Effects of hyperglycemia on hepatic gluconeogenic flux during glycogen phosphorylase inhibition in the conscious dog

Morning Engagement of Hepatic Insulin Receptors Improves Afternoon Hepatic Glucose Disposal and Storage

Glucose tolerance improves significantly upon consuming a second, identical meal later in the day (second meal phenomenon). We previously established that morning hyperinsulinemia primes the liver for increased afternoon hepatic glucose uptake (HGU). Although the route of insulin delivery is an important determinant of the mechanisms by which insulin regulates liver glucose metabolism (direct hepatic vs indirect insulin action), it is not known if insulin's delivery route affects the second meal response. To determine whether morning peripheral insulin delivery (as occurs clinically (subcutaneous)) can enhance afternoon HGU, conscious dogs were treated in the morning with insulin delivered via the portal vein, or peripherally (leg vein), while glucose was infused to maintain euglycemia. Consequently, arterial insulin levels increased similarly in both groups, but relative hepatic insulin deficiency occurred when insulin was delivered peripherally. In the afternoon, all animals were challenged with the same hyperinsulinemic-hyperglycemic clamp to simulate identical postprandial-like conditions. The substantial enhancement of HGU in the afternoon caused by morning portal vein insulin delivery was lost when insulin was delivered peripherally. This indicates that morning insulin does not cause the second meal phenomenon via its indirect actions on the liver, but rather through direct activation of hepatic insulin signaling.

Subclinical Reactive Hypoglycemia with Low Glucose Effectiveness-Why We Cannot Stop Snacking despite Gaining Weight

Obesity has grown worldwide owing to modern obesogenic lifestyles, including frequent snacking. Recently, we studied continuous glucose monitoring in obese/overweight men without diabetes and found that half of them exhibit glucose levels less than 70 mg/dL after a 75-g oral glucose load without notable hypoglycemic symptoms. Interestingly, people with "subclinical reactive hypoglycemia (SRH)" snack more frequently than those without it. Since the ingestion of sugary snacks or drinks could further induce SRH, a vicious cycle of "Snacking begets snacking via SRH" can be formed. Glucose effectiveness (Sg) is an insulin-independent mechanism that contributes to most of the whole-body glucose disposal after an oral glucose load in people without diabetes. Our recent data suggest that both higher and lower Sg are associated with SRH, while the latter but not the former is linked to snacking habits, obesity, and dysglycemia. The present review addresses the possible role of SRH in snacking habits in people with obesity/overweight, taking Sg into account. It is concluded that, for those with low Sg, SRH can be regarded as a link between snacking and obesity. Prevention of SRH by raising Sg might be key to controlling snacking habits and body weight.

Mild Physiologic Hyperglycemia Induces Hepatic Insulin Resistance in Healthy Normal Glucose-Tolerant Participants

CONTEXT

Chronic hyperglycemia worsens skeletal muscle insulin resistance and β-cell function. However, the effect of sustained physiologic hyperglycemia on hepatic insulin sensitivity is not clear.

OBJECTIVE

To examine the effect of sustained physiologic hyperglycemia (similar to that observed in patients with type 2 diabetes) on endogenous (primarily reflecting hepatic) glucose production (EGP) in healthy individuals.

DESIGN

Volunteers participated in a three-step hyperinsulinemic (10, 20, 40 mU/m2 per minute) euglycemic clamp before and after a 48-hour glucose infusion to increase plasma glucose concentration by ∼40 mg/dL above baseline. EGP was measured with 3-3H-glucose before and after chronic glucose infusion.

PARTICIPANTS

Sixteen persons with normal glucose tolerance [eight with and eight without a family history (FH) of diabetes] participated in the study.

MAIN OUTCOME MEASURE

EGP.

RESULTS

Basal EGP increased following 48 hours of glucose infusion (from a mean ± SEM of 2.04 ± 0.08 to 3.06 ± 0.29 mg/kgffm⋅ min; P < 0.005). The hepatic insulin resistance index (basal EGP × fasting plasma insulin) markedly increased following glucose infusion (20.1 ± 1.8 to 51.7 ± 6.6; P < 0.005) in both FH+ and FH- subjects.

CONCLUSION

Sustained physiologic hyperglycemia for as little as 48 hours increased the rate of basal hepatic glucose production and induced hepatic insulin resistance in health persons with normal glucose tolerance, providing evidence for the role of glucotoxicity in the increase in hepatic glucose production in type 2 diabetes.

Exercise-induced improvements in glucose effectiveness are blunted by a high glycemic diet in adults with prediabetes

AIMS

Glucose effectiveness (GE) refers to the ability of glucose to influence its own metabolism through insulin-independent mechanisms. Diminished GE is a predictor of progression to type 2 diabetes. Exercise training improves GE, however, little is known about how dietary interventions, such as manipulating the glycemic index of diets, interact with exercise-induced improvements in GE in at-risk populations.

