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Uterine estrogen receptor (ER) and thymidine kinase (TK) activity were investigated in response to an injection of estradiol (E2) (1.43microgram/100g body weight) in SD-strain immature female rats aged 21 days and spayed-adult rats aged 104 days (14 days after castration). In immature rats, uterine cytosol ER decreased abruptly after the injection of E2 and increased gradually from 4 hours later. On the other hand, nuclear ER increased rapidly and shaped a peak 1 hour after the injection of E2 and decreased gradually afterward. Uterine TK activity began to increase rapidly from 18 hours and shaped a peak 24 hours after the injection of E2 and was reduced to the same level as the controls 48 hours later. No differences in behavior of uterine cytosol and nuclear ER after the injection of E2 between immature and spayed-adult rats were noticed. Although uterine TK activity in immature rats was stimulated 30 times 24 hours after the injection of E2, that of spayed-adult rats increased by only twice. Uterine TK  was separated into 3 fractions (A, B and C) by means of a DEAE-cellulose column chromatography. In immature rats, 95% of the great TK activity induced by E2 was involved in fraction A, which was not inhibited by dCTP. In contrast, uterine TK isozymes in spayed-adult rats were at fraction B and C, whose activities were inhibited by dCTP, without relation to the administration of E2. The molecular weight of fraction A was estimated to be approximately 125,000 and that of B was 100,000 by the method of gel filtration on a Sepharose CL-6B column. No differences in uterine ER levels between immature and spayed-adult rats were found, but E2-induced different TK isozymes seem to depend on the stage of uterine growth.

The Effects of Estradiol on Uterine Estrogen Receptor and Thymidine Kinase in Immature and Spayed-adult Rats