Mónica Morales

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 a 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.

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...

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.

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 (27–36)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.

Possible participation of an islet B-cell calcium-sensing receptor in insulin release.

The calcium-sensing receptor gene was recently shown to be expressed in rat pancreatic islets and purified islet B-cells. In this study, we investigated the possible role of this receptor in the regulation of insulin release from isolated rat pancreatic islets. Poly-l-arginine (0.2–0.3 μM) and poly-l-lysine (0.03–0.1 μM) increased insulin output evoked by d-glucose (8.3 mM). This positive effect faded out at higher concentrations of the basic peptides. Likewise, the release of insulin evoked by 8.3 mMd-glucose was significantly lower at high (1.0 mM) than low (0.05–0.1 mM) concentrations of neomycin. The insulinotropic action of Ba2+ in Ca2+-deprived islets was potentiated in rats pretreated with pertussis toxin. However, Gd3+ inhibited insulin release evoked by dd-glucose in islets prepared from normal rats or animals pretreated with pertussis toxin and incubated in the absence or presence of either theophylline or forskolin. Gd3+ (0.3 mM) failed to affect effluent radioactivity from islets prelabeled with myo-[2-3H]inositol and cyclic AMP net production in islets incubated in the absence or presence of forskolin. Gd3+ decreased, however, 45Ca efflux from prelabeled islets perifused in the absence or presence of extracellular Ca2+. It is speculated that a negative insulinotropic action mediated by the calcium-sensing receptor, and possibly attributable to a fall in cytosolic Ca2+ concentration, may prevent excessive insulin secretion in pathological situations of hypercalcemia.