Effect of calcium withdrawal on mechanical threshold in skeletal muscle fibres of the frog.

1. Voltage-clamp experiments were performed on frog skeletal muscle fibres using two intracellular micro-electrodes. The threshold for the Na current and the strength-duration curve for mechanical threshold were determined. 2. The change in threshold for the Na current was studied as a function of the external Ca and Mg concentrations which ranged from 0.1 to 50 mM. 3. The resting potential, effective resistance and threshold for the Na current were unchanged when 1.8 mM-Ca was replaced by 3 mM-Mg, indicating that the surface potential and the electrical properties of the fibres were not modified. The additon of 5 mM-EGTA did not affect these parameters. 4. In Ca-free saline (3 mM-Mg and 5 mM-EGTA) the mechanical threshold was significantly increased for short pulses (less than or equal to 20 msec.). In isolated single muscle fibres this effect was observed shortly after applying the Ca-free saline, and was rapidly reversed upon the return to control saline. 5. In isotonic EGTA (85 mM-EGTA) the muscle fibres were depolarized and were unable to contract even if they were hyperpolarized to --90 mV for 12 min prior to stimulation. If 3 mM-Mg was added, most fibres contracted locally. 6. In single muscle fibres caffeine contractures were unmodified after a 30 min exposure to Ca-free saline. 7. It can be concluded that external Ca withdrawal impairs Ca release from the sarcoplasmic reticulum and that external Ca is not essential for triggering contraction.