RU486 blocks the secondary surge of follicle-stimulating hormone in the rat without blocking the drop in serum inhibin.

The stimulatory effect of progesterone on the release of the primary surges of serum LH and FSH is well characterized. Less is known about the relationship of progesterone to the secondary FSH surge. We used the antiprogesterone, RU486, to block the action of progesterone and studied the effects on the primary surges of LH and FSH, and especially on the secondary surge of FSH. Proestrous rats were treated with RU486 at 1,200 h. Rats were killed on proestrus and estrus, and serum levels of LH, FSH, and inhibin were measured. In all RU486-treated rats, the primary surges of LH and FSH were significantly attenuated, and the secondary FSH surge was completely abolished, despite a drop in inhibin levels following the primary surges. A high replacement dose of progesterone, delivered immediately after RU486 treatment, did not restore the primary surges of LH and FSH, or the secondary surge of FSH. These data suggest that other factors in addition to a drop in inhibin are responsible for producing the secondary FSH surge.