Effects of glucagon-like peptide 1 on the kinetics of glycogen synthase a in hepatocytes from normal and diabetic rats.

Glucagon-like peptide 1(7-36)amide (GLP-1) is currently under investigation as a possible tool in the treatment of non-insulin-dependent diabetes mellitus. In addition to enhancing nutrient-stimulated insulin release, the peptide also favors glycogen synthesis and glucose use in liver, muscle, and adipose tissue. GLP-1 also activates glycogen synthase a in hepatocytes from both normal and diabetic rats. In the present study, the kinetic aspects of such an activation were investigated in hepatocytes from normal rats and from animals rendered diabetic induced by injection of streptozotocin, either in the adult age (insulin-dependent diabetes mellitus model) or in days 1 or 5 after birth (non-insulin-dependent diabetes mellitus models). GLP-1 increased, in a dose-dependent manner, glycogen synthase a activity in the hepatocytes from all groups studied. The activation of the enzyme reached a steady state within 1 min exposure to GLP-1, which, at 10(-12) M, caused a half-maximal activation. When comparing fed vs. overnight-starved normal rats, a somewhat lower basal activity of glycogen synthase a in fasted animals (P < 0.05) coincided with a greater relative increment in reaction velocity in response to GLP-1. The basal activity of glycogen synthase a and the relative extent of its inhibition by glucagon or activation by insulin and GLP-1 were modulated by the extracellular concentration of D-glucose. The activation of glycogen synthase a by either insulin or GLP-1 resulted not solely in an increase in maximal velocity but also in a decrease in affinity of the enzyme for uridine diphosphate-glucose; in diabetic animals, the capacity of insulin or GLP-1 to increase the maximal velocity and Michaelis-Menten constant were less marked than in normal rats. In conclusion, this study indicates that the GLP-1-induced activation of glycogen synthase a displays attributes of rapidity, sensitivity, and nutritional dependency that are well suited for both participation in the physiological regulation of enzyme activity and therapeutic purpose.

Preserved GLP-I effects on glycogen synthase a activity and glucose metabolism in isolated hepatocytes and skeletal muscle from diabetic rats.

To search if biological effects of GLP-I on glucose metabolism in extrapancreatic tissue are present in diabetic states, we have studied the action of GLP-I and insulin on glycogen-enzyme activity, glycogen synthesis, and glucose metabolism in isolated hepatocytes and soleus muscle from adult streptozotocin (STZ)- and neonatal STZ-treated diabetic rats. This work confirms the previously reported insulin-like effects of GLP-I on glucose metabolism in both muscle and liver tissue from normal rats (control). The present study extends those observations to the muscle and liver tissue of diabetic animals. In both muscle and liver tissue, the metabolism of D-glucose, in the absence of added peptides, was more severely affected in adult STZ (IDDM model) than in neonatal STZ (nSTZ; NIDDM model) rats, and the magnitude of hormonal effect on metabolic variables was lower in diabetic rats than in control rats, as a rule. Nevertheless, in liver and muscle tissue of diabetic rats, GLP-I was able to increase glycogen synthase activity, augment the net rate of D-[U-14C]glucose incorporation into glycogen, and increase D-[5-3H]glucose utilization, D-[U-14C]glucose oxidation, and lactate production. In conclusion, GLP-I exerts insulin-like effects on D-glucose metabolism in both muscle and liver tissue in IDDM or NIDDM animal models, and present observations reinforce the view that GLP-I may represent a most promising tool in the treatment of diabetic patients.

Exendin-4 agonist and exendin(9-39)amide antagonist of the GLP-1(7-36)amide effects in liver and muscle.

The GLP-1 structurally related peptides exendin-4 and exendin(9-39)amide were found to act, in rat liver and skeletal muscle, as agonist and antagonist, respectively, of the GLP-1(7-36)amide effects on glucose metabolism. Thus, like GLP-1(7-36)amide, exendin-4 increased glycogen synthase a activity and glucose incorporation into glycogen in both tissues and also stimulated exogenous D-glucose utilization and oxidation in muscle. These effects of GLP-1(7-36)amide and exendin-4 were inhibited by exendin(9-39)amide. Our findings provide further support to the proposed use of GLP-1, or exendin-4, as a tool in the treatment of diabetes mellitus. Thus, in addition to the well-known insulinotropic action of the peptides, they act both in liver and in muscle in a manner most suitable for restoration of glucose homeostasis, with emphasis on their positive effects upon glycogen synthesis in the two tissues and on the stimulation of exogenous glucose catabolism in muscle.

Inositolphosphoglycans and diacyglycerol are possible mediators in the glycogenic effect of GLP-1(7-36)amide in BC3H-1 myocytes.

A potent glycogenic effect of GLP-1(7-36)amide has been found in rat hepatocytes and skeletal muscle, and specific receptors for this peptide, which do not seem to be associated with the adenylate cyclase-cAMP system, have been detected in these tissue membranes. On the other hand, inositolphosphoglycan molecules (IPGs) have been implicated as second messengers of the action of insulin. In this work, we have found, in differentiated BC3H-1 myocytes, specific binding of [125I]GLP-1(7-36)amide, and a stimulatory effect of the peptide on glycogen synthesis, confirming the findings in rat skeletal muscle. Also, GLP-1(7-36)amide modulates the cell content of radiolabelled glycosylphosphatidylinositols (GPIs) and increases the production of diacylglycerol (DAG), in the same manner as insulin acts, indicating hydrolysis of GPIs and an immediate and short-lived generation of IPGs. Thus, IPGs and DAG could be mediators in the glycogenic action of GLP-1(7-36)amide in skeletal muscle.

Inositolphosphoglycans are possible mediators of the glucagon-like peptide 1 (7-36)amide action in the liver.

A potent glycogenic effect for GLP-1(7-36)amide has been found in rat hepatocytes and skeletal muscle, and the specific receptors detected for GLP-1(7-36)amide in these tissue membranes do not seem to be associated to adenylate cyclase. On the other hand, inositolphosphoglycan molecules (IPGs) have been implicated as second messengers in the action of insulin. In a human hepatoma cell line (HEP G-2), we have observed the presence of [125I]GLP-1(7-36)amide specific binding, and a stimulatory effect of the peptide upon glycogen synthesis, confirming the findings in isolated rat hepatocytes. Also, GLP-1(7-36)amide modulates the cell content of radiolabelled glycosylphosphatidylinositols (GPIs), in the same manner as insulin, indicating hydrolysis of GPIs and an immediate and short-lived generation of IPGs. Thus, IPGs could be mediators in the GLP-1(7-36)amide glycogenic action in the liver.

Glucagon-like peptide 1: a potent glycogenic hormone.

GLP-1(7-36)amide is an insulinotropic peptide derived from the intestinal post-translational proglucagon process, the release of which is increased mainly after a carbohydrate meal; also, its anti-diabetogenic effect in normal and diabetic states has been reported. In this study, GLP-1(7-36)amide stimulates the formation of glycogen from glucose in isolated rat hepatocytes, such a glycogenic effect being achieved with physiological concentrations of the peptide. The GLP-1(7-36)amide-induced glycogenesis is abolished by glucagon, and it is accompanied by stimulation of the glycogen synthase alpha activity and by a decrease in the basal and glucagon-stimulated cyclic AMP content. These findings could explain, at least in part, the GLP-1(7-36)amide insulin-independent plasma glucose lowering effect.