Hypothalamic glucagon signaling inhibits hepatic glucose production.

Glucagon activates hepatic protein kinase A (PKA) to increase glucose production, but the gluco-stimulatory effect is transient even in the presence of continuous intravenous glucagon infusion. Continuous intravenous infusion of insulin, however, inhibits glucose production through its sustained actions in both the liver and the mediobasal hypothalamus (MBH). In a pancreatic clamp setting, MBH infusion with glucagon activated MBH PKA and inhibited hepatic glucose production (HGP) in rats, as did central glucagon infusion in mice. Inhibition of glucagon receptor-PKA signaling in the MBH and hepatic vagotomy each negated the effect of MBH glucagon in rats, whereas the central effect of glucagon was diminished in glucagon receptor knockout mice. A sustained rise in plasma glucagon concentrations transiently increased HGP, and this transiency was abolished in rats with negated MBH glucagon action. In a nonclamp setting, MBH glucagon infusion improved glucose tolerance, and inhibition of glucagon receptor-PKA signaling in the MBH enhanced the ability of intravenous glucagon injection to increase plasma glucose concentrations. We also detected a similar enhancement of glucose concentrations that was associated with a disruption in MBH glucagon signaling in rats fed a high-fat diet. We show that hypothalamic glucagon signaling inhibits HGP and suggest that hypothalamic glucagon resistance contributes to hyperglycemia in diabetes and obesity.

Choline supplementation promotes hepatic insulin resistance in phosphatidylethanolamine N-methyltransferase-deficient mice via increased glucagon action.

Biosynthesis of hepatic choline via phosphatidylethanolamine N-methyltransferase (PEMT) plays an important role in the development of type 2 diabetes and obesity. We investigated the mechanism(s) by which choline modulates insulin sensitivity. PEMT wild-type (Pemt(+/+)) and knock-out (Pemt(-/-)) mice received either a high fat diet (HF; 60% kcal of fat) or a high fat, high choline diet (HFHC; 4 g of choline/kg of HF diet) for 1 week. Hepatic insulin signaling and glucose and lipid homeostasis were investigated. Glucose and insulin intolerance occurred in Pemt(-/-) mice fed the HFHC diet, but not in their Pemt(-/-) littermates fed the HF diet. Plasma glucagon was elevated in Pemt(-/-) mice fed the HFHC diet compared with Pemt(-/-) mice fed the HF diet, concomitant with increased hepatic expression of glucagon receptor, phosphorylated AMP-activated protein kinase (AMPK), and phosphorylated insulin receptor substrate 1 at serine 307 (IRS1-s307). Gluconeogenesis and mitochondrial oxidative stress were markedly enhanced, whereas glucose oxidation and triacylglycerol biosynthesis were diminished in Pemt(-/-) mice fed the HFHC diet. A glucagon receptor antagonist (2-aminobenzimidazole) attenuated choline-induced hyperglycemia and insulin intolerance and blunted up-regulation of phosphorylated AMPK and IRS1-s307. Choline induces glucose and insulin intolerance in Pemt(-/-) mice through modulating plasma glucagon and its action in liver.

The expression of GLP-1 receptor mRNA and protein allows the effect of GLP-1 on glucose metabolism in the human hypothalamus and brainstem.

In the present work, several experimental approaches were used to determine the presence of the glucagon-like peptide-1 receptor (GLP-1R) and the biological actions of its ligand in the human brain. In situ hybridization histochemistry revealed specific labelling for GLP-1 receptor mRNA in several brain areas. In addition, GLP-1R, glucose transporter isoform (GLUT-2) and glucokinase (GK) mRNAs were identified in the same cells, especially in areas of the hypothalamus involved in feeding behaviour. GLP-1R gene expression in the human brain gave rise to a protein of 56 kDa as determined by affinity cross-linking assays. Specific binding of 125I-GLP-1(7-36) amide to the GLP-1R was detected in several brain areas and was inhibited by unlabelled GLP-1(7-36) amide, exendin-4 and exendin (9-39). A further aim of this work was to evaluate cerebral-glucose metabolism in control subjects by positron emission tomography (PET), using 2-[F-18] deoxy-D-glucose (FDG). Statistical analysis of the PET studies revealed that the administration of GLP-1(7-36) amide significantly reduced (p < 0.001) cerebral glucose metabolism in hypothalamus and brainstem. Because FDG-6-phosphate is not a substrate for subsequent metabolic reactions, the lower activity observed in these areas after peptide administration may be due to reduction of the glucose transport and/or glucose phosphorylation, which should modulate the glucose sensing process in the GLUT-2- and GK-containing cells.

