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1Histamine produced dose-dependent contractile responses on both isolated perfused ear arteries and aortic strips of the rabbit. These responses were blocked by mepyramine and potentiated by both metiamide and dithiothreitol. 2 In the presence of maximum potentiation by metiamide, dithiothreitol still potentiated the contractile response to histamine of both preparations. 3 In the presence of mepyramine, histamine produced dose-dependent reductions in the contractile response to noradrenaline. This vasodilator action of histamine was abolished by metiamide but was unaffected by dithiothreitol. 4 The vasodilator action of histamine on the human isolated perfused temporal artery and the positive inotropic effect of histamine on the isolated spontaneously beating atria of the rabbit were blocked by metiamide but unaffected by dithiothreitol. 5It is concluded that the rabbit aorta, like the ear artery, contains both HI and H2 histamine

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Neuropeptide Y (NPY 10(-10)-10(-7) mol/l) had little or no effect on the perfusion pressure of rabbit isolated ear arteries but potentiated the brief contractile responses to injections of noradrenaline, histamine or brief periods of electrical stimulation (2-20 Hz for 5 s). This effect was slowly reversible. Similar potentiation was seen following long periods of electrical stimulation (2-40 Hz for 1-5 min) which produced well sustained increases in perfusion pressure. Following even longer periods of electrical stimulation (10-30 min) during which the perfusion pressure was not maintained, responses to noradrenaline and histamine were potentiated but the responses to electrical stimulation for 5 s were greatly reduced. The increase in sensitivity following prolonged electrical stimulation may be due to NPY released as a cotransmitter.