Inhibition of Ca2+ inflow at nerve terminals of frog muscle blocks facilitation while phasic transmitter release is still considerable.

Action potentials were triggered in the motor nerve by a suction electrode and calcium currents (iCa) in the nerve terminals were measured by means of a perfused macro-patch-clamp electrode on the distal portion of the end-plates. Postsynaptic currents were blocked by adding d-tubocurarine, whereas presynaptic Na+ (iNa) and K+ (iK) currents were blocked by adding tetrodotoxin (TTX), tetraethylammonium and 3,4-diaminopyridine, respectively, to the perfusate of the electrode. The current components which could be suppressed by addition of Cd2+ to the perfusate were taken as presynaptic iCa. The observed effects on the presynaptic current components were very similar to those reported previously. If the electrode was perfused with Ringer's solution containing the blockers for iNa and iK, the same, obviously complete block of iCa was obtained by 50 and 100 microM Cd2+, an average of 96% block by 20 microM Cd2+ and 50% block by about 5 microM Cd2+. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (Fd) were elicited by variable-duration (0.5-3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20 microM Cd2+ added to the perfusate reduced the rate of phasic release of quanta insignificantly for short depolarizing pulses and by a factor of about 10 for longer pulses. Fd was blocked almost completely. Addition of 50 microM Cd2+ to the perfusate had a greater depressive effect on release after short depolarizing pulses and reduced release after longer pulses by a factor of about 100.(ABSTRACT TRUNCATED AT 250 WORDS)