Effects of bufetolol and propranolol on active and passive membrane properties of dog papillary muscle.

Effects of bufetolol and propranolol, adrenergic beta-receptor blocking and anti-arrhythmic drugs, on active and passive membrane properties of the dog papillary muscle were investigated with microelectrode and sucrose-gap methods. Bufetolol (10(-5) to 10(-4) g/ml) and propranolol (10(-6) g/ml) significantly decreased the maximum rate of rise of the action potential. The maximum responsive frequency to driving stimulus was decreased in the presence of bufetolol (3 X 10(-5) g/ml) and propranolol (10(-5 g/ml), whereas the effective refractory period was not affected. The critical threshold potential was shifted to more positive potential in the presence of the drugs. The passive membrane property, the space constant (lambda), the time constant (tau) and the current-voltage relations of the muscle membrane were not significantly altered by the drugs. It is concluded that bufetolol and propranolol suppress the excitability of the muscle membrane and this action may be ascribed to the decrease in the sodium conductance (gNa) and to the rise of gNa onset potential without alteration in the passive membrane property.