No abstract

Human myometrium contains hCG/LH receptors. There are fewer of these receptors during labor compared to no labor at preterm or term deliveries. Exogenous hCG can directly inhibit oxytocin-stimulated human myometrial contractions. These findings suggest that hCG may directly maintain myometrial quiescence during pregnancy. As maintenance of uterine quiescence may involve down-regulation of myometrial gap junctions, we investigated the effect of hCG on connexin-43 (CX-43) gene expression from RNA to protein and morphological gap junctions. The addition of 5 or 10 nM highly purified hCG to subconfluent cultures of pregnant myometrial smooth muscle cells resulted in a significant decrease in CX-43 protein levels. Higher hCG concentrations (100 and 1000 nM), however, had no effect. The maximal effect of hCG was seen at 4-8 h of culture, followed by recovery after a longer duration of culture. hCG treatment also concomitantly decreased CX-43 messenger RNA and morphological gap junctions. The hCG effect on CX-43 protein levels is hormone specific and mediated by protein kinase-A signaling. Estradiol and oxytocin increased, whereas progesterone decreased, CX-43 protein levels and morphological gap junctions. The oxytocin-induced increase was reversed by cotreatment with hCG. Although RU 486 alone had no effect on CX-43 protein levels, it prevented the down-regulating action of hCG and progesterone. In summary, our results demonstrate that hCG can directly decrease CX-43 messenger RNA, protein, and morphological gap junctions in cultured pregnant human myometrial smooth muscle cells. The hCG action is concentration and time dependent, hormone specific, and mediated by protein kinase-A signaling and appears to involve progesterone receptors. These data lend support to the concept that hCG could be one of the hormones responsible for maintaining uterine quiescence by down-regulating myometrial gap junctions during pregnancy.

Our laboratory previously demonstrated that the human myometrium contains functional hCG/LH receptors. The present study investigated whether hCG can directly regulate oxytocin-stimulated human myometrial contractions. Uterine specimens were obtained from 30- to 40-yr-old women undergoing hysterectomy for leiomyomata, metrorrhagia, or prolapse. Myometrial strips from the lower uterine segment were primed for 24 h with 2.2 nmol/L estradiol. Then, the slices were incubated for 4 h at 37 C with or without 10 nmol/L hCG and stimulated with 1 mumol/L oxytocin, and the contractions were measured. The results showed that hCG inhibited the amplitude while paradoxically increasing the frequency of contractions. The effect of hCG was seen in proliferative, but not secretory, phase myometrial specimens. hCG had no effect on rat hepatic portal vein smooth muscle contractions, suggesting that the hCG action was tissue specific. Oxytocin treatment of human myometrial smooth muscle cells resulted in a dose-dependent increase in intracellular free Ca2+ levels. Pretreatment with hCG resulted in an attenuation of the oxytocin response, suggesting that the action of hCG was mediated by decreasing intracellular free Ca2+ levels. In summary, our results demonstrate that hCG can directly inhibit the amplitude of oxytocin-stimulated contractions of human myometria from the proliferative phase of the cycle. The hCG action is tissue specific and appears to be mediated by decreasing intracellular free Ca2+ levels in myometrial smooth muscle cells.

Human myometrium contains functional hCG/LH receptors. The purpose of this study was to determine whether or not leiomyomas that arise from myometrium also contain these receptors. Northern blotting demonstrated that leiomyomas and normal adjacent myometria contained hCG/LH receptor mRNA transcripts. Western immunoblotting showed that leiomyomas and corresponding normal myometria also contained 60- and 50-kDa receptor proteins. Ligand blotting revealed that only the 50-kDa receptor protein in leiomyomas and corresponding normal myometria could bind 125I-hCG and that this binding inhibited by excess unlabeled hCG. In situ hybridization and immunocytochemistry revealed that smooth muscle cells in leiomyomas and corresponding normal myometria contained the hCG/LH receptor mRNA transcripts and receptor proteins. The receptor levels were lower in leiomyomas than in corresponding normal myometria. However, the receptors were functional, as treatment with hCG resulted in a decrease of connexin-43 protein levels in leiomyomas as in normal myometria. In summary, human uterine leiomyomas express a functional hCG/LH receptor gene at a reduced level compared with that for normal corresponding myometria. This finding could be relevant to an understanding of the growth control mechanisms in leiomyomas and the manner in which medical therapy for leiomyomas might work.

No abstract