Local anaesthetic and anti-arrhythmic actions of alprenolol relative to its effect on intracellular potentials and other properties of isolated cardiac muscle.

1. Alprenolol, a beta-adrenoceptor blocking drug reported to have the same potency as propranolol in vitro and in vivo, was found to be four times more active than procaine as a local anaesthetic on frog sciatic nerve.2. At doses of 0.125 mg/kg and above alprenolol protected anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.3. In isolated rabbit atria, resting potentials were unchanged, and the duration of the action potential was not prolonged, by concentrations of alprenolol up to 10.5 x 10(-6)M. The maximum rate of depolarization (MRD), however, was reduced by 30% at a concentration of 0.525 x 10(-6)M. This was 214 times less than the concentration which reduced the frog action potential height by 25%.4. The concentration of alprenolol required to produce more than 15% decrease in contraction or maximum driven frequency was 3.5 x 10(-6)M.5. As a test of the direct action of alprenolol on isolated cardiac muscle, MRD was also a more sensitive test than the measurement of electrical threshold or conduction velocity.