Dose-dependent relationships between the effects of epinephrine and glucose on the activity of key enzymes of glycogen metabolism in the rat liver.

The interactions of the effects of i.v. injections of 1.00, 0.50 or 0.25 g kg-1 glucose with those of i.v. administration of 0.5 or 1.0 microgram kg-1 epinephrine applied 10 min after glucose on the activity of liver glycogen phosphorylase and glycogen synthase were studied one minute after the hormone injection in adult male rats which had fasted for 24 h. The administration of 0.5 microgram kg-1 epinephrine failed to influence the activity of either enzyme. After the dose of 1.0 microgram kg-1, however, phosphorylase activity increased, that of synthase decreased. This effect of epinephrine on the activity of both enzymes was counteracted by 1.00 and 0.50 g kg-1 of glucose, that of synthase even by 0.25 g kg-1 of glucose. A stimulatory effect of glucose on synthase a activity was confirmed. A similar effect on total synthase activity is being described both in fed rats and those which had fasted for 24 h. In fed animals a decreased sensitivity of synthase against physiological levels of catecholamines and the paradoxical rebound of the sensitivity after an i.v. glucose load of 1.00 g kg-1 were observed.