The cellular mechanism of active chloride secretion in vertebrate epithelia: studies in intestine and trachea

Abstract

The cellular mechanism of active chloride secretion, as it is manifested in the intestine and trachea, appears to possess the following elements: (1) NaCl co-transport across the basolateral membrane; (2) Cl ^- accumulation in the cell above electrochemical equilibrium due to the Na ⁺ gradient; (3) a basolateral Na ⁺ -K ⁺ pump that maintains the Na ⁺ gradient; (4) a hormone-regulated Cl ^- permeability in the apical membrane; (5) passive Na ⁺ secretion through a paracellular route, driven by the transepithelial potential difference; and (6) an increase in basolateral membrane K ⁺ permeability occurring in conjunction with an increase in Na ⁺ -K ⁺ pump rate. Electrophysiological studies in canine trachea support this model. Adrenalin, a potent secretory stimulus in that tissue, increases apical membrane conductance through a selective increase in Cl ^- permeability. Adrenalin also appears to increase basolateral membrane K ⁺ permeability. Whether or not adrenalin also increases paracellular Na ⁺ permeability is unclear. Some of the testable implications of the above secretion model are discussed.