Hormone Ontogeny in the Ovine Fetus. XXIV. Porcine Follicular Fluid “Inhibins” Selectively Suppress Plasma Follicle-Stimulating Hormone in the Ovine Fetus*

Abstract

The gonads of the late gestational ovine fetus synthesize inhibin, and under the influence of FSH the concentration and content of inhibin bioactivity in fetal testes and ovaries increases. In the present study we administered inhibin-rich charcoal-treated porcine follicular fluid (pff) as a bolus to eight chronically catheterized ovine fetuses between 116 and 128 days gestation. FSH levels decreased significantly to 81.3 +/- 4.0% of baseline values after 210 min and decreased further to 71.7 +/- 4.0% throughout the sampling period (5 h). LH levels were not affected by this treatment. An estimate of the secretion rate by integration of the response curve showed a significant decrease in the concentration of plasma ovine (o) FSH after pff compared with saline injection (-28.5 +/- 10.9 U after pff vs. +19.7 +/- 8.8 U after saline; P less than 0.01), while oLH secretion remained unaltered (116.6 +/- 102.1 U after pff vs. 174.5 +/- 99.4 U after saline; P = NS). These data show that in the late gestation ovine fetus pituitary secretion of oFSH, but not oLH, is selectively decreased by inhibin-rich pff, recognizing that the net FSH-suppressing activity of pff is the sum of the actions of FSH-stimulating (e.g. activin) and -suppressing (inhibin and follistatins) factors. Thus, the inhibin-FSH feedback mechanism is potentially functional at least by 0.8d gestational age, raising the possibility of a role for inhibin in the decline of circulating fetal FSH toward term.