The Effects of Histamine and Some Histamine‐like Agonists on Blood Pressure in the Cat

Abstract

The effects of histamine on blood pressure have been compared with the effects caused by four histamine‐like agonists in anaesthetized cats. It has been confirmed that the depressor responses to histamine involve both H1‐ and H2‐receptors: depressor responses also follow the administration of selective H1‐ and H2 ‐receptor agonists. 2‐Methylhistamine, in doses up to 1 × 10−7 mol/kg, lowers blood pressure by interaction with H1‐receptors. Larger doses of 2‐methylhistamine also lower blood pressure but this may involve H2‐receptors. 4‐Methylhistamine, in doses up to 1 × 10−7 mol/kg, lowers blood pressure by interaction with H2‐receptors. Larger doses of 4‐methylhistamine also lower blood pressure but this may involve H1‐receptors. 2‐(2‐Aminoethyl)pyridine and 2‐(2‐aminoethyl)thiazole both lower blood pressure by interaction with H1‐receptors only. The potential value and limitations of these compounds as tools to investigate the cardiovascular effects of histamine are discussed.

Histamine Receptors in Peripheral Vascular Beds in the Cat

“…The values for both 2-methylhistamine and 4-methylhistamine are similar to the values reported by Black et al (1972) Owen (1975) on cat blood pressure.…”

“…Both 2-(2-aminoethyl)pyridine and 2-(2-aminoethyl)thiazole have been shown previously to be selective H1 receptor agonists (Lee & Jones, 1949;Grossman et al, 1952;Durant et al, 1974). Both compounds caused dose-dependent depressor responses by interaction with Hl-receptors (Owen, 1975) and have now been shown to cause dose-dependent vasodilatation by interaction with HI -receptors.…”

“…2-Methylhistamine has relatively higher agonist activity on H1-receptors than on H2-receptors (Black et al, 1972, Durant et al, 1974Owen, Each of the histamine-like agonists mimicked the histamine response. Three of the agonists did so by interaction with HI -receptors and the fourth, 4-methylhistamine, did so by interaction with H2-receptors.…”

THE CARDIOVASCULAR RESPONSE TO DIMAPRIT, A SELECTIVE HISTAMINE H2‐RECEPTOR AGONIST

“…Until recently, the most selective histamine H2-receptor agonist described was 4-methylhistamine (Black, Duncan, Durant, Ganellin & Parsons, 1972) and, although this compound shows marked selectivity for H2-receptors some residual HI-receptor activity remains (Durant, Ganellin & Parsons, 1975) and the depressor responses to large doses are due in part to interaction with histamine Hl-receptors (Owen, 1975).…”

“…The identification of histamine receptors in the cardiovascular system, and elsewhere, has been based predominantly on the use of selective histamine receptor antagonists and confirmed with relatively selective receptor agonists (e.g. Owen, 1975;Powell & Brody, 1976.…”