CALCIUM DEPENDENT ELECTRICAL-ACTIVITY IN TWITCH MUSCLE-FIBERS OF FROG

When twitch muscle fibers of the frog were equilibrated in chloride free saline with 2.5-20 mM tetraethylammonium sulfate [(TEA)2 SO4], the action potential was followed by a long depolarizing response. This response was greatly reduced by adding Cl- (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 Ca since the phenomenon was abolished by removing external Ca or by adding Co or D-600 (.alpha.-isopropyl-.alpha.-[N-methyl-N-homoveratryl-.gamma.-aminopropyl]-3,4,5-trimethoxyphenylacetonitrile), and was not greatly affected by reducing or removing external Na or by adding tetrodotoxin.