Vasodilating β-adrenoceptor blocking drugs do not redistribute myocardial flow during acute coronary ligation in dogs

The effects of two nonselective β-adrenoceptor blocking drugs, additionally endowed with vasodilating properties with different mechanisms: labetalol which blocks α-adrenoceptors, and bucindolol which nonspecifically relaxes vascular smooth muscle, were investigated on regional myocardial blood flow (RMBF) distribution in ischemic and nonischemic areas and on ST-segment elevation in ischemic areas during intermittent coronary artery occlusion in dogs. Both labetalol and bucindolol reduced heart rate, arterial blood pressure and myocardial oxygen consumption. However, they did not affect coronary vascular resistance and did not induce any favorable coronary blood flow redistribution phenomenon either from the epicardium to the endocardium or from the nonischemic to the ischemic areas. These results indicate that the inability of labetalol and bucindolol to favorably redistribute RMBF was due to the fact that their vasodilating properties counteract one major determinant of RMBF redistribution. i.e. the β-adrenoceptor blockade-induced increase in regional coronary vascular resistance. Finally, both labetalol and bucindolol dose dependently decreased ST-segment elevation in ischemic myocardial areas.