Role of Calcium in the Actions of Agents Affecting Membrane Permeability

Abstract

It has been apparent from the earliest electrophysiological measurements that extracellular Ca²⁺ plays a role in maintaining membrane stability and in long-term control of membrane permeability. Endogenous neurotransmitters and hormones, known to affect membrane permeability in a number of tissues, do not affect the level of extracellular Ca²⁺ (though surface binding may be affected), but quite often modulate the intracellular Ca²⁺ concentration. Thus, if Ca²⁺ were to play a role in alteration (rather than maintenance) of membrane permeability, then an action of intracellular Ca²⁺ should be evident. Recent investigations have demonstrated that this is indeed the case. The membranes of diverse cell types have been shown to respond to elevation of intracellular Ca²⁺ usually with an elevation in permeability to K⁺. In the red cell, this relationship has been demonstrated by several techniques although no endogenous receptor mechanisms capable of controlling this process under physiological conditions have been identified. Excitable cells behave similarly and (as discussed below) changes in intracellular Ca²⁺ may mediate receptor-dependent as well as voltage-dependent changes in K⁺ permeability.