Cellular functions of a cell in a metastable equilibrium state

Abstract

A unifying hypothesis which might replace some of the many ion pumps which are invoked to describe distribution of ions across living cell membranes is developed quantitatively. Resting cells are assumed to be in a metastable state such that ions are in equilibrium between an extracellular aqueous phase, in which water has the properties of the bulk liquid, and an intracellular aqueous phase in which water has enhanced structure and strongly modified solvent properties. Partition coefficients or medium effects for Na +, K + and Cl − are calculated for several cell types. It is shown that in such a hypothetical cell, possessing no ion pumps there is an amplified Donnan potential between the two phases, its sign determined by the net charge on intracellular proteins, and its magnitude increased by a separation of ions induced by the difference in solvent properties of the water in the two phases. It is shown that a cell in such a metastable state is excitable and can generate an action potential with an inward surge of Na+ followed by an outward surge of K+. An explanation is offered for the transient release of Caa+ from the sarcoplasmic reticulum following excitation of a muscle fibre. Regulation of cellular volume is shown to be a necessary result of the presence in the extracellular solution of a high concentration of Na+, an ion with a very low affinity for intracellular water. It is concluded that the principal cellular functions that are commonly attributed to the sodium pump are also a feature of a cell in a metastable equilibrium state.