Histaminergic agents can modulate adrenergic smooth muscle neurotransmission.

The effects of histamine and some agonists and antagonists of the two types of histaminergic receptors on adrenergic smooth muscle neurotransmission were studied. The experiments were carried out on electrical stimulated (0.1 Hz, 1 pulse, 1 msec pulse duration, supramaximal voltage) prostatic part of rat vasa deferentia. Histamine, 2-(aminoethyl) thiazole and 2-methylhistamine (H1-agonists) as well as dimaprit (H2-agonist) had a biphasic effect on smooth muscle contractions induced by electrical stimulation (ES): low drug concentrations potentiated and high concentrations markedly inhibited the ES-contractions. The agonist of H1-receptors pyridylethylamine in low concentrations did not affect the smooth muscle contractions and in high concentrations decreased their amplitude. Diphenhydramine and mepyramine (H1-antagonists) strongly potentiated (up to 250%) the ES-contractions, while the H2-antagonist cimetidine did not change them. These effects might be due to the interaction of the drugs tested with different populations of the histaminergic receptors.