Plasma progesterone and its relationship to serum gonadotrophins in immature female rats

In immature female rats, low values for concentrations of plasma progesterone were generally found from days 6-15 and from days 25-32 of life. Maximum progesterone concentrations (13-0-14-1 ng/ml), comparable to metoestrous values in the adult rat, occurred on days 20-22. The progesterone appeared to be of ovarian origin since after ovariectomy, on day 18, low progesterone concentrations were found 1 and 2 days later (2-5 ng/ml and 1-3 ng/ml) as compared with control values of 10-7 ng/ml and 14-1 ng/ml. However, adrenalectomy also lowered progesterone concentrations, 1 and 2 days later (6-4 and 4-9 ng/ml). The effect of progesterone, either alone or in combination with oestradiol benzoate (OB), on serum gonadotrophins was studied in rats ovariectomized on day 18. The highest dose of progesterone (0-15 mg) only slightly diminished the rise in serum luteinizing hormone (LH) after ovariectomy and had no effect on serum follicle-stimulating hormone FSH). Oestradiol benzoate in a dose of 0-025 mug/100 g body weight was highly effective in preventing the rise in both LH and FSH concentrations, and OB treatment (resulting in a near-physiological oestradiol concentration) combined with progesterone treatment was more effective than treatment with OB alone. The results suggest that the amounts of progesterone and oestradiol present in the 20-day-old rat are adequate to cause the decrease in FSH level normally observed in immature female rats around this age.