CHARACTERIZATION OF VASCULAR P-2 PURINOCEPTORS IN THE RAT ISOLATED-PERFUSED KIDNEY

In isolated, constant-pressure perfused rat kidneys at basal vascular tone, injected P-2 purinoceptor agonists evoked vasoconstriction (alph a, beta-methylene ATP > beta,gamma-methylene ATP > ATP-gamma-S > 2-met hylthio ATP > ATP > ADP = UTP). In kidneys with raised tone, the nucle otides produced vasodilatation at low doses (2-methylthio ATP > ADP = ATP = ATP-gamma-S > UTP; alpha,beta-methylene ATP and beta,gamma-methy lene ATP, inactive), and constriction at high doses (alpha,beta-methyl ene ATP > beta,gamma-methylene ATP > ATP-gamma-S > 2-methylthio ATP > ADP = ATP > UTP). Removal of the endothelium abolished the dilator res ponses to the agonists. NG-Nitro-L-arginine methylester (L-NAME, 5 x 1 0(-5) M) abolished vasorelaxation in response to 2-methylthio ATP, a r esponse which could be restored by additional L-arginine (3 x 10(-3) M ). Both vasodilatation and constriction due to the nucleotides remaine d unaffected by indomethacin (3 x 10(-6) M), S-(p-nitrobenzyl)-6-thioi nosine (3 x 10(-5) M) and 8-phenyltheophylline (3 x 10(-6) M). Pyridox alphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1-3 x 10(-6) M) , inhibited vasoconstriction caused by alpha,beta-methylene ATP, 2-met hylthio ATP and UTP, but not by ATP. Suramin (3 x 10(-5) M) caused a r ightward shift of the dose-response curves for constriction caused by alpha,beta-methylene ATP (27-fold) and 2-methylthio ATP (5-fold), wher eas the ATP curve was shifted to the left (20-fold). With Evans blue ( 10(-5) M), vasodilatation due to the nucleotides was abolished and the dose-response curves for vasoconstriction caused by ATP and UTP were shifted left more than 100-fold, the effect to both could not be antag onized by PPADS (3 x 10(-6) M). These results suggest: (1) the differe nt rank orders of P-2 purinoceptor agonist potencies for constrictor a nd dilator responses in perfused rat kidney are consistent with mediat ion via P-2X and P-2Y purinoceptors, respectively; (2) P-2X purinocept ors, selectively sensitive to blockade by PPADS, are located on vascul ar smooth muscle; (3) endothelial P-2Y purinoceptor stimulation result s in vasodilatation involving NO synthesis but not release of prostano ids; (4) Evans blue, which appears to combine selective P-2Y purinocep tor blockade and strong inhibition of ecto-nucleotidases, potentiates vasoconstriction in response to the degradable nucleotides, ATP, 2-met hylthio ATP and UTP; (5) additionally, Evans blue unmasks a PPADS-inse nsitive P-2U purinoceptor where the nearly equipotent nucleotides, ATP and UTP, can produce vasoconstriction.