Ca-dependent K channels with large unitary conductance in chromaffin cell membranes

Abstract

Injection of Ca ions increases the membrane permeability of many types of cells (reviewed by Meech¹), and in molluscan neurones, removal of Ca ions from the external solution suppresses a voltage-dependent K conductance^2,3. These two phenomena may be related through a potential–dependent Ca influx². The question then arises whether the voltage dependence of the corresponding K permeability system is an intrinsic property of this system or whether it is merely a consequence of the increase of Ca influx with depolarization. The resolution of this and similar problems has been hampered by technical difficulties, due to the presence of voltage–dependent Ca channels, of Ca–independent K conductances and of strong intra-cellular buffering for Ca ions. In the present study patches of membranes containing functional, Ca-dependent K channels have been isolated from chromaffin cells. It is shown that application of low Ca concentrations to the inner side of the membrane affects the properties of the channels, while Ca ions are ineffective on the outer side of the membrane. The method used here overcomes several of the limitations of the previous studies using cellular recording.