Limitation of cardiac output by a coronary α₁-constrictor tone during exercise in dogs

This study was performed to examine whether an α₁-constrictor tone, which limits coronary functional hyperemia during exercise, imposes a significant limitation on global cardiac performance as determined by cardiac output (CO). Seven dogs were chronically instrumented to measure left ventricular pressure (LVP), maximum rate of rise of LVP (dP/dt(max)), heart rate (HR), mean aortic pressure (AoP), circumflex blood flow velocity (CFV), and CO at rest and during submaximal exercise. Either the selective α₁- adrenergic antagonist prazosin (0.5 mg) or the vasodilator adenosine was administered into the circumflex artery during exercise at 6.4 kilometers per hour (kph)/16% treadmill incline. Exercise caused significant increase in mean AoP, HR, LVP, dP/dt(max), CFV, stroke volume (SV), and CO, whereas systemic vascular resistance (SVR) was significantly reduced. After intracoronary α₁-blockade with prazosin, CFV, dP/dt(max), SV, and CO increased further (17 ± 2, 19 ± 3, 16 ± 2, and 17 ± 2%, respectively) without changing mean AoP, HR, or SVR. Comparable increases were observed when CFV was increased by a similar degree using the direct vasodilator adenosine. These results indicate that increasing coronary flow by removing a coronary α₁-constrictor tone with prazosin or by direct vasodilation with adenosine during submaximal exercise leads to an increase in myocardial oxygen supply and, as a result, cardiac pump performance (SV and CO).