INHIBITION OF CA-2+ 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 μM Cd2+, an average of 96% block by 20 μM Cd2+ and 50% block by about 5 μM 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 μM 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 μM 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. The double-logarithmic slope of the dependence of the rate of release on pulse duration was reduced from values of about 8 in the controls to about 4 in 50 μM Cd2+. Fd was completely blocked. The effects of Cd2+ on release and Fd were reversible. If the Ca2+ in the perfusate of the electrode was lowered, the block by Cd2+ became more effective. Comparison of the effects of Cd2+ on presynaptic iCa and on release and facilitation shows that there is considerable phasic release at Cd2+ concentrations which block iCa, while facilitation is reduced approximately in proportion to the extent of block of iCa. The latter observation is in line with the "residual Ca2+" concept of facilitation. Phasic release in spite of block of iCa by Cd2+ is assumed to be mediated by the non-Ca, depolarization controlled activator in concert with a considerable maintained Ca2+ concentration in the terminal. © 1990 Springer-Verlag.