Role of extracellular metal cations in the potential dependence of force inactivation in skeletal muscle fibres.

The present experiments were designed to further characterize a metal ion binding site at the voltage sensor in the T-tubular (TT) membrane which controls the release of Ca2+ from the sarcoplasmic reticulum. For this purpose the potential dependence of force inactivation was measured under voltage clamp control in short toe muscle fibres of the frog. External solutions contained in each case one species of metal ion (Ca2+, Ba2+, Na+ and Li+, respectively). Assuming that the metal ion binds with different affinities to the resting and active state of the sensor and that the metal ion free sensor is inactivated, we estimated the dissociation constants by using the inactivation midpoint voltages (V) at different concentrations of one species of metal ion. For Ca2+ the analysis resulted in a low apparent dissociation constant KD1 (binding to the resting state) of approximately 5 x 10(-8) M and a high apparent dissociation constant KD2 > 23 mM (binding to the active state). The corresponding values for Ba2+ were: KD1 = 5 x 10(-5) M and KD2 > 125 mM. For different reasons, the data for Na+ and Li+ proved to be inconclusive.