Evidence for two types of P2-purinoceptor in longitudinal muscle of the rabbit portal vein.

The actions of ATP, 2-methylthioATP, alpha, beta-methyleneATP, beta, gamma-methyleneATP, adenosine and non-adrenergic, non-cholinergic inhibitory nerve stimulation were compared on the ergotamine-contracted longitudinal muscle of the isolated rabbit portal vein with the vessel endothelium either intact or removed by mechanical rubbing. Non-adrenergic, non-cholinergic inhibitory nerve stimulation produced frequency-dependent relaxations. The purines (except alpha, beta-methyleneATP) also caused relaxation with the following potency order: 2-methylthioATP greater than ATP greater than beta, gamma-methyleneATP = adenosine. 8-Phenyltheophylline, a potent P1-purinoceptor antagonist, antagonised relaxations to adenosine but not those to ATP indicating that ATP and its analogues act directly via a P2-purinoceptor and not via a P1-purinoceptor after breakdown to adenosine. alpha, beta-MethyleneATP produced contractions which were not maintained and was tachyphylactic. The contractions were unaffected by tetrodotoxin and therefore unlikely to be due to release of a contractile substance from a neuronal source following initiation of an action potential. Removal of the endothelium did not affect the actions of non-adrenergic, non-cholinergic inhibitory nerve stimulation, ATP, alpha, beta-methyleneATP, beta, gamma-methyleneATP or adenosine. Thus purine action does not have an endothelium-dependent component in the rabbit portal vein longitudinal muscle. The results indicate the possibility that there may be more than one type of P2-purinoceptor in this tissue.