Glucagon-like peptide 1 excites hypocretin/orexin neurons by direct and indirect mechanisms: implications for viscera-mediated arousal.

Glucagon-like peptide 1 (GLP-1) is produced by neurons in the caudal brainstem that receive sensory information from the gut and project to several hypothalamic regions involved in arousal, interoceptive stress, and energy homeostasis. GLP-1 axons and receptors have been detected in the lateral hypothalamus, where hypocretin neurons are found. The electrophysiological actions of GLP-1 in the CNS have not been studied. Here, we explored the GLP-1 effects on GFP (green fluorescent protein)-expressing hypocretin neurons in mouse hypothalamic slices. GLP-1 receptor agonists depolarized hypocretin neurons and increased their spike frequency; the antagonist exendin (9-39) blocked this depolarization. Direct GLP-1 agonist actions on membrane potential were abolished by choline substitution for extracellular Na+, and dependent on intracellular GDP, suggesting that they were mediated by sodium-dependent conductances in a G-protein-dependent manner. In voltage clamp, the GLP-1 agonist Exn4 (exendin-4) induced an inward current that reversed near -28 mV and persisted in nominally Ca2+-free extracellular solution, consistent with a nonselective cationic conductance. GLP-1 decreased afterhyperpolarization currents. GLP-1 agonists enhanced the frequency of miniature and spontaneous EPSCs with no effect on their amplitude, suggesting presynaptic modulation of glutamate axons innervating hypocretin neurons. Paraventricular hypothalamic neurons were also directly excited by GLP-1 agonists. In contrast, GLP-1 agonists had no detectable effect on neurons that synthesize melanin-concentrating hormone (MCH). Together, our results show that GLP-1 agonists modulate the activity of hypocretin, but not MCH, neurons in the lateral hypothalamus, suggesting a role for GLP-1 in the excitation of the hypothalamic arousal system possibly initiated by activation by viscera sensory input.

Treatment of type 2 diabetes mellitus with agonists of the GLP-1 receptor or DPP-IV inhibitors.

Glucagon-like peptide-1 (GLP-1) is a peptide hormone from the gut that stimulates insulin secretion and protects beta-cells, inhibits glucagon secretion and gastric emptying, and reduces appetite and food intake. In agreement with these actions, it has been shown to be highly effective in the treatment of Type 2 diabetes, causing marked improvements in glycaemic profile, insulin sensitivity and beta-cell performance, as well as weight reduction. The hormone is metabolised rapidly by the enzyme dipeptidyl peptidase IV (DPP-IV) and, therefore, cannot be easily used clinically. Instead, resistant analogues of the hormone (or agonists of the GLP-1 receptor) are in development, along with DPP-IV inhibitors, which have been demonstrated to protect the endogenous hormone and enhance its activity. Agonists include both albumin-bound analogues of GLP-1 and exendin-4, a lizard peptide. Clinical studies with exendin have been carried out for > 6 months and have indicated efficacy in patients inadequately treated with oral antidiabetic agents. Orally active DPP-IV inhibitors, suitable for once-daily administration, have demonstrated similar efficacy. Diabetes therapy, based on GLP-1 receptor activation, therefore, appears very promising.

The hypothalamus is involved in the anorexic effect of glucagon-like peptide-1 in chicks.

