BLOOD-FLOW REGULATION DURING ACUTE REGIONAL ISCHEMIA IN FELINE HEARTS - IMPORTANCE OF POSTJUNCTIONAL ALPHA-1-ADRENOCEPTORS AND ALPHA-2-ADRENOCEPTORS

Influence of postjunctional alpha-1- and subsequent alpha-2-adrenergic antagonism on myocardial blood flow was measured in a group of anesthetized cats with acute occlusion of the left anterior descending coronary artery (LAD) and a control group (n = 10 for both). The relatively selective postjunctional alpha-1-(doxazosin) and alpha-2-adrenergic (SK&F 104078) antagonists were applied after beta-adrenergic blockade (propranolol). Regional myocardial blood flow was obtained with radiolabeled microspheres. Major hemodynamic determinants for perfusion were kept constant both within and between groups by right atrial pacing and aortic obstruction. Mean coronary resistance in nonischemic myocardium was permanently lower in the occlusion group as compared with controls (p < 0.01). Subsequent alpha-2-adrenergic antagonism reduced mean coronary resistance in controls only (p < 0.05). Cardiac output (CO) and dP/dt was reduced in LAD-occluded hearts after alpha-2-adrenergic blockade (p < 0.01, p < 0.05). The study demonstrates the significance of postjunctional alpha-2-adrenergic-mediated vasoconstriction in well-perfused myocardium of control hearts, whereas such vasoconstriction was deteriorated in LAD-occluded hearts. A role for myocardial alpha-2-adrenoceptors for maintenance of global cardiac function in acute regional ischemia was also indicated.