Characterization of sodium and chloride conductances in preneoplastic and neoplastic murine colonocytes

Glucocorticoids, such as dexamethasone, induce amiloride-sensitive Na+ conductances in rat distal colon epithelium. The activity of these conductances diminishes from the surface to the base of the crypt whereas cAMP-stimulated Cl- secretion decreases from the crypt base to the surface. These gradients are likely to be perturbed during carcinogenesis. 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 dimethylhydrazine (DMH) (18 mg/kg) for 5 weeks. Before sacrifice animals were treated for 3 days with dexamethasone. Colonocyte populations from the surface to the crypt base (C1-C5) were harvested from the distal colon by a Ca2+-chelating procedure. The activity of Na+ conductances was determined by uptake of 22Na+ by surface and crypt colonocyte populations and by membrane vesicles in the presence and absence of 10 microM amiloride. In control rats Na+ conductance was highest in surface colonocytes and absent in the crypt base. As early as 2 weeks after initiation of DMH treatment amiloride-inhibited Na+ uptake was virtually absent in the upper crypt. Transcriptional assessment of the alpha-, beta- and gamma-subunits that constitute the epithelial Na+ channel revealed that DMH treatment reduces the expression of beta-subunit mRNA. We then examined 36Cl- efflux from isolated colonocytes of normal and carcinogen-treated rats in response to forskolin (0.01 mM). Forskolin induced a marked rise in cAMP in lower crypt cells concomitant with a significant stimulation of 36Cl- efflux. Intracellular cAMP increased in upper crypt cells in response to forskolin without an increase in 36Cl- efflux. By contrast, upper crypt colonocytes from DMH-treated rats showed forskolin-stimulated efflux beginning 4 weeks after initiation of treatment. We conclude that induction of Na+ conductances by glucocorticoids 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 compartments during different stages of chemical carcinogenesis.