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Possible Existence Of A Long-loop Feedback System Between FSH And Inhibin In Female Rats

L. V. DePaolo, L. D. Anderson, A. N. Hirshfield

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Experiments were designed in which peripheral plasma inhibin levels were presumably altered in an attempt to investigate an interdependency between pituitary follicle-stimulating hormone (FSH) and ovarian inhibin secretion. In the first study, unilateral ovariectomy (ULO) was performed on 4-day cycling female rats under ether anesthesia at 0800 h on diestrous day 1 (D1). Inhibin-like activity [FSH-inhibiting activity(FSH-IA)] in untreated ovarian venous plasma (OVP) collected from the remaining ovary was assessed by an in vitro pituitary bioassay system. Both plasma FSH levels and FSH-IA significantly increased between 4 and 12 h after ULO. Thereafter, plasma FSH declined between 12 and 32 h after ULO, whereas FSH-IA remained elevated during this same time interval. Compared to sham-operated rats, plasma FSH was significantly elevated 4, 12, and 24 h after ULO, whereas FSH-IA was statistically higher only at 32 h after ULO. In a second experiment, rats were injected with charcoal-treated porcine follicular fluid (PFF) on proestrus and estrus. Control rats received saline. The data indicate that increased plasma FSH levels on D1 in PFF-treated rats (FSH rebound) may be a consequence of reduced endogenous inhibin secretion on estrus. As well, return of FSH to control levels on D2 in PFF-treated rats may have resulted from an FSH-associated increase in FSH-IA on D1 and D2.