ABSTRACT
Insulin-like effects of glucagon-like peptide-1(7-36)amide (GLP-1) in rat liver, skeletal muscle and fat, and also the presence of GLP-1 receptors in these extrapancreatic tissues, have been documented. In skeletal muscle and liver, the action of GLP-1 is not associated with an activation of adenylate cyclase, and in cultured murine myocytes and hepatoma cell lines, it was found that GLP-1 provokes the generation of inositolphosphoglycan molecules (IPGs), which are considered second messengers of insulin action. In the present work, we document in isolated normal rat adipocytes and hepatocytes that GLP-1 exerts a rapid decrease of the radiolabelled glycosylphosphatidylinositols (GPIs)--precursors of IPGs--in the same manner as insulin, indicating their hydrolysis and the immediate short-lived generation of IPGs. Thus, IPGs could be mediators in the GLP-1 actions in adipose tissue and liver, as well as in skeletal muscle, through GLP-1 receptors which are, at least functionally, different from that of the pancreatic B-cell.
Subject(s)
Adipocytes/metabolism , Glycosylphosphatidylinositols/metabolism , Liver/metabolism , Peptide Fragments/metabolism , Adipocytes/drug effects , Adipose Tissue/cytology , Adipose Tissue/metabolism , Animals , Cells, Cultured , Glucagon , Glucagon-Like Peptide 1 , Glucagon-Like Peptides , Liver/cytology , Male , Peptide Fragments/pharmacology , Rats , Rats, WistarABSTRACT
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.
Subject(s)
Diglycerides/metabolism , Glucagon/metabolism , Inositol Phosphates/metabolism , Insulin Antagonists/metabolism , Muscle Fibers, Skeletal/metabolism , Peptide Fragments/metabolism , Polysaccharides/metabolism , Protein Precursors/metabolism , Animals , Glucagon/pharmacology , Glucagon-Like Peptide 1 , Glycogen/biosynthesis , Glycogen/metabolism , Glycosylphosphatidylinositols/metabolism , Insulin/metabolism , Insulin/pharmacology , Mice , Mice, Inbred Strains , Muscle Fibers, Skeletal/drug effects , Muscle, Skeletal/cytology , Muscle, Skeletal/metabolism , Peptide Fragments/pharmacology , Protein Binding/physiology , Protein Precursors/pharmacologyABSTRACT
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.
Subject(s)
Glycosylphosphatidylinositols/metabolism , Liver/drug effects , Neurotransmitter Agents/pharmacology , Peptide Fragments/pharmacology , Animals , Cell Line, Transformed , Glucagon , Glucagon-Like Peptide 1 , Glucagon-Like Peptides , Glucose/metabolism , Glycogen/biosynthesis , Glycogen Synthase/metabolism , Humans , Liver/metabolism , RatsABSTRACT
We have found [125I]glucagon-like peptide (GLP)-1(7-36)amide specific binding activity in rat liver and isolated hepatocyte plasma membranes, with an M(r) of approximately 63,000, estimated by cross-linking and SDS-PAGE. The specific binding was time- and membrane protein concentration-dependent, and equally displaced by unlabelled GLP-1(7-36)amide and by GLP-1(1-36)amide, achieving its ID50 at 3 x 10(-9) M of the peptides. GLP-1(7-36)amide did not modify the basal or the glucagon (10(-8) M)-stimulated adenylate cyclase in the hepatocyte plasma membranes. These data, together with our previous findings of a potent glycogenic effect of GLP-1(7-36)amide in isolated rat hepatocytes, led us to postulate that the insulin-like effects of this peptide on glucose liver metabolism could be mediated by a type of receptor probably different from that described for GLP-1 in pancreatic B-cells or, alternatively, by the same receptor which, in this tissue as well as in muscle, uses a different transduction system.
