Measurement of estrogen receptors in the ovariectomized ewe is affected by body condition and secondary binding sites

Two studies were carried out to examine possible causes of the variability that occurs in studies on estrogen receptors in sheep. In the first study, the concentration of estrogen receptors in the uterus of a group of 16 ovariectomized ewes was inversely related to their live weight (r = -0.52, p less than 0.05). A second study examined 8 ovariectomized controls and 7 ewes that were permanently infertile after prolonged exposure to estrogenic clover pasture. The concentration of estrogen receptors in the pituitary of these ewes was inversely related to a measure of body fatness (r = -0.67, p less than 0.05), and there were no differences between control and clover-affected ewes. In addition to the classical estrogen receptor with a dissociation constant of 0.03 X 10(-9) mole/L, there were secondary binding sites with dissociation constants of about 0.9 X 10(-9) mole/L in the pituitary. The amount of this binding varied among sheep and appeared to increase as the cytosol was diluted. It is suggested that a failure to distinguish the presence of these sites may have led to apparent differences between experimental groups in the amount of affinity of receptors in previous studies. The effect of body condition on the number of cytosolic receptors may also have confounded previous studies.