Progesterone secretion by granulosa cells from rats with four- or five-day estrous cycles: the development of responses to follicle-stimulating hormone, luteinizing hormone, and testosterone.

Changes in granulosa cell sensitivity and responsiveness to gonadotropins during the rat estrous cycle were studied by measuring progesterone (P) secretion in vitro in response to treatment with increasing doses of LH or FSH (0, 0.1, 1, 10, 100 ng/ml). The effect of testosterone [(T); 0.5 microM] on response to gonadotropins was also examined. Granulosa cells were isolated from the largest ovarian follicles of rats with 4- and 5-day estrous cycles at 0800 h, 1400 h, and 2000 h on proestrus and on the preceding day of diestrus at 1200 h and 2000 h. In rats with 5-day cycles, granulosa cells were also obtained at 1200 h on the first day of diestrus. Fifty percent maximal P production and 50% effective dose (ED50-dose of gonadotropin which elicited 50% maximal P production) were calculated from dose-response curves for LH and FSH and were used as measures of responsivity and sensitivity to gonadotropins, respectively. Basal P secretion and 50% maximal P secretion increased progressively as cells were isolated at later stages of follicular growth in both 4- and 5-day cycles. In cells from 5-day rats, however, basal and gonadotropin-stimulated P secretion were higher on the second day of diestrus than in cells from 4-day rats. By proestrus responsiveness was equal. Granulosa cell sensitivity to FSH was constant during 4- and 5-day cycles, as indicated by a lack of change in the ED50. Granulosa cell sensitivity to LH was lower than sensitivity to FSH on diestrus of both 4- and 5-day cycles. However, by the morning of proestrus sensitivity to LH increased and was similar to that for FSH. T increased basal P production only slightly, but synergized with both LH and FSH to stimulate 2-fold increases in 50% maximal P production by granulosa cells isolated at all times except 2000 h on proestrus, after the endogenous LH surge. T had no effect on the sensitivity (ED50) of granulosa cells to LH or FSH. In summary, granulosa cell responsiveness to LH and FSH increased in parallel during the final stages of follicular growth, but increased sensitivity was noted only for LH. The development of granulosa cell capacity to secrete P appears to be more advanced in 5-day rats than in 4-day rats relative to the next estrus. Because T synergized with LH and FSH to increase P secretion without altering sensitivity to gonadotropins, it probably acts at a site distal to gonadotropin receptors.