Effect of aging on the dissociation kinetics and estradiol receptor nuclear interactions in mouse uteri: correlation with biological effects.

The dissociation kinetics of uterine estradiol receptor (UER) from young, middle-aged, and old mice have been studied in vitro and correlated with interactions of the steroid receptor with nuclear suspensions in a cell-free system. Furthermore, we have determined in these various age groups of mice the concentration of uterine cytosolic progesterone receptors and the activity of glucose-6-phosphate dehydrogenase. Compared to UER from young mice, the receptor from middle-age and old mice displayed similar first order dissociation kinetics, but a significantly reduced fraction of the slow component resulting from transformation of the receptor into a state of higher affinity for estradiol. In all age groups, sodium molybdate markedly inhibited this heat activation of UER. Recombination studies using heat-treated cytosolic UER and enriched nuclear fractions of various age groups suggested a markedly reduced ability for nuclear binding with advancing age, despite unchanged affinity of activated UER for nuclear acceptor sites that ranged from 3-5 X 10(9) M-1. Cross-incubation studies of heat-activated cytosolic UER with nuclei from young, middle-aged, and old mice suggested that the activation defect of cytosolic UER was already present at middle age and that a reduced nuclear ability to support receptor binding followed the onset of reproductive acyclicity. In parallel with these endocrine defects, we observed, with aging, decreases in mean baseline progesterone receptor concentrations and glucose-6-phosphate activity in uteri of both intact and castrated mice. The diminution of these two parameters was present at middle age, further increased at old age, and persisted after administration of low (0.2 mg) or high (2.0 mg) doses of estradiol for 3 days before study. Our observations suggest that decreased receptor activation is primarily responsible for the decreased effects of estrogen in aging mice.