P-1 purinoceptor antagonism by clonidine in the rat caecum.

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.