Evidence for the presence of P1- and P2-purinoceptors in rat caecum.

Adenosine, 2-chloroadenosine, adenosine triphosphate (ATP) and adenosine tetraphosphate induced concentration-dependent relaxation of rat caecum. Caffeine as well as theophylline, at concentrations (25-100 microM) lower than that known to produce significant phosphodiesterase inhibition, shifted the concentration-response curve for adenosine to the right in a parallel fashion without diminishing the maximal response. The pA2 values for caffeine and theophylline against adenosine were found to be 4.6 +/- 0.26 (slope = 0.97 +/- 0.05) and 4.9 +/- 0.28 (slope = 0.98 +/- 0.07), respectively. Though the relaxant effect of adenosine tetraphosphate was blocked in a non-competitive manner by quinidine (5 microM) and phenoxybenzamine (25 microM), theophylline or caffeine (50 microM) failed to block this response. Similarly, quinidine also blocked the relaxant effect of ATP in a non-competitive manner. Besides, the relaxant effect due to adensine was rapid in onset as compared to a slow relaxant effect of ATP. These observations suggest the presence of P1- and P2-purinoceptors in the caecum. The activation of these receptors by adenosine and ATP, respectively, leads to relaxation of the tissue.