Tissue Distribution and Specific Binding of Tritiated Dexamethasone in Vivo: Autoradiographic and Cell Fractionation Studies in the Mouse*

Abstract

The mouse is frequently used in studies concerned with glucocorticoids, but it has not been studied extensively in terms of its glucocorticoid receptor profile, with the exception of cells of the immune system. Therefore, adrenalectomized mice (C57/B strain) were injected iv with 5 X 10-10 mol tritiated dexamethasone ([3H]DM), either alone or with a 100- fold excess of nonradioactive dexamethasone (DM), and the following were determined in liver, kidney, heart, and brain: 1) total tissue radioactivity 3, 5, 10, and 30 min after injection, 2) the time course of total and nonspecific cytoplasmic binding of [3H]DM 3–30 min after injection, 3) the nuclear binding of [3H]DM 30 min after injection and its displaceability by excess DM, and 4) the cellular distribution of radioactivity within these four tissues by autoradiography 30 min after injection. Tissue fractionation studies showed clearly the presence of displaceable cytoplasmic binding of [3H]DM in heart, liver, and kidney, and of displaceable nuclear binding in all four tissues. Autoradiography showed displaceable binding only in nuclei of the four tissues. The conclusions drawn from this study are: 1) [3H]DM is bound to high affinity, low capacity sites in liver, kidney, heart, and brain; 2) after in vivo injection of [3H]DM, nuclear binding of the steroid can be studied more precisely in heart and brain than cytoplasmic binding; 3) the bulk distribution of [3H]DM between different tissues is significantly altered by the presence of excess DM, a finding to be considered when interpreting autoradiographic results; and 4) autoradiography, taken together with tissue fractionation studies, permits the within-tissue localization of cells showing preferential, displaceable, nuclear binding of labeled steroid to putative physiological receptors.