Accelerated communication H1 contractile and H2 relaxant receptors in canine gastric muscularis mucosae

Histamine had two effects on the contractility of canine gastric muscularis mucosae in vitro: relaxation at or below 10 microM and contraction at higher concentrations. Selective agonists and antagonists were used to test the possibility that these effects were mediated by different receptor subtypes. The H1-selective agonist 2-pyridylethylamine (2-PEA) and the H2-selective agonist dimaprit contracted and relaxed this muscle, respectively, while the H3-selective agonist R-alpha-methylhistamine had no effect. The H1- and H2-selective antagonists mepyramine and ranitidine selectively blocked 2-PEA-mediated contractions and dimaprit-mediated relaxations, respectively. Agonist responses, were unaltered by tetrodotoxin, suggesting a site of action other than nerves. Our results indicate that canine gastric corpus muscularis mucosae possesses both contractile H1 and relaxant H2 receptors.