A slow calcium‐dependent component of charge movement in Rana temporaria cut twitch fibres

1 Charge movement was studied in highly stretched frog cut twitch fibres in a double Vaseline‐gap voltage‐clamp chamber, with the internal solution containing either 0.1 mM EGTA or 20 mM EGTA plus 1.8 mM total Ca 2+ . 2 Fibres were stimulated with TEST pulses lasting 100‐400 ms. Replacement of the external Cl − with an ‘impermeant’ anion, such as SO 4 2‐ , CH 3 SO 3 − , gluconate or glutamate, greatly reduced the calcium‐dependent Cl − current in the ON segment and generated a slowly decaying inward OFF current in charge movement traces. 3 Application of 20 mM EGTA to the internal solution abolished the slow inward OFF current, implying that the activation of the current depended on the presence of Ca 2+ in the myoplasm. The possibility that the slow inward OFF current was carried by cations flowing inwards or anions flowing outwards was studied and determined to be unlikely. 4 During a long (2000 ms) TEST pulse, a slowly decaying ON current was also observed. When the slow ON and OFF currents were included as parts of the total charge movement, ON‐OFF charge equality was preserved. This slow capacitive current is named I δ . 5 When Cl − was the major anion in the external solution, the OFF I δ was mostly cancelled by a slow outward current carried by the inflow of Cl − . 6 The OFF I δ component showed a rising phase. The average values of the rising time constants in CH 3 SO 3 − and SO 4 2‐ were similar and about half of that in gluconate. 7 The OFF I δ component in CH 3 SO 3 − had a larger magnitude and longer time course than that in SO 4 2‐ . The maximum amount of Q δ in CH 3 SO 3 − was about three times as much as that in SO 4 2‐ , whereas the voltage dependence of Q δ was similar in the two solutions. 8 Since the existence of Q δ depends on the presence of Ca 2+ in the myoplasm, it is speculated that Q δ could be a function of intracellular calcium release.