CHARACTERIZATION OF SWELLING-INDUCED ION-TRANSPORT IN HT-29CL.19A CELLS - ROLE OF INORGANIC AND ORGANIC OSMOLYTES DURING REGULATORY VOLUME DECREASE

Combined intracellular and transepithelial potential and resistance me asurements were performed to localize the ion conductances activated b y hypo-osmotic shock of cultured human colonic carcinoma cells (HT-29C l.19A). Furthermore, the effect of cell swelling induced by a hypo-osm otic solution on the intracellular Ca2+ activity [Ca2+](i) and release of amino acids into the extracellular solution was examined, Applicat ion of a 40% hypo-osmotic solution on both sides of confluent monolaye rs induced a hyperpolarization of the intracellular potential caused b y increased K+ conductance of the basolateral membrane, followed by a sustained depolarization due to increased Cl- conductance in the apica l and basolateral membranes. Usually no transepithelial current occurr ed, presumably because of random distribution of Cl- channels. However , in some monolayers cell swelling induced a transepithelial Cl- curre nt because of a more pronounced expression of volume-sensitive Cl- cha nnels in the apical membrane. Exposure to hypo-osmotic solution increa sed [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), whi ch prevented carbachol-induced Ca2+ release, suggesting that cell swel ling 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 ex trusion of amino acids.