Abstract
Summary
Young cycling rats 60 to 70 days of age, and old irregular-cycling or con-stant-estrous rats 10 to 11 months of age were ovariectomized. At 25 or 52 days after ovariectomy, they were each given a single injection of 8 μg of estradiol benzoate/100 g of body weight, followed 72 hr later by a single injection of 0.8 mg of progesterone/ 100 g of body weight. Radioimmunoassay of serum LH from blood samples removed 4,6,8, and 10 hr after progesterone administration showed that LH rose significantly more in the young than in the old, former constant-estrous rats 25 days after ovariec-tomy, and more than in either group of old rats at 55 days after ovariectomy. These results indicate that the positive feedback of estrogen and progesterone on LH release is reduced in old as compared to young cycling female rats, and is believed to account at least in part for the irregular cycling, and failure of ovulation in the old constant-estrous rats.
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