Separation of sodium and calcium currents in the somatic membrane of mollusc neurones. With an Appendix by Yu A. Shakhovalov

Abstract

Characteristics of the transmembrane ionic currents under controlled changes in ionic composition of extra- and intracellular medium were studied in isolated neurons from the ganglia of mollusks, Helix pomatia, Lymnaea stagnalis and Planorbis corneus. The neurons were investigated by a new technique which allows dialysis of their interior, and clamping of the potential at the surface membrane without using micro-electrodes. Replacement of K+ by Tris inside the neurons eliminated the outward K current so that the actual time course of the inward current could be measured. The latter was separated into 2 additive components, 1 of which was carried by Na+ and the other by Ca2+. Both inward currents were unaltered by tetrodotoxin (TTX); however, Ca current could be separately blocked by externally applied Cd ions (Kd = 7.2 .times. 10-5 M) and by the use of F- as an intracellular anion. No reversal of Na inward current could be achieved in neurons dialyzed with Na-free solution, indicating the absence of outward current carrying ions through the corresponding channels. With 5 mM-Na inside the cell, the equilibrium potential was close to the value predicted by the Nernst equilibrium. A non-specific outward current could be detected in K-free cells at membrane potentials exceeding 20-40 mV. Its time course was proportional to 1-exp (-t/.tau.ns). Cd ions depressed this current. The presence of the non-specific outward current made an exact measurement of the equilibrium potential for the Ca inward current impossible. The kinetics of Na inward currents could be described by m3h and those of the Ca current by m2h law. The corresponding values for Vm = 0 are: .tau.m(Na) = 1.1 .+-. 0.5 ms, .tau.m(Ca) = 2.4 .+-. 1.0 ms, .tau.h(Na) = 7.9 .+-. 2.0 ms. The inactivation of Ca current included 2 first-order kinetic processes with .tau.h1 = 50 .+-. 10 ms and .tau.h = 320 .+-. 30 ms. The data are considered proof of the existence of separate systems of Na+ and Ca2+-conducting channels in the nerve cell membrane.