Study on tubular transient current and mechanical activation in skeletal muscle of frog

The relation between membrane current and mechanical activity during excitation of frog muscle fibers was studied using two-microelectrode voltage clamp technique. The current flow upon excitation of membrane consisted of transient initial inward and subsequent outwards currents which were carried by Na+ influx and K+ efflux respectively. Blocking of the outward K+ current by 3,4-diaminopyridine resulted in appearance of a late inward current associated with marked potentiation of twitch tension. Detubulation of the fiber by treatment with hypertonic glycerol suppressed the outward K+ current as well as the late inward current and abolished the contractile force. Dantrolene also gave similar effects on membrane currents and reduced twitch tension markedly. These results suggest that the late inward current through the T-system and not the outward K+ current may be the first step to lead the muscle cell to mechanical activation.