Variations in the Effectiveness with which Rat Follicle- Stimulating Hormone Can Stimulate Its Own Secretion during the Rat Estrous Cycle*

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Abstract
The ability of exogenous rat FSH to elevate endogenous FSH in plasma was tested at different times during the 4-day estrous cycle of the rat. Rats kept in a room with the light on from 0500-1900 h daily were administered phenobarbital at 1200–1300 h. A cannula was then inserted in the right atrium for subsequent blood collections and iv injection of saline or 0.5–2 μg highly purified rat FSH. Plasma FSH remained low after iv saline in all rats. Intravenous FSH at 1400 h on proestrus caused plasma FSH during proestrus to rise and then fall to levels about 4-fold above values observed in saline-injected controls. It then remained high during the morning of estrus. Plasma FSH was low or undetectable during estrus in rats hypophysectomized before FSH injection. The divergence in plasma FSH patterns in FSH-injected intact and hypophysectomized rats suggested that exogenous rat FSH caused endogenous FSH release within a maximum of 5 h. A similar pattern in plasma FSH was observed after injection of rat FSH at 2100 h on proestrus. Injection of rat FSH at 1700 h on any day of the estrous cycle caused a plasma FSH pattern similar to that observed in intact rats given rat FSH at 1400 or 2100 h on proestrus. However, the plasma FSH elevations the next morning were greatest in rats given FSH on diestrous day II or on proestrus and smallest in rats given FSH on estrus. The results suggest that FSH-positive feedback 1) has a maximal latency of onset of 5 h and 2) may become less effective in maintaining elevated plasma FSH during estrus, resulting in the termination of the periovulatory FSH surge in plasma. (Endocrinology106: 476, 1980)