Myofibrillar ATPase activity in rat heart after chronic propranolol administration

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Abstract
Chronic chemical sympathectomy brought about by 6-hydroxydopamine injections results in a depression in myocardial contractile function which is accompanied by reduced myofibrillar ATPase activity. To determine whether chronic .beta.-adrenergic receptor blockade elicits similar alterations in cardiac contractile-protein ATPase activity, adult rats were given twice-daily injections of propranolol 7 days/wk for 2 wk. Effective .beta.-adrenergic receptor blockade was verified by the lack of hemodynamic responsiveness to isoproterenol infusion. Myofibrils were prepared from left ventricular tissue and analyzed for ATPase activity. Myofibrillar ATPase activity was 295 .+-. 8 nmol Pi .cntdot. mg-1 .cntdot. min-1 in controls. Enzyme activity was not significantly different in propranolol-injected rats. Chronic propranolol administration does not alter the ATPase activity of cardiac myofibrils. The altered contractile-protein enzymatic properties resulting from chronic chemical sympathectomy may not occur as the result of a reduced level of cardiac .beta.-adrenergic receptor stimulation.