Endogenous Glucose Production In Humans Research Articles

Central KATP Channels Modulate Glucose Effectiveness in Humans and Rodents.

Hyperglycemia is a potent regulator of endogenous glucose production (EGP). Loss of this “glucose effectiveness” is a major contributor to elevated plasma glucose concentrations in type 2 diabetes (T2D). KATP channels in the central nervous system have been shown to regulate EGP in humans and rodents. We examined the contribution of central KATP channels to glucose effectiveness. Under fixed hormonal conditions (studies using a pancreatic clamp), hyperglycemia suppressed EGP by ∼50% in both humans without diabetes and normal Sprague-Dawley rats. By contrast, antagonism of KATP channels with glyburide significantly reduced the EGP-lowering effect of hyperglycemia in both humans and rats. Furthermore, the effects of glyburide on EGP and gluconeogenic enzymes were abolished in rats by intracerebroventricular administration of the KATP channel agonist diazoxide. These findings indicate that about half of the suppression of EGP by hyperglycemia is mediated by central KATP channels. These central mechanisms may offer a novel therapeutic target for improving glycemic control in subjects with T2D.

227-OR: Novel Method of Assessing Hypothalamic KATP Channel Activation in Humans Using Magnetic Resonance Imaging (MRI)

Hypothalamic KATP channels play an important role in regulating metabolism: activation of these channels with diazoxide (DZX) reduces endogenous glucose production (EGP) in rodents. When insulin is maintained at basal levels, oral DZX similarly suppresses EGP in humans (JCI 121:4916, 2011). Given the need to directly examine central regulation of EGP in humans, we developed a novel non-invasive measure of hypothalamic KATP channel activation. Since cerebral blood flow (CBF) and neuronal activity are coupled, we mapped hypothalamic neuronal activity using pseudo-continuous arterial spin labeling (pCASL) to measure CBF using 3 Tesla MRI with high resolution 3-dimensional (3D) acquisition and published analyses approaches (MRM 73: 102, 2015). The hypothalamus was extracted from each 3D map (3x3x3mm) using a hypothalamus region-of-interest (ROI) mask delineated upon the Johns Hopkins Template. To directly assess hypothalamic KATP channel activity using pCASL, we examined the time dependent effects of DZX (6mg/kg) vs. placebo in a randomized, double blinded fashion. Eight healthy nondiabetic individuals (42±4 yr, BMI 24±1 kg/m2, 7M) underwent a baseline scan, with repeat scans at 2.5 and 5 hours following DZX or placebo. This time course was determined based on previous observations that DZX appearance in rat CSF precedes EGP suppression, which is maximal at ∼6 hours in rats and humans. DZX induced an ∼11% drop in hypothalamic CBF, from 59.5±2.5 to 53.0±3.1 ml/100g/min at 2.5 hours (p=0.06), returning to baseline at 5 hours (57.7±2.7 ml/100g/min, p=0.3). Following placebo there was no change in CBF (p=0.8 by repeated measures ANOVA). No change in CBF was observed in other brain regions (e.g., cerebral cortex, thalamus, or hippocampus). In conclusion, pCASL provides a novel image-based measure of hypothalamic response to KATP channel activation in vivo, and can thus be used to examine the metabolic significance of hypothalamic KATP channel activation in humans. Disclosure S. Sharma: None. S. Aleksic: None. R. Maginley: None. E. Lontchi Yimagou: None. L. Upadhyay: None. S. Bhansali: None. O. Sandu: None. C.A. Branch: None. M. Hawkins: Other Relationship; Self; Novo Nordisk Inc. Funding National Institutes of Health (R01DK069861); Albert Einstein College of Medicine

