[A quantitative study on the relationship between the specific binding of glucocorticoid receptor and glucocorticoid...

The inhibitory effect of dexamethasone (Dex) on the incorporation of [3H] Urd and specific binding of [3H] Dex with glucocorticoid receptor (GR) were measured in order to quantitatively study the relationship between these two indices in rat thymocytes. The results showed that there existed a close correlation between the two indices in the range of 10(-9)-10(-5) mol/L Dex. When thymocytes were treated with RU486 in the concentration range of 10(-10)-10(-6) mol/L, both [3H] Dex specific binding and inhibitory effect of Dex were blocked with different fractions. The specific binding was linearly related to Dex biopotency. When extrapolated 100% of specific binding corresponded to 100% of biopotency while 20% of specific binding corresponded approximately to 0% of biopotency. It is likely that GR exhibited an occupancy threshold of about 20% total receptors, with only few spare ones in rat thymocytes.