Responses of the aorta of the garter snake (Thamnophis sirtalis parietalis) to purines
Authors: Knight Gillian E. , Burnstock Geoffrey 
Published: July 2012

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.
Adenosine, d‐5′‐(N‐ethylcarboxamide) adenosine (NECA), R‐ and S‐N6‐(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 pD2 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 A1 and A2 receptors and cannot satisfactorily be classified according to the P1‐(adenosine) purinoceptor subtypes established in mammalian preparations.
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; pD2 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.
Suramin (0.1 mm) 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, pD2 values could not be calculated.
Pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS; 30 μm), a P2X‐purinoceptor antagonist, antagonized constrictions to α,β‐MeATP only. Reactive blue 2 (RB2; 30 μm), a P2Y‐purinoceptor antagonist, inhibited vasoconstrictions to 2MeSATP only.
Indomethacin (30 μm) 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.
Repeated administration of α,β‐MeATP (3 μm) caused desensitization of the receptor responsible for the constriction due to α,β‐MeATP whereas the responses to ATP and 2MeSATP were unaltered.
In summary, both P1‐purinoceptors mediating vasodilatation and P2‐purinoceptors mediating vasoconstriction are present on the garter snake aorta. However, in contrast to mammalian vessels, both P2X and P2Y subtypes mediate vasoconstriction. There was no evidence for vasodilatation to ATP or analogues. Stimulation of the P2‐purinoceptor by ATP and 2MeSATP caused the synthesis of a prostanoid. In addition, the possibility of a receptor activated by ATP, separate from P2X and P2Y 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.