Alpha 1-adrenergic coronary constriction during exercise and ischemia.

This paper reviews work primarily from our laboratories, examining an alpha 1-adrenergic receptor-mediated coronary constriction during exercise and myocardial ischemia in dogs. It was demonstrated that in the quiescent conscious dog, the coronary circulation is devoid of an alpha 1-coronary constriction. Furthermore, it was shown by the intracoronary injection of selective agonists that both alpha 1- and alpha 2-receptor subtypes are present in coronary vessels. However, during exercise or ischemia only the selective alpha 1-antagonist prazosin caused an increase in coronary inflow, indicating that only alpha 1-receptors were activated. During both conditions, the increase in flow caused by alpha 1-blockade was associated with an increased contractile function in subendocardium. In experiments on anesthetized dogs, it was shown that prazosin caused an equal increase in perfusion of subepicardial and subendocardial layers during stellate ganglion stimulation. However, contractile function was increased only in subendocardium. It was proposed that only in deeper muscle layers does an alpha 1-coronary constriction impose a flow-limitation on contractile function. Finally, recent results indicate that myocardial ischemia, produced either by partial coronary stenosis or by maintenance of coronary inflow at the resting level during exercise, may initiate a vicious cycle with a further increase in alpha 1-adrenergic coronary constriction. Abolition of this positive feedback mechanism may partially explain the anti-infarction effects of chronic ventricular sympathectomy, as previously observed in our laboratories.