AN ANALYSIS OF THE DEPRESSOR RESPONSES TO HISTAMINE IN THE CAT AND DOG: INVOLVEMENT OF BOTH H1‐ AND H2‐RECEPTORS

Abstract

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 × 10⁻⁶ mol/kg i.v. The displacement was maximal with a dose-ratio of less than ten. 3 Metiamide alone, up to 2 × 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 × 10⁻⁷ and 2 × 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.