID) from steelhead trout (Oncorhynchus mykiss) acclimated to 1, 5, and 10°C. At 10°C, the vessels showed a concentration-dependent dilation to adenosine (ADE; 61 ± 8%), sodium nitroprusside (SNP; 35 ± 10%), and serotonin (SER; 27 ± 2%) (all values maximum responses). A biphasic response (mild contraction then dilation) was observed in vessels exposed to increasing concentrations of epinephrine (EPI; 34 ± 9% dilation) and norepinephrine (NE; 32 ± 7% dilation), whereas the effect was less pronounced with bradykinin (BK; 12.5 ± 3.5% constriction vs. 6 ± 6% dilation). Finally, a mild constriction was observed after exposure to acetylcholine (ACh; 6.5 ± 1.4%), while endothelin (ET)-1 caused a strong dose-dependent increase in tone (79 ± 5% constriction). Acclimation temperature had varying effects on the responsiveness of vessels. The dilations induced by EPI, ADE, SER, and SNP were reduced/eliminated at 5°C and/or 1°C as compared with 10°C. In contrast, acclimation to 5 and 1°C increased the maximum constriction induced by ACh and the sensitivity of vessels to ET-1 (but not the maximum response) at 1°C was greater. Acclimation temperature had no effect on the response to NE, and responsiveness to BK was variable.

Vasodilation Responses to Non-Selective α-Adrenergic Blockage of Coronary Bypass Grafts in Diabetic and Non-Diabetic Patients: In Vitro Study

BACKGROUND

Adrenergic tonus is increased in atherosclerotic coronary arteries. In this study, we aimed to demonstrate in vitro effects of phentolamine, a reversible nonselective alpha (α) adrenergic blocker, on coronary artery bypass grafts (CABG) and compare its effects in diabetic and nondiabetic patients.

METHODS

A total number of 30 patients (15 diabetic and 15 nondiabetic) who were assigned to elective CABG surgery were enrolled into the study. For both groups of patients, 16 internal mammarian artery (IMA) samples, 16 saphenous vein (SV) samples and 16 radial artery (RA) samples were collected and studied in the tissue bath system. The vasodilatation responses to increasing doses of phentolamine were recorded.

RESULTS

When grafts were compared in terms of amount of vasodilatation to phentolamine, IMA had the most prominent vasodilatation followed by RA and SV respectively. Although the vasodilatation responses in nondiabetic patients were numerically higher than diabetic patients, there was no statistically difference between the groups.

CONCLUSION

Phentolamine, a nonselective α adrenergic blocker, is proven to have equal vasodilatory effects in diabetic and nondiabetic CABG grafts and can safely be used both intravenously and topically in the perioperative period.

Reprint of: the paradox of α-adrenergic coronary vasoconstriction revisited

Activation of coronary vascular α-adrenoceptors results in vasoconstriction which competes with metabolic vasodilation during sympathetic activation. Epicardial conduit vessel constriction is largely mediated by α(1)-adrenoceptors; the constriction of the resistive microcirculation largely by α(2)-adrenoceptors, but also by α(1)-adrenoceptors. There is no firm evidence that α-adrenergic coronary vasoconstriction exerts a beneficial effect on transmural blood flow distribution. In fact, α-blockade in anesthetized and conscious dogs improves blood flow to all transmural layers, during normoperfusion and hypoperfusion. Also, in patients with coronary artery disease, blockade of α(1)- and α(2)-adrenoceptors improves coronary blood flow, myocardial function and metabolism. This article is part of a Special Issue entitled "Coronary Blood Flow".

The ischemic metabolite lysophosphatidylcholine increases rat coronary arterial tone by endothelium-dependent mechanisms

Lysophosphatidylcholine (LPC), a hydrolysis product of phospholipid degradation, accumulates in the ischemic myocardium. Using isolated hearts or rat coronary septal arteries, we tested the impact of LPC in modulating basal function or the responses to vasoactive agents. Sustained perfusion of hearts with LPC augmented coronary perfusion pressure (CPP) and reduced left ventricular developed pressure (LVDP). By mechanisms that have yet to be identified, these effects on CPP and LVDP were exaggerated when LPC was removed from the perfusate. Although LPC (or its washout) had no direct effect on vascular tone in the isolated coronary artery, it selectively potentiated the receptor-coupled vasoconstrictor response to U-46619, a thromboxane A(2) mimetic. Interestingly, when LPC was washed out, the potentiation to U-46619 was even more pronounced. Both the immediate and residual effects of LPC were endothelium-dependent. EDHF was likely the sole mediator responsible for the direct effects of LPC on U-46619-vasoconstriction, whereas the augmented vasoconstrictor responses following LPC washout may in part be related to an increase in ET-1, and a striking reduction in the bioavailability of NO. Our data suggest that in addition to reducing the accumulation of LPC to prevent ischemia-reperfusion (I/R) damage, efforts targeting an improved endothelium-dependent regulation of vascular tone could be an attractive approach to limit the cardiac damage induced by I/R.

