Inhibitory histamine H2-receptor in the guinea-pig urinary bladder

The histamine H2-receptor in the guinea-pig urinary bladder was characterized by determining the effects of histamine and impromidine on contractions induced by electrical transmural stimulation (ETS). The contractile responses to ETS (0.5 ms, 15 V, for 15 s) at frequencies of 1 to 30 Hz were abolished by treatment with tetrodotoxin, and were partly inhibited by scopolamine, indicating that the ETS-induced contraction has scopolamine-sensitive and -resistant components. Histamine and impromidine inhibited the scopolamine-resistant contraction induced by ETS but not the ETS-induced scopolamine-sensitive contraction and nicotine- and acetylcholine (ACh)-induced contractions. The inhibitory effects of histamine and impromidine were antagonized by cimetidine, but not by diphenhydramine and mepyramine. Thus, the inhibitory effect of histamine may be mediated through H2-receptors. As impromidine did not affect the tetrodotoxin-sensitive and Ca2+-dependent ETS-evoked release of ACh and noradrenaline (NA) from the isolated urinary bladder preloaded with [3H]choline and [3H]NA, respectively, the H2-receptor may not be involved in the cholinergic and adrenergic mechanisms. These results indicate that histamine H2-receptors are present in the guinea-pig urinary bladder. The H2-receptor located on non-cholinergic excitatory neurons may be involved in the inhibitory action produced by histamine.