Calcium channels in skeletal muscle fibres of the frog.

The vaseline gap voltage clamp technique was used to study slow and fast calcium currents in phasic and tonic muscle fibres of the frogs Rana temporaria (R.t.) and Xenopus laevis (X.l.). At physiological Ca2+ and Na+ concentrations three inward currents could be recorded in R.t. twitch muscle fibres: a fast Ca current, a slow Ca current, and a non-specific (INS) current carried by Na ions. The last current appeared as a sole component at a low threshold membrane potential (MP -55 mV) and was present until the slow Ca current appeared. In tonic R.t. muscle and twitch X. laevis fibres only the fast and the slow Ca currents could be recorded; however the inactivation of the slow Ca current was very prolonged in the presence of Na ions. A modulatory effect of Na ions on Ca2+ currents was observed. In phasic fibres the fast Ca currents inactivated either very slowly or rapidly.