Subject(s)
Glucagon/metabolism , Liver/metabolism , Peptide Fragments/metabolism , Animals , Autoradiography , Binding, Competitive , Cell Membrane/metabolism , Glucagon-Like Peptide 1 , Glucagon-Like Peptides , Insulin/metabolism , Liver/cytology , Peptides/metabolism , Protein Binding , Rats , Rats, WistarABSTRACT
We have found [125I]glucagon-like peptide-1(7-36)-amide-specific binding activity in rat skeletal muscle plasma membranes, with an estimated M(r) of 63,000 by cross-linking and SDS-PAGE. The specific binding was time and membrane protein concentration dependent, and displaceable by unlabeled GLP-1(7-36)-amide with an ID50 of 3 x 10(-9) M of the peptide; GLP-1(1-36)-amide also competed, whereas glucagon and insulin did not. GLP-1(7-36)-amide did not modify the basal adenylate cyclase activity in skeletal muscle plasma membranes. These data, together with our previous finding of a potent glycogenic effect of GLP-1(7-36)-amide in rat soleus muscle, and also in isolated hepatocytes, which was not accompanied by a rise in the cell cyclic AMP content, lead use to believe that the insulin-like effects of this peptide on glucose metabolism in the muscle could be mediated by a type of receptor somehow different to that described for GLP-1 in pancreatic B cells, where GLP-1 action is mediated by the cyclic AMP-adenylate cyclase system.
Subject(s)
Glucagon/metabolism , Muscle, Skeletal/metabolism , Peptide Fragments/metabolism , Protein Precursors/metabolism , Receptors, Glucagon/metabolism , Adenylyl Cyclases/metabolism , Animals , Binding, Competitive , Cell Membrane , Glucagon-Like Peptide 1 , Glucagon-Like Peptide-1 Receptor , Glucagon-Like Peptides , Insulin/metabolism , Rats , Rats, Wistar , Receptor, Insulin/drug effects , Receptor, Insulin/metabolism , Receptors, Glucagon/isolation & purificationABSTRACT
Specific binding of [125I]glucagon-like peptide-1(7-36)amide ([125I]GLP-1(7-36)amide) to solubilized rat adipose tissue membranes was found to be dependent on temperature, time, and membrane protein concentration and readily dissociated. GLP-1(1-36)amide, GLP-2, or glucagon (10(-6) M) did not compete with [125I]GLP-1(7-36)amide binding. Half-maximal binding was achieved with 8 x 10(-10) M unlabeled GLP-1(7-36)amide, and the Scatchard plot revealed the presence of high and low affinity binding sites with Kd values of approximately 0.6 and 20 nM, respectively. The binding capacity of [125I]GLP-1(7-36)amide was about 3 times higher than that of [125I]glucagon, while the high affinity Kd and the half-maximal binding of the two peptides were similar. The presence and abundance of GLP-1(7-36)amide receptors in fat tissue together with the previous findings that the peptide stimulates glycerol and cAMP production in rat adipocytes and stimulates fatty acid synthesis in explants of rat adipose tissue open the possibility that this insulinotropic intestinal peptide may also be involved in the regulation of lipid metabolism in health and disease.
Subject(s)
Adipose Tissue/metabolism , Peptide Fragments/metabolism , Receptors, Glucagon , Animals , Binding, Competitive , Cell Membrane/metabolism , Glucagon , Glucagon-Like Peptide 1 , Glucagon-Like Peptide-1 Receptor , Glucagon-Like Peptides , Kinetics , Male , Rats , Rats, Wistar , Receptors, Cell Surface/metabolism , Solubility , TemperatureABSTRACT
Currently, in certain clinical situations there is an increasing trend towards using dosage regimens involving aminoglycoside antibiotics based on the administration of a single dose of the drug per day instead of administering the same amount in two or three administrations. The aim of the present study was to discover the pharmacokinetic profile and the nephrotoxic potential of this new form of administration in experimental animals receiving gentamicin. The study was conducted on two groups of rabbits, one of which received a single dose of the drug at 7 mg/kg i.v. and the other 7 mg/kg administered every 12 hours, allometrically equivalent to gentamicin dosing at 5 mg/kg every 24 hours to human subjects. The number of doses administered was 20. From the pharmacokinetic point of view, the results point to the existence of a significant degree of accumulation of the antibiotic in renal cortex as a result of the dosage regimen, no important modifications occurring in the pharmacokinetic parameters of gentamicin calculated from its plasma kinetics. This shows that the two compartment model employed predicts drug levels in accessible tissues but not in deep ones where gentamicin is accumulated for long periods of time. From the toxicological point of view, the treatment caused appreciable damage of the renal tubules during the first phases of the treatment which was not detectable from the serum creatinine levels or the kinetic behaviour of the aminoglycoside.
