Inward calcium current in twitch muscle fibres of the frog.

Abstract

Voltage clamp experiments using the 3 micro-electrode voltage clamp technique were performed on sartorius muscles of the frog. By blocking K currents with tetraethylammonium and replacing Cl- with SO4 a slow inward current was detected. The slow inward current is mainly carried by Ca, since it is abolished by Co and D-600 [.alpha.-isopropyl-.alpha.-[(N-methyl-N-homoveratryl)-.alpha.-aminopropyl]-3,4,5-trimethoxyphenylacetonitrile], it depends on external Ca, and is not affected by removing external Na or by tetrodotoxin (TTX). The slow inward current has a mean threshold of -40 mV, reaches a mean maximum value at approximately 0 mV of 81 .mu.Acm-2 and has a mean reversal potential of +38 mV. The Ca current is inactivated by the application of 2 s conditioning prepulses according to a sigmoid curve with Vh = -42 mV and k = 6.2 mV. The slow time course of this Ca current makes it unlikely that it participates in contraction during a twitch, but it might be activated during long depolarizations as K contractures.