The effects of metal ions and caffeine on electro-mechanical coupling in crayfish muscle fibers.

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1. The effects of divalent metal ions on electro-mechanical coupling were examined in crayfish muscle fibers, using the voltage clamp method. Metal ions were added by substitution for Na. The relation between tension and membrane potential was directly figured by leading tension and potential to the Y and X axes of the oscilloscope and by depolarizing the membrane very slowly with the linearly rising pulse.2. Replacement of NaCl with equimolar choline chloride shifted the tension-membrane potential relation toward a more negative potential.3. Divalent metal ions elevated the mechanical threshold potential and decreased the tension at the given membrane potential. They not only shifted the curve relating tension and membrane potential along the potential axis but also decreased the slope of the curve. The order of the inhibitory action was Cd>Co≥Mn>Ni>Ca≥Mg>Sr>Ba. Their inhibitory effects appeared fairly rapidly after addition of metal ions and disappeared, fully reversibly, after their removal.4. The time courses of tension rise after the start of depolarization and its fall after the end of depolarization were explained as the sum of two or three exponential components. The time constants were not changed by metal ions.5. Caffeine lowered the mechanical threshold potential and shifted the tension-membrane potential relation toward a more negative potential.6. It is suggested that the inhibitory effect of metal ions on tension results from their stabilizing action on the Ca-permeability of the internal membrane system.