Vol. 284, Issue 1, 75-82, 1998

Amiodarone Inhibits the Na⁺-K⁺ Pump in Rabbit Cardiac Myocytes after Acute and Chronic Treatment¹

David F. Gray², Anastasia S. Mihailidou, Peter S. Hansen³, Kerrie A. Buhagiar, Nerida L. Bewick, Helge H. Rasmussen and David W. Whalley

Cardiology Department, Royal North Shore Hospital, St. Leonards, 2065 NSW, and University of Sydney, 2006 NSW, Australia

Amiodarone has been shown to affect cell membrane physicochemical properties, and it may produce a state of cellular hypothyroidism. Because the sarcolemmal Na⁺-K⁺ pump is sensitive to changes in cell membrane properties and thyroid status, we examined whether amiodarone affected Na⁺-K⁺ pump function. We measured Na⁺-K⁺ pump current (I_p) using the whole-cell patch-clamp technique in single ventricular myocytes isolated from rabbits. Chronic treatment with oral amiodarone for 4 weeks reduced I_p when myocytes were dialyzed with patch-pipettes containing either 10 mM Na⁺ or 80 mM Na⁺. In myocytes from untreated rabbits, acute exposure to amiodarone in vitro reduced I_p when patch pipettes contained 10 mM Na⁺ but had no effect on I_p at 80 mM Na⁺. Amiodarone had no effect on the voltage dependence of the pump or the affinity of the pump for extracellular K⁺ either after chronic treatment or during acute exposure. We conclude that chronic amiodarone treatment reduces overall Na⁺-K⁺ pump capacity in cardiac ventricular myocytes. In contrast, acute exposure of myocytes to amiodarone reduces the apparent Na⁺ affinity of the Na⁺-K⁺ pump. An amiodarone-induced inhibition of the hyperpolarizing Na⁺-K⁺ pump current may contribute to the action potential prolongation observed during treatment with this drug.