LIMITATION OF CARDIAC-OUTPUT BY A CORONARY ALPHA(1)-CONSTRICTOR TONE DURING EXERCISE IN DOGS

This study was performed to examine whether an alpha(1)-constrictor to ne, which limits coronary functional hyperemia during exercise, impose s a significant limitation on global cardiac performance as determined by cardiac output (CO). Seven dogs were chronically instrumented to m easure left ventricular pressure (LVP), maximum rate of rise of LVP (d P/dt(max)), heart rate (HR), mean aortic pressure (AoP), circumflex bl ood flow velocity (CFV), and CO at rest and during submaximal exercise . Either the selective alpha(1)-adrenergic antagonist prazosin (0.5 mg ) or the vasodilator adenosine was administered into the circumflex ar tery during exercise at 6.4 kilometers per hour (kph)/16% treadmill in cline. Exercise caused significant increase in mean AoP, HR, LVP, dP/d t(max,) CFV, stroke volume (SV), and CO, whereas systemic vascular res istance (SVR) was significantly reduced. After intracoronary alpha(1)- blockade with prazosin, CFV, dP/dt(max), SV, and CO increased further (17 +/- 2, 19 +/- 3, 16 +/- 2, and 17 +/- 2%, respectively) without ch anging mean AoP, HR, or SVR. Comparable increases were observed when C FV was increased by a similar degree using the direct vasodilator aden osine. These results indicate that increasing coronary flow by removin g a coronary alpha(1)-constrictor tone with prazosin or by direct vaso dilation with adenosine during submaximal exercise leads to an increas e in myocardial oxygen supply and, as a result, cardiac pump performan ce (SV and CO).