Effects of thyroid hormone on the insulin receptor in rat liver membranes

Although many studies have investigated the complex interrelationships between thyroid hormone levels and insulin-glucose secretion and action, there is at present a lack of information concerning the effects of various thyroid hormone levels on insulin receptors. In the present study, rat liver membranes were prepared from control, hyperthyroid [10 micrograms L-T4 (T4)/100 g BW for 14 days], and hypothyroid rats and the binding characteristics of [125I]iodoinsulin to these membranes were studied. In addition, serum T3, T4, glucose, and insulin levels were measured. The mean (+/- SD) serum T4 and T3 levels were higher (P less than 0.05) in the T4-injected rats than in the hypothyroid rats (T4, 9.1 +/- 0.6 vs. 1.7 +/- 0.4 microgram/dl; T3, 144 +/- 18 vs. 30 +/- 16 ng/dl). When compared to controls, glucose levels were higher in the hypothyroid rats (116 +/- 11 vs. 141 +/- 14 mg/dl; P less than 0.05) and were statistically unaltered in thyrotoxicosis. Plasma insulin levels were increased in hypothyroidism (6.5 +/- 0.8 vs. 10.8 +/- 2.9 microU/ml; P less than 0.05) and decreased in thyrotoxicosis (7.5 +/- 0.5 vs. 28.3 +/- 15 microU/ml; P less than 0.05). Despite these alterations, membrane binding and the derived Scatchard plots were not significantly different in the hyperthyroid or hypothyroid groups. In summary, the present studies indicate that plasma membrane insulin receptor number and affinity are unaltered regardless of the thyroid state of the rat.