Thyroid hormone administration in vivo regulates the activity of hepatic glycogen phosphorylase phosphatase

Hort term (48 h) administration in vivo of either T₄ or T₃, but notthe biologically inactive D-isomer of T₃, was associated with a decrease in basal glycogen phosphorylase a activity and an increase in phosphorylase a phosphatase activity of rat hepatocytes. This influence of thyroid hormones on hepatic phosphorylase a and phosphorylase phosphataseactivities 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%.