SPECIFIC BINDING OF LH-RH TO THE ANTERIOR PITUITARY GLAND DURING THE OESTROUS CYCLE IN THE RAT

Abstract

The specific binding of [125I]LH-RH [luteinizing hormone releasing hormone] to partially purified homogenates of the anterior pituitary gland varied during various stages of the estrous cycle in the rat. The quantity of [125I]LH-RH bound was maximum at proestrus (150 fmol/mg protein) and minimum at estrus (20 fmol/mg protein) and metestrus (16 fmol/mg protein). Specific binding of LH-RH to the pituitary was undetectable in rats ovariectomized during the afternoon of diestrus I or morning of estrus but restored by treatment with estradiol-17.beta. alone or with progesterone. Peripheral plasma levels of LH-RH measured during the estrous cycle by radioimmunoassay were 5.0 pg/ml at diestrus I and increased to 20 pg/ml at proestrus; plasma LH-RH levels at diestrus II, estrus and metestrus were not significantly elevated. The mean plasma LH levels as measured by radioreceptor assay were the highest (16.7 ng/ml) at 4-5 p.m. of proestrus, somewhat lower (4 ng/ml) at 1-2 p.m. of proestrus, and lowest (1-2 ng/ml) at other times. Pre-ovulatory LH surge was not observed in rats ovariectomized at diestrus or killed at the expected time of proestrus. Estradiol treatment alone elevated plasma LH levels in such rats and progesterone administration augmented this effect. The ovulatory surge of LH at proestrus in the rat involves the hypothalamic release of LH-RH and increased LH-RH binding in the pituitary gland, both of which may be stimulated by the pre-ovulatory rise in estrogen and/or progesterone levels.