The voltage and temperature dependence of the end-plate current in frog skeletal muscle

The effect of membrane potential (V) on the half-time (t 1/2) of the falling phase of the end-plate current (e.p.c.) was found to obey the equation t 1/2 = A X eBV + C, where A, B and C are constants. The temperature dependence of t 1/2 was found to follow the Arrhenius equation. The activation energy (Ea) varied from about 50 kJ/mol to about 120 kJ/mol. At membrane potentials between about -40 mV and -140 mV, the Ea/V relation was similar in all end-plates investigated: Ea increased if membrane potential was made more negative. At membrane potentials between about +60 mV and -40 mV, however, the Ea/V relation was different in different end-plates: If membrane potential was made more negative, Ea was either increased, or not affected, or decreased. It is concluded that at negative levels of membrane potential the decay of the e.p.c. depends on average life-time of ionic channels, opened up by the action of acetylcholine on junctional receptors. At strongly positive levels of membrane potential, however, the decay of the e.p.c. can be determined by the average life-time of ionic channels or by the clearance of transmitter from the synaptic cleft, or both. Either of these processes can be reflected in the value of constant C in the above equation.