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 dimethyl-hydrazine (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 μM 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 α-, β- and γ- subunits that constitute the epithelial Na⁺ channel revealed that DMH treatment reduces the expression of β-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 β- subunit of the channel. By contrast, Cl^- transport is stimulated both in surface and lower crypt cell compartments during different stages of chemical carcinogenesis.