- It has been previously demonstrated that adenosine induces natriuresis when administered directly into the renal circulation of the rat. It was postulated that the mechanism was inhibition of tubule Na+ reabsorption. In the current study, the hypothesis was tested that adenosine inhibits ion reabsorption across the inner medullary collecting duct (IMCD), a tubule segment which is rich in adenosine receptors. IMCD epithelium from rat kidney was grown in primary culture as a confluent monolayer on Costar filters, allowing selective access to the basolateral and apical surfaces of the cells. Transepithelial resistance was taken as a measure of epithelial permeability and ion conductance. Na+ uptake was studied using 22Na+ and used to determine the permeability of the epithelial monolayer specifically to Na+. Exposure of the basolateral aspect of the monolayer to adenosine (10−8–10−7 M) increased transepithelial resistance in a dose- and time-dependent manner; in parallel, adenosine (10−7–10−6 M) reduced apical Na+ uptake from 20±5 to 10±2 nmol/cm2. 1,3-Dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX, 5×10−9 M), an adenosine antagonist with selectivity for the A1 receptor, inhibited the rise in transepithelial resistance and the decrease in Na+ uptake following the addition of adenosine. The effects of adenosine on transepithelial resistance were reproduced with the A1 receptor selective adenosine analogue N6-cyclohexyladenosine (CHA, 10−8 –10−7 M) but not with the A2 selective analogues, 5′-N-ethylcarboxamidoadenosine (NECA) or CGS 21680. CHA (10−7 M) inhibited apical Na+ uptake by 50%, an effect abolished by PACPX. The effects of adenosine on transepithelial resistance and Na+ uptake were inhibited, but only in part, by amiloride. These data suggest that adenosine inhibits ion movement, specifically apical Na+ uptake, across the IMCD epithelium and that this effect is mediated by A1 receptors from the basolateral aspect of the cell. The results are consistent with the hypothesis that adenosine inhibits Na+ reabsorption across the IMCD.