title: P-1 purinoceptor antagonism by clonidine in the rat caecum.
abstract: Adenosine as well as 2-chloroadenosine induced concentration-dependent relaxation of the rat caecum. Although clonidine (10(-9)-10(-6)M) did not exhibit any effect per se yet it shifted the concentration-response curve for adenosine as well as 2-chloroadenosine to the right in a parallel fashion without diminishing the maximal response. The pA2 value for the interaction of clonidine with adenosine was found to be 8.71 (slope 0.92). Guanfacine, another alpha 2 agonist, did not modify the adenosine-, 2(2)-chloroadenosine-induced relaxant effects. These observations indicated that clonidine blocks P1- purinoceptors competitively without any involvement of alpha 2-receptors.
mesh_terms: D000241:Adenosine / Q000037:antagonists & inhibitors; D000818:Animals; D002432:Cecum / Q000187:drug effects; D003000:Clonidine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D016316:Guanfacine; D006146:Guanidines / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D010648:Phenylacetates / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D011956:Receptors, Cell Surface / Q000187:drug effects*; D011983:Receptors, Purinergic
chemical_list: D006146:Guanidines; D010648:Phenylacetates; D011942:Receptors, Adrenergic, alpha; D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D016316:Guanfacine; D000241:Adenosine; D003000:Clonidine