Central KATP Channels Modulate Glucose Effectiveness

Glucose effectiveness describes the restraining effect of hyperglycemia on endogenous glucose production (EGP), which is lost in type 2 diabetes (T2D). Since central nervous system KATP channels likely regulate EGP in humans and rats, we examined whether these channels contribute to glucose effectiveness. Paired 4-hour hyperglycemic ’pancreatic clamp’ studies were performed in 9 healthy humans (age 37 ± 15 years; BMI 26 ± 4 kg/m2) 3 hours following glyburide 10 mg (GLB) or placebo (PLC). With PLC, hyperglycemia suppressed EGP by 59% (euglycemia: 2.38 ± 0.229 vs. hyperglycemia: 0.98 ± 0.17, p < 0.001). GLB resulted in 46% less suppression of EGP by hyperglycemia from baseline euglycemia compared to PLC (GLB: 1.56 ± 0.29 mg/kg/min vs. PLC: 0.98 ± 0.17 mg/kg/min, p = 0.047). To confirm that GLB’s effects were centrally mediated, parallel hyperglycemic studies were conducted in normal rats after oral gavage of normal saline (NS) (n=8), GLB (n=14), and oral GLB with intracerebroventricular (ICV) infusion of KATP channel agonist diazoxide (DZX) (n=8). GLB attenuated hyperglycemia’s suppressive effect on EGP (GLB: 9.4 ± 0.3 mg/kg/min vs. NS: 6.9 ± 0.3 mg/kg/min; p = 0.047), while ICV DZX abolished the effects of GLB (DZX + GLB: 5.5 ± 0.5 mg/kg/min vs. GLB, p = 0.003)(Figure 1). This suggests that ∼50% of the suppression of EGP by hyperglycemia is mediated by central KATP channels. Targeting these channels may offer a new therapeutic approach to improve glycemic control in T2D. Disclosure W.G. Mitchell: None. E. Lontchi-Yimagou: None. S.A. Reda: None. M. Carey: None. K. Zhang: None. S. Aleksic: None. D. Huffman: None. M. Hawkins: None.

The Effect of Ingested Glucose Dose on the Suppression of Endogenous Glucose Production in Humans.

Insulin clamp studies have shown that the suppressive actions of insulin on endogenous glucose production (EGP) are markedly more sensitive than for stimulating glucose disposal (Rd). However, clamp conditions do not adequately mimic postprandial physiological responses. Here, using the variable infusion dual-tracer approach, we used a threefold range of ingested glucose doses (25, 50, and 75 g) to investigate how physiological changes in plasma insulin influence EGP in healthy subjects. Remarkably, the glucose responses were similar for all doses tested, yet there was a dose-dependent increase in insulin secretion and plasma insulin levels. Nonetheless, EGP was suppressed with the same rapidity and magnitude (∼55%) across all doses. The progressive hyperinsulinemia, however, caused a dose-dependent increase in the estimated rates of Rd, which likely accounts for the lack of a dose effect on plasma glucose excursions. This suggests that after glucose ingestion, the body preferentially permits a transient and optimal degree of postprandial hyperglycemia to efficiently enhance insulin-induced changes in glucose fluxes, thereby minimizing the demand for insulin secretion. This may represent an evolutionarily conserved mechanism that not only reduces the secretory burden on β-cells but also avoids the potential negative consequences of excessive insulin release into the systemic arterial circulation.

Evaluation of the Effect of Enteral Lipid Sensing on Endogenous Glucose Production in Humans.

Administration of lipids into the upper intestine of rats has been shown to acutely decrease endogenous glucose production (EGP) in the preabsorptive state, postulated to act through a gut-brain-liver axis involving accumulation of long-chain fatty acyl-CoA, release of cholecystokinin, and subsequent neuronal signaling. It remains unknown, however, whether a similar gut-brain-liver axis is operative in humans. Here, we infused 20% Intralipid (a synthetic lipid emulsion) or saline intraduodenally for 90 min at 30 mL/h, 4 to 6 weeks apart, in random order, in nine healthy men. EGP was assessed under pancreatic clamp conditions with stable isotope enrichment techniques. Under these experimental conditions, intraduodenal infusion of Intralipid, compared with saline, did not affect plasma glucose concentration or EGP throughout the study period. We conclude that Intralipid infusion into the duodenum at this rate does not elicit detectable effects on glucose homeostasis or EGP in healthy men, which may reflect important interspecies differences between rodents and humans with respect to the putative gut-brain-liver axis.

