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

Activation of histamine H-3 receptors by histamine (0.1 to 10 mu M), (R)alpha-methylhistamine and N-alpha-methylhistamine (0.01 to 0.3 mu M) was shown to inhibit cholinergic nerve transmission in the guinea-pig ileum. Iodoaminopotentidine (IAP 300 nM), a potent H-2 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 mu M). Dimaprit (0.1 to 10 mu M) an H-2 receptor agonist/H-3 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 mu M) an H-2 receptor antagonist did not modify the inhibitory effect of histamine. These results suggest that IAP may inhibit H-3 receptors in the ileum at similar concentrations reported to inhibit H-2 receptors in functional studies.