REGULATION OF NA+-3HCO3- COTRANSPORT IN RABBIT PROXIMAL CONVOLUTED TUBULE VIA ADENOSINE-A(1) RECEPTOR

We investigated the role of adenosine A1-receptor in the regulation of basolateral Na+-3HCO3- cotransporter in the rabbit proximal convoluted tubule (PCT) microperfused in vitro by monitoring basolateral membrane potential and intracellular pH. FK-453, a highly specific A1 antagonist, inhibited basolateral HCO3- conductance in a concentration-dependent manner (10(-10)-10-(5) M). Other A1 antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) at 10(-5) M and theophylline at 10(-3) M, also had similar effects. N6-cyclohexyladenosine (CHA) at 10(-7) M attenuated the effect of low concentration (10(-8)M) of FK-453. Either enhancement of the degradation of adenosine by 0.1 U/ml adenosine deaminase (ADA) or inhibition of adenosine release from the cells by 10(-6) M S-(4-nitrobenzyl)-6-thioinosine (NBTI) mimicked the effects of A, antagonists. These observations suggest that endogenous adenosine is released from PCT cells and stimulates Na+-3HCO3- cotransporter. Both 10(-4) M 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP) and 10(-6) M forskolin also inhibited basolateral HCO3- conductance. Both 10(-6) M FK-453 and 10(-4) M CPT-cAMP decreased the initial rate as well as the magnitude of intracellular acidification induced by reduction of peritubular HCO3- concentration from 25 to 0 mM. Neither 10(-6) M FK-453 nor 10(-7) M CHA changed intracellular Ca2+ concentration as measured by fura-2 fluorescence. These results indicate that adenosine might stimulate HCO3- exit across the basolateral membrane through Na+-3HCO3- cotransporter by decreasing intracellular cAMP via A1-receptor activation. Neither addition of amiloride nor removal of Na+ from the luminal fluid influenced the basolateral HCO3- conductance, indicating that A1 antagonists and the agents that increase intracellular cAMP directly affect the basolateral HCO3- conductance through Na+-3HCO3- cotransporter. These observations suggest that endogenous adenosine is released from the PCT cells and stimulates the basolateral Na+-3HCO3- cotransporter via an A1-receptor-mediated mechanism. This may, at least in part, account for the mechanisms of diuretic effect elicited by A1-receptor antagonists.