Na+ reabsorption in cultured rat epididymal epithelium via the Na+/nucleoside cotransporter

The effect of nucleoside on Na+ reabsorption via Na+/nucleoside cotransport er in cultured rat epididymal epithelia was studied by short-circuit curren t (Isc) technique. Guanosine added apically stimulated Isc in a dose-depend ent manner, with a median effective concentration (EC50) of 7 +/- 2 muM (me an +/- SEM). Removal of Na+ from the apical bathing solution or pretreatmen t with a nonspecific Na+/nucleoside cotransporter inhibitor, phloridzin, co mpletely blocked the Isc response to guanosine. Moreover, the guanosine res ponse was abolished by pretreatment of the tissue with ouabain, a Na+/K+-AT Pase inhibitor, suggesting the involvement of Na+/nucleoside cotransporter on the apical side and Na+/K+-ATPase on the basolateral side in Na+ reabsor ption, in contrast, the Isc response to guanosine was not affected after de sensitization of purinoceptors by ATP. Addition of the Na+/K+/2Cl(-) sympor t inhibitor bumetanide to the basolateral side or the nonspecific Cl- chann el blocker diphenylamine-2-carboxylate to the apical side showed no effect on the tsc response to guanosine, excluding stimulation of Cl- secretion by guanosine as the cause of the guanosine-induced Isc. The Isc response to p urine nucleoside (guanosine and inosine) was much higher than that to pyrim idine nucleoside (thymidine and cytidine). Consistent with substrate specif icity, results of reverse transcription-polymerase chain reaction revealed mRNA for concentrative nucleoside transporter (CNT2), which is a purine nuc leoside-selective Na+/nucleoside cotransporter in the epididymis, but not f or CNT1. It is suggested that the Na+/nucleoside cotransporter (i.e., CNT2) may be one of the elements involved in Na+ and fluid reabsorption in the e pididymis, thereby providing an optimal microenvironment for the maturation and storage of spermatozoa.