Alterations in hepatic nuclear binding of triiodothyronine in experimental diabetes mellitus in rats

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Abstract
The interrelationship between hepatic nuclear T3 [triiodothyronine] receptors and glucose metabolism was studied in 8 diabetic rats and 8 paired control animals. Serum glucose (mean .+-. SEM [standard error of the mean], normal vs. diabetic, 167 .+-. 11 vs. 470 .+-. 41 mg/100 ml, P < 0.001) and plasma glucagon (183 .+-. 8.5 vs. 370 .+-. 29 pg/ml, P < 0.001) were higher in diabetic animals than in controls; serum insulin was not significantly lower (59 .+-. 19 vs. 24 .+-. 12 .mu.U/ml). Serum T4 [thyroxine] (4.1 .+-. 0.53 vs. 0.8 .+-. 0.27 .mu.g/100 ml, P < 0.005) and T3 (77.3 .+-. 3.2 vs. 41.7 .+-. 12.1 ng/100 ml, P < 0.05) were lower in diabetic rats than in controls. Hepatic concentration of non-protein sulfhydryl-groups was also moderately (.apprx. 19%) lower in diabetic rats than in controls (4.62 .+-. 0.11 vs. 3.75 .+-. 0.24 .mu.mol/g, P < 0.02). The maximal binding capacity (MBC) of the binding of [125I]T3 to isolated rat liver nuclei was significantly decreased in the diabetic rats (368 .+-. 37 vs. 232 .+-. 36 f[femto]mol/mg DNA, P < 0.01; mean decrease 38%); the affinity constant Ka) was also lower but not significantly (1.60 .+-. 0.13 vs. 1.25 .+-. 0.13 109 l/M, 0.05 < P < 0.1). Addition of 1 mM dithiothreitol (DTT) enhanced the Ka of nuclear binding o T3 similarly in the controls and the diabetics. However, it did not restore the decreased MBC of nuclear binding of T3 in diabetic rats to the level of the controls. The decrease in MBC of hepatic nuclei for T3 did not correlate (P < 0.05) with the decrease in serum T3, serum T4, or hepatic non-protein sulfydryl-groups. There was, however, a significant negative correlation of MBC of T3 with serum glucose and with plasma glucagon. A regulatory role of glucose metabolism (or glucagon) in modulation of binding of T3 by hepatic nuclear T3 receptors is suggested.