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

The depressor responses to histamine, in anaesthetized cats and dogs, have been shown to involve both histamine H1‐ and H2‐ receptors. 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. Metiamide alone, up to 2 × 10−6 mol kg−1 min−1, had no significant effect on the histamine dose‐response curve. 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. 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.