Extragonadal gonadotropin receptors, their distribution and function.

Luteinizing hormone (LH) and a second pituitary gonadotropin, follicle-stimulating hormone (FSH), together control steroid secretion and gamete development in both males and females. LH and its agonist human chorionic gonadotropin (hCG), share a common receptor in gonadal cells. There is increasing evidence for the existence of extraovarian LH/hCG binding sites in mammalian females. High affinity, low capacity LH/hCG receptors have been detected in pigs, rabbits and rats. Gonadotropin receptors in the human uterus were also demonstrated using immunocytochemical techniques. The presence of LH/hCG receptors, both in the endometrium and myometrium, has been so far found in pigs and humans. Receptors in the endometrium are present in glandular and luminal epithelium and stromal cells. In the myometrium, they are present in circular and longitudinal myometrial smooth muscle and vascular smooth muscle. Recently, LH/hCG receptor gene expression was confirmed both in the endometrium and myometrium of the pig. The amount of receptors is higher in the luteal phase as compared to the follicular phase of the estrous cycle indicating possible regulation of these receptors by ovarian hormones. Treatment with estradiol benzoate increased the number of LH/hCG binding sites compared with ovariectomized gilts receiving corn oil. However, administration of progesterone caused an elevation of these receptors, when compared with estradiol. Combined administration of estradiol and progesterone increased receptor capacity similar to progesterone alone. It seems that there are species differences in the uterine binding response to estradiol since the positive response to estradiol by uterine receptors in the ovariectomized pig is quite different from that observed in rabbits and rats. In the endometrium, hCG and/or LH may regulate glandular and luminal epithelial cell function via cAMP modulation or by increasing the local synthesis of steroid hormones. Stimulation of LH receptors with hCG in estrogen primed ovariectomized gilts had a quiescent effect on myometrial contractility in vitro. We examined also the effect of hCG on electromyographic activities of the uterus in ovariectomized and estrogen treated pigs. The hCG treatment caused a significant reduction of total duration of electrical activity and mean burst duration. Based on our earlier and recent results we suggest that the role of LH/hCG receptors in the myometrium is the regulation of uterine contractility, though the second messenger system remains to be elucidated.