EVIDENCE FOR THE PRESENCE OF 2 TYPES OF P-2 PURINOCEPTOR IN THE GUINEA-PIG ILEAL LONGITUDINAL SMOOTH-MUSCLE PREPARATION

The effects of some agonists acting at P-2 purinoceptors on guinea-pig isolated ileum longitudinal smooth muscle have been examined. The preparation contracted in response to ATP, alpha,beta-methylene ATP and 2-methylthio ATP. but not UTP. In this respect, alpha,beta-methylene ATP and 2-methylthio ATP were approximately equipotent and both were 10-50 times more active than ATP. Responses to alpha,beta-methylene ATP, but not 2-methylthio ATP or ATP, were antagonised by atropine and tetrodotoxin, suggesting that alpha,beta-methplene ATP activates cholinergic nerves in the ileum, whilst the other two compounds act on the smooth muscle. Two other purine nucleotide analogues, beta,gamma-methylene ATP and beta,gamma-imido ATP, did not cause contraction. However, both compounds antagonised responses to alpha,beta-methylene ATP, but not those to 2-methylthio ATP. Suramin antagonised responses to both alpha,beta-methylene ATP and 2-methylthio ATP, whilst Cibacron blue was without effect on responses to either agonist. We conclude that the purinoceptor on cholinergic nerves has some of the characteristics of the P-2x purinoceptor, whilst the purinoceptor on ileal smooth muscle has some of the characteristics of the P-2y purinoceptor. However, further work will be necessary before definitive classification is possible.