A VOLTAGE-DEPENDENT AND CALCIUM-PERMEABLE ION-CHANNEL IN FUSED PRESYNAPTIC TERMINALS OF TORPEDO

1. We used a preparation of fused presynaptic nerve terminals of Torpe do electromotor nerve and the patch-clamp technique for characterizati on of single ion channels. We report here of a large, nonselective ion channel which is highly voltage dependent. 2. The slope conductance o f the I-V relation was estimated by either direct measurement of the s ingle-channel current amplitude at different voltages (850 +/- 18 pS ( SE); n = 9) or by variance analysis (834 +/- 23 pS; n = 5). 3. The vol tage dependence was examined in three ways. At steady-state DC voltage conditions, NPo (the open probability times the number of channels in the patch) was estimated. At potentials <0 mV, the probability of the channel to open is negligible and increases dramatically, within a ve ry narrow voltage range, to >50% at +8 mV (n = 8). 4. In pulse experim ents, the activation time delay is shorter as the voltage step reaches more positive values. The mean time for half activation (T-1/2) decre ases from 15 ms at +10 mV to 4 ms at +30 mV (n = 5). 5. Ensemble curre nts exhibit rectification in response to voltage ramps at negative pot entials (n = 10). 6. The channel was found to be nonselective. Its per meability to Na+, K+, Cl-, glutamate, Ba+2, and Ca+2, relative to Na+, was 1.00, 1.00, 1.22, 1.07, 0.85, and 0.62, respectively. 7. Based on the transport number of calcium, the calculated driving force, and th e mean channel open time, we estimated the number of calcium ions ente ring the nerve terminal upon depolarization. This number is not substa ntially different from the number of ions entering through voltage-dep endent, calcium-selective channels in other cells. 8. We speculate tha t this nonselective ion channel, may serve as a calcium entry route in to the nerve terminal and hence be involved in transmitter release.