(R)-alpha-methylhistamine augments neural, cholinergic bronchospasm in guinea pigs by histamine H1 receptor activation.

In the airways, activation of histamine H3-receptors with (R)-alpha-methylhistamine inhibits neurally induced cholinergic contractions in vitro and peptidergic responses in vivo. The role of histamine H3-receptors on the cholinergic bronchoconstriction induced by electrical stimulation of the dorsal medulla in guinea pigs was assessed in this study. There was no evidence for an H3-receptor mediated inhibition of cholinergic bronchospasm in vivo. However, there was potentiation of central cholinergic bronchoconstriction by (R)-alpha-methylhistamine or histamine by a mechanism involving H1-receptors. I.v. (R)-alpha-methylhistamine (0.3-3 mg/kg) or histamine (0.001-0.01 mg/kg) produced a transient bronchospasm and potentiated the bronchoconstriction due to medullary stimulation. These effects of (R)-alpha-methylhistamine and histamine were blocked by the histamine H1-antagonist, chlorpheniramine (30 micrograms/kg i.v.) but not by H2- or H3-receptor antagonists. (R)-alpha-Methyl-histamine did not potentiate the bronchoconstriction due to i.v. methacholine. Other bronchoconstrictor agents such as methacholine and serotonin did not potentiate the CNS-induced bronchospasm.