RESPONSES OF THE AORTA OF THE GARTER SNAKE (THAMNOPHIS-SIRTALIS-PARIETALIS) TO PURINES

1 Isolated aortic rings from the garter snake (Thamnophis sirtalis par ietalis) were investigated in order to identify and classify responses to adenosine and adenosine 5'-triphosphate (ATP) and their analogues as part of a comparative study of vertebrate purinoceptors. 2 Adenosin e, D-5'-(N-ethylcarboxamide) adenosine (NECA), R- and S-N-6-(2-phenyli sopropyl) adenosine (R- and S-PIA) and 2-chloradenosine (2-CA) all con centration-dependently relaxed aorta preconstricted with noradrenaline (NA). The order of potency was: NECA > R-PIA = 2-CA > adenosine > S-P IA. Individual pD(2) values for the analogues were: NECA 7.12 +/- 0.13 (9), R-PIA 5.93 +/- 0.25 (7), 2-CA 5.64 +/- 0.40 (5), adenosine 5.04 +/- 0.10 (13) and S-PIA 4.26 +/- 0.10 (7). The order of potency has ch aracteristics of both A(1) and A(2) receptors and cannot satisfactoril y be classified according to the P-1-(adenosine) purinoceptor subtypes established in mammalian preparations. 3 ATP, alpha,beta-methylene AT P (alpha,beta-MeATP), 2-methylthio ATP (2MeSATP), beta,gamma-methylene ATP (beta,gamma-MeATP) and uridine 5'-triphosphate (UTP) all concentr ation-dependently constricted the isolated aorta. The order of potency was alpha,beta-MeATP = 2MeSATP > ATP > beta,gamma-MeATP > UTP. Only A TP, alpha,beta-MeATP and 2MeSATP consistently produced a maximum respo nse; pD(2) values were: ATP 3.98 +/- 0.07 (10), alpha,beta-MeATP 5.86 +/- 0.15 (12) and 2MeSATP 6.06 +/- 0.23 (9). In vessels preconstricted with NA neither ATP nor 2MeSATP caused relaxation in the presence or absence of the endothelium. 4 Suramin (0.1 mM) inhibited vasoconstrict ion to ATP, alpha,beta-MeATP, 2MeSATP and beta gamma-MeATP; however, s ince contractions to ATP and analogues did not reach a maximum respons e in the presence of this and other antagonists, pD(2) values could no t be calculated. 5 Pyridoxalphosphate-6-azophenyl-2',4'-dis acid (PPAD S; 30 mu M), a P-2X-purinoceptor antagonist, antagonized constrictions to alpha,beta-MeATP only. Reactive blue 2 (RB2; 30 mu M), a P-2Y-puri noceptor antagonist, inhibited vasoconstrictions to 2MeSATP only. 6 In domethacin (30 mu M) inhibited vasoconstriction in response to ATP and 2MeSATP, but not alpha,beta-MeATP, suggesting that the presence of an unaltered phosphate chain on the ATP analogue was necessary to stimul ate the production of a prostanoid. 7 Repeated administration of alpha ,beta-MeATP (3 mu M) caused desensitization of the receptor responsibl e for the constriction due to alpha,beta-MeATP whereas the responses t o ATP and 2MeSATP were unaltered. 8 In summary, both P-1-purinoceptors mediating vasodilatation and P-2-purinoceptors mediating vasoconstric tion are present on the garter snake aorta. However, in contrast to ma mmalian vessels, both P-2X, and P-2Y subtypes mediate vasoconstriction . There was no evidence for vasodilatation to ATP or analogues. Stimul ation of the P-2-purinoceptor by ATP and 2MeSATP caused the synthesis of a prostanoid. In addition, the possibility of a receptor activated by ATP, separate from P-2X and P-2Y subtypes is discussed since contra ctions to ATP proved to be insensitive to both PPADS and RB2. A compar ison is made of purinoceptors in the garter snake aorta with those in other vertebrate vessels.