The effect of
bunitrolol, a
beta-adrenergic blocking agent having intrinsic
sympathomimetic activity, on ischemic myocardial metabolism was examined in anesthetized dog hearts.
Ischemia was induced by ligating the left anterior descending coronary artery for 3 or 30 min.
Bunitrolol (0.3 or 1.0 mg/kg of
body weight) was injected intravenously 5 min before the onset of
ischemia.
Ischemia reduced myocardial high-energy
phosphate levels and changed the levels of glycolytic intermediates.
Bunitrolol at a dose of 0.3 mg/kg significantly decreased heart rate, whereas the
drug at a dose of 1.0 mg/kg did not. This result suggests that intrinsic
sympathomimetic activity is observed only at a high dose of
bunitrolol. Pretreatment with
bunitrolol at 0.3 but not 1.0 mg/kg attenuated the myocardial metabolic changes caused by 3 min of
ischemia. After 30 min of
ischemia, a beneficial effect of
bunitrolol on the ischemic myocardium was not observed. These results suggest that
bunitrolol may lessen the ischemic influence on the myocardium in the early stage of
ischemia and that intrinsic
sympathomimetic action may not have a beneficial effect on the ischemic myocardium.