Possible involvement of histamine H2-receptors in the vasodilator effect of clonidine on isolated and perfused canine coronary arteries.

The mechanism of clonidine-induced vasodilatation was investigated in the isolated and perfused canine coronary arteries. Clonidine produced a slight vasoconstriction at small doses whereas it induced a dose-related vasodilatation at large doses (more than 3 micrograms). The vasodilatation was reversed to vasoconstriction by treatment with 100 micrograms of cimetidine (a selective H2-receptor antagonist) but was not affected by 100 micrograms of chlorpheniramine (a selective H1-receptor antagonist). These results suggest that clonidine can dilate the isolated and perfused canine coronary arteries by acting on histamine H2-receptors.