The present study was carried out to investigate whether the hypothalamus is involved in the anorexic effect of glucagon-like peptide-1 (GLP-1) in chicks. To examine this, Fos expression in the chick hypothalamus were immunohistochemically detected after intracerebroventricular (ICV) injection of 30-pmol GLP-1. ICV injection of GLP-1 stimulated the expression of Fos-like immunoreactive (FLI) cells in the ventromedial hypothalamic nucleus (VMN). When 15-pmol GLP-1 was directly injected into the chick VMN, the chick's food intake was significantly decreased compared with the control treatment. Microinjection of GLP-1 into the (LHA) also inhibited feeding in chicks, although ICV injection of GLP-1 did not stimulate FLI expression in the brain area. These results suggest that VMN and some brain regions are involved in the anorexic effect of GLP-1 in chicks.

Glucagon-like peptide 2 decreases mortality and reduces the severity of indomethacin-induced murine enteritis.

Glucagon-like peptides (GLPs) are secreted from enteroendocrine cells in the gastrointestinal tract. GLP-1 actions regulate blood glucose, whereas GLP-2 exerts trophic effects on intestinal mucosal epithelium. Although GLP-1 actions are preserved in diseases such as diabetes, GLP-2 action has not been extensively studied in the setting of intestinal disease. We have now evaluated the biological effects of a human GLP-2 analog in the setting of experimental murine nonsteroidal antiinflammatory drug-induced enteritis. Human (h)[Gly(2)]GLP-2 significantly improved survival whether administered before, concomitant with, or after indomethacin. h[Gly(2)]GLP-2-treated mice exhibited reduced histological evidence of disease activity, fewer intestinal ulcerations, and decreased myeloperoxidase activity in the small bowel (P < 0.05, h[Gly(2)]GLP-2- vs. saline-treated controls). h[Gly(2)]GLP-2 significantly reduced cytokine induction, bacteremia, and the percentage of positive splenic and hepatic bacterial cultures (P < 0.05). h[Gly(2)]GLP-2 enhanced epithelial proliferation (P < 0.05 for increased crypt cell proliferation in h[Gly(2)]GLP-2- vs. saline-treated mice after indomethacin) and reduced apoptosis in the crypt compartment (P < 0.02). These observations demonstrate that a human GLP-2 analog exerts multiple complementary actions that serve to preserve the integrity of the mucosal epithelium in experimental gastrointestinal injury in vivo.

Is there appetite after GLP-1 and PACAP?

Anitobesity drugs must increase the sensitivity of the hypothalamic satiety center towards leptin and antagonize the synthesis and action of NPY. The array of pharmacologic tools available is vast and presently ineffective. Among peptide analogs considered for evaluation [NPY-5 antagonists and CCK-A, bombesin, amylin and melanocyte-stimulating hormone-4 (or melanin-concentrating hormone?) agonists], is there a place for GLP-1 and PACAP? GLP-1 receptors present in ARC, PVN, VMN, and SON are the target for both central and blood-borne GLP-1 in those hypothalamic neurons endowed with GLUT-2 and glucokinase. GLP-1, hypersecreted by L-cells after a meal, is a potent insulinotropic agent and, together with glucose, reduces food intake and induces c-fos in the ARC. PACAP is present in the ARC, PVN, and SCH, and its hypothalamic type I receptor elevates cAMP and inositol triphosphate in the PVN, where it may perhaps antagonize NPY-induced food intake and hyperinsulinemia. However, irrelevant neuroendocrine, autonomic, and circadian functions are also activated by this peptide, making it a less than ideal base on which to build an obesity treatment.

Regulation and function of insulin-like growth factor-binding protein-1.

