Possible existence of a long-loop feedback system between FSH and inhibin in female rats

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Abstract
Experiments were designed in which peripheral plasma inhibin levels were presumably altered to investigate an interdependency between pituitary follicle-stimulating hormone (FSH) and ovarian inhibin secretion. 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 4-12 h after ULO. Plasma FSH then declined between 12-32 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. Rats were injected with charcoal-treated porcine follicular fluid (PFF) on proestrus and estrus. Control rats received saline. Increased plasma FSH levels on D[day]1 in PFF-treated rats (FSH rebound) may be a consequence of reduced endogenous inhibin secretion on estrus. Return of FSH to control levels on D2 in PFF-treated rats may also result from an FSH-associated increase in FSH-IA on D1 and D2.

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