The Relationship Between Progestins and Gonadotropins During the Late Luteal Phase of the Menstrual Cycle in Rhesus Monkeys

The effects of progesterone on blood levels of FSH and LH during the late luteal and early follicular phase of the cycle were studied in 10 regular cycling rhesus monkeys. Progesterone administered for 8 days in silastic capsules, beginning 12 days after peak concentrations of estrogen in serum, prolonged the luteal phase levels of this hormone in 6 animals. Blood levels of progesterone were not affected in 4 control animals given cholesterol. Throughout two consecutive cycles, progesterone, 20α-hydroxypregn- 4-ene-3-one, estradiol, FSH, and LH were measured by radioimmunoassay at selected intervals. The length of time between the LH peaks in cycle I and II differed significantly (p < 0.05) between animals treated with progesterone (35.8 ± 1.21, SD, days) and those treated with cholesterol (31.0 ± 2.16, SD, days). The concentrations of FSH in the late luteal and early follicular phase of the cycle continued to rise significantly (p < 0.01 by regression analysis) in both treated and control animals in spite of the fact that progesterone levels in the systemic circulation were maintained between 6 to 10 ng/ml in the experimental group. The slopes of the regression curves for FSH during the 8-day treatment period did not differ significantly between control and experimental animals. On days 2 and 3 after treatment, however, significantly more FSH was found in animals treated with progesterone than in controls (p < 0.05). The concentrations of 20α-hydroxypregn-4-ene-3-one were maintained between 1.5 and 2.5 ng/ml in animals treated with progesterone but not in controls (animals with a cholesterol implant). These data suggest that the rising levels of FSH in primates during the late luteal and early follicular phase of the cycle are not due to declining levels of progesterone or 20α-hydroxypregn-4-ene-3-one from the degenerating corpus luteum. (Endocrinology94: 128, 1974)