Membrane currents related to configuration changes in the action potential of frog atrial muscle in Na- and Ca-free conditions

Abstract

Changes in membrane currents and action potential in frog atrial muscle under Ca-free or Na-deficient conditions were investigated using double sucrose-gap techniques. In Ca-free Ringer, the instantaneous current ( I k1 and others) decreased and the delayed outward current ( I x ) was markedly retarded. The slow inward current ( I slow ) diminished in the absence of the calcium inward current ( I Ca ). The remaining slow sodium inward current ( I Na slow ) was less than one fourth in amplitude of the control I slow , and showed a faster activation and a slower inactivation. In Na-deficient Ringer when LiCl or sucrose replaced NaCl, the I slow increased in amplitude, despite the elimination of I Na slow . The I k1 also increased, but the amplitude of fully activated I x diminished although the activation of I x appeared much faster than in the control. Thus, it became clear that in a Ca-free condition, the overshoot of the action potential (AP) decreased due to the absence of I Ca , and the AP-duration was prolonged due to the pronounced delay of activation of I x and the depression of I k1 . The I Na slow was too small to contribute to the delayed repolarization of AP. In Na-deficient conditions, the AP-amplitude increased due to enhancement of I Ca , and the AP-duration was shortened mainly as a result of the fast activation of I x and due to the increase of I k1 .