Adenosine stimulation of Na+ transport is mediated by an A1 receptor and a [Ca2+]i-dependent mechanism

Adenosine stimulation of Na + transport by an A 1 receptor and a [Ca 2+ ] i -dependent mechanism. Studies were performed to determine the primary signal transduction mechanism that mediates adenosine stimulation of electrogenic sodium transport in renal epithelial cells. Experiments were performed on cultured amphibian A 6 cells with an adenosine analogue that preferentially binds to the A 1 receptor, cyclohexyladenosine (CHA). Sodium transport was assessed by the equivalent short circuit current (Ieq). CHA was found to stimulate Ieq via activation of an A 1 receptor because (1) the threshold concentration was 1 nM compared to that of 10 µM for the specific A 2 agonist CGS21680, (2) CHA inhibited vasopressin (AVP)-stimulated cAMP production by a pertussis toxinsensitive mechanism, and (3) the action of CHA was inhibited by the A 1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). CHA increased intracellular Ca 2+ ([Ca 2+ ] i ) and stimulated phosphoinositide turnover at concentrations that increased Ieq and in a time course that paralleled the increase in Ieq. Ion transport was stimulated by a Ca 2+ -dependent mechanism because the CHA induced increase in Ieq was inhibited by chelating [Ca 2+ ] i with 5,5′dimethyl BAPTA in a dose-dependent manner, with a K i of approximately 10 µM. The increase in Ieq was also dose-dependently inhibited by the specific PKC inhibitors dihydroxychlorpromazine and chelerythrine, and by trifluoperazine which inhibits PKC and calmodulin. Further studies indicated that CHA-stimulated Ieq was independent of cAMP generation because CHA did not induce an increase in cAMP accummulation parallel to the increase in Ieq in a dose-response analysis, and the adenylate cyclase inhibitor 2′,5′ dideoxyadenosine (DDA) did not affect the CHA-induced increase in Ieq. These studies indicate, therefore, that adenosine stimulation of Ieq occurs, at least in part, through calcium-dependent signal transduction events and not through regulation of adenylate cyclase.