Abstract
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 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.
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Received: 26 March 1997 / Received after revision: 20 June 1997 / Accepted: 7 July 1997
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Fraser, G., Portnoy, M., Bleich, M. et al. Characterization of sodium and chloride conductances in preneoplastic and neoplastic murine colonocytes. Pflügers Arch 434, 801–808 (1997). https://doi.org/10.1007/s004240050468
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DOI: https://doi.org/10.1007/s004240050468