I mc-Methylene ATP and ATP both produced concentration-dependent contractions of the isolated mesenteric artery of the rabbit that were not inhibited by reactive blue 2. 2 In preparations where the tone had been raised with noradrenaline, ATP and 2-methylthio ATP, but not a,-methylene ATP, produced relaxations of the vessel. These relaxations were inhibited in the presence of reactive blue 2. 3 Reactive blue 2 did not inhibit the contractions to noradrenaline, and only slightly inhibited relaxations to adenosine and acetylcholine.4 The rank order of potency of purine nucleotide analogues in contracting the vessel was: a,-methylene ATP > P,'y-methylene ATP = 2-methylthio ATP > ATP, and in relaxing the vessel at raised tone was: 2-methylthio ATP > ATP > ,y-methylene ATP > a,-methylene ATP.5 It is concluded from this study that in the isolated mesenteric artery ofthe rabbit, purine nucleotides act via P2y-purinoceptors to cause the muscle to relax and via P2x-purinoceptors to cause the muscle to contract. The results also suggest that reactive blue 2 selectively inhibits responses mediated via the P2y-purinoceptor, at least within a limited concentration range.

1 Mechanical responses to transmural electrical stimulation were recorded in isolated transverse ring preparations of rabbit saphenous artery. Electrical stimulation for a period of 1 s produced a rapid monophasic contraction and, for a period of 1 min, a biphasic contraction consisting of a rapid constriction followed by a slower sustained constriction. All contractions were abolished in the presence of tetrodotoxin (1 glg ml-') or guanethidine (4 JiM).2 After desensitization of the P2-purinoceptor with aj-methylene ATP, contractions to electrical stimulation for 1 s were reduced significantly at all frequencies tested: responses evoked by stimulation at 4 Hz were usually almost completely inhibited, whereas those evoked by stimulation at 64 Hz were only partially inhibited. On the other hand, in the presence of the a-adrenoceptor antagonist, prazosin, neurogenic contractions were only partially reduced: at 4 Hz there was no significant reduction in the neurogenic contractions while at 32 and 64 Hz, contractions were reduced by an average of20 and 28% respectively. Usually all contractions were abolished by a combination of the two drugs. 3 Prazosin antagonized contractions of the vessel to exogenously applied noradrenaline but not to ATP, whereas desensitization of the P2-purinoceptor with a,-methylene ATP blocked responses to ATP but not those to noradrenaline. The concentration-response curve to histamine was not affected by treatment of the vessel with prazosin, or after desensitization of the P2-purinoceptor with ax,fmethylene ATP. 4 These results suggest that noradrenaline and ATP are co-released from sympathetic nerves supplying the rabbit saphenous artery, both substances being involved in the mechanical contractions of this tissue. Further, the ratio of ATP to noradrenaline involved in these mechanical contractions is dependent upon the frequency of stimulation, but at all frequencies tested the purinergic component is greater than the adrenergic component.

1 The effects of reserpine and of 6-hydroxydopamine on the contractions of the rabbit isolated saphenous artery produced by stimulation of the sympathetic nerves were studied. 2 In vessels exposed to reserpine, substantial contractions to nerve stimulation were recorded despite a 95.7% reduction in the noradrenaline content of the tissue. These responses of the vessel were not significantly affected by the t,-antagonist, prazosin, whereas after desensitization of the P2-purinoceptor with a,-methylene ATP, no response to nerve stimulation remained.3 In vessels exposed to 6-hydroxydopamine, no nerve-mediated responses were observed. 4 Noradrenaline-containing nerves were observed by fluorescence histochemistry in control tissues, but were not observed in tissues treated with reserpine or 6-hydroxydopamine. 5 The potencies of ATP and histamine were not significantly affected by reserpine or 6-hydroxydopamine treatment. However, there was a slight supersensitivity to noradrenaline in reserpine-treated and 6-hydroxydopamine-treated vessels compared with that of control vessels. Prazosin was selective for a-adrenoceptors, while a,p-methylene ATP was selective for P2-purinoceptors.6 These results substantiate the finding that ATP and noradrenaline are sympathetic cotransmitters in the rabbit isolated saphenous artery, and demonstrate that ATP can act as a transmitter independently of noradrenaline in this vessel.