Characterization of swelling-induced ion transport in HT-29Cl.19A cells. Role of inorganic and organic osmolytes during regulatory volume decrease

Abstract

 Combined intracellular and transepithelial potential and resistance measurements were performed to localize the ion conductances activated by hypo-osmotic shock of cultured human colonic carcinoma cells (HT-29Cl.19A). Furthermore, the effect of cell swelling induced by a hypo-osmotic solution on the intracellular Ca²⁺ activity [Ca²⁺]_i and release of amino acids into the extracellular solution was examined. Application of a 40% hypo-osmotic solution on both sides of confluent monolayers induced a hyperpolarization of the intracellular potential caused by increased K⁺ conductance of the basolateral membrane, followed by a sustained depolarization due to increased Cl^–conductance in the apical and basolateral membranes. Usually no transepithelial current occurred, presumably because of random distribution of Cl^–channels. However, in some monolayers cell swelling induced a transepithelial Cl^–current because of a more pronounced expression of volume-sensitive Cl^–channels in the apical membrane. Exposure to hypo-osmotic solution increased [Ca²⁺]_i transiently. The increase of [Ca²⁺]_i was also observed to occur in the presence of the muscarinic receptor agonist carbachol or the inhibitor of the microsomal Ca²⁺-ATPase thapsigargin (TG), which prevented carbachol-induced Ca²⁺ release, suggesting that cell swelling recruits Ca²⁺ from a different source compared to carbachol or TG. Following incubations with hypo-osmotic solutions, about 60% of the intracellular free amino acids including aspartate, glutamate, glycine and taurine was released. It is concluded that the regulatory volume decrease (RVD) in HT-29Cl.19A colonocytes is achieved by activation of K⁺ and Cl^–conductances, resulting in net loss of salt, as well by extrusion of amino acids.