The units of calcium conduction in Helix neurones.

Abstract

VOLTAGE-DEPENDENT membrane calcium currents, ICa, occur in muscle1,2, axons3,4 and nerve cell bodies5–9 but are not well characterised because of difficulties in separating them from other ionic currents, and inadequate spatial and temporal control of the clamp voltages used to examine them. As a result, information on the conduction process underlying ICa is limited. Recently two new techniques have been reported6,9,10 which overcome most of the problems associated with experimental investigations of ICa. We have used the suction pipette technique9,10, which combines methods of internal perfusion and voltage clamp, to study ICa in isolated nerve cell bodies of the snail, Helix aspersa. The time course of ICa in response to step changes in membrane potential was readily examined and, in addition, very small fluctuations (noise) in current were also observed. Assuming that the fluctuations represent current contributions from a population of independent two-state (conducting/nonconducting) unit conductances, 〈ICa〉, the average value of the fluctuations in the steady state, and σ2, the variance of the fluctuations can be used to estimate a single unit conductance, γca. Analysis of spontaneous current noise has been used previously to obtain γ values for the Na and K systems in several axon membranes11–14, and γ values for acetylcholine-sensitive conductances at the neuromuscular junction15,16 and in molluscan neurones17, but γCa is unknown. Unit ion conductances of a few picosiemens or greater have been reported and where comparisons can be made, these values are similar to those estimated using quite different techniques such as tetrodotoxin binding18–21. We found γCa to be less than 1 pS however, and we suggest that the low value results from the effect of a Ca2+ binding site in the conductance.