Evidence of H3 receptor inhibition by iodoaminopotentidine in the guinea pig ileum.

Activation of histamine H3 receptors by histamine (0.1 to 10 microM), (R)alpha-methylhistamine and N(alpha)-methylhistamine (0.01 to 0.3 microM) was shown to inhibit cholinergic nerve transmission in the guinea-pig ileum. Iodoaminopotentidine (IAP 300 nM), a potent H2 receptor antagonist, was found to decrease this effect but had no significant effect (P>0.05) on contractile responses produced by exogenous acetylcholine (0.2 microM). Dimaprit (0.1 to 10 microM) an H2 receptor agonist/H3 receptor antagonist, produced no significant effect (P>0.05) on the response to cholinergic nerve stimulation but reduced the effect of N(alpha)-methylhistamine. Furthermore, ranitidine (10 microM) an H2 receptor antagonist did not modify the inhibitory effect of histamine. These results suggest that IAP may inhibit H3 receptors in the ileum at similar concentrations reported to inhibit H2 receptors in functional studies.