EVIDENCE IN SUPPORT OF THE P1/P2 PURINOCEPTOR HYPOTHESIS IN THE GUINEA‐PIG TAENIA COLI

Abstract

1 The relaxations induced by adenosine 5′-triphosphate (ATP), adenosine 5′-diphosphate (ADP), adenosine 5′-monophosphate (AMP) and adenosine on the carbachol-contracted taenia coli of the guinea-pig have been studied. ATP and ADP produce similar responses which differ in nature and time course from those of AMP and adenosine. 2 Theophylline, at concentrations (25–200μm) lower than those which produce significant phosphodiesterase inhibition, blocks the effects of AMP and adenosine but fails to antagonize the responses elicited by ATP and ADP. The antagonism of adenosine by theophylline appears to be competitive. 3 Apamin (1–100 nM) blocks the inhibitory effects of ATP and ADP but fails to antagonize the responses to AMP and adenosine. The antagonism by apamin is non-competitive. 4 The results indicate that ATP and adenosine relax the taenia coli by activating different receptors and are consistent with the P₁ P₂ purinoceptor hypothesis.