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Possible inhibitory effects of insulin on epinephrine-induced changes in the enzymes of glycogen metabolism in skeletal muscle were tested using a perfused rat hindlimb preparation. Epinephrine and/or insulin were infused over a wide range of concentrations. Insulin at 6 X 10(-9) M increased the activity ratio (--Glc-6-P/+Glc-6-P) of glycogen synthase from a basal value of 0.09 +/- 0.01 to 0.13 +/- 0.01 and caused a 23% decrease in the Ka for Glc-6-P. In contrast, epinephrine at 10(-7) M decreased the activity ratio to 0.05 +/- 0.01 and increased the Ka for Glc-6-P 6.3-fold. Insulin was without effect on the concentration of cAMP or the activity ratio (-cAMP/+cAMP) of cAMP-dependent protein kinase and caused a small decrease in the activity ratio (-AMP/+AMP) of phosphorylase, whereas epinephrine caused large increases in all these parameters. Insulin at 6 X 10(-11) to 6 X 10(-8) M had no inhibitory effect on the actions of 10(-8) or 10(-7) M epinephrine on glycogen synthase, phosphorylase or cAMP-dependent protein kinase at 30 min or at earlier times. Insulin (6 X 10(-9) M) also did not alter th concentration of cAMP in the presence of 10(-8) or 10(-7) M epinephrine. These data are not consistent with the view that insulin activates glycogen synthase by producing an inhibitor of cAMP-dependent protein kinase. Nor do they support the hypothesis that insulin acts by decreasing the activity of an inhibitor of a multisubstrate phosphoprotein phosphatase.

The effects of hypothyroidism on glycogen metabolism in rat skeletal muscle were studied using the perfused rat hindlimb preparation. Three weeks after propylthiouracil treatment, serum thyroxine was undetectable and muscle glycogen and Glc-6-P were decreased. Basal and epinephrine-stimulated phosphorylase a and phosphorylase b kinase activities were also significantly reduced, as were epinephrine-stimulated cAMP accumulation and cAMP-dependent protein kinase activity. Conversely, basal and epinephrine-stimulated glycogen synthase I activities were significantly higher while the Ka of the enzyme for Glc-6-P was lower in hypothyroid animals. Propylthiouracil-treated rats also had increased phosphoprotein phosphatase activities towards phosphorylase and glycogen synthase and decreased activity of phosphatase inhibitor 1. beta-Adrenergic receptor binding and basal and epinephrine-stimulated adenylate cyclase activities were reduced in muscle particulate fractions from hypothyroid rats. Administration of triiodothyronine to rats for 3 days after 3 weeks of propylthiouracil treatment restored the altered metabolic parameters to normal. It is proposed that the decreased beta-adrenergic responsiveness of the enzymes of glycogen metabolism in hypothyroid rat skeletal muscle is due to increased activity of phosphoprotein phosphatases and to reduced beta-adrenergic receptors and adenylate cyclase activity.

Hepatic glycogen metabolism was studied in rats during the period of transition from the fed to fasted states. Glycogenic activity was measured in vivo based on the incorporation of [14C]glucose into liver glycogen. Its changes were almost parallel to the changes in glycogen synthase activity. Progressive accumulation of liver glycogen that occurred in the fed state was associated with a proportional increase in glycogenic activity. Within 4 h after the cessation of food intake, glycogenic activity showed a precipitous fall from the peak to its nadir without significant changes in glycogen content. Meanwhile, the glucose concentration in the portal vein decreased. Upon further development of fasting, glycogenic activity displayed a progressive regain, reciprocally as glycogen contents gradually decreased. The precipitous fall of glycogenic activity during the transition from the fed to fasted states was associated with a transient increase in plasma glucagon, and was partly overcome by the injection of anti-glucagon serum. It is concluded that the fall of portal venous concentration of glucose and secretion of glucagon act as a signal to initiate liver glycogen metabolism characteristic of the fasted or postabsorptive state.

Using the perfused rat hindlimb preparation, the role of insulin in the regulation of glycogen metabolism in voluntary skeletal muscle has been characterized. A maximally effective concentration of insulin (1 mU/ml) caused a threefold increase in glucose clearance by 5 min. However, the -glucose-6-P/+glucose-6-P activity ratio of glycogen synthase was not significantly increased before 20 min. Insulin concentrations as low as 0.1 mU/ml significantly modified the glycogen synthase activity ratio and the half-maximal activation constant (A0.5) for glucose-6-P at 30 min, but had no effect on tissue cAMP. These changes were not dependent on the presence of glucose and were not modified by fasting. These results indicate that high physiological concentrations of insulin activate glycogen synthase in voluntary skeletal muscle and that this effect is independent of changes in glucose uptake or tissue cyclic AMP.

We studied a 3 year old girl with mental retardation and limb muscle weakness. The muscle glycogen content was 17.4 mg/g tissue, which was approximately three times higher than normal. There were no other known abnormalities noted in this child that could explain the cause of glycogen storage disease. Our in vitro glycolysis study showed marked increase of pyruvate, but no increase in lactate levels. The observed results suggested to us that an abnormal lactate dehydrogenase might account for the abnormal accumulation of glycogen in the muscle.