Role of the amplitude of the gonadotropin surge in the rat.

Ovaries that contained pregnant mare's serum gonadotropin-induced preovulatory follicles were perifused for 21 hours with (1) control media, (2) 5%, (3) 30%, or (4) 85% of the gonadotropin surge. All of the gonadotropin surges induced oocyte maturation and enhanced progesterone (P) secretion. Ovulated oocytes were observed in one of four ovaries in the 85% surge group. These data demonstrate that only a small percentage of the gonadotropin surge is required to induce a maximal P secretion and oocyte maturation. Because neither P secretion nor oocyte maturation is enhanced with increasing gonadotropin levels, these two follicular responses must be mediated through a threshold-dependent mechanism such that once the threshold level is obtained, a maximum response is induced. Greater than 85% of the surge appears to be required to induce follicular rupture, suggesting that the threshold for the follicular rupture is greater than that for either P secretion or oocyte maturation.