Follicle-Stimulating Hormone and Luteinizing Hormone Affect the Endogenous Release of Pituitary Follicle-Stimulating Hormone and the Ovarian Secretion of Inhibin in Rats

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Abstract
FSH-inhibiting activity (FSH-IA) has been demonstrated in ovarian vein plasma (OVP) of cyclic rats, and such activity varies inversely with plasma FSH concentrations. Since FSH-IA rapidly declines in OVP during proestrous preovulatory LH and FSH surges, it is possible that these gonadotropins may affect the ovarian secretion of such material. The present study has examined the parallel changes produced in endogenous plasma FSH concentrations at 0900 h on estrus concomitant with alterations in OVP FSH-IA after the iv pulse injection of phosphate-buffered saline, 0.4 or 1.0 μg highly purified rat FSH, or 8.0 μg highly purified rat LH into phenobarbital-blocked proestrous rats. Phenobarbital administration on proestrus (1245 h) effectively blocked the spontaneous release of LH and FSH that afternoon. At 0900 h on estrus, plasma FSH levels in salinetreated proestrous control rats were elevated above values at 1400 h on proestrus, whereas they were basal in rats previously injected with phenobarbital. The pulse injection of 0.4 or 1.0 μg rat FSH into phenobarbital-blocked proestrous rats at 1300 h increased plasma FSH at 1400 h from basal (μg FSH and to 171 ± 19 ng/ml in animals which received 1.0 μg FSH. At 0900 h on estrus, both groups of FSH-treated rats had significantly higher plasma FSH levels than the phenobarbital-treated controls, and in rats which received 1.0 μg FSH, endogenous plasma FSH levels on estrus were significantly higher than those in saline-treated controls. Similarly, endogenous plasma FSH levels were elevated significantly above phenobarbital-blocked controls at 0900 h on estrus in rats which received LH on proestrus. Saline-treated control rats ovulated 7.1 ± 0.5 eggs and LHtreated rats ovulated 6.8 ± 0.5 ova. In contrast, animals which received 0.4 μg FSH did not ovulate, whereas 15 of 17 animals injected with 1.0 μg FSH ovulated 4.9 ± 0.7 ova. Plasma progesterone concentrations 1 and 4 h after the injection of 0.4 or 1.0 μg FSH on proestrus remained basal. However, animals injected with LH exhibited elevated plasma progesterone levels at 1400 h but not at 1700 h. OVP was obtained from all groups at 0900 h on estrus, was charcoal-treated to remove steroids, and was assessed for FSHIA by its ability to suppress the basal 18-h secretion of FSH and LH in two separate primary pituitary cell cultures. Ovariectomized rat plasma (20%) was added to the control wells of the culture. OVP from control rats reduced the 18-h secretion of FSH in the two cell cultures by 25.7% and 17.6%, respectively. In the phenobarbital-blocked group, FSH-IA in OVP was high (75.1% and 52.3% inhibition of basal secretion). The injection of 1.0 μg FSH or 8.0 μg LH on proestrus resulted in elevated endogenous plasma FSH levels at 0900 h on estrus, and OVP from such animals produced only 16.6–33.3% inhibition of basal FSH secretion. Basal LH secretion was not affected by the addition of OVP to the cultures. We conclude that the endogenous proestrous surge of FSH and LH affects ovarian function by reducing the ovarian secretion of material with FSH-IA into the peripheral circulation. With the decline in such plasma FSH-IA, pituitary gonadotrophs may be released from negative feedback inhibition and increase their secretion of FSH into the blood during late proestrus and the morning of estrus. (Endocrinology106: 1047, 1980).

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