Regulation of rat liver glycogen synthesis and activities of glycogen cycle enzymes by glucose and galactose.

Direct regulation of rat liver glycogen metabolism by glucose and galactose was studied using an isolated liver perfusion system. Activation of glycogen synthase and net glycogen synthesis increased linearly when perfusate glucose concentration was increased from 125 to 500 mg/100 ml. Galactose, rapidly taken up by isolated rat liver regardless of circulating glucose concentration, increased these responses to glucose. In the presence of galactose (greater than or equal to 5 mg/100 ml), activation of synthase and glycogen synthesis were 1.5-fold higher at any given glucose concentration. The addition of insulin did not appreciably after synthase activation by glucose and galactose. Phosphorylase activity, low at circulating glucose levels above 125 mg/100 ml, was further decreased as glucose was increased or when galactose was added to the perfusate. Release of glucose into the perfusate in response to aglycemia was increased in the presence of galactose.