The actions of histamine on the separated layers of the guinea-pig uterus: the influence of ovarian steroids.

The effects of histamine have been studied on separated circular and longitudinal myometrial layers from the guinea-pig. Virgin guinea-pigs in dioestrus were (1) untreated, (2) treated for 14 days thrice weekly with oestradiol cypionate 20 micrograms/kg s.c., or (3) treated as in (2) but in addition with progesterone 3 mg/animal s.c. daily for 4 days before use. Preparations were field-stimulated (60 V 2 ms 30 Hz for 5 s every 100 s) to elicit regular contractions. Histamine was equipotent in producing enhancement of evoked contractions of circular myometrium from animals in each treatment group (pD2 = 5.03, 5.05 and 4.95 in groups 1, 2 & 3 respectively), and enhanced contractility in all but two longitudinal myometrial preparations. Treatment with oestradiol alone, and with oestradiol and progesterone following oestradiol priming enhanced the magnitude of maximal response and produced a small decrease in the potency of histamine on the longitudinal myometrium. pA2 estimates for mepyramine as an antagonist were similar (8.6) in preparations of longitudinal myometrium from each group of animals. Metiamide (1-10 mumol/l) did not antagonise the effects of histamine in this layer. These results, taken together, indicate that ovarian steroids act selectively on the longitudinal myometrial layer to amplify the action of histamine, but do not alter the histamine receptor subtype which in this layer is exclusively H1.