Calcium dependent electrical activity in twitch muscle fibres of the frog

When twitch muscle fibres of the frog were equilibrated in chloride free saline with 2.5–20 mM tetraethylammonium sulphate [(TEA)2SO4], the action potential was followed by a long depolarizing response. This response was greatly reduced by adding chloride ions (10 mM). In 20 mM (TEA)2SO4 the response consisted of an initial depolarization of – 23 mV lasting several seconds, followed by a slow delayed spike reaching + 23 mV. After the delayed spike the cell remained continuously depolarized creeping to a steady depolarization of + 25 mV (mean values). The response can be attributed to an increase in membrane conductance to calcium since the phenomenon was abolished by removing external calcium or by adding cobalt or D-600, and was not greatly affected by reducing or removing external sodium or by adding tetrodotoxin.