Effect of Thyroid Hormones on the Activity of Hepatic Glucose-6-Phosphatase in Fed and Fasted Rats

Summary

The action of thyroid hormones on hepatic glucose-6-phos-phatase was studied in rats. Fed and 24-h fasted rats received T₃ (10 ug/day) or T₄ (25 ug/day) 1 h, 1 or 3 days before sacrificing. In addition a group of fed rats was treated with T₄ for 7 and 14 days. The glucose-6-phosphatase activity was measured in the isolated microsomes prepared from the liver. The intactness of the microsomal preparation was checked using 2 mM mannose-6-phosphate as a substrate.

In fed rats a single injection of T₃ or T₄ augmented the activities of the translocase and hydrolase components of glucose-6-phosphatase provided that the rats were killed 24 h after the administration of hormone. This effect was more pronounced in animals treated for 3-14 days. As expected, fasting per se increased the activities of both components of the enzyme. Moreover, in fasted rats treatment with T₃ and T₄ for 3 days further augmented the activities of the translocase and the hydrolase components of glucose-6-phosphatase. In fed animals T₃ and T₄ increased the latency of the enzyme whereas in fasted animals thyroid hormones increased the activities of the trans-locase and hydrolase components in parallel, maintaining the level of latency of the enzyme system. Administration of T₃ and T₄ increased blood glucose level in fasted rats after one day, while in fed rats a significant hyperglycaemia appeared after 7-14 days of treatment. In conclusion, T₃ and T₄ increase the activities of the translocase and hydrolase components of hepatic glucose-6-phosphatase in fed and fasted rats. This in turn may mediate the enhanced flow of glucose through the glucose cycle (glucose- > glucose-6-P- > glucose.

Key-Words

Fasting - Glucose-6-Phosphate Translocase - Glucose-6-Phsophate Hydrolase - Glucose Cycle