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 μM 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⁺.