Charge movement in cut twitch fibres of Rana temporaria containing 0.1 mM EGTA

1. Charge movement was studied in the cut twitch fibres of Rana temporaria with a double Vaseline-gap voltage-clamp technique. 2. In fibres containing 0.1 mM EGTA, the I gamma hump component of charge movement was either unresolvable or had a brief duration at all potentials. In the steady-state charge-voltage (Q-V) plots that were separable into the less voltage dependent (Q beta) and more voltage dependent (Q gamma) components, the amount of Q gamma was 0.9 +/- 0.1 nC microF-1. (mean +/- S.E.M., n = 18). 3. Fibres containing 20 mM EGTA showed broad I gamma humps at about -45 mV. The addition of 1.8 mM total Ca2+ to the end-pools accelerated the rising phase and abbreviated the duration of the hump. 4. I gamma in fibres containing 0.1 mM EGTA resembled those in intact fibres when recorded at similar low temperatures. 5. These results explain the controversy concerning the variable appearance of I gamma humps in cut fibres.