title: Evidence for the presence of P1- and P2-purinoceptors in rat caecum.
abstract: 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.
mesh_terms: D015762:2-Chloroadenosine; D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002432:Cecum / Q000187:drug effects; D000242:Cyclic AMP / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009126:Muscle Relaxation / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects*; D011802:Quinidine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000187:drug effects*; D011983:Receptors, Purinergic; D013806:Theophylline / Q000494:pharmacology
chemical_list: D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D015762:2-Chloroadenosine; D002110:Caffeine; D000255:Adenosine Triphosphate; D013806:Theophylline; D000242:Cyclic AMP; D011802:Quinidine; D000241:Adenosine