Capacity of Old Versus Young Female Rats to Secrete LH, FSH and Prolactin

Serum LH, FSH and prolactin levels were measured by radioimmunoassays before and after ovariectomy and during and after estradiol benzoate (EB) treatment in 4-5 month old young mature female rats, in 22-24 month old constant estrous and pseudopregnant rats, and in 26-30 month old anestrous rats. Serum LH in intact old constant estrous rats was significantly higher than in young rats on the day of estrus, about the same in intact old pseudopregnant rats as in young rats in diestrus, and was undetectable in the old anestrous rats. By 7 weeks after ovariectomy, serum LH rose about 26 fold in young rats, only about 3 fold in the old constant estrous rats, about 14 fold in the old pseudopregnant rats, and remained unmeasurable in the old anestrous rats. After daily EB injection for 8 days, the relative fall in serum LH in old constant estrous and pseudopregnant rats was significantly smaller than in the young rats, and 2 weeks after estrogen treatment was terminated the rise in serum LH was significantly less in the old constant estrous and pseudopregnant than in young rats. Serum LH in the old anestrous rats remained undetectable. Intact old constant estrous and pseudopregnant rats showed about the same serum FSH values as young rats on the day of estrus, but exhibited a significantly smaller FSH rise after ovariectomy and a smaller FSH fall after 8 days of EB administration than the young rats. Intact old anestrous rats had as low serum FSH levels as young rats in diestrus, showed only a small rise in serum FSH after ovariectomy and no change after estrogen injections. After estrogen treatment was withdrawn, the rise in serum FSH in the old constant estrous and pseudopregnant rats was significantly less than in the young rats, and showed no increase in the old anestrous rats. The intact old constant estrous rats had 2 to 3 times more serum prolactin than young rats, and the intact old pseudopregnant rats had about as much serum prolactin as young rats on the day of estrus. Serum prolactin in intact old anestrous rats was twice as high as in old constant estrous rats and 4 to 8 times greater than in old pseudopregnant or young rats. After ovariectomy, serum prolactin decreased significantly in the old constant estrous and young rats, but not in the old pseudopregnant or anestrous rats. EB administration increased serum prolactin in all categories of rats, and fell after estrogen treatment was terminated. These observations suggest that the hypothalamo-pituitar