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Na⁺ Reabsorption in Cultured Rat Epididymal Epithelium via the Na⁺/Nucleoside Cotransporter¹

^a Department of Physiology, Faculty of Medicine, Chinese University of Hong Kong, Shatin, N.T., Hong Kong ^b Department of Medicine, Division of Gastroenterology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

ABSTRACT

The effect of nucleoside on Na⁺ reabsorption via Na⁺/nucleoside cotransporter in cultured rat epididymal epithelia was studied by short-circuit current (Isc) technique. Guanosine added apically stimulated Isc in a dose-dependent manner, with a median effective concentration (EC₅₀) of 7 ± 2 µM (mean ± SEM). Removal of Na⁺ from the apical bathing solution or pretreatment with a nonspecific Na⁺/nucleoside cotransporter inhibitor, phloridzin, completely blocked the Isc response to guanosine. Moreover, the guanosine response was abolished by pretreatment of the tissue with ouabain, a Na⁺/K⁺-ATPase inhibitor, suggesting the involvement of Na⁺/nucleoside cotransporter on the apical side and Na⁺/K⁺-ATPase on the basolateral side in Na⁺ reabsorption. In contrast, the Isc response to guanosine was not affected after desensitization of purinoceptors by ATP. Addition of the Na⁺/K⁺/2Cl^- symport inhibitor bumetanide to the basolateral side or the nonspecific Cl^- channel blocker diphenylamine-2-carboxylate to the apical side showed no effect on the Isc response to guanosine, excluding stimulation of Cl^- secretion by guanosine as the cause of the guanosine-induced Isc. The Isc response to purine nucleoside (guanosine and inosine) was much higher than that to pyrimidine nucleoside (thymidine and cytidine). Consistent with substrate specificity, results of reverse transcription-polymerase chain reaction revealed mRNA for concentrative nucleoside transporter (CNT2), which is a purine nucleoside-selective Na⁺/nucleoside cotransporter in the epididymis, but not for 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 epididymis, thereby providing an optimal microenvironment for the maturation and storage of spermatozoa.

FOOTNOTES

First decision: 21 August 2000.

¹ Supported by the Research Grant Council of Hong Kong.

² Correspondence. FAX: 852 2603 5022; patrickwong{at}cuhk.edu.hk

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