Regulation of coronary vascular tone via redox modulation in the alpha1-adrenergic-angiotensin-endothelin axis of the myocardium

We hypothesized that alpha(1)-adrenoceptor stimulation of cardiac myocytes results in the production of an endothelin (ET)-releasing factor that stimulates the coronary vasculature to release ET and, by manipulating the redox state of cardiac and vascular cells, may influence the extent of alpha(1)-adrenergic-ET-1 vasoconstriction. Dihydroethidium (DHE) and dichlorodihydrofluorescein (DCF) intensities were increased by phenylephrine stimulation in isolated rat cardiac myocytes, which were enhanced by the mitochondrial electron transport chain complex I inhibitor rotenone (DHE: 20.4 +/- 1.2-fold and DCF: 25.2 +/- 0.9-fold, n = 8, P < 0.01, respectively) but not by the NADPH oxidase inhibitor apocynin. Olmesartan, an angiotensin II type 1 receptor antagonist, and enalaprilate did not change DHE and DCF intensities by phenylephrine. Next, we measured the vasoconstriction of isolated, pressurized rat coronary arterioles (diameter: 74 +/- 8 microm) in response to supernatant collected from isolated cardiac myocytes. The addition of supernatant from phenylephrine-stimulated myocytes to a 2-ml vessel bath (n = 8 each) caused volume-dependent vasoconstriction (500 microl: -14.8 +/- 2.2%). Olmesartan and TA0201, an ET type A receptor antagonist, converted vasoconstriction into vasodilation (8.5 +/- 1.2% and 10.5 +/- 0.5%, P < 0.01, respectively) in response to supernatant from phenylephrine-stimulated myocytes, which was eliminated with catalase. Vasoconstriction was weakened using supernatant from phenylephrine with rotenone-treated myocytes. Treatment of arterioles with apocynin to myocyte supernatant converted vasoconstriction into vasodilation (7.8 +/- 0.8%, P < 0.01). These results suggest that alpha(1)-adrenergic stimulation in cardiac myocytes produces angiotensin I and H(2)O(2) and that angiotensin releases ET-1 through NADPH oxidase in coronary arterioles. Thus, coronary vasoconstriction via the alpha-adrenergic-angiotensin-ET axis appears to require redox-mediated signaling in cardiac and vascular cells.

Increased diastolic time fraction as beneficial adjunct of α1-adrenergic receptor blockade after percutaneous coronary intervention

The effect of α1-receptor blockade with urapidil on coronary blood flow and left ventricular function has been attributed to relief of diffuse coronary vasoconstriction following percutaneous coronary intervention (PCI). We hypothesized that an increase in diastolic time fraction (DTF) contributes to the beneficial action of urapidil. In eleven patients with a 63% (SD 13) diameter stenosis, ECG, aortic pressure (Pa) and distal intracoronary pressure (Pd), and blood flow velocity were recorded at baseline and throughout adenosine-induced hyperemia. Measurements were obtained before and after PCI and after subsequent α1-receptor blockade with urapidil (10 mg ic). DTF was determined from the ECG and the Pa waveform. Functional parameters such as coronary flow velocity reserve, fractional flow reserve, and an index of hyperemic microvascular resistance (HMR) were assessed. Urapidil administration after PCI induced an upward shift in the DTF-heart rate relationship, resulting in a 3.1% (SD 2.7) increase in hyperemic DTF at a constant heart rate ( P < 0.005) due to a shorter duration of systole. Hyperemic Pa and Pd decreased, respectively, by 6.1% (SD 6.6; P < 0.05) and 5.7% (SD 5.8; P < 0.01) after α1-blockade. Although epicardially measured functional parameters were on average not altered by α1-blockade due to concurrent changes in pressure and heart rate, HMR decreased by urapidil in those patients where coronary pressure remained constant. In conclusion, α1-receptor blockade after PCI produced a modest but significant prolongation of DTF at a given heart rate, thereby providing an adjunctive beneficial mechanism for improving subendocardial perfusion, which critically depends on DTF.

