Effects of and adrenergic blockade on coronary arterial microvessels in the beating canine heart

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.

Effects of low doses of endothelin-1 on basal vascular tone and autoregulatory vasodilation in canine coronary microcirculation in vivo

The plasma level of endothelin-1 (ET-1) increases in several cardiovascular disorders. The present study examined whether threshold doses of ET-1 affect vascular tone and autoregulatory vasodilation during a reduction in perfusion pressure in the coronary microcirculation in vivo. In anesthetized open-chest dogs, arterial microvessels in the epimyocardium were observed through a microscope equipped with a floating objective. In 6 dogs, ET-1 (10(-13) to 10(-8)mol/L) was superfused onto the epimyocardium in a cumulative fashion. In another set of dogs (n= 16), the perfusion pressure of the observed vascular bed was reduced to 60 mmHg (mild stenosis) and to 40 mmHg (severe stenosis) by a hydraulic occluder, and the microvascular responses were observed in the presence (n=9) or absence (n=7) of ET-1 (10(-12) or 10(-11) mol/L). ET-1 > or =10(-11) mol/L constricted coronary arterioles (< or =100 microm in diameter) and small arteries (>100 microm in diameter) in a dose-dependent fashion. ET-1 of 10(-12) mol/L affected neither the basal diameters nor the dilation of vessels during the pressure reduction. ET-1 of 10(-11) mol/L decreased the diameters of arterioles and small arteries before and during the mild and severe stenosis. However, ET-1 did not attenuate the percentage dilation of arterioles from the baseline in response to the mild and severe stenosis. The data indicates the following: (1) ET-1 at doses > or =10(-11) mol/L similarly constricts coronary arterioles and small arteries; (2) ET-1 at 10(-11) mol/L, which is slightly higher than the pathophysiological plasma level, increases the basal vascular tone, but does not attenuate the autoregulatory vasodilation of the coronary microcirculation.

Pertussis toxin-sensitive G protein mediates coronary microvascular control during autoregulation and ischemia in canine heart

GTP-binding regulatory proteins (G proteins) regulate various biological functions, but their participation in controlling coronary microvascular tone has not been established yet. The goal of the present study was to elucidate the role of pertussis toxin (PTX)-sensitive G protein in regulating coronary microvascular tone during autoregulation and ischemia. In 42 open-chest dogs, coronary arterial microvessels on the surface of the left ventricle were directly observed by epi-illuminated fluorescence microangiography using a floating objective system. PTX (300 ng/mL) was superfused onto the surface of the left ventricle for 2 hours to block Gi and G(o) protein in epimyocardial coronary microvessels in vivo. PTX superfusion caused no change in the resting diameters of microvessels and significantly blocked the vasoconstriction induced by BHT 920 (a selective alpha 2-agonist). After pretreatment with PTX or its vehicle, the left anterior descending coronary artery (LAD) was occluded by a hydraulic occluder to reduce coronary perfusion pressure (CPP) in a stepwise fashion. A mild stenosis (CPP, 60 mm Hg), a severe stenosis (CPP, 40 mm Hg), and complete occlusion were sequentially produced. Coronary flow velocity in the LAD distal to the stenotic site was continuously monitored. In both PTX and vehicle groups, flow velocity did not significantly decrease during mild stenosis, proving that transmural coronary autoregulatory function was well preserved in the preparation. During severe stenosis and complete occlusion, the coronary flow velocity significantly decreased. In the vehicle group, microvessels < 100 microns in inner diameter significantly dilated in response to the reduction in perfusion pressure (mild stenosis, 6.2 +/- 1.9%; severe stenosis, 21.1 +/- 4.4%; and complete occlusion, 16.8 +/- 5.9%; P < .05 versus baseline diameters). In the PTX group, microvessels did not dilate during each occlusion level (mild stenosis, -2.0 +/- 0.9%; severe stenosis, -3.9 +/- 1.9%; and complete occlusion, -13.4 +/- 2.9%; P < .05 versus vehicle group). PTX did not affect the microvascular dilation caused by nitroprusside. The present data indicate that PTX-sensitive G protein is crucially involved in microvascular control during autoregulation and ischemia.

Effect of calcitonin gene-related peptide on coronary microvessels and its role in acute myocardial ischemia

BACKGROUND

Calcitonin gene-related peptide (CGRP) is a potent dilator of epicardial conduit vessels and is released during myocardial ischemia in humans. However, the effect of CGRP on coronary arterial microvessels is still unclear, and it is unknown if CGRP modulates the tone of coronary arterial microvessels during acute myocardial ischemia.

METHODS AND RESULTS

Epimyocardial microvessels were observed through a microscope equipped with a floating objective system in anesthetized open-chest dogs. Heart rate and aortic pressure were maintained at control levels. Flow velocity of the left anterior descending coronary artery (LAD) was measured with a suction-cup Doppler probe. When CGRP was cumulatively infused into the LAD (0.05, 0.5, 5.0, and 50 pmol/kg per minute) or superfused (0.03, 0.3, 3.0, and 30 nmol/L) over the left ventricular surface, arterial control microvessels > 100 microns in diameter dilated dose dependently at dosages of 0.5 to 50 pmol/kg per minute (infused) or 0.3 to 30 nmol/L (superfused), but those < 100 microns dilated only at the highest dose, and those > 100 microns had greater dilation in both groups. Only the highest dose of CGRP (infused) significantly increased coronary flow. The superfusion of CGRP(8-37) (CGRP receptor antagonist, 300 nmol/L) did not affect the control diameters of coronary arterial microvessels but completely abolished CGRP-induced vasodilation at the same doses (infused and superfused). However, 300 nmol/L of CGRP(8-37) did not affect the response of coronary arterial microvessels to the LAD occlusion in any size.

CONCLUSIONS

CGRP preferentially dilates the coronary arterial microvessels > 100 microns in diameter but has only a small effect on those < 100 microns. Endogenous CGRP does not modulate the tone of coronary arterial microvessels during acute myocardial ischemia in beating canine hearts.