Thyroid hormones regulate hepatic glycogen synthase.

Short term (48 h) in vivo administration of either T3 or T4 was associated with an increase in hepatic glycogen synthase activity in the rat. Administration of 0.25 mg T3/kg BW 48, 24, and 2 h before enzyme preparation increased the total glycogen synthase activity by approximately 50% and increased the percentage of synthase in the I or active form from 19 +/- 1% to 63 +/- 3%. These effects of thyroid hormones on glycogen synthase were rapid (expressed within 24 h) and dose dependent with respect to T3 or T4. Fasting rats for 24 h increased hepatic glycogen synthase levels in control but not thyroid hormone-treated rats. Total glycogen synthase activity of rats administered thyroid hormone and fasted for 24 h remained 1.4-fold greater than that of their fasted euthyroid controls. The percentage of synthase in the I form was 79 +/- 4% for the fasted T3-treated rats compared to 51 +/- 2% for their fasted euthyroid counterparts. Thus, both total hepatic glycogen synthase activity and the percentage of synthase in the I form are increased by short term administration of thyroid hormones in vivo.