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HISTAMINE RECEPTORS IN PERIPHERAL VASCULAR BEDS IN THE CAT

1 The vasodilator activity of histamine has been studied in anaesthetized cats. 2 Histamine causes dose‐dependent vasodilatation in the vasculature of the hind‐limb and mesentery, perfused with blood at constant flow. 3 Experiments using the selective antagonists mepyramine and metiamide indicate the involvement of both H1‐ and H2‐receptors in the vasodilator responses to histamine. Mepyramine (2.5 × 10−6 mol/kg), causes displacement of the histamine dose‐response curve. This displacement is maximum with a dose‐ratio of about 10. Further dose‐dependent displacement of the dose‐response curve occurred after metiamide (4 × 10−7 mol kg−1 min−1 and 2 × 10−6 mol kg−1 min−1), although these does of metiamide had no effect on histamine responses in the absence of mepyramine. 4 Vasodilator responses could also be elicited by the selective H1‐receptor agonists, 2‐methylhistamine, 2‐(2‐aminoethyl)pyridine and 2‐(2‐aminoethyl)thiazole and the selective H2‐receptor agonist, 4‐methylhistamine. 5 The selectivity of mepyramine and metiamide as histamine receptor antagonists was confirmed by their failure to reduce the vasodilator responses to acetylcholine, isoprenaline and bradykinin.