Insulin-like growth factor-binding protein (IGFBP)-1 is one of six homologous proteins that specifically bind and modulate the mitogenic and metabolic actions of insulin-like growth factor (IGF)-I and IGF-II. Of the six IGFBP, IGFBP-1 is the only one that displays rapid dynamic regulation in vivo, with serum levels varying 10-fold or more in relation to meals. The complementary cDNA for IGFBP-1 was first reported in 1988. The predicted 234-amino acid sequence has a molecular mass of 25.3 kDa. The N-terminal and C-terminal regions are highly homologous among rat, human, and bovine sequences, and contain 18 conserved cysteines which are postulated to provide a framework for ligand binding. The 65-residue midregion is less homologous and does not contain cysteines, but does include a Pro-Glu-Ser-Thr (PEST) domain that is typical of rapidly metabolized proteins. The gene for IGFBP-1 has been localized to human chromosome region 7p12-p14, where it is contiguous with the gene for IGFBP-3. IGFBP-1 mRNA and protein expression have been identified in human liver and uterine decidua, and in nonhuman kidney. In vitro and in vivo studies indicate that insulin is the primary regulator of IGFBP-1 expression in these tissues, and that the primary effect of insulin is rapid inhibition of transcription. On the other hand, cortisol, glucagon, and cAMP stimulate IGFBP-1 production. Limited data also show a potent stimulatory effect of phorbol esters. A detailed review of IGFBP-1 levels and physiology in vivo and in vitro is presented. The function of IGFBP-1 is not completely defined. However, several studies demonstrate that IGFBP-1 inhibits IGF binding to cell surface receptors and thereby inhibits IGF-mediated mitogenic and cell metabolic actions. Furthermore, IGFBP-1 regulation by insulin and glucoregulatory hormones in vitro and limited in vivo data are consistent with a role for IGFBP-1 in glucose counterregulation.

Testing a mechanism of control in human cholesterol metabolism: relation of arginine and glycine to insulin and glucagon.

Eight men were given 2 casein meals, one with and one without a supplement of arginine and glycine, to measure the effect on plasma amino acids, insulin and glucagon. Supplementation resulted in increased levels of plasma glucagon, glycine and arginine, a tendency to decreased insulin and significantly lower insulin/glucagon ratio, tryptophan and tyrosine. The data suggest that insulin and glucagon, which control cholesterol metabolism, respond to dietary and postprandial plasma amino acid levels of arginine and glycine.

Hormonal control of substrate cycling in humans.

Recent studies have established the existence of substrate cycles in humans, but factors regulating the rate of cycling have not been identified. We have therefore investigated the acute response of glucose/glucose-6P-glucose (glucose) and triglyceride/fatty acid (TG/FA) substrate cycling to the infusion of epinephrine (0.03 microgram/kg.min) and glucagon. The response to a high dose glucagon infusion (2 micrograms/kg.min) was tested, as well as the response to a low dose infusion (5 ng/kg.min), with and without the simultaneous infusion of somatostatin (0.1 microgram/kg.min) and insulin (0.1 mU/kg.min). Additionally, the response to chronic prednisone (50 mg/d) was evaluated, both alone and during glucagon (low dose) and epinephrine infusion. Finally, the response to hyperglycemia, with insulin and glucagon held constant by somatostatin infusion and constant replacement of glucagon and insulin at basal rates, was investigated. Glucose cycling was calculated as the difference between the rate of appearance (Ra) of glucose as determined using 2-d1- and 6,6-d2-glucose as tracers. TG/FA cycling was calculated by first determining the Ra glycerol with d5-glycerol and the Ra FFA with [1-13C]palmitate, then subtracting Ra FFA from three times Ra glycerol. The results indicate that glucagon stimulates glucose cycling, and this stimulatory effect is augmented when the insulin response to glucagon infusion is blocked. Glucagon had minimal effect on TG/FA cycling. In contrast, epinephrine stimulated TG/FA cycling, but affected glucose cycling minimally. Prednisone had no direct effect on either glucose or TG/FA cycling, but blunted the stimulatory effect of glucagon on glucose cycling. Hyperglycemia, per se, had no direct effect on glucose or TG/FA cycling. Calculations revealed that stimulation of TG/FA cycling theoretically amplified the sensitivity of control of fatty acid flux, but no such amplification was evident as a result of the stimulation of glucose cycling by glucagon.

Complications of insulin-dependent diabetes mellitus: management of insulin reactions and acute illness.

