Comparative effects of propranolol and practolol in the early stages of experimental canine myocardial infarction

1 The effects of propranolol and practolol, at equivalent myocardial beta-adrenoceptor blocking doses, (as assessed by the degree of shift of isoprenaline dose-response curves) were investigated in anaesthetized greyhounds before and after acute coronary artery ligation. 2 When administered intravenously to the intact close-chest dog, propranolol (0.1 mg/kg) and practolol (0.5 mg/kg) caused similar decreases in heart rate, left ventricular dP/dt max, myocardial blood flow and cardiac output. Only propranolol increased peripheral vascular resistance. 3 When administered 2-3h after acute coronary artery ligation, propranolol (0.1 mg/kg) significantly decreased blood flow in both normally perfused and ischaemic regions of the heart. There was also electrocardiographic evidence of further deterioration after propranolol; two out of seven Animals died following this treatment. 4 Practolol (0.5 mg/kg) when administered after coronary artery ligation also decreased normal myocardial blood flow but flow in the ischaemic area remained unchanged. Evidence was obtained from electrocardiographic, myocardial temperature, myocardial O2 consumption and lactate measurements that the administration of practolol, in contrast to propranolol, benefited the ischaemic myocardium. 5 Analysis of the results suggests that this beneficial action of practolol may be related to at least two mechanisms. Firstly the ability of practolol to increase the period during diastole when perfusion of the subendocardium is possible, without decreasing the transventricular pressure during this period. Secondly that practolol does not unmask alpha-adrenoceptor vasoconstriction in the ischaemic region.