A voltage dependent and calcium permeable ion channel in fused presynaptic
terminals of Torpedo.
Meir, Alon and Rami Rahamimoff.
Department of Physiology and the Bernard Katz Minerva Center for Cell
Biophysics, Hebrew University- Hadassah Medical School, POBox 12272, Jerusalem
91120, Israel.
APStracts 2:0324N, 1995.
SUMMARY AND CONCLUSIONS
1. We used a preparation of fused presynaptic nerve terminals of Torpedo
electromotor nerve and the patch clamp technique for characterization of
single ion channels . We report here of a large, non selective ion channel
which is highly voltage dependent. 2. The slope conductance of the i-V
relation was estimated by either direct measurement of the single channel
current amplitude at different voltages (85018 pS, meanS.E.M, n=9) or by
variance analysis (83423 pS, meanS.E.M, n=5). 3. The voltage dependence was
examined in three ways. At steady state DC voltage conditions NP O ( the open
probability times the number of channels in the patch) was estimated. At
potentials below 0 mV, the probability of the channel to open is negligible
and increases dramatically, within a very narrow voltage range, to more than
50% at +8 mV (n=8). 4. In pulse experiments the activation time delay is
shorter as the voltage step reaches more positive values. The mean time for
half activation (T 1/2 ) decreases from 15 ms at +10 mV to 4 ms at +30 mV
(n=5). 5. Ensemble currents exhibit rectification in response to voltage ramps
at negative potentials (n=10). 6. The channel was found to be non selective.
Its permeability 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. On the
basis of the transport number of calcium, the calculated driving force and the
mean channel open time, we estimated the number of calcium ions entering the
nerve terminal upon depolarization; this number is not substantially different
from the number of ions entering through voltage dependent calcium selective
channels in other cells. 8. We speculate that this non selective ion channel,
may serve as a calcium entry route into the nerve terminal and hence be
involved in transmitter release.
Received 12 September 1995; accepted in final form 30 October 1995.
APS Manuscript Number J610-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95