Differential blockade of estrogen-induced uterine responses by the antiestrogen nafoxidine.

We have used an experimental design described by Gardner et al. [6] for dissociating early and late uterine responses to estradiol, involving pretreatment of immature rats with 5 micrograms nafoxidine (Upjohn U-11, 100 A, UA) for 24 h, before administrating estradiol. In these conditions the authors showed that responses occurring 4-h after estradiol administration were not blocked, while 24-h responses were abolished. These findings were defined and extended in the present investigation which shows that: (1) The overall wet weight response of the uterus to estradiol in UA-pretreated animals is decreased when compared to saline pretreated rats. (2) The early increase in cGMP content induced at 2-4 h by estrogen is also decreased but not abolished by the pretreatment with UA, contrary to the late increase in cGMP, which is abolished. (3) The late estrogen-induced proliferative response, measured by the [3H]thymidine labeling index, in the myometrium, stroma and luminal epithelium is maintained after pretreatment with UA. It is remarkable that this occurs in the absence of any estrogen induced uterine hypertrophy as measured by the 24-h increase in uterine weight and RNA or protein content. These results strongly support the hypothesis proposed by Gardner et al. [6] that different control mechanisms might regulate early and late uterine responses to estrogen. Our data suggest the existence of the following dissociable groups of response: (1) Wet weight increase and early increase in cGMP content, (2) Late hypertrophy and second rise in cGMP content and (3) Proliferative response; which are respectively, moderately depressed, abolished or unaffected by UA pretreatment.