Progesterone (P4) inhibits FSH production by 60–70% in cell cultures of pituitaries from normal ewes. This communication reports that P4 is much less effective in inhibiting FSH production in cultures prepared from pituitaries of ovariectomized ewes. P4 (10−7 m) inhibited 61 ± 5% of basal FSH secretion in cultures from intact anestrous ewes. When cultures were prepared from pituitaries collected on day 9, 21, or 42 after ovariectomy, P4 inhibited FSH secretion by only 46 ± 3%, 16 ± 2%, and 10 ± 2%, respectively. One group of ewes received implants of 17β-estradiol (E2) at the time of ovariectomy. When cultures were prepared from these ewes 35 days after ovariectomy, P4 inhibited FSH secretion 56 ± 5%, not significantly different (P > 0.05) from inhibition in normal cultures. Furthermore, under similar culture conditions, E2 and a porcine ovarian inhibin preparation inhibited FSH secretion regardless of the length of time after ovariectomy. These in vitro results suggest that ovariectomy causes a time-dependent decrease in pituitary responsiveness to P4 in vivo. Since E2 can maintain P4 sensitivity in pituitaries from ovariectomized ewes, E2 may be the only ovarian factor required to maintain pituitary responsiveness to P4 in vivo.
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