Negative feedback action of progesterone on tonic luteinizing hormone secretion in the ewe: dependence on the ovaries.

The ability of physiological increments in circulating progesterone to inhibit tonic LH secretion in the ewe was investigated in several experimental settings. Progesterone was administered sc by means of Silastic implants containing the steroid. The implants maintained circulating progesterone at levels which were similar to those observed during the mid-luteal phase of the estrous cycle. In long-term ovariectomized ewes, a sustained elevation in circulating progesterone effected a significant reduction in serum LH concentrations, but mean LH levels during progesterone treatment remained more than 20-fold greater than those observed during the mid-luteal phase of the estrous cycle. In other ewes in which this treatment was initiated immediately following ovariectomy, physiological increments in circulating progesterone prevented the typical post-castration rise in serum LH concentrations and maintained circulating LH at midluteal phase levels. This response was shortlived, however, as circulating LH incr...