Effect of insulin on the altered thyroid function and adrenergic responsiveness in the diabetic rat

Abstract

We have previously reported the tail skin temperature response to isoproterenol as being significantly attenuated in male rats 4 weeks after the induction of diabetes with streptozotocin. The current study evaluated the time course of this altered adrenergic responsiveness and the role of thyroid hormone and insulin treatment in the tail skin temperature response. In the first study, the tail skin temperature response to isoproterenol was monitored weekly, for 6 weeks. The tail skin temperature response was similar in control and diabetic animals after 1 week of streptozotocin treatment. However, after 2, 4, 5, and 6 weeks of diabetes the tail skin temperature response was significantly reduced. Total T3, T4, and free T4 concentrations were also significantly reduced in these diabetic animals. In a second study, the effects of graded doses of insulin treatment on thyroid hormone levels was assessed. The reduced thyroid hormone concentrations observed in untreated streptozotocin-diabetic rats were restored towards control levels in animals receiving the lowest dose of insulin (1 U/day), whereas higher doses of insulin were required to more closely restore euglycemia and lower glycated hemoglobin. A subsequent study, utilizing a 1-U/day dose of insulin, resulted in a normalization of the tail skin temperature response to isoproterenol, during 8 weeks of treatment in the treated diabetic rat. In a final study utilizing the spontaneous BB diabetic rat maintained on daily insulin treatment, no differences in the tail skin temperature response or thyroid hormone levels were observed. Collectively, these results suggest that treatment with doses of insulin that do not completely normalize blood glucose and glycated hemoglobin nearly restore thyroid status and reduce β-adrenergic responsiveness associated with the diabetic state.Key words: β-adrenergic responsiveness, tail skin temperature, streptozotocin, BB rat, diabetes, insulin.