Redox-dependent coronary metabolic dilation

We have observed that hydrogen peroxide (H2O2), the dismutated product of superoxide, is a coronary metabolic dilator and couples myocardial oxygen consumption to coronary blood flow. Because the chemical activity of H2O2 favors its role as an oxidant, and thiol groups are susceptible to oxidation, we hypothesized that coronary metabolic dilation occurs via a redox mechanism involving thiol oxidation. To test this hypothesis, we studied the mechanisms of dilation of isolated coronary arterioles to metabolites released by metabolically active (paced at 400 min) isolated cardiac myocytes and directly compared these responses with authentic H2O2. Studies were performed under control conditions and using interventions designed to reduce oxidized thiols [0.1 microM dithiothreitol (DTT) and 10 mM N-acetyl-L-cysteine (NAC)]. Aliquots of the conditioned buffer from paced myocytes produced vasodilation of isolated arterioles (peak response, 71% +/- 6% of maximal dilation), whereas H2O2 produced complete dilation (92% +/- 7%). Dilation to either the conditioned buffer or to H2O2 was significantly reduced by the administration of either NAC or DTT. The location of the thiols oxidized by the conditioned buffer or of H2O2 was determined by the administration of the fluorochromes monochlorobimane (20 microM) or monobromotrimethylammoniobimane (20 microM), which covalently label the reduced total or extracellular-reduced thiols, respectively. H2O2 or the conditioned buffer predominantly oxidized intracellular thiols since the fluorescent signal from monochlorobimane was reduced more than that of monobromotrimethylammoniobimane. To determine whether one of the intracellular targets of thiol oxidation that leads to dilation is the redox-sensitive kinase p38 mitogen-activated protein (MAP) kinase, we evaluated dilation following the administration of the p38 inhibitor SB-203580 (10 microM). The inhibition of p38 attenuated dilation to either H2O2 or to the conditioned buffer from stimulated myocytes by a similar degree, but SB-203580 did not attenuate dilation to nitroprusside. Western blot analysis for the activated form of p38 (phospho-p38) in the isolated aortae revealed robust activation of this enzyme by H2O2. Taken together, our results show that an active component of cardiac metabolic dilation, like that of H2O2, produces dilation by the oxidation of thiols, which are predominantly intracellular and dependent activation on the p38 MAP kinase. Thus coronary metabolic dilation appears to be mediated by redox-dependent signals.

Basal and hyperaemic myocardial blood flow in regionally denervated canine hearts: an in vivo study with positron emission tomography

PURPOSE

Positron emission tomography (PET) studies in patients with diabetic autonomic neuropathy (DAN) have demonstrated the impact of this disease on cardiac sympathetic innervation and myocardial blood flow (MBF). To investigate the effects of selective partial sympathetic denervation of the left ventricle (LV) on baseline and hyperaemic MBF, we measured myocardial presynaptic catecholamine re-uptake (uptake-1), beta-adrenoceptor (beta-AR) density and MBF non-invasively by means of PET in a canine model of regional sympathetic denervation.

METHODS

In 11 anaesthetised dogs, the sympathetic nerves of the free wall and septum of the LV were removed by means of dissection and phenol painting. Three weeks later, the animals were studied with PET. MBF was measured at baseline and following i.v. adenosine (140 microg kg(-1) min(-1)) and dobutamine (20 microg kg(-1) min(-1)) using(15)O-labelled water. Sympathetic denervation was confirmed by an 80+/-12% decrease in the volume of distribution (V(d)) of [(11)C]hydroxyephedrine (HED) compared with innervated regions. Myocardial beta-AR density was measured using [(11)C]CGP12177.

RESULTS

Innervated and denervated regions showed no differences in MBF at baseline and during adenosine or dobutamine. [(11)C]HED V(d)was inversely correlated with MBF in both regions at baseline, and the correlation was lost during hyperaemia in denervated regions. However, for any given value of MBF, [(11)C]HED V(d)was significantly lower in the denervated regions. beta-AR density was comparable in denervated and innervated regions (17.9+/-4.2 vs 18.4+/-3.3 pmol g(-1); p=NS).

CONCLUSION

In this experimental model, selective, regional sympathetic denervation of the LV, which results in a profound reduction in [(11)C]HED V(d), did not affect baseline or hyperaemic MBF. In addition, we demonstrated that, under baseline conditions, there was a significant inverse correlation between [(11)C]HED V(d)and MBF in both denervated and innervated regions.

