Beta-adrenoceptor blocking and electrophysiological effects of bufetolol in the guinea pig atria

The potency of blockade of bufetolol, a beta-adrenoceptor blocking drug, and effects of bufetolol on the action potential, contractile force and various electrophysiological properties of the atrium were investigated in comparison with propranolol and quinidine. Bufetolol had a pA2 of 8.65 against the positive chronotropic action of isoproternol on the guinea pig sinus node. Bufetolol, 10(-7) g/ml, did not affect the action potential of the atrial muscle, while the drug, 3 X 10(-5) g/ml, significantly decreased the overshoot potential, the amplitude and the maximum rate of rise of the action potential and prolonged the times for 50% and 90% repolarization. The contractile force was reduced by bufetol, 3 X 10(-5) g/ml. The maximum responsive frequency to the driving stimulus was decreased by bufetol, 3 X 10(-5) g/ml. The excitability of the muscle membrane was suppressed by bufetolo, 3 X 10(-5) g/ml, as indicated by changes in membrane responsiveness, membrane reactivation and strength-duration curve. Propranolol, 10(-5) g/ml and quinidine, 10(-5) g/ml showed similar effects on excitability. The authors suggest that these effects of bufetolol are responsible for its antidyshythmic effects.