Control of gonadotropin secretion in the ovine fetus. III. Effect of castration on serum follicle-stimulating hormone levels during the last trimester of gestation.

Gonadal involvement in fetal FSH regulation was examined by studying FSH levels in 13 female (7 castrate and 6 sham control) and 13 male (8 castrate and 5 sham control) chronically catheterized ovine fetuses operated upon in utero at 106-115 days gestation (term = 147 days). These fetuses had been studied previously for pulsatile LH secretion every 2-7 days over a 2- to 38-day period until fetal delivery or death. From each study day, 3 1-h spaced blood samples (1.5-2.0 ml) were taken for FSH determination by RIA (NIH FSH-S8 standard), and the results were averaged. The overall mean was then calculated for each fetus. In female fetuses, there was no significant difference in mean serum FSH levels between castrates [53.4 +/- 5.0 ng/ml (+/- SEM)] and controls (52.5 +/- 14.4 ng/ml). In contrast, serum FSH levels in the eugonadal males were significantly (P less than 0.001) lower (23.4 +/- 8.0 ng/ml) than those in castrate males (56.9 +/- 7.1 ng/ml, a value comparable to those observed in both female groups). Mean serum FSH levels declined significantly (P less than 0.001) in castrate fetuses of both sexes after 125 days (61.5 +/- 5.6 vs. 42.4 +/- 7.7 ng/ml in females; 64.1 +/- 6.1 vs. 51.5 +/- 8.6 ng/ml in males). In the males, the FSH decline did not reach sham control levels, which remained unchanged with advancing gestation. Moreover, mean serum FSH levels were significantly higher in a group of 4 male fetuses (62.2 +/- 13.7 ng/ml) castrated at 121-130 days gestation compared to values in 3 age-matched sham castrate controls (22.1 +/- 2.6 ng/ml; P less than 0.001). The increment in serum FSH levels in castrate compared to sham castrate male fetuses demonstrates an important role for the fetal testis in FSH regulation from 106 days gestation until term. The lack of a detectable castration effect on the relatively high serum FSH levels in eugonadal females indicates that the fetal ovary does not play a similar role and suggests that in females, FSH is secreted in a functionally castrate mode. The decline in FSH levels after 125 days in castrate fetuses of both sexes may result at least in part from the previously reported coincident rise in circulating levels of feto-placental sex steroids and/or PRL.(ABSTRACT TRUNCATED AT 400 WORDS)