Eosinophils as the source of uterine nuclear type II estrogen binding sites

Rat uterine nuclei have been reported to contain two types of estrogen binding sites (I and II). Type I is the classical high affinity, low capacity translocatable estrogen receptor, while type II is a lower affinity non-translocatable binding site. Correlation of data on hormonal specificity of induction and inhibition, tissue and cellular localization, and ontogenic appearance for type II binding sites and eosinophils suggested the hypothesis that these binding sites are associated with eosinophils. Although type II sites are reported to be highly correlated with uterine growth, premature growth can be elicited in newborn rats by five daily estradiol injections without the appearance of type II sites. Under these conditions, no eosinophils, as measured by peroxidase activity, are found. However, multiple estradiol injections on postnatal days 6-10 or 10-14 increased the levels of both type II sites and eosinophils. Eosinophils, purified from a peritoneal lavage, were found to contain a low affinity binding site with characteristics similar to type II binding sites. Short term estrogen-treated rat uteri contain only type I nuclear receptor and low levels of eosinophils while long term estrogen-treated rat uteri contain both type I and type II nuclear binding sites as well as approximately 6 X 10(5) eosinophils/uterus. The addition of this number of purified eosinophils to short term treated uteri resulted in saturation curves and Scatchard plots identical to those seen in long term treated uteri. These data indicate that the type II nuclear binding site is transported into the uterus with eosinophils following estrogen treatment.