Bioactive Follicle-Stimulating Hormone Levels in Serum and Urine of Male and Female Rats from Birth to Prepubertal Period1

Using a sensitive in vitro granulosa cell aromatase bioassay (GAB), w e determined serum and urinary levels of bioactive follicle-stimulating hormone (bio-FSH) in male and female rats from birth to Day 40 of age. In addition, serum immunoreactive FSH (immuno-FSH) was measured by radioimmunoassay to determine the bio- to immuno-(B/I) ratio of FSH. During the neonatal period (Days 1–7 of age), both sexes bud detectable serum bio-FSN levels. In the infantile period (Days 7–21), serum bio-FSH levels initially decreased ut Day 10 f o r both sexes, and then rose steadily, reaching maximum concentrations at Day 14 (males: 68.7 ng/ml; females: 114.6 ng/ml). Subsequently, FSH levels in the females decreased from Day 16 throughout the juvenile (Days 21–35) and prepubertal (Days 35–40) periods. In contrast, FSH levels in the males fluctuated during these periods. In the males, immuno-FSH reflected the bioactive profiles, with a B/I ratio of 2.2 ± 0.2. In the females, the B/I ratio was approximately 2.5 during the neonatal and infantile periods but declined to approximately 1.0 during the juvenile and prepubertal periods, consistent with earlier observations of heterogeneous forms of pituitary FSH in immature female rats. Morning urine samples were also collected daily and bio-FSH concentrations were determined. In both sexes, urinary bio-FSH profiles were highly correlated ( r = 0.93) with serum FSH throughout development. However, the urine concentrations were about 50-fold higher than serum. This study demonstrates that (I) urine bio-FSH levels reflect serum bio-FSH levels; (2) serum bio-FSH levels in male rats, in general, reflect immuno-FSH profiles; and (3) serum B/I ratios of FSH in females decrease during the juvenile and prepubertal periods, suggesting the transition from birth to sexual maturity is accompanied by qualitative changes in the circulating forms of FSH.