Shifts in the voltage dependence of synaptic release due to changes in the extracellular calcium concentration at nerve terminals on muscle of crayfish and frogs

Abstract

The rate of release of transmitter quanta, elicited by variable depolarization pulses applied to a nerve terminal by means of a macro-patch-clamp electrode, was measured in muscles of crayfish and frogs. The electrode was perfused with solutions containing different Ca concentrations, Ca_e. The bath was superfused separately, usually with solutions containing nominally no Ca_b and elevated Mg_b. A fixed depolarization pulse followed the variable test pulse within 7–10 ms, and facilitation, F_c, of release after the fixed pulse was determined as a measure of Ca-inflow during the test pulse. As described before, F_c always showed a peak, F_c, at depolarization amplitudes of the test pulse below the saturation level of release. When Ca_e was changed, the depolarization levels generating F_c shifted in a negative direction if Ca_e was lowered, and in a positive direction if Ca_e was increased. These shifts agreed with the known dependence of the effective membrane potential (controlling e.g. Ca inward current) on Ca_e which is due to shielding of surface changes by Ca²⁺ (cf. Hille 1984). Changes of Ca_b, at constant Ca_e, did not affect the depolarization dependence of F_c. It is concluded that Ca inflow is not the only factor controlling quantal release, and that at least in depolarizations beyond those eliciting F_c another potential dependent factor increases release while Ca inflow presumably falls.