Intranasal insulin suppresses endogenous glucose production in humans compared with placebo in the presence of similar venous insulin concentrations.

Intranasal insulin (INI) has been shown to modulate food intake and food-related activity in the central nervous system in humans. Because INI increases insulin concentration in the cerebrospinal fluid, these effects have been postulated to be mediated via insulin action in the brain, although peripheral effects of insulin cannot be excluded. INI has been shown to lower plasma glucose in some studies, but whether it regulates endogenous glucose production (EGP) is not known. To assess the role of INI in the regulation of EGP, eight healthy men were studied in a single-blind, crossover study with two randomized visits (one with 40 IU INI and the other with intranasal placebo [INP] administration) 4 weeks apart. EGP was assessed under conditions of an arterial pancreatic clamp, with a primed, constant infusion of deuterated glucose and infusion of 20% dextrose as required to maintain euglycemia. Between 180 and 360 min after administration, INI significantly suppressed EGP by 35.6% compared with INP, despite similar venous insulin concentrations. In conclusion, INI lowers EGP in humans compared with INP, despite similar venous insulin concentrations. INI may therefore be of value in treating excess liver glucose production in diabetes.

Direct effect of GLP-1 infusion on endogenous glucose production in humans

Glucagon-like peptide-1 (GLP-1) lowers glucose levels by potentiating glucose-induced insulin secretion and inhibiting glucagon release. The question of whether GLP-1 exerts direct effects on the liver, independently of the hormonal changes, is controversial. We tested whether an exogenous GLP-1 infusion, designed to achieve physiological postprandial levels, directly affects endogenous glucose production (EGP) under conditions mimicking the fasting state in diabetes. In 14 healthy volunteers, we applied the pancreatic clamp technique, whereby plasma insulin and glucagon levels are clamped using somatostatin and hormone replacement. The clamp was applied in paired, 4 h experiments, during which saline (control) or GLP-1(7-37)amide (0.4 pmol min⁻¹ kg⁻¹) was infused. During the control study, plasma insulin and glucagon were maintained at basal levels and plasma C-peptide was suppressed, such that plasma glucose rose to a plateau of ~10.5 mmol/l and tracer-determined EGP increased by ~60%. During GLP-1 infusion at matched plasma glucose levels, the rise of EGP from baseline was fully prevented. Lipolysis (as indexed by NEFA concentrations and tracer-determined glycerol rate of appearance) and substrate utilisation (by indirect calorimetry) were similar between control and GLP-1 infusion. GLP-1 inhibits EGP under conditions where plasma insulin and glucagon are not allowed to change and glucose concentrations are matched, indicating either a direct effect on hepatocytes or neurally mediated inhibition.

Activation of KATP channels suppresses glucose production in humans

Increased endogenous glucose production (EGP) is a hallmark of type 2 diabetes mellitus. While there is evidence for central regulation of EGP by activation of hypothalamic ATP-sensitive potassium (K(ATP)) channels in rodents, whether these central pathways contribute to regulation of EGP in humans remains to be determined. Here we present evidence for central nervous system regulation of EGP in humans that is consistent with complementary rodent studies. Oral administration of the K(ATP) channel activator diazoxide under fixed hormonal conditions substantially decreased EGP in nondiabetic humans and Sprague Dawley rats. In rats, comparable doses of oral diazoxide attained appreciable concentrations in the cerebrospinal fluid, and the effects of oral diazoxide were abolished by i.c.v. administration of the K(ATP) channel blocker glibenclamide. These results suggest that activation of hypothalamic K(ATP) channels may be an important regulator of EGP in humans and that this pathway could be a target for treatment of hyperglycemia in type 2 diabetes mellitus.

