Interaction of clonidine with chronotropic agents on isolated right atria

Clonidine was administered to isolated guinea pig right atria in order to characterize its chronotropic activity and its interaction with other chronotropic agents at the postjunctional level. Clonidine either had no significant effect (10−7–10−4 M) or decreased (10−3 M) atrial rate. Pretreatment of the atria with clonidine noncompetitively antagonized (10−6–10−4 M) the positive chronotropic actions of isoproterenol, and competitively antagonized (10−4 M) the negative chronotropic actions of pilocarpine. At doses of 10−6 or 3 × 10−6 M, clonidine also noncompetitively antagonized the positive chronotropic effects of 4-methylhistamine and glucagon. The results show that clonidine antagonizes both adrenergic and cholinergic influences on atrial rate at the postjunctional level and suggest that the antagonism of adrenergic influences does not involve a direct interaction with β-adrenergic receptors.