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Effect of LH-RH on LH release from the dispersed pituitary cells of pregnant rats.

In order to study the mechanism which induces a sudden drop in serum LH levels between day 11 and 13 of pregnancy in rats, LH responses of the pituitary to LH-RH were investigated using an in vitro incubation system of the dispersed pituitary cells. Dispersed pituitary cells, obtained on day 10 or 15 of pregnancy, were incubated for 3 hours with synthetic LH-RH. The addition of 5ng LH-RH significantly increased the release of LH from the pituitary cells both on days 10 and 15 of pregnancy. The response of LH to LH-RH was significantly greater on day 10 than 15. A combination treatment of estradiol and progesterone blocked the stimulatory effect of LH-RH on LH release on day 10 but not on day 15. These results indicated that LH responsiveness of the pituitary was decreased between day 10 and day 15 of pregnancy, and this would play an important role in the lowering of serum LH levels during mid-pregnancy in rats. A possible role of progesterone and estradiol in the regulation of the responsiveness of the pituitary to LH-RH was also suggested.