Female rats were treated with Metrodin (highly purified urinary FSH from menopausal women) or saline during the oestrous cycle. On the day of pro-oestrus they were anaesthesized with phenobarbital and received four repetitive LHRH injections 1 h apart. This treatment with FSH suppressed the unprimed LH response to the first LHRH injection. During the subsequent injections the maximal LHRH self-priming was delayed by 3 h till the fourth LHRH stimulation. At this time, LH release in response to LHRH was equally as high as shown in the saline controls after the second LHRH injection. Ovariectomized rats did not show the self-priming effect and FSH treatment was ineffective in suppressing LHRH-induced LH release. Administration of FSH followed by an additional 4- or 24-h period before LHRH stimulation were equally effective in suppressing the unprimed LH release and delaying (up to 3 h) the maximal priming of LH release by LHRH. Even 4–20-fold increased amounts of LHRH did not affect the suppressed unprimed release of LH after FSH treatment. Treatment with FSH did not change oestradiol and progesterone levels.
It was concluded that FSH treatment suppresses the unprimed LHRH-induced LH release and delays maximal LHRH self-priming by enhancing the release of an ovarian factor.