Periovulatory Hormonal Dynamics: Relationship of Immunoassayable Gonadotropins and Ovarian Steroids to the Bioassayable Luteinizing Hormone Surge in Rhesus Monkeys

The relationship between ovarian steroids and LH during the midcycle gonadotropin surge is controversial. Recent demonstration of temporal and quantitative differences in immunoassayable LH (I-LH) and bioassayable LH (B-LH) at midcycle have further clouded this issue. To evaluate the relationship of I-LH, FSH, and ovarian steroids to the onset of the midcycle B-LH surge, blood samples were obtained from five chronically catheterized rhesus monkeys at 2-h intervals for 5-6 days. The plasma was assayed for FSH, LH, 17β-estradiol (E₂), progesterone (P₄), and 17±-hydroxyprogesterone (17-OHP) by RIA and for LH by a rat interstitial cell testosterone in vitro bioassay. The initiation of the B-LH surge served as time zero (t₀) for the temporal analysis of changes in plasma hormone levels. The I-LH and FSH surges were initiated 6.4 ± 2.2 h (mean ±SEM) and 5.2 ± 1.9 h, respectively, after the onset of the B-LH surge. Although the duration of the ascending limb of the surge was similar for B-LH, I-LH, and FSH, the mean ± SEM total duration of the B-LH surge (34.5 ±3.5 h) was significantly longer (P <0.025) than those of I-LH (24.4 ± 5.0 h) and FSH (27.6 ± 2.3 h). Before the onset of the B-LH surge (t₀ - 4 h), the ratio of B-H to I-LH was unity; however, during the acme of gonadotropin secretion (t₀ + 12- 16 h), the B-LH to I-LH ratio approached 6:1. Doubling times for B-LH, I-LH, and FSH were similar during the ascending phase of the surge. Plasma E₂ concentrations increased continuously from t₀ - 40 h to t₀, with (a mean ±SEM doubling time of 32.6 ±4.7 h. Peak E₂ concentrations occurred within 6 h after the onset of the B-LH surge. Plasma P₄ concentrations began to increase at t₀ - 6 h in four monkeys and at t₀ in one monkey. Plasma P₄ concentration plateaued from t₀ to t₀ + 24 h, then increased rapidly, with a mean ± SEM doubling time of 20.5 ± 2.9 h. Although there were significant individual variations in plasma 17-OHP concentrations, a definite increase in 17-OHP occurred by t₀ - 10 h, and peak concentrations occurred at t₀ + 1 h. Plasma 17-OHP concentrations infsequently declined and remained at a nadir until t₀ + 68 h, at which time they began to increase, with a mean ± SEM doubling time of 30.9 ± 2.6 h. These data demonstrate that the midcycle B-LH surge is qualitatively and quantitatively distinct from that of I-LH; further, P₄ and 17-OHP may facilitate the onset of both B-LH and I-LH surges.