Physiologic Influences of Transepithelial K⁺ Secretion

Abstract

Cellular ionic balance relies on ion channels and coupled transporters to maintain and use the transmembrane electrochemical gradients of the cations Na⁺ and K⁺. High intracellular K⁺ concentration provides a ready reserve within the body allowing epithelia to secrete K⁺ into the fluid covering the apical membrane in the service of numerous physiologic activities. A major role for transepithelial K⁺ secretion concerns the balance of total body K⁺ such that excretion of excess K⁺ in the diet safeguards against disturbances to cellular balance. Accomplishing this transepithelial flow involves two archetypical cellular mechanisms, Na⁺ absorption and Cl⁻ secretion. Ion channels for K⁺, Na⁺, and Cl⁻, as well as cotransporters, exchangers, and pumps contribute to produce transepithelial flow by coupling electrochemical gradients such that K⁺ flow enters across the basolateral membrane and exits through the apical membrane. Beyond excretion, transepithelial K⁺ secretion serves to create the high K⁺ concentration of endolymph in the inner ear that supports sensation of sound and body orientation. For several epithelia such as those in airways and gastric mucosa, the elevated K⁺ concentration of apical fluid may occur largely as a consequence of supporting the secretion of other ions such as Cl⁻or H⁺. Less well-appreciated consequences of K⁺ secretion may result as in saliva and colonic luminal fluid where a high K⁺ concentration likely influences interactions with the resident microbiome. Independent control of K⁺ secretion also allows for specific adjustments in rate that serve the physiology of organs large and small.