A postsynaptic inhibitory histamine H2-receptor in the mouse isolated vas deferens.

The effect of histamine on adrenergic neurotransmission in the mouse isolated vas deferens preparation was investigated. Concentrations of histamine ranging from 0.2 to 650 microM depressed, in a dose-related manner, not only the contractile response elicited by field stimulation but also the response caused by the addition of exogenous noradrenaline and acetylcholine. However, the release of [3H]-NA evoked by field stimulation or by high K+ remained unchanged in the presence of these concentrations of histamine. The inhibitory effect of histamine on the contractile responses caused by various stimuli was reduced or completely antagonized by cimetidine, a histamine H2-receptor antagonist but not by mepyramine, a conventional antihistamine. The inhibitory effect of histamine was found to be inversely proportional to both the Ca2+ concentration in the bathing medium and to the frequency of field stimulation. Further, the inhibitory effect of histamine was markedly reduced when Mg2+ was omitted from the bathing medium. It is concluded that the mouse vas deferens preparation contains a post-junctional inhibitory H2-receptor. The stimulation of H2-receptors by histamine inhibits the contractile response of the vas deferens, possibly by decreasing the availability of Ca2+ required for contraction by depressing the influx of Ca2+.