Effects of epinephrine on coronary microvascular diameters.

This study was designed to examine the hypothesis that epinephrine has nonuniform effects on coronary microvascular diameters. Measurements of coronary microvascular diameter were completed in anesthetized, open-chest cat preparations in which the epicardial microcirculation was viewed through an intravital microscope using stroboscopic epi-illumination. Images of coronary microvessels were digitized and analyzed on a video monitor. With arterial pressure controlled, measurements in the absence and presence of beta-adrenergic blockade (propranolol 1 mg/kg) were obtained during epinephrine infusion (1-2 micrograms/kg/min). In the absence of beta-adrenergic blockade, epinephrine produced a 25% increase in myocardial perfusion. Under these conditions, coronary vasodilation was observed in all classes of coronary arterial and arteriolar vessels. In the presence of beta-adrenergic blockade, epinephrine produced a significant decrease in myocardial perfusion (-20%). Nonuniform effects on diameter were observed in arterial and venous segments of the coronary circulation. These data are consistent with the view that in the absence of beta-adrenergic blockade, the functional coronary hyperemia associated with epinephrine administration is produced by uniform coronary arterial and arteriolar dilation. In the presence of beta-adrenergic blockade, with metabolic effects controlled, epinephrine produced a decrease in myocardial perfusion, which is related to a nonuniform decrease in coronary microvascular diameters. Such heterogeneous effects on microvascular diameters result in a redistribution of coronary microvascular resistance.