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 (i_Ca) 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⁺ (i_Na) and K⁺ (i_K) 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 i_Ca. 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 i_Na and i_K, the same, obviously complete block of i_Ca was obtained by 50 and 100μM Cd²⁺, an average of 96% block by 20μM Cd²⁺ and 50% block by about 5μM Cd²⁺. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (F_d) were elicited by variable-duration (0.5–3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20μM 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. F_d was blocked almost completely. Addition of 50μM 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 50μM Cd²⁺. F_d was completely blocked. The effects of Cd²⁺ on release and F_d 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 i_Ca and on release and facilitation shows that there is considerable phasic release at Cd²⁺ concentrations which block i_Ca, while facilitation is reduced approximately in proportion to the extent of block of i_Ca. The latter observation is in line with the “residual Ca²⁺” concept of facilitation. Phasic release in spite of block of i_Ca 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.