Effect of alpha1-adrenergic receptors in cardiac pathophysiology

Compelling evidence now exists that proves adrenergic blockade is at the center of neurohormonal antagonism in heart failure (HF). Catecholamines are well known to act through both beta- and alpha-adrenergic receptors (ARs), which mediate their effects through distinct receptor pathways. Beta-AR blockers are commonly used in the treatment of HF and have distinct receptor affinity profiles. The recent COMET trial comparing 2 important beta-blocking drugs showed a distinct advantage for carvedilol in decreasing the risk of mortality from HF. The mechanism of action for carvedilol differs from metoprolol tartrate in its ability to block both alpha- and beta-ARs, leading to renewed interest in the potential role of alpha-ARs in the progression of HF. In contrast, however, the ALLHAT study discontinued use of doxazosin, an alpha1-receptor blocker because of an increase in cardiovascular events among patients using this drug. The results of these studies appear to be in contrast with respect to the role of alpha-ARs in regards to cardiovascular pathophysiology. Further study of the alpha-receptor and understanding the role of alpha-ARs in HF is necessary to understand the therapeutic effect of alpha-blockade. This article reviews our understanding of the alpha-AR in HF.

Beating myocardium counteracts myogenic tone of coronary microvessels: involvement of ATP-sensitive potassium channels

Myogenic tone is intrinsic to vascular tissue and plays an important role in determining basal coronary resistance. However, the effect of the beating heart on myogenic tone is unknown. We investigated the effects of myocardium-derived vasoactive factors on the myogenic tone of coronary microvessels in the resting condition and during increased metabolism. Pressurized isolated coronary vessels (detector vessel, DV) of rabbits (n = 33, maximal inner diameter 201 +/- 8 microm) were gently placed on beating hearts of anesthetized dogs and observed with an intravital microscope equipped with a floating objective. To shut off the myocardium-derived vasoactive signals, we placed plastic film between DV and the heart. The intravascular pressure was changed from 120 to 60 cmH(2)O, and pressure-diameter curves were obtained with and without the contact of DV and the myocardium. The direct contact shifted the pressure-diameter curve upward (P < 0.05 vs. without contact), and myogenic tone was reduced by approximately 40%. When endothelium of DV was denuded, the shift persisted, but the degree of shift was reduced to 10% (P < 0.05 vs. with endothelium). The shift was abolished by glibenclamide, an ATP-sensitive potassium (K(ATP)) channel blocker. A similar upward shift was induced by rapid pacing, but the shift was not blocked by glibenclamide. We conclude that the beating myocardium counteracts myogenic tone by releasing transferable vasoactive signals that affect the endothelium and the vascular smooth muscle, and that the signals are solely mediated by the activation of K(ATP) channels, unlike the rapid pacing-induced vasoactive factors.

Selective right, but not left, coronary endothelial dysfunction precedes development of pulmonary hypertension and right heart hypertrophy in rats

We investigated a causal role for coronary endothelial dysfunction in development of monocrotaline (MCT)-induced pulmonary hypertension and right heart hypertrophy in rats. Significant increases in pulmonary pressure and right ventricular weight did not occur until 3 wk after 60 mg/kg MCT injection (34 ± 4 vs. 19 ± 2 mmHg and 37 ± 2 vs. 25 ± 1% septum + left ventricular weight in controls, respectively). Isolated right coronary arteries (RCA) showed significant decreases in acetylcholine-induced NO dilation in both 1-wk (33 ± 3% with 0.3 μM; n = 5) and 3-wk (18 ± 3%; n = 11) MCT rats compared with control rats (71 ± 8%, n = 10). Septal coronary arteries (SCA) showed a smaller decrease in acetylcholine dilation (55 ± 8% and 33 ± 7%, respectively, vs. 73 ± 8% in controls). No significant change was found in the left coronary arteries (LCA; 88 ± 6% and 81 ± 6%, respectively, vs. 87 ± 3% in controls). Nitro-l-arginine methyl ester-induced vasoconstriction, an estimate of spontaneous endothelial NO-mediated dilation, was not significantly altered in MCT-treated SCA or LCA but was increased in RCA after 1 wk of MCT (−41 ± 6%) and decreased after 3 wk (−18 ± 3% vs. −27 ± 3% in controls). A marked enhancement to 30 nM U-46619-induced constriction was also noted in RCA of 3-wk (−28 ± 6% vs. −9 ± 2% in controls) but not 1-wk (−12 ± 7%) MCT rats. Sodium nitroprusside-induced vasodilation was not different between control and MCT rats. Together, our findings show that a selective impairment of right, but not left, coronary endothelial function is associated with and precedes development of MCT-induced pulmonary hypertension and right heart hypertrophy in rats.

Alterations of alpha-adrenergic modulations of coronary microvascular tone in dogs with heart failure

BACKGROUND

It remains unclear whether coronary microvascular response to alpha-adrenergic activation alters in chronic heart failure (CHF).

