Augmentation of coronary flow improves myocardial function in exercise

We compared the effects of increasing coronary blood flow (CBF) by intracoronary alpha 1-adrenergic blockade or by a direct vasodilator in six chronically instrumented dogs undergoing submaximal exercise. On different days, CBF, left ventricular pressure (LVP), regional myocardial segment length (SL), and arterial blood pressure (ABP) were measured at rest, during strenuous exercise before administration of vasodilating agents, and during exercise after administration of the vasodilating agents prazosin (0.5 mg) or adenosine (10-100 micrograms/min continuous infusion) through an indwelling circumflex artery catheter. Exercise resulted in a significant increase in CBF, which was accompanied by significant increases in maximum rate of LVP generation (dP/dtmax), percentage of SL shortening (%SL) and maximum rate of SL shortening (dL/dtmax) in both the anterior and posterior regions of the left ventricle. As compared with the exercise level alone, prazosin administration or adenosine infusion during exercise significantly increased CBF by 22 and 26%, respectively. Furthermore, these increases in CBF were followed by significant increases in both dP/dtmax and dL/dtmax over exercise levels without prazosin administration or adenosine infusion. These results suggest that myocardial contractile function may be flow-limited under conditions of submaximal exercise and that coronary vasodilation, either by alpha 1-adrenergic blockade or by a direct vasodilator, improves contractile function.