Depolarizing pulses to neuromuscular terminals of frogs can elicit graded, phasic transmitter release in the absence of Ca influx.

Quantal synaptic currents were recorded by means of a macro-patch-clamp electrode, through which the terminal could be also depolarized by current pulses. The tip of the electrode was perfused rapidly, applying either Ringer's solution or an EGTA-buffered less than 0.1 microM Ca Ringer's. Muscle and nerve outside the electrode were superfused with normal or 10 mM Ca Ringer which served to keep the resting intracellular Ca concentration, Cair, in the terminal below the electrode relatively high. When Ca inflow was prevented by decreasing the Ca concentration to less than 0.1 microM, release was depressed, but still measurable, for low depolarizations, and much less or not at all depressed for large depolarizations to positive membrane potentials. The time course of the depressed release without Ca-inflow was the same as that in the controls with Ca inflow. It appears that the voltage dependent activator proposed in the Ca-voltage theory of release can elicit maximal release in the absence of Ca inflow, provided Cair is sufficiently high. The voltage dependencies of this activator as well as that of Ca inflow can be estimated from the results.