Role of Cell Sodium in Regulation of Transepithelial Sodium Transport

Abstract

One of the basic properties of living cells is that they maintain an internal electrolyte composition that is optimal for their metabolism and specific functions. For epithelial cells, whose specific function is active electrolyte transport, the problem arises how to preserve the internal ionic milieu and at the same time generate widely varying transcellular ion fluxes. Since transcellular transport involves movements across the two limiting membranes of epithelial cells, large changes in transcellular transport would be expected to profoundly affect the internal electrolyte composition and volume of the epithelial cells and hence threaten their survival, if the pumps and leaks in the membranes were invariant and independent. However, the internal ionic milieu may be kept within relatively narrow limits despite varying rates of transcellular transport, if the rates of movement across the apical or luminal membrane and the contraluminal or basolateral membrane are somehow coupled. In other words, when the rate of ion movement across one membrane changes, the internal composition will not be markedly altered, if similar changes in ion movement take place at the other membrane. Hence, maintenance of a relatively constant intracellular composition of electrolytes requires a mechanism which rapidly coordinates the pumps and leaks at the apical and basolateral cell membranes.