METHODS AND RESULTS

We investigated the alpha-adrenergic receptor-mediated effects on coronary pressure-flow relationship (CPFR) in a tachycardia-induced canine heart failure model. The dogs studied were male (29 of 31) and the drugs were given intracoronary. The slope of CPFR during long diastole was evaluated as an index of coronary vascular resistance, during alpha1- or alpha2-adrenergic stimulation or inhibition under anesthesia in the baseline and failing state after 3 weeks of rapid ventricular pacing. Resting coronary blood flow and CPFR did not change in the failing state from the baseline state. Neither alpha1 nor alpha2 stimulation changed the slope of CPFR in the baseline state. However, in the failing state, alpha1 stimulation decreased the slope of CPFR by 23 +/- 5% (P < .05), whereas alpha2 stimulation increased it by 73 +/- 10% (P < .05), which was nearly abolished by pretreatment with NG-nitro-L-arginine methyl ester.

CONCLUSION

Alpha2-mediated vasodilatory action, presumably via endothelium-derived nitric oxide release, would be enhanced in the coronary microvascular bed, which may antagonize enhanced alpha1-induced vasoconstriction in CHF.

Role of endothelin in α-adrenoceptor coronary vasoconstriction

It has been proposed that α-adrenoceptor vasoconstriction in coronary resistance vessels results not from α-adrenoceptors on coronary smooth muscle but from α-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the α-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs ( n = 10), the endothelin ETA/ETB receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the α-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and α-adrenoceptor blockade results indicates that α-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the α-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if α-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.

Cardiac myocytes control release of endothelin-1 in coronary vasculature

Alpha-adrenergic vasoconstriction in the coronary circulation is mediated through alpha-adrenoceptors on cardiac myocytes and subsequent release of endothelin, a very potent, long-lasting vasoconstrictor. Recent studies found that adult cardiac myocytes do not express the preproendothelin gene. Thus we hypothesized that alpha-adrenoceptor stimulation on the cardiac myocytes results in the production of an endothelin-releasing factor, which stimulates the coronary vasculature to produce endothelin. We tested this hypothesis by using an in vitro model in which isolated adult rat cardiac myocytes can be stimulated with an alpha-adrenoceptor agonist (phenylephrine). Their bathing fluid is then transferred to isolated coronary arterioles, and vasoactive responses are measured. To identify the source of endothelin, the endothelin-converting enzyme inhibitor phosphoramidon was added to either the myocytes or the isolated arterioles. Phenylephrine enhanced the vasoconstrictor properties of the myocyte bathing fluid. Administration of phosphoramidon (in either the presence or the absence of phenylephrine) to the myocytes had no effect on the vasoactive properties of the bathing fluid. In contrast, administration of phosphoramidon to the isolated arteriole before administration of the bathing fluid converted vasoconstriction to vasodilation, similar to the effect of the endothelin A receptor antagonist JKC-301, indicating that the endothelin is indeed produced by the coronary vasculature. Administration of the angiotensin type 1 receptor antagonist losartan to the vessel bath enhanced vasodilation to the bathing fluid of the phenylephrine-treated but not control myocytes. In conclusion, during alpha-adrenergic activation cardiac myocytes release a factor, probably angiotensin II, that stimulates the vascular production of endothelin. Although the physiological implications of this mechanism are not obvious, this may represent a protective mechanism that integrates neuronal vasoconstrictor mechanisms with myocardial metabolism, which minimizes periods of both coronary underperfusion and overperfusion.

Matching coronary blood flow to myocardial oxygen consumption

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

Effects of xylazine on canine coronary artery vascular rings

OBJECTIVE

To determine the effects of xylazine on canine coronary artery smooth muscle tone.

SAMPLE POPULATION

Hearts of 26 healthy dogs.

PROCEDURE

Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation.

RESULTS

Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.

α-Adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in experimental diabetes mellitus

This study tested whether α-adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in diabetes mellitus. Experiments were conducted in dogs instrumented with catheters in the aorta and coronary sinus and with a flow transducer around the circumflex coronary artery. Diabetes was induced with alloxan monohydrate ( n = 8, 40 mg/kg iv). Arterial plasma glucose concentration increased from 4.7 ± 0.2 mM in nondiabetic, control dogs ( n = 8) to 21.4 ± 1.9 mM 1 wk after alloxan injection. Coronary blood flow, myocardial oxygen consumption (MV̇o2), aortic pressure, and heart rate were measured at rest and during graded treadmill exercise before and after infusion of the α-adrenoceptor antagonist phentolamine (1 mg/kg iv). In untreated diabetic dogs, exercise increased MV̇o2 2.7-fold, coronary blood flow 2.2-fold, and heart rate 2.3-fold. Coronary venous Po2 fell as MV̇o2 increased during exercise. After α-adrenoceptor blockade, exercise increased MV̇o2 3.1-fold, coronary blood flow 2.7-fold, and heart rate 2.1-fold. Relative to untreated diabetic dogs, α-adrenoceptor blockade significantly decreased the slope of the relationship between coronary venous Po2 and MV̇o2. The difference between the untreated and phentolamine-treated slopes was greater in the diabetic dogs than in the nondiabetic dogs. In addition, the decrease in coronary blood flow to intracoronary norepinephrine infusion was significantly augmented in anesthetized, open-chest, β-adrenoceptor-blocked diabetic dogs compared with the nondiabetic dogs. These findings demonstrate that α-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during physiological increases in MV̇o2.

