Estrogen dynamics in the female rhesus monkey

Area of Science:

• Endocrinology
• Reproductive Biology
• Primate Physiology

Background:

• Estrone (E1) and estradiol (E2) are key estrogens regulating the female reproductive cycle.
• Understanding their metabolic clearance is crucial for comprehending hormonal dynamics during the menstrual cycle.
• Rhesus monkeys serve as a valuable model for studying human reproductive physiology.

Purpose of the Study:

• To investigate the metabolic clearance rates (MCR) and interconversions of E1 and E2 throughout the menstrual cycle in female rhesus monkeys.
• To determine if changes in protein binding or enzyme activity correlate with altered MCR during the cycle.
• To elucidate the hormonal regulation of estrogen metabolism in a primate model.

Main Methods:

• Reproduced menstrual cycles in ovariectomized rhesus monkeys using Silastic capsules containing E2 and progesterone.
• Administered constant infusions of radiolabeled estradiol ([3H] E2) and estrone ([14C] E1) on Days 9, 14, and 23 of the cycle.
• Measured serum E1 and E2 levels via radioimmunoassay and calculated MCR and interconversion rates.

Main Results:

• The MCR of E2 was significantly higher on Day 14 (mid-cycle peak) compared to Days 9 and 23.
• Similarly, the MCR of E1 was significantly elevated on Day 14 versus Days 9 and 23.
• No significant changes were observed in the percentage of free or albumin-bound E2, or in sex hormone-binding globulin levels across the studied days.
• Interconversion rates between E1 and E2 remained unaffected by the day of the menstrual cycle.

Conclusions:

• Elevated E2 levels during the mid-cycle peak lead to increased MCR for both E2 and E1 in female rhesus monkeys.
• These MCR changes occur independently of alterations in steroid protein binding or 17 beta-hydroxysteroid dehydrogenase activity.
• The study highlights a dynamic regulation of estrogen metabolism tied to specific phases of the reproductive cycle.