Male and female rats were gonadectomized and subsequently (1-3 months later) treated with steroids (estradiol benzoate plus progesterone or testosterone propionate) preliminary to the intracarotid injection of either saline or a highly purified preparation of porcine LH-RH. Groups of animals were killed at 3, 5, 10 or 20 min after the injection. Serum and pituitary LH were measured by radioimmunoassay and serum FSH was examined by bioassay or radioimmunoassay. Representative pituitary glands from control groups and from groups responding strongly to LH-RH were studied by electron microscopy. The pituitary gonadotrophic cells were highly modified as a result of gonadectomy but no clear distinction into 2 types could be made. Ovariectomized, estrogen-progesterone (OEP) pretreated animals gave a greater response to the LH-RH injections than testosterone treated castrates of either sex or castrated male rats given ovarian steroids. At 5 min after the injection of 100 ng of LH-RH into OEP rats, serum levels of LH were elevated approximately 13-fold over control levels while serum levels of FSH were elevated less than 2- fold, if at all. In spite of this evidence of intensive release of LH from the pituitary no significant depletion of pituitary LH stores was seen and no morphologic signs of accelerated LH release were detected.
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