Resistance to blockade of vasodepressor responses to tolazoline by chlorpheniramine and metiamide in rabbit hindlimb vasculature.

In perfused hindquarters of anesthetized rabbits, metiamide alone and a combination of metiamide and chlorpheniramine failed to modify tolazoline-induced depressor responses, indicating a lack of histamine H1 and H2-receptor involvement. Failure of atropine and propranolol to alter tolazoline responses precludes cholinergic and beta-adrenergic mechanisms. On the other hand, graded pressor responses to histamine were reversed to depressor responses by chlorpheniramine, whereas a combination of chlorpheniramine and metiamide completely abolished the effects of histamine. The latter results confirm that in the rabbit hindlimb vasculature histamine H1 and H2-receptors mediate pressor and depressor responses, respectively.