Myogenic responses of isolated adipose tissue arterioles

Previous in vivo studies indicate that vascular autoregulation does take place in adipose tissue. We tested the hypothesis that adipose tissue arterioles can develop a myogenic response to increases in transmural pressure. Arterioles, isolated from the inguinal fat pad of male Wistar rats, were placed in a microvessel chamber containing a Kreb's bicarbonate-buffered solution (pH 7.4) gassed with 10% O2 (5% CO2; 85% N2). Vessels were cannulated and pressurized to 100 mm Hg and studied under no-flow conditions. Control diameters were obtained at 100 mm Hg. Changes in arteriolar diameter were observed and measured by television microscopy and video caliper. Diameters, in response to 20 mm Hg step increases in transmural pressure, were measured before and after removal either of extracellular calcium or of the endothelium, and administration of indomethacin (10(-5) M) or L-NAME (3 x 10(-4) M). Removal of calcium resulted in an increase in control diameter of 81% and completely eliminated the myogenic response. In contrast, administration of indomethacin increased control diameter by 13%. L-NAME significantly enhanced the myogenic response; however, neither endothelium removal nor indomethacin had any significant effect. These results indicate that adipose tissue arterioles are capable of eliciting a myogenic response that could contribute to the regulation of blood flow in vivo. Furthermore, it appears that calcium is essential for the myogenic response and that nitric oxide significantly contributes to the modulation of baseline myogenic tone, as well as the myogenic response.

Delayed arteriolar relaxation after prolonged agonist exposure: functional remodeling involving tyrosine phosphorylation

Although arteriolar contraction is dependent on Ca2+-induced myosin phosphorylation, other mechanisms including Ca2+ sensitization and time-dependent phenomena such as cytoskeletal and cellular reorganization may contribute to contractile events. We hypothesized that if arteriolar smooth muscle exhibits time-dependent behavior this may be manifested in differences in relaxation after short- and long-term exposure to contractile agonists. Studies were conducted in isolated arterioles pressurized to 70 mmHg. In initial experiments (n = 10), rate of relaxation was measured after acute (5 min) or prolonged (4 h) exposure to 5 microM norepinephrine (NE). Prolonged exposure to NE resulted in significantly (P < 0.05) increased time for relaxation in physiological salt solution. Rapid relaxation of vessels exposed to NE for 4 h was observed after superfusion with 0 mM Ca2+ buffer, indicating that the alteration in relaxation was reversible and Ca2+ dependent. A similarly impaired dilation was not observed with 4-h exposure to KCl (75 mM). To determine mechanisms contributing to the effects of prolonged NE exposure, studies were performed in the presence of the microtubule depolymerizing agent demecolcine (10 microM) or a series of tyrosine phosphorylation inhibitors. Although demecolcine caused significant vasoconstriction (P < 0.05) and potentiated NE vasoconstriction, it did not prevent the effect of long-term NE exposure on relaxation. Genistein, although having no effect on acute NE-induced contraction, concentration-dependently inhibited prolonged NE constriction. Similarly, Src (PP1) and p42/44 MAP kinase (PD-98059) inhibitors prevented maintenance of long-term NE contraction. The data indicate that prolonged exposure to NE induces biochemical alterations that impair relaxation after removal of the agonist. The contractile effects are Ca2+ dependent and involve tyrosine phosphorylation but do not appear to involve the polymerization state of the microtubule network.

Metabolic regulation of coronary vascular tone: role of endothelin-1

Coronary tone is determined by a balance between endogenously produced endothelin and metabolic dilators. We hypothesized that coronary vasodilation during augmented metabolism is the net result of decreased endothelin production and increased production of vasodilators. Isolated rat myocytes were stimulated at 0, 200, and 400 beats/min to modify metabolism. Supernatant from these preparations was added to isolated coronary arterioles with and without blocking vasoactive pathways (adenosine, bradykinin, and endothelin). Chronically instrumented swine were studied while resting and running on a treadmill before and after endothelin type A (ET(A)) receptor blockade. The vasodilatory properties of the supernatant increased with increased stimulation frequencies. Combined blockade of adenosine and bradykinin receptors abolished vasodilation in response to supernatant of stimulated myocytes. ET(A) blockade increased vasodilation to supernatant of unstimulated myocytes but did not affect dilation to supernatant of myocytes stimulated at 400 beats/min. In vivo, ET(A) blockade resulted in coronary vasodilation at rest, which waned during exercise. Thus endothelin has a tonic constrictor influence through the ET(A) receptor at low myocardial metabolic demand but its influence decreased during increased metabolism.

