Pre- and postjunctional effects of histamine on the guinea pig urinary bladder: evidence for heterogeneity in the H1-receptor population?

Histamine was tested on the guinea pig isolated urinary bladder to determine both the direct effect on the muscle and the influence on the contractions induced by field stimulation. Histamine (10(-7)-10(-3) M) caused contraction of the bladder and enhancement of the atropine-resistant response to field stimulation. These effects were sensitive to the H1-receptor antagonist pyrilamine (pA2 = 8.55 and 7.07, respectively). The H2-antagonists cimetidine and famotidine were ineffective on both parameters. It is concluded that predominantly H1-receptors are present in the guinea pig urinary bladder and that they are localized both on the muscle and on non-cholinergic nerve terminals. The significant difference between the pA2 values for pyrilamine is likely to suggest a heterogeneity in the H1-receptor population.