1. Electrophoretic and enzyme techniques have been used to study the structure and function of the contractile protein system in the myocardium of dogs before and after β-adrenoceptor blockade. Animals were examined after acute β-adrenoceptor blockade by using intravenous atenolol (0.2 mg/kg) and following chronic therapy with oral atenolol (100 mg twice daily) for a mean period of 106 days.

2. Two-dimensional polyacrylamide-gel electrophoretic techniques were used to examine the myocardial contractile and regulatory proteins present in endomyocardial biopsy specimens obtained after acute and chronic β-adrenoceptor blockade. No differences in charge, molecular weight or the relative proportions of actin, myosin light chains, tropomyosin or troponin-C were seen after either acute or chronic β-adrenoceptor blockade.

3. The maximal activity and calcium sensitivity of the myofibrillar adenosine triphosphatase (ATPase) was also unchanged after acute and chronic atenolol therapy.

4. It is concluded that β-adrenoceptor blockade has no significant adaptive effect on the structural or functional properties of the myofibril.

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