The units of calcium conduction in Helix neurones

Abstract

VOLTAGE-DEPENDENT membrane calcium currents, I_Ca, occur in muscle^1,2, axons^3,4 and nerve cell bodies^5–9 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 I_Ca is limited. Recently two new techniques have been reported^6,9,10 which overcome most of the problems associated with experimental investigations of I_Ca. We have used the suction pipette technique^9,10, which combines methods of internal perfusion and voltage clamp, to study I_Ca in isolated nerve cell bodies of the snail, Helix aspersa. The time course of I_Ca 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, 〈I_Ca〉, 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, γ_ca. Analysis of spontaneous current noise has been used previously to obtain γ values for the Na and K systems in several axon membranes^11–14, and γ values for acetylcholine-sensitive conductances at the neuromuscular junction^15,16 and in molluscan neurones¹⁷, but γ_Ca 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 binding^18–21. We found γ_Ca 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.