The effects of hypophysectomy and administration of pituitary hormones on luteal function and uptake of high density lipoproteins by luteinized ovaries and adrenals of the rat.

The role of plasma lipoproteins and hypophyseal hormones in the maintenance of progesterone secretion by the rat corpus luteum was investigated. In the first experiment, rats were treated daily from days 1-6 of pregnancy with 5 mg/kg 4-aminopyrozolopyramidine (4APP), a blocker of hepatic lipoprotein secretion, or with 5 mg/kg 4APP and 1 or 2 mg ovine PRL or 0.1 ml 0.5% phosphoric acid (4APP vehicle). The administration of 4APP reduced serum cholesterol and progesterone levels on days 2-6 of pregnancy and ovarian progesterone on day 6. The reduced progesterone secretion had no effect on embryo implantation. PRL, in the doses used, was incapable of abrogating the effects of 4APP on circulating or ovarian progesterone levels. Ovaries and adrenals, but not kidneys, of pseudopregnant rats exhibited specific and saturable uptake of porcine high density lipoprotein (HDL). Time-course studies indicated that the uptake of HDL was rapid in ovaries compared to that in adrenals. Ovaries from rats not only exhibited uptake of porcine HDL, but also were capable of using it for progesterone synthesis. Immature rats were assigned to 7 groups of 16 rats each; 8 rats from each group received 4 mg/kg 4APP, and 8 received 4APP vehicle from day 1 of pseudopregnancy. Four groups underwent hypophysectomy on day 1 and received one of the following: 0.1 mg (30 IU/mg) ovine PRL, 0.1 mg ovine LH or 0.1 mg synthetic ACTH daily, or no replacement therapy. Three sham-hypophysectomized groups received 0.1 mg PRL or LH twice daily or no hormone treatment. On day 5 of pseudopregnancy, rats received intravascular HDL, as described above, and were killed 1 h later. Treatment with 4APP increased the adrenal uptake of HDL, but ovarian uptake was not different from that in the control group. Hypophysectomy reduced both adrenal and ovarian uptake of HDL. In adrenals only ACTH at the dose employed ameliorated reduction of HDL uptake induced by hypophysectomy, while in the ovaries, both PRL and LH reversed the effect of hypophysectomy. The effect of PRL on uptake was specific to [125I]HDL and did not alter [125I]albumin uptake. It is concluded that: 1) hypophysectomy reduces HDL uptake in the luteinized rat ovary; and 2) PRL and LH replacement therapy maintain ovarian uptake of HDL, suggesting a direct effect of these luteotropins on lipoprotein uptake.