Blockade by burimamide of the effects of clonidine on cardiac contractility phosphorylase activation and cyclic adenosine monophosphate in isolated guinea pig heart.

Clonidine in a dose-range of 2.5 microgram to 80 microgram caused positive inotropic effect, which was accompanied by increase in the cyclic AMP levels and phosphorylase-activation of the isolated perfused guinea pig heart. Clonidine-induced biochemical and mechanical effects were blocked by burimamide, an H2-receptor antagonist Propranolol (1 x 10(-6)M), phentolamine (1 x 10(-6)M) or reserpine pretreatment, did not affect the clonidine responses on the perfused guinea pig heart. Clonidine reduced the 4-methyl-histamine (H2-agonist) responses of guinea pig heart. Our data suggest that the cardiac effects of clonidine may be due to stimulation of H2-type of receptors.