Comparison of the Regulation of Luteinizing Hormone (LH) Secretion in Immature and Adult Rats

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Abstract
A careful study of some factors influencing LH secretion and storage has been made in immature and adult rats of both sexes. The ovarian ascorbic acid assay for LH has been utilized throughout. The LH content of the anterior lobe was 19.8 μg/gland in immature females and 22.3 μg/gland in adults. In immature males the content (7.0 μg /gland) was lower than that in immature females and rose markedly to a value of 32.9 μg/gland in adults, a value which was greater than that found in adult females. No detectable LH was found in plasma from rats with intact gonads except in the case of adult males, in which minimal levels were detectable. Two to 3 weeks following gonadectomy, pituitary LH content had risen significantly in both adult females and immature males, whereas it remained unchanged in immature females and adult males. By contrast with the variable response to gonadectomy seen in the hypophysis, plasma LH activity increased significantly in all gonadectomized groups. When expressed on a weight basis, equivalent LH-releasing activity was found in stalk median eminence extracts from immature and adult males on iv injection into immature females which had been pretreated with gonadotrophins. Extracts prepared from immature and adult rats of both sexes elevated the plasma LH activity in ovariectomized, estrogen-progesterone blocked rats. From the above results it appeared that all LH-releasing machinery of adults was present in immature animals. Daily sc injections of estradiol benzoate in immature and adult oophorectomized females lowered plasma LH below that found in controls, but immature animals were 2–3 times more sensitive to this effect of estrogen than adults. Estrogen also reduced hypophysial LH content, which indicates that it blocks synthesis as well as secretion of LH. The dose of estrogen required to reduce plasma LH in immature and adult rats appeared to be somewhat greater than that secreted by the normal animal, as judged by effects of this estrogen replacement therapy on uteri and vaginas of the recipients. It appears possible that in intact females plasma LH levels are kept down by the combined inhibitory effect of estrogen and progesterone. One factor initiating puberty may be resetting of a hypothalamic gonadostat so that circulating levels of gonadal hormones which are effective in holding gonadotrophin secretion in check in immature animals are rendered ineffective at puberty. This results in increased rates of gonadotrophin secretion and resultant gonadal development.