Further characterization of the sorbitol permease in PAP-HT25 cells.

The sorbitol permease enables the efflux of sorbitol from cultured rabbit papillary cells (PAP-HT25) in response to a reduction in osmolality. The anion transport inhibitor 5-nitro-2-(3-phenylpropylamino)benzoate (100 microM) inhibited efflux by 92%, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.5 mM) reduced efflux by 40%. 2,4-Dinitrobenzenesulfonate and p-chloromercuriphenylsulfonic acid had no effect. The protease trypsin (0.05 mg/ml) reduced sorbitol efflux by 53%, pronase (0.01 mg/ml) by 47%, and papain (0.1 mg/ml) by 49%; chymotrypsin had no effect. Sugars and sugar alcohols at different concentrations (10-200 mM) in the bathing solution did not influence sorbitol efflux. Determination of the osmotically induced influx of sugar alcohols showed that xylitol uptake was faster than that of sorbitol; 6-deoxysorbitol was slower; L-sorbitol, arabitol, galactitol, and 2-deoxysorbitol entered at the same rate as sorbitol; and maltitol did not enter the cells. Sorbitol and 6-deoxysorbitol at 9 mM competitively inhibited [14C]sorbitol influx by 24 and 32%, respectively, whereas xylitol, taurine, betaine, and myo-inositol showed no inhibition. We conclude that 1) a specific inhibitor of the permease was not found, 2) the sorbitol permease or associated regulator is a protein, and 3) the C-6 atom of sorbitol is important in the selectivity of the permease.