Summary. Granulosa cell responsiveness at an early (1–2 h) or late (14–16 h) stage of differentiation following the onset of oestrus [and presumably the LH surge] was studied in 16 cows. Follicular fluid collected at the early stage (8 preovulatory follicles) had a higher concentration of testosterone (P < 0·05), oestradiol (P < 0·01) and oestrone (P < 0·01) than did follicular fluid collected at the late stage of oestrus (8 preovulatory follicles). No difference in follicular fluid progesterone was noted between follicles collected at the early and late stages of oestrus. Granulosa cells collected at the early stage of oestrus had a higher in-vitro response (progesterone production) to LH (P < 0·05), forskolin (P < 0·08) and diacylglycerol (P < 0·05) than did granulosa cells collected at the late stage of oestrus. However, later stage granulosa cells produced more (P < 0·01) progesterone after culture with prostaglandin E-2 than did earlier stage granulosa cells. These results show that follicular fluid oestrogen decreases, which suggests a loss of aromatase activity as oestrus progresses, and that granulosa cells become refractory (low progesterone production) to in-vitro LH, forskolin, and diacylglycerol challenge, yet acquire responsiveness to prostaglandin E-2 as oestrus progresses.
Keywords: cow; granulosa cells; LH; ovary; progesterone