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

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Abstract
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 – 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 subsequently 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.

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