The calcium current of helix neuron

Abstract

Calcium Current, ICa, was studied in isolated nerve cell bodies of Helix aspersa after suppression of Na+ and K+ currents. The suction pipette method described in the preceding paper was used. Ica rises to a peak value and then subsides exponentially and has a null potential of 150 mV or more and a relationship with [Ca+] that is hyperbolic over a small range of [Ca]s. When [Ca], is increased > Ca is reduced disproportionately, but the effect is not hyperbolic. 7ca is blocked by extracellular Ni+, La+, Cd++, and Co++ and is greater when and Sr++ carry the current. Saturation and blockage are described by a Langmuir adsorption relationship similar to that found in Balanus. Thus, the calcium conductance probably contains a site which binds the ions referred to. The site also appears to be voltage-dependent. Activation and inactivation of 7ca are described by first order kinetics, and there is evidence that the processes are coupled. For example, inactivation is delayed slighdy in its onset andr inactivation depends upon the method of study. However, the currents are described equally well by either a noncoupled Hodgkin-Huxley mh scheme or a coupled reaction. Facilitation of 7ca by prepulses was not observed. For times up to 50 ms, currents even at small depolarizations were accounted for by suitable adjustment of the activation and inactivation rate constants. © 1978, Rockefeller University Press., All rights reserved.