Glycogen Formation In Muscle Research Articles

Glyconeogenic and oxidative lactate utilization in skeletal muscle.

In this article we present a synthesis of recent information concerning the fate of lactate in skeletal muscle. This is important since lactate is continuously produced by skeletal muscle at rest and at all levels of exercise. Therefore, the disposal of lactate as an 'intermediary' metabolite is discussed. The two primary fates of lactate in skeletal muscle are (1) oxidation and (2) glycogen synthesis (glyconeogenesis). From recent evidence it seems relatively clear that glycogen formation in muscle is primarily dependent on glucose, although in fast twitch muscles a considerable proportion of lactate can account for muscle glycogen formation, especially immediately after exercise when circulating lactate levels are elevated. Exactly how lactate is converted to glycogen is not known yet, but an extramitochondrial pathway that is divergent from the hepatic gluconeogenic pathway seems likely. Oxidation of lactate is quantitatively the most important means of disposing of lactate, whether in exercising or nonexercising muscle. The lactate gradient between muscle and blood may be an important factor dictating whether lactate is taken up or released by muscle, independent of whether the muscle is active or not. Finally a novel role for epinephrine is considered that may be important for the mitochondrial oxidation of lactate.

Effects of fat on insulin-stimulated carbohydrate metabolism in normal men.

We have examined the onset and duration of the inhibitory effect of an intravenous infusion of lipid/heparin on total body carbohydrate and fat oxidation (by indirect calorimetry) and on glucose disappearance (with 6,6 D2-glucose and gas chromatography-mass spectrometry) in healthy men during euglycemic hyperinsulinemia. Glycogen synthase activity and concentrations of acetyl-CoA, free CoA-SH, citrate, and glucose-6-phosphate were measured in muscle biopsies obtained before and after insulin/lipid and insulin/saline infusions. Lipid increased insulin-inhibited fat oxidation (+40%) and decreased insulin-stimulated carbohydrate oxidation (-63%) within 1 h. These changes were associated with an increase (+489%) in the muscle acetyl-CoA/free CoA-SH ratio. Glucose disappearance did not decrease until 2-4 h later (-55%). This decrease was associated with a decrease in muscle glycogen synthase fractional velocity (-82%). The muscle content of citrate and glucose-6-phosphate did not change. We concluded that, during hyperinsulinemia, lipid promptly replaced carbohydrate as fuel for oxidation in muscle and hours later inhibited glucose uptake, presumably by interfering with muscle glycogen formation.

Direct observation of glycogen synthesis in human muscle with 13C NMR.

On the basis of previous indirect measurements, skeletal muscle has been implicated as the major site of glucose uptake and it has been suggested that muscle glycogen formation is the dominant pathway. However, direct measurements of the rates of glycogen synthesis have not been possible by previous techniques. We have developed 13C NMR methods to measure directly the rate of human muscle glycogen formation from infused, isotopically labeled [1-13C]glucose. We show that under conditions of imposed hyperglycemia and hyperinsulinemia, a majority of the infused glucose was converted to muscle glycogen in a normal man. This directly shows that muscle is the major site of glucose disposal under these conditions, and provides quantitation of the glucose flux to muscle glycogen.

Muscle enzyme activity and insulin sensitivity in type 1 (insulin-dependent) diabetes mellitus.

The mechanisms of insulin insensitivity in diabetes are poorly understood. We have therefore assessed the relationship between glucose disposal during a euglycaemic clamp, muscle glycogen formation, and the activities of insulin regulated enzymes within skeletal muscle in five Type 1 (insulin-dependent) diabetic patients, both on conventional injection therapy (HbA1 11.0 +/- 1.0 (SD) %) and after 6 weeks continuous subcutaneous insulin infusion (HbA1 7.6 +/- 1.4%, p less than 0.01). On both regimens, overnight euglycaemia before the clamp was maintained with an intravenous insulin infusion. The increase in clamp glucose requirements (insulin 0.1 U X kg-1 X h-1) between injection therapy and continuous subcutaneous insulin infusion was significant (6.2 +/- 0.9 (SE) to 7.0 +/- 0.9 mg X kg-1 X min-1, p less than 0.05), but small compared to differences between subjects. Glucose requirement remained lower than in control subjects (10.4 +/- 0.7 mg X kg-1 X min-1, p less than 0.05). The increase in muscle glycogen with the clamp was slightly higher on continuous subcutaneous insulin infusion (9.5 +/- 2.5 mg/g protein) than on injection therapy (8.5 +/- 2.4 mg/g, p less than 0.05), but less than in control subjects (17.9 +/- 2.1 mg/g, p less than 0.05). The expressed activity of glycogen synthase and pyruvate dehydrogenase increased significantly between fasting and the end of the clamps in the patients (p less than 0.001 and less than 0.005), but was not significantly different between the two treatment regimens. Expressed glycogen synthase activity at the end of the clamp was lower on both treatments than in control subjects (p less than 0.05). Both enzyme activities were, however, highly correlated with glucose requirement between patients, (r = 0.89-0.94, p less than 0.05-0.02), and glycogen synthase was similarly correlated in the control subjects (r = 0.84, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic effects of insulin in the European eel, Anguilla anguilla L

Eels were injected intraperitoneally with insulin (100 IU/kg) and during 14 days the effects on different metabolites in blood, liver and muscle were studied. Exogenous insulin had a marked effect on carbohydrate metabolism. The observed changes (hypoglycemia, depletion of hepatic glycogen and an initial increase in muscle glycogen formation) indicate that insulin in eels induces effects on carbohydrate metabolism qualitatively similar to those found in mammals. Insulin also appears to have an important function in the regulation of lipid metabolism in eels, indicated by a rapid decrease in plasma FFA and a somewhat delayed decrease in other blood lipids. Protein metabolism and plasma inorganic ions were only slightly affected after insulin injection.

Action of deoxycorticosterone acetate on glycogen. II. Relationship of muscle glycogen formation to potassium metabolism.

1. Daily administration of DOCA to intact rats resulted in inhibition of muscle glycogen formation, accompanied by an approximately proportional decrease in potassium concentration of both skeletal muscle and blood serum. 2. By forcing an increase in potassium intake of DOCA treated rats, potassium concentrations of both muscle and blood serum were maintained at essentially normal levels, and glycogen production was not significantly inhibited. 3. These data indicated that the DOCA induced inhibition of muscle glycogen formation was secondary to potassium depletion.

The influence in vitro of deoxycorticosterone on glycogen formation in muscle

The influence <i>in vitro</i> of deoxycorticosterone on glycogen formation in muscle