Responses of the aorta of the garter snake ( Thamnophis sirtalis parietalis ) to purines

1 Isolated aortic rings from the garter snake ( Thamnophis sirtalis parietalis ) 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 Adenosine, d ‐5′‐(N‐ethylcarboxamide) adenosine (NECA), R‐ and S‐N 6 ‐(2‐phenylisopropyl) adenosine (R‐ and S‐PIA) and 2‐chloroadenosine (2‐CA) all concentration‐dependently relaxed aorta preconstricted with noradrenaline (NA). The order of potency was: NECA > R‐PIA = 2‐CA > adenosine > S‐PIA. 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 characteristics of both A 1 and A 2 receptors and cannot satisfactorily be classified according to the P 1 ‐(adenosine) purinoceptor subtypes established in mammalian preparations. 3 ATP, α,β‐methylene ATP (α,β‐MeATP), 2‐methylthio ATP (2MeSATP), β,γ‐methylene ATP (β,γ,‐MeATP) and uridine 5′‐triphosphate (UTP) all concentration‐dependently constricted the isolated aorta. The order of potency was α,β‐MeATP = 2MeSATP > ATP > β,γ‐MeATP > UTP. Only ATP, α,β‐MeATP and 2MeSATP consistently produced a maximum response; pD 2 values were: ATP 3.98 ± 0.07 (10), α,β‐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 m m ) inhibited vasoconstriction to ATP, α,β‐MeATP, 2MeSATP and β,γ‐MeATP; however, since contractions to ATP and analogues did not reach a maximum response in the presence of this and other antagonists, pD 2 values could not be calculated. 5 Pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS; 30 μ m ), a P 2X ‐purinoceptor antagonist, antagonized constrictions to α,β‐MeATP only. Reactive blue 2 (RB2; 30 μ m ), a P 2Y ‐purinoceptor antagonist, inhibited vasoconstrictions to 2MeSATP only. 6 Indomethacin (30 μ m ) inhibited vasoconstriction in response to ATP and 2MeSATP, but not α,β‐MeATP, suggesting that the presence of an unaltered phosphate chain on the ATP analogue was necessary to stimulate the production of a prostanoid. 7 Repeated administration of α,β‐MeATP (3 μ m ) caused desensitization of the receptor responsible for the constriction due to α,β‐MeATP whereas the responses to ATP and 2MeSATP were unaltered. 8 In summary, both P 1 ‐purinoceptors mediating vasodilatation and P 2 ‐purinoceptors mediating vasoconstriction are present on the garter snake aorta. However, in contrast to mammalian vessels, both P 2X and P 2Y subtypes mediate vasoconstriction. There was no evidence for vasodilatation to ATP or analogues. Stimulation 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 contractions to ATP proved to be insensitive to both PPADS and RB2. A comparison is made of purinoceptors in the garter snake aorta with those in other vertebrate vessels.