Hypotonic cell swelling stimulates permeability to cAMP in a rat colonic cell line

This study characterized the membrane permeability to cAMP in a cell line derived from the rat colon (CC531(mdr+)) by comparison of fluxes of 3H-cAMP, 3H-8-bromo-cAMP, 3H-taurine, 3H-adenosine and 3H-5'AMP under various experimental conditions including cell membrane depolarization and hypotonic cell swelling. Cell volume was modified by changing the osmolality and composition of the extracellular medium. Incubation in iso- and hypotonic KCl media induced graded increases in cell volume and stable activation of volume-sensitive channels that was reflected in an increased efflux of 3H-taurine. Incubation in hypotonic KCl solution also enhanced the efflux of 3H-8-Br-cAMP (a non-hydrolysable analogue of cAMP). Both the efflux of 3H-taurine and of 3H-8-Br-cAMP were inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB, 100 microM) suggesting the involvement of volume-sensitive anion channels. To gain further insight into the route mediating cAMP permeability, the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine were determined over short (5-min) periods. Uptakes of these substrates demonstrated close similarities: comparable increases were observed that correlated with the increases in cell volume in iso- and hypoosmotic KCl media; they were inhibited strongly by NPPB (100 microM) and metabolic inhibitors (deoxyglucose, 20 mM together with the mitochondrial uncoupler carbonylcyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 10 microM) while barely reduced by dipyridamole (100 microM) and they were not affected by adenosine (1 mM). In contrast, the uptakes of 3H-adenosine and 3H-5'AMP had strikingly different properties; they were insensitive to cell swelling; barely inhibited by NPPB (100 microM) and metabolic inhibitors (deoxyglucose and FCCP) while strongly reduced by dipyridamole (100 micro M). Unlike the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine, the uptakes of 3H-adenosine and 3H-5'AMP were reduced in Na(+)-free media, suggesting the presence in this cell line of two different adenosine carriers, one sodium-dependent and one sodium-independent. Taken together the present data show that in this rat colonic cell line, cAMP permeability is increased by cell swelling in hypotonic KCl medium and inhibited by NPPB and metabolic inhibitors. The similarity of these characteristics to those of taurine permeability suggests the involvement of a volume-sensitive anion pathway.