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

ABSTRACT

The relative importance of LH vs. FSH in stimulating estradiol secretion by granulosa cells during follicular development was assessed in Sprague-Dawley rats with regular 4- or 5-day estrous cycles. At various times during diestrus (D-1200 h, D-2000 h) and proestrus (P-0800 h, P-1400 h, P-2000 h) granulosa cells were isolated from the presumptive preovulatory follicles. The cells were cultured with 0.5 microM testosterone and various doses of LH or FSH (0, 0.1, 1, 10, or 100 ng/ml). Media were collected and replaced daily for 3 days and measured for estradiol by RIA. Estradiol secretion in the absence of gonadotropins (endogenous aromatase activity) increased progressively with stage of the cycle. At earlier times of cell isolation (D-1200, D-2000, P-0800) secretion by cells from 5-day rats was greater relative to 4-day rats. Since estradiol production in the absence of gonadotropins declined progressively over each 3-day culture period, effects of the gonadotropins were most evident on the third day of culture when endogenous aromatase activity was low. On this day FSH consistently increased estradiol secretion above control levels. Sensitivity to FSH, as measured by the 50% maximally effective dose (ED50; approximately 1-3 ng FSH/ml), did not vary with cycle type or stage of the cycle and the response to FSH, in terms of 50% maximal estradiol secretion, was also relatively constant. In contrast, the effects of LH varied with cycle type and time of cell isolation. As follicles developed, cells from both 4- and 5-day rats became more sensitive to LH, as evidenced by a decline in the ED50 from approximately 32 ng LH/ml to approximately 3 ng/ml. This increase in sensitivity to LH is consistent with previous reports that the number of LH receptors on granulosa cells increases progressively during the final stages of follicular development. However, the increase in sensitivity to LH occurred earlier in cells from 5-day rats, relative to the expected next estrus (later, relative to the preceding estrus). Responsiveness to LH (50% maximal estradiol production) was relatively constant, except that it was lower in cells from 4-day rats isolated at D-1200. These results indicate that the LH receptors acquired by granulosa cells during diestrus and proestrus are functionally linked with aromatase activity and may, therefore, be important to the production of estradiol levels sufficient to elicit the LH surge.(ABSTRACT TRUNCATED AT 400 WORDS)