Effects of lanthanum on contractile inactivation and D600-induced paralysis in twitch muscle fibres of the frog.

The effects of the trivalent ion La upon force inactivation were investigated in voltage-clamped toe muscle fibres of the frog Rana temporaria. In the presence of La, the contracture plateau was remarkably lengthened and spontaneous relaxation which reflects force inactivation was reduced. Subsequent depolarizations led to a further decrease in the rate of relaxation, indicating that La slowed down force inactivation in a depolarization-dependent manner. Moreover, the development of paralysis in muscle fibres was also affected by La. In Ca-free solutions, paralysis was induced after a sequence of no more than three pulses to 0 mV; in the presence of D600 only one pulse was sufficient. Addition of La suppressed paralysis in Ca-free solutions and decelerated the action of D600. It is assumed that La alters the kinetics of force inactivation by binding to and replacing Ca at a site linked to a molecule in the T-tubular membrane which functions as voltage sensor in excitation-contraction (EC) coupling.