Vasodilator signals from the ischemic myocardium are transduced to the coronary vascular wall by pertussis toxin-sensitive G proteins: a new experimental method for the analysis of the interaction between the myocardium and coronary vessels

OBJECTIVES

We sought to detect cross-talk between the beating heart and coronary vascular bed during myocardial ischemia and to test the hypothesis that the cross-talk is mediated by pertussis toxin (PTX)-sensitive G proteins (G(PTX)) in vessels.

BACKGROUND

Coronary flow is closely related to the myocardial metabolic state, indicating the existence of a close interaction between cardiac muscle and coronary vascular beds. Experimental methods for the analysis of the interaction, however, have not been established.

METHODS

Coronary detector vessels (DVs) were isolated from rabbit hearts. One end of the vessel was cannulated to a micropipette, and the other end was ligated. After the DV was pressurized (60 cm H(2)O), it was gently placed on the myocardium, which was perfused by the left anterior descending coronary artery (LAD) of anesthetized, open-chest dogs (n = 23). The LAD was occluded, and the DV diameter was observed using an intravital microscope with a floating objective system. To evaluate the involvement of G(PTX), the DV was pre-incubated with PTX (100 ng/ml).

RESULTS

The LAD occlusion of the beating heart produced significant dilation of DVs (241 +/- 25 microm) by 10%. The DVs pretreated with PTX (250 +/- 27 microm) did not dilate in response to myocardial ischemia. N(omega)-nitro-L-arginine (100 micromol/l), but not glibenclamide (5 micromol/l), abolished the ischemia-induced DV dilation.

CONCLUSIONS

We have established experimental methods for direct analysis of the interaction between the myocardium and coronary microvessels. We conclude that the ischemic myocardium releases transferable vasodilator signals that are transduced by means of the G(PTX) located in the vascular walls. The nitric oxide pathway is involved in the signal transduction.

Diaphragm arterioles are less responsive to alpha1- adrenergic constriction than gastrocnemius arterioles

The sympathetic nervous system has greater influence on vascular resistance in low-oxidative, fast-twitch skeletal muscle than in high-oxidative skeletal muscle (17). The purpose of this study was to test the hypothesis that arterioles isolated from low-oxidative, fast-twitch skeletal muscle [the white portion of gastrocnemius (WG)] possess greater responsiveness to adrenergic constriction than arterioles isolated from high-oxidative skeletal muscle [red portion of the gastrocnemius muscle (RG) and diaphragm (Dia)]. Second-order arterioles (2As) were isolated from WG, RG, and Dia of rats and reactivity examined in vitro. Results reveal that Dia 2As constrict less to norepinephrine (NE) (10(-9) to 10 (-4) M) than 2As from RG and WG, which exhibited similar NE-induced constrictions. This difference was not endothelium dependent, because responses of denuded 2As were similar to those of intact arterioles. The blunted NE-induced constrictor response of Dia 2As appears to be the result of differences in alpha1-receptor effects because 1) arterioles from Dia also responded less to selective alpha1-receptor stimulation with phenylephrine than RG and WG arterioles; 2) arterioles from Dia, RG, and WG dilated similarly to isoproterenol (10(-9) to 10(-4) M) and did not respond to selective alpha2-receptor stimulation with UK-14304; and 3) endothelin-1 produced similar constriction in 2As from Dia, RG, and WG. We conclude that differences in oxidative capacity and/or fiber type composition of muscle tissue do not explain different NE responsiveness of Dia 2As compared with 2As from gastrocnemius muscle. Differences in alpha1-adrenergic constrictor responsiveness among arterioles in skeletal muscle may contribute to nonuniform muscle blood flow responses observed during exercise and serve to maintain blood flow to Dia during exercise-induced increases in sympathetic nerve activity.

