In order to elucidate a possible self-regulation for hCG synthesis in placenta, effects of estradiol on hCG production and secretion were evaluated by culturing early placental tissue in the presence or absence of estradiol. The cellular level of mRNAs encoding hCG (α, β) and hPL were estimated by mean grain count per syncytial nucleus on the placental sections hybridized in situ with labeled cDNA probes corresponding to these mRNAs. Immunoreactive hCG, hCGα and hPL in the media and explanted tissues were measured by the homologous RIAs. Addition of estradiol at concentration of 1-10ng/ml into the medium caused an increase in the cellular levels of hCGαmRNA after 24-hour cultured, and exhibited significant increases in immunoreactive hCGα levels in the media and explanted tissues after 72-hour culture. The addition of estradiol neither affected the cellular levels of mRNA encoding hCGβ and hPL nor immunoreactive hCG and hPL levels in the media and tissues. The appropriated concentration of estradiol (1-10ng/ml) used in above experiments was found to be similar to the tissue concentration in normal placenta.
These findings suggest that the physiological concentration of estradiol selectively stimulates hCGα synthesis and secretion by normal placenta. Thus, estradiol in placenta may be a factor responsible for the increase of hCGα in maternal serum and placental tissue with the progress of gestation.