Physiologic Influences of Transepithelial K+ Secretion

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 _r secretion. Ion channels for K+, Na+, and Clr , 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 _r 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.