Control of coronary blood flow during exercise

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

Influence of dual ET(A)/ET(B)-receptor blockade on coronary responses to treadmill exercise in dogs

We hypothesized that endothelin (ET) release during exercise may be triggered by alpha-adrenergic-receptor activation and thereby influence coronary hemodynamics and O(2) metabolism in dogs. Exercise resulted in coronary blood flow increases (to 1.88+/-0.26 from 1.10+/- 0.12 ml x min(-1) x g(-1)) and in a fall (P<0.01) in coronary sinus O(2) saturation (17.4+/-1.5 to 9.6+/-0.7 vol%), whereas myocardial O(2) consumption (MVO(2)) increased (109+/-13% from 145+/-16 microl O(2) min(-1) x g(-1)). Tezosentan, a dual ET(A)/ET(B)-receptor blocker, slightly reduced mean arterial pressure (MAP) and increased heart rate throughout exercise. The relationship between coronary sinus O(2) saturation and MVO(2) was shifted upward (P<0.05) after tezosentan administration; i.e., as MVO(2) increased during exercise, coronary sinus O(2) saturation was disproportionately higher after ET-receptor blockade. After propranolol, tezosentan resulted in significant decreases (P<0.05) in left ventricular pressure, the first derivative of left ventricular pressure over time, and MAP during exercise. As MVO(2) increased during exercise, coronary sinus O(2) saturation levels after tezosentan became superimposable over those observed before ET-receptor blockade. Thus dual blockade of ET(A)/ET(B) receptors alters coronary hemodynamics and O(2) metabolism during exercise, but ET activity failed to increase beyond baseline levels.

Limitation of coronary reserve after successful angioplasty is prevented by oral pretreatment with an alpha1-adrenergic antagonist

Coronary vasoconstriction that occurs after percutaneous transluminal coronary angioplasty (PTCA) is abolished by intracoronary phentolamine. An impairment of coronary vasodilator reserve (CVR) has been observed < or = 7 days after successful PTCA. To ascertain whether pretreatment with the alpha1-adrenergic receptor blocker doxazosin could prevent the limitation of CVR after PTCA, we carried out a randomised, double-blind, controlled study on 26 patients with significant (> 75%) single vessel disease undergoing PTCA. Twelve patients received doxazosin 4 mg daily in addition to their standard treatment, while 14 patients received matching placebo, starting 11 days before PTCA. Myocardial blood flow (MBF) at baseline and after i.v. dipyridamole (0.56 mg/kg) was measured within 5 days after PTCA using positron emission tomography (PET) with oxygen-15-labelled water. Angioplasty was successful in all patients with a residual stenosis < or = 35%. At PET scanning, hemodynamic parameters were comparable in the two groups. In the territory subtended by the dilated artery, CVR was significantly higher in patients treated with doxazosin compared with those receiving placebo (2.78 +/- 0.1.21 vs. 1.95 +/- 0.68; p < 0.01). Conversely, CVR in the remote territories subtended by angiographically normal arteries was similar in the two groups (2.53 +/- 0.92 and 2.48 +/- 0.80, respectively; p = NS). Treatment with oral doxazosin in addition to standard antianginal therapy can prevent the impairment of CVR frequently observed despite successful PTCA.

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.

Endothelin antagonists block alpha1-adrenergic constriction of coronary arterioles

We have previously observed that intracoronary administration of the alpha1-adrenergic agonist phenylephrine (PE) over a period of minutes induced both an immediate and long-lasting (2 h) vasoconstriction of epicardial coronary arterioles. Because it is unlikely that alpha1-adrenergic constriction would persist for hours after removal of the agonist, this observation supports the view that another constrictor(s) is released during alpha1-adrenergic activation and induces the prolonged vasoconstriction. Therefore, we hypothesized that the prolonged microvascular constriction after PE is due to the production of endothelin (ET). We focused on ET not only because this peptide produces potent vasoconstriction but also because its vasoconstrictor action is characterized by a long duration. To test this hypothesis, the diameters of coronary arterioles (<222 micrometers) in the beating heart of pentobarbital-anesthetized dogs with stroboscopic intravital microscopy were measured during a 15-min intracoronary infusion of PE (1 microgram. kg-1 . min-1) and at 15-min intervals for a total of 120 min. All experiments were performed in the presence of beta-adrenergic blockade with propranolol. At 120 min, arterioles in the PE group were constricted (-23 +/- 9% change in diameter vs. baseline). Pretreatment with the ET-converting enzyme inhibitor phosphoramidon or the ETA-receptor antagonist FR-139317 prevented the PE-induced constriction at 120 min (-1 +/- 3 and -6 +/- 3%, respectively, P < 0.01 vs. PE). Pretreatment with the selective alpha1-adrenergic antagonist prazosin (Prz) also prevented the sustained constriction (0 +/- 2%, P < 0.01 vs. PE) but Prz given 60 min after PE infusion did not (-13 +/- 3%). In the aggregate, these results show that vasoconstriction of epicardial coronary arterioles via alpha1-adrenergic activation is blocked by an ET antagonist and an inhibitor of its production. From these data, we conclude that alpha1-adrenergic activation promotes the production and/or release of ET, which produces or facilitates microvascular constriction of epicardial canine coronary arterioles.