Effects of progesterone on the estradiol-induced follicle-stimulating hormone (FSH) surge and FSH beta messenger ribonucleic acid in the rat

This study was designed to investigate the effects of progesterone on the estradiol (E2)-induced FSH surge and FSH beta messenger RNA (mRNA) using immature rat models developed previously to demonstrate inhibition or facilitation of the LH surge by progesterone. Twenty-eight day-old rats that received E2 implants at 0900 h had FSH surges about 1700 h on day 29 (32 h). In rats treated with E2 alone, serum FSH was 15.1 +/- 1.6 ng/ml at this time, while in those animals treated concurrently with E2 and progesterone, serum FSH was significantly suppressed (8.3 +/- 0.7 ng/ml, P less than 0.001). For demonstration of progesterone facilitation, rats were primed for 24 h with E2 before progesterone treatment. This led to premature and enhanced FSH secretion: at 1400 h on day 29 serum FSH was 45.5 +/- 2.7 ng/ml compared to 6.4 +/- 0.5 ng/ml in rats treated with E2 alone. To examine the effects of these dual actions of progesterone on FSH synthesis, steady state concentrations of FSH beta mRNA were measured by Northern analysis. FSH beta mRNA generally increased in parallel with FSH release. Levels of this mRNA were about 1.5-fold higher in rats undergoing E2-induced FSH surges than in rats in which the surge was blocked by progesterone. Also, at the onset of the progesterone-facilitated FSH surge, FSH beta mRNA was about 5-fold higher in animals treated with E2 and progesterone than in those treated with E2 only. On the morning after the FSH surge (48 h after E2 treatment) FSH beta mRNA was low to undetectable. In contrast, levels of FSH beta mRNA were 7- to 8-fold higher at this time in rats in which the surge was blocked by progesterone. Serum inhibin concentrations were significantly elevated (P less than 0.05) in animals treated with E2 alone for 32 h (3077 +/- 260 fmol/ml) or 48 h (2344 +/- 148 fmol/ml) compared to those treated with E2 and progesterone in the inhibition paradigm (2469 +/- 106, 1896 +/- 114 fmol/ml, respectively). After 32 h of E2 treatment in the facilitation paradigm, serum inhibin was comparable (P greater than 0.2) in rats treated for 8 h with blank implants (2592 +/- 168 fmol/ml) and those treated for 8 h with progesterone (2720 +/- 188 fmol/ml).(ABSTRACT TRUNCATED AT 400 WORDS)