Occasional mild hypoglycemia is an unavoidable and usually acceptable side effect of intensive insulin therapy. Patients with insulin-dependent diabetes mellitus may have impaired glucose counterregulation, which may increase the risk of hypoglycemia and justify less ambitious glycemic goals. A conservative but flexible approach to the treatment of insulin reactions is appropriate in order to avoid hyperglycemia. Insulin requirements are often increased during acute illness, and frequent self-monitoring of blood glucose concentrations is necessary to determine the need for supplementation with regular insulin. Frequent supplementation, together with modification of diet and maintenance of fluid intake, should not only minimize the need for hospitalization but also prevent severe deterioration in glycemic control.

A mechanism by which primary or secondary hypothalamic involvement results in the development of insulin-dependent diabetes mellitus (IDDM).

A literature survey and hypothesis is presented in which it is concluded that an intracellular ventromedial hypothalamic (VMH) glucopenia results in a bibrachial response consisting of adenohypophysial release of growth hormone and ACTH as well as sympathetic discharge, both of which act to elevate plasma glucose and remove the VMH glucopenia. This glucopenia may occur as a result of either a deficiency of circulating insulin or alterations in the kinetic properties of the VMH cellular insulin receptor. Two mechanisms for the development of insulin dependent diabetes mellitus (IDDM) are presented: (1) a defect in VMH glucose transport and/or metabolism such that a VMH glucopenia occurs with a subsequent bibrachial response. The result of this is glucose overproduction and a chronic excess glucose stimulus will eventually cause B-cell exhaustion (primary hypothalamic involvement). (2) reduction of the B-cell population by chemical, genetic and/or viral interactions with a consequential insulopenia results in a VMH glucopenia (secondary to a reduced glucose transport into the VMH cells) and causes a bibrachial response. This VMH response may temporarily restore plasma glucose balance but a chronically enhanced counter-regulatory response to maintain this balance will eventually stress the remaining B-cell population and cause further reductions in B-cell numbers (secondary hypothalmic involvement).

Glucagon and the circulation.

Glucagon is a vasodilator substance that reduces blood pressure via a decreased vascular resistance in the splanchnic and hepatic vasculature. Species differences in the response of various vascular beds to glucagon have been documented. In the kidney, glucagon in relatively large doses increased renal plasma flow, glomerular filtration, and electrolyte excretion. It has been shown that intraarterial injection of glucagon into the renal artery can produce an increase in electrolyte excretion on the side that received an injection with minimal or no changes in glomerular filtration. This indicated a direct tubular effect of this polypeptide. This effect may be related to the increased glomerular filtration observed in poorly controlled diabetics where insulin concentrations are low and glucagon concentrations are high. The tubular effects of glucagon are probably mediated via cAMP and prostaglandin formation in renal tubular cells, especially the ascending limbs of Henle and collecting ducts. Glucagon increases the RNA concentration in glomerular tissue, and this effect is probably independent of cAMP. The latter effect of glucagon has been related to the glomerular enlargement and membrane thickening observed in poorly controlled insulin-dependent diabetics. Starvation natriuresis has been related to increased concentrations of glucagon in blood. The likely mechanism is that glucagon increased the renal excretion of organic acids, possibly by inhibiting the renal tubular reabsorption of these acids. Little is known concerning the effects of glucagon on the cAMP content of vascular smooth muscle. Indirect evidence suggests that such effects may be mediated via the production of cAMP. If this can be established, it would be likely that the glucagon-induced vasodilation is due to a cAMP-dependent phosphorylation of the myosin light chain kinase. This kinase shows reduced sensitivity to the Ca++ calmodulin complex when it is phosphorylated by the cAMP-dependent kinase and thus may produce relaxation of smooth muscle. In cardiac muscle, glucagon produced positive inotropic and chronotropic effects. These effects show species differences and in some species activate only the auricle with minimal effects of ventricular muscle. The effects of glucagon in general resemble those of a beta-adrenergic agent; however, glucagon seems to be nonarrhythmogenic in a variety of cardiac preparations and its effects are not blocked by propranolol. In some of these experimental conditions the chronotropic effects of glucagon play an important role in the antiarrhythmogenic effects, although direct cardiac membrane effects have been postulated. Several factors can modify the