Renal glucose production compensates for the liver during the anhepatic phase of liver transplantation.

The extent of the renal contribution to postabsorptive endogenous glucose production (EGP) in humans is controversial. We measured EGP in the absence of the liver during the anhepatic phase (AH) of liver transplantation in five patients (aged 46.4+/-10.2 years, two women). Stable labeling of plasma glucose (PG) was achieved for a 2-h period before the AH by primed continuous infusion of di-deuterated 6,6[2H2]glucose (1.7 mg/min) and continued throughout the AH. PG was maintained above the fasting level (6.1+/-2.73 mmol/l) with 5% dextrose labeled with 6,6[2H2]glucose throughout the AH (mean level during the AH 0.98+/-0.45 mg x kg(-1) x min(-1)). Isotopic enrichment remained stable at 0.84+/-0.21% atom percent excess throughout. EGP, calculated by use of a modified Steele equation, decreased from 2.6+/-1.24 at baseline to 0.97+/-0.9 mg x kg(-1) x min(-1) (36% baseline, P = 0.045) but recovered at approximately 30 min to reach 1.38+/-0.83 mg x kg(-1) x min(-1) (54% baseline) by 60 min. Epinephrine, lactate, free fatty acid, and glycerol levels increased significantly (0.79+/-0.74 to 3.65+/-2.1 nmol/l, P = 0.005; 1.88+/-0.43 to 3.46+/-0.9 mmol/l, P = 0.024; 543.9+/-215.5 to 705.5+/-219.2 micromol/l, P = 0.012; 75.6+/-30.2 to 139+/-96.3 micromol/l, P = 0.003, respectively). These data show that postabsorptive nonhepatic glucose production in humans may contribute to greater than one-third of overall EGP, increasing when required, and that it is associated with a stress response and increased gluconeogenic substrate availability. We conclude that extrahepatic tissues, most notably those of the kidney, make a significant contribution to EGP in humans.

Mechanism by which glucose and insulin inhibit net hepatic glycogenolysis in humans.

13C NMR spectroscopy was used to assess flux rates of hepatic glycogen synthase and phosphorylase in overnight-fasted subjects under one of four hypoglucagonemic conditions: protocol I, hyperglycemic (approximately 10 mM) -hypoinsulinemia (approximately 40 pM); protocol II, euglycemic (approximately 5 mM) -hyperinsulinemia (approximately 400 pM); protocol III, hyperglycemic (approximately 10 mM) -hyperinsulinemia (approximately 400 pM); and protocol IV; euglycemic (approximately 5 mM) -hypoinsulinemia (approximately 40 pM). Inhibition of net hepatic glycogenolysis occurred in both protocols I and II compared to protocol IV but via a different mechanism. Inhibition of net hepatic glycogenolysis occurred in protocol I mostly due to decreased glycogen phosphorylase flux, whereas in protocol II inhibition of net hepatic glycogenolysis occurred exclusively through the activation of glycogen synthase flux. Phosphorylase flux was unaltered, resulting in extensive glycogen cycling. Relatively high rates of net hepatic glycogen synthesis were observed in protocol III due to combined stimulation of glycogen synthase flux and inhibition of glycogen phosphorylase flux. In conclusion, under hypoglucagonemic conditions: (a) hyperglycemia, per se, inhibits net hepatic glycogenolysis primarily through inhibition of glycogen phosphorylase flux; (b) hyperinsulinemia, per se, inhibits net hepatic glycogenolysis primarily through stimulation of glycogen synthase flux; (c) inhibition of glycogen phosphorylase and the activation of glycogen synthase are not necessarily coupled and coordinated in a reciprocal fashion; and (d) promotion of hepatic glycogen cycling may be the principal mechanism by which insulin inhibits net hepatic glycogenolysis and endogenous glucose production in humans under euglycemic conditions.