Thyroid Hormone Administration in Vivo Regulates the Activity of Hepatic Glycogen Phosphorylase Phosphatase*

Short term (48 h) administration in vivo of either T4 or T4, but not the biologically inactive D-isomer of T3, was associated with a decrease in basal glycogen phosphorylase alpha activity and an increase in phosphorylase alpha phosphatase activity of rat hepatocytes. This influence of thyroid hormones on hepatic phosphorylase alpha and phosphorylase phosphatase activities was shown to be dose dependent. As little as 0.0025 mg T3/kg BW administered in vivo at 48, 24, and 3 h before liver excision increased the phosphatase activity by 20%. The administration of 0.25 mg T3/kg BW on the same schedule increased the phosphatase activity by nearly 100%.