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.