Correlation with Biochemical Responses

X 1010 sites/100 ,ug protein, and the other with an apparent Kd of 25 nM and a binding capacity of 1.4 X 1012 sites/100 ,g protein. Measurement of the ability of several thyronine analogues to inhibit the binding of [1251I]T3 revealed the following rank order of potency: L-T3 > L-T4 > D-T3 = D-T4 > L-3,5-T2 > rT3 > D,Lth*ronine. Binding of T3 was inhibited by the omission of calcium from the medium or by the addition of the This work was supported in part by grant AM-18416 from the National Institute of Arthritis, Metabolism, and Digestive Diseases, and grant AGO-1346 from the National Institute of Aging, National Institutes of Health, Bethesda, MD. Received for publication 13 April 1982 and in revised form 2 August 1982. beta adrenergic antagonist alprenolol. As judged from studies of the lower affinity binding site, these manipulations decreased the affinity, but not the number, of binding sites for T3. The relative potencies of thyronine analogues to inhibit the binding of ['25I]T3 were generally parallel to their previously reported potencies in stimulating the uptake of the sugar analogue 2-deoxy-glucose (2-DG) in intact rat thymocytes in vitro. Further, the inhibition of T3-binding produced by L-alprenolol or by excluding calcium from the medium resembled the previously reported inhibition that these manipulations produce with respect to T3induced enhancement of 2-DG uptake. These findings suggest that the binding sites for T3 present in the plasma membrane of rat thymocytes act as functional receptors linked to the stimulation of 2-DG uptake that T3 induces in these cells.