The mechanism of the vasodilator effects of histamine and 4-methylhistamine in the hindquarters of the cat.

Intraarterial administration of histamine and 4-methylhistamine induced a fall in perfusion pressure of perfused hindquarters of the chloralose-anesthetized cat. Mepyramine which is a histamine H1-receptor blocker competitively inhibited the depressor effect of histamine without altering that of 4-methylhistamine. Continuous intravenous infusion of metiamide, which is a histamine H2-receptor blocker, to the mepyramine treated cats caused a further inhibition in response to histamine as well as 4-methylhistamine. These results indicate that both histamine H1- and H2-receptors are present in the vasculature of the cat hindquarters and both of them are responsible for the vasodilator effect of histamine.