METHODS

We enrolled 33 adults with obesity and pre-diabetes (17 males, 65.7 ± 4.3 years, 34.9 ± 4.2 kg m^-2) into a 12-week exercise training program (1 h day^-1 and 5 day week^-1 at ~ 85% of maximum heart rate) while being randomized to concurrently receive either a low (EX-LOG: 40 ± 0.3 au) or high (EX-HIG: 80 ± 0.6 au) glycemic index diet. A 75-g oral-glucose-tolerance test (OGTT) was performed before and after the intervention and GE was calculated using the Nagasaka equation. Insulin resistance was estimated using a hyperinsulinemic-euglycemic clamp and cardiorespiratory fitness using a VO_2max test.

RESULTS

Both EX-LOG and EX-HIG groups had similar improvements in weight (8.6 ± 5.1 kg, P < 0.001), VO_2max (6 ± 3.5 mL kg^-1 min^-1, P < 0.001) and clamp-measured peripheral insulin resistance (1.7 ± 0.9 mg kg^-1 min^-1, P < 0.001), relative to baseline data. GE in EX-LOG and EX-HIG was similar at baseline (1.9 ± 0.38 vs. 1.85 ± 0.3 mg dL^-1 min^-1, respectively; P > 0.05) and increased by ~ 20% post-intervention in the EX-LOG arm (∆GE: 0.07-0.57 mg dL^-1 min^-1, P < 0.05). Plasma free fatty acid (FFA) concentrations also decreased only in the EX-LOG arm (∆FFA: 0.13 ± 0.23 mmol L^-1, P < 0.05).

CONCLUSIONS

Our data suggest that a high glycemic index diet may suppress exercise-induced enhancement of GE, and this may be mediated through plasma FFAs.

Effects of 11β-hydroxysteroid dehydrogenase-1 inhibition on hepatic glycogenolysis and gluconeogenesis

The aim of this study was to determine the effect of prolonged 11β-hydroxysteroid dehydrogenase-1 (11β-HSD1) inhibition on basal and hormone-stimulated glucose metabolism in fasted conscious dogs. For 7 days prior to study, either an 11β-HSD1 inhibitor (HSD1-I; n = 6) or placebo (PBO; n = 6) was administered. After the basal period, a 4-h metabolic challenge followed, where glucagon (3×-basal), epinephrine (5×-basal), and insulin (2×-basal) concentrations were increased. Hepatic glucose fluxes did not differ between groups during the basal period. In response to the metabolic challenge, hepatic glucose production was stimulated in PBO, resulting in hyperglycemia such that exogenous glucose was required in HSD-I (P < 0.05) to match the glycemia between groups. Net hepatic glucose output and endogenous glucose production were decreased by 11β-HSD1 inhibition (P < 0.05) due to a reduction in net hepatic glycogenolysis (P < 0.05), with no effect on gluconeogenic flux compared with PBO. In addition, glucose utilization (P < 0.05) and the suppression of lipolysis were increased (P < 0.05) in HSD-I compared with PBO. These data suggest that inhibition of 11β-HSD1 may be of therapeutic value in the treatment of diseases characterized by insulin resistance and excessive hepatic glucose production.

Estimating hepatic glucokinase activity using a simple model of lactate kinetics

OBJECTIVE

Glucokinase (GCK) acts as a component of the "glucose sensor" in pancreatic β-cells and possibly in other tissues, including the brain. However, >99% of GCK in the body is located in the liver, where it serves as a "gatekeeper", determining the rate of hepatic glucose phosphorylation. Mutations in GCK are a cause of maturity-onset diabetes of the young (MODY), and GCKR, the regulator of GCK in the liver, is a diabetes susceptibility locus. In addition, several GCK activators are being studied as potential regulators of blood glucose. The ability to estimate liver GCK activity in vivo for genetic and pharmacologic studies may provide important physiologic insights into the regulation of hepatic glucose metabolism.

RESEARCH DESIGN AND METHODS

Here we introduce a simple, linear, two-compartment kinetic model that exploits lactate and glucose kinetics observed during the frequently sampled intravenous glucose tolerance test (FSIGT) to estimate liver GCK activity (K(GK)), glycolysis (K(12)), and whole body fractional lactate clearance (K(01)). RESULTS To test our working model of lactate, we used cross-sectional FSIGT data on 142 nondiabetic individuals chosen at random from the Finland-United States Investigation of NIDDM Genetics study cohort. Parameters K(GK), K(12), and K(01) were precisely estimated. Median model parameter estimates were consistent with previously published values.

CONCLUSIONS

This novel model of lactate kinetics extends the utility of the FSIGT protocol beyond whole-body glucose homeostasis by providing estimates for indices pertaining to hepatic glucose metabolism, including hepatic GCK activity and glycolysis rate.

Contribution of organ blood flow, intrinsic tissue clearance and glycaemia to the regulation of glucose use in obese and type 2 diabetic rats: a PET study

BACKGROUND AND AIMS

Chronic hyperglycaemia aggravates obesity and diabetes mellitus. The use of glucose by body organs depends on several factors. We sought to investigate the role of blood flow, intrinsic tissue glucose clearance and blood glucose levels in regulating tissue glucose uptake under fasting conditions (FCs) and in response to acute hyperglycaemia (AH) in obese and type 2 diabetic rats.

METHODS AND RESULTS

Thirty-six