Interrelationships Between Circulating Levels of Estradiol-17β, Progesterone, FSH and LH Immediately After Unilateral Ovariectomy in the Cyclic Rat

At various times between 0 and 32 h after unilateral ovariectomy (ULO) or sham operation of female rats at the diestrous-2 stage of the 5-day cycle, levels of FSH and LH in serum and of estradiol-17β (E 2 ) and progesterone (P) in plasma were estimated by radioimmunoassay and numbers of Graafian follicles were counted in serial sections of the remaining ovary, in order to study temporal relationships between changes in these parameters. FSH concentrations were increased over control values from 4-28 h after ULO while P levels appeared decreased from 2-24 h after ULO. LH and E 2 levels were generally within the range of values in sham-operated rats. The numbers of healthy Graafian follicles were increased over control values from 24 h after ULO onwards. These findings suggest a relationship between the increase of FSH levels and either decreased P levels and/or decreased secretion of another factor by the decreased number of Graafian follicles. In a second experiment, ULO was followed by administration of either P or steroid-free bovine follicular fluid (BFF) and FSH levels were estimated 4 h later. Administration of P via s.c. implantation of P-containing silicone tubing resulted in doubling of peripheral P levels, nevertheless FSH levels increased. After injection (i.p.) of BFF, FSH levels did not rise. These data seem to indicate that increased FSH levels after ULO and at diestrus-2 are induced by a transient decrease in peripheral levels of a nonsteroidal inhibin-like ovarian factor as present in BFF.