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The effect of the gonadal steroids on the time-course of release and synthesis of LH, FSH and prolactin was studied in vitro. Pituitary glands from ovariectomized rats were incubated for four sequential periods of 1 h in the presence or absence of 1·84 µmol oestradiol-17β/l, 3·44 µmol 5a-dihydrotestosterone/l or 31·80 µmol progesterone/l.

The rate of release of LH was not affected by oestradiol or dihydrotestosterone, but was enhanced by progesterone after the third period of incubation. Synthesis of LH was increased by the three steroids tested, from 1 to 4 h of incubation, the effect being more marked for oestradiol than for the other steroids. The rate of release of FSH was depressed after 3 h whereas its synthesis was increased between 1 and 2 h, only in the presence of dihydrotestosterone. Synthesis of FSH was also stimulated by oestradiol after 2 h incubation but its release was not affected. Progesterone showed no effect on either the release or the synthesis of FSH. Although oestradiol and dihydrotestosterone induced a rise in both LH and FSH synthesis, the onset, magnitude and duration of the responses were different, indicating separate regulatory mechanisms. Oestradiol stimulated the rates of both release and synthesis of prolactin. The effect was already evident after 1 h of incubation and increased thereafter. On the contrary, progesterone treatment inhibited the release and synthesis of prolactin. The rate of synthesis decreased after 1 h of incubation, whereas release was depressed after 3 h. Dihydrotestosterone had no effect on the release and synthesis of prolactin.

The evidence provided by this study indicates that the effect of the steroid hormones in vitro was predominantly on the synthesis of LH, FSH and prolactin. When changes in release of LH, FSH and prolactin occurred they were always preceded by alterations in hormone synthesis.