CHARACTERIZATION OF SODIUM AND CHLORIDE CONDUCTANCES IN PRENEOPLASTICAND NEOPLASTIC MURINE COLONOCYTES

Glucocorticoids, such as dexamethasone, induce amiloride-sensitive Na conductances in rat distal colon epithelium. The activity of these co nductances diminishes from the surface to the base of the crypt wherea s cAMP-stimulated Cl- secretion decreases from the crypt base to the s urface. These gradients are likely to be perturbed during carcinogenes is. We therefore determined the magnitude of Na+ and Cl- conductances in colonocytes isolated from normal and carcinogen-treated rats. Colon carcinogenesis was induced by injection of dimethyl-hydrazine (DMH) ( 18 mg/kg) for 5 weeks. Before sacrifice animals were treated for 3 day s with dexamethasone. Colonocyte populations from the surface to the c rypt base (C1-C5) were harvested from the distal colon by a Ca2+-chela ting procedure. The activity of Na+ conductances was determined by upt ake of Na-22(+) by surface and crypt colonocyte populations and by mem brane vesicles in the presence and absence of 10 mu M amiloride. In co ntrol rats Na+ conductance was highest in surface colonocytes and abse nt in the crypt base. As early as 2 weeks after initiation of DMH trea tment amiloride-inhibited Na+ uptake was virtually absent in the upper crypt. Transcriptional assessment of the alpha-, beta- and gamma-subu nits that constitute the epithelial Na+ channel revealed that DMH trea tment reduces the expression of beta-subunit mRNA. We then examined Cl -36(-) efflux from isolated colonocytes of normal and carcinogen-treat ed rats in response to forskolin (0.01 mM). Forskolin induced a marked rise in cAMP in lower crypt cells concomitant with a significant stim ulation of Cl-36(-) efflux. Intracellular cAMP increased in upper cryp t cells in response to forskolin without an increase in Cl-36(-) efflu x. By contrast, upper crypt colonocytes from DMH-treated rats showed f orskolin-stimulated efflux beginning 4 weeks after initiation of treat ment. We conclude that induction of Na+ conductances by gluco-corticoi ds is inhibited during the early stages of chemical carcinogenesis due to lack of induction of the beta-subunit of the channel. By contrast, Cl- transport is stimulated both in surface and lower crypt cell comp artments during different stages of chemical carcinogenesis.