Comparison of the effects of Ca2+ and those of Mg2+ and La3+ on endplate currents in mouse interosseus muscles.

Acetylcholine-induced currents were measured in dissociated muscle fibres from the mouse interosseus. Fast and slow desensitization could be distinguished, the latter ending in a steady-state current. The time constants of the two phases of current decrease and the steady-state current were decreased when the superfusing solution contained di- or trivalent cations in addition to the 0.2 mM Mg2+ present in the control solution. When Ca2+, Mg2+ or La3+ was added at the time at which the current had settled to a near steady-state, there was a fall of current, the time course of which was much faster than that during desensitization. The amplitude and the time course of current change were nearly the same for Ca2+ and Mg2+, showing that the effect was not specific for any particular cation. No evidence of intracellular action of the cations was found. The [ACh]-current relationship was shifted to a higher [ACh] by raising [Mg2+] or [La3+]. Recovery of the ability to react to ACh was slowed by increasing [La3+], but the initial recovery of fast and slow time constants of desensitization was not much changed. Recovery was impeded by the presence of a low (conditioning) [ACh]; this effect was enhanced by La3+. It was concluded that the effects of polyvalent cations are similar to those of non-specific blockers.