[Calcium and calcium-activated potassium currents of motor nerve endings in the frog].
Evoked electrical responses of nerve endings were recorded in experiments on frog cutaneous-pectoris muscle under visual control. During superfusion by Ca2+-free solution with tubocurarine a late inward current was found in the evoked response of nerve endings recorded by CaCl2 filled electrode. With addition of the external solution of 4-aminopyridine the late current was changed into an outward one. This outward current depended on Ca2+ concentration in the electrode, decreased after local increase of K+ concentration and was abolished by Co2+ ions. Local ionophoretic application of tetraethyl-ammonium eliminated the outward current and revealed a strong and prolonged inward current. Similar currents were recorded by microelectrodes filled with SrCl2, BaCl2 and MgCl2. It is concluded that the late inward current is carried through potential-dependent calcium channels, while the late outward one--through Ca2+-activated K+ channels. The ratio of calcium and Ca2+-activated potassium currents in motor nerve endings and their role in the transmitter release are discussed.