Effects of bromocriptine and dopamine on the binding of estradiol to its uterine cytoplasmic receptors.

The in vivo and in vitro effects of bromocriptine on the binding of estradiol 17 beta to its specific uterine receptors were studied in both mature and immature rats. A single i.p. injection of bromocriptine reduced the specific estradiol-receptor interaction in adult rats, while it resulted ineffective in prepubertal animals. The effect of bromocriptine in mature rats was dose-dependent and evident only when using a dose of at least 0.70 mg/kg. Experiments in vitro also showed that in mature rats bromocriptine affected the estradiol-receptor interaction inducing a decrease in binding, which is well correlated to the concentration used. On the contrary, no effect of bromocriptine in vitro was observed when using cytosol obtained from uteri of immature rats. Very similar results were obtained in experiments in vitro when using dopamine instead of bromocriptine. Our results suggest that apparently there are two forms of estrogen receptors, one present both in prepubertal and pubertal age and the other present only in mature animals. The latter is the type of receptor sensitive to both bromocriptine and dopamine and presumably develops under the control of hormonal factors, which are only present at puberty.