An analysis of the depressor responses to histamine in the cat and dog: involvement of both H1- and H2-receptors.

1 The depressor responses to histamine, in anaesthetized cats and dogs, have been shown to involve both histamine H1- and H2-receptors. 2 In both species, histamine caused dose-dependent falls in blood pressure. The dose-response curve could be displaced to the right by administration of mepyramine 2.5 times 10(-6) mol/kg i.v. The displacement was maximal with a dose-ratio of less than ten. 3 Metiamide alone, up to 2 times 10(-6) mol kg-1 min-1, had no significant effect on the histamine dose-response curve. 4 When administered in the presence of mepyramine, metiamide, 4 times 10(-7) and 2 times 10(-6) mol kg-1 min-1, caused dose-dependent displacements to the right, of the histamine dose-response curve greater than could be achieved with mepyramine alone. 5 The results indicate the presence of both histamine H1- and H2-receptors in the cardiovascular system of the cat and dog. Both receptors produce a common response, i.e. a fall in blood pressure.