Suggestive evidence for inhibitory action of amiodarone on Na+, K+-pump activity in guinea pig heart.

1. Effects of amiodarone, a powerful antiarrhythmic agent, on Na+, K+-pump activity were examined on ventricular papillary muscle of guinea pig, by means of conventional microelectrode technique. 2. The activity of Na+, K+-pump was measured by two methods. One of them was to measure the amplitudes of depolarization observed during overdrive stimulation (3.3 Hz) and of hyperpolarization observed after the overdrive stimulation (post-overdrive hyperpolarization), and the other, to measure the hyperpolarization observed following introduction of 10 mM K+, after exposure to K+ free solution for a certain duration. 3. Amiodarone significantly decreased the amplitude of depolarization during overdrive stimulation and the amplitude of post-overdrive hyperpolarization. 4. In the latter method, the deactivation process of hyperpolarization recorded by the introduction of 10 mM K+ following K+ depletion slowed down by amiodarone. 5. These findings suggest that amiodarone may inhibit, at least in part, the Na+, K+-pump activity in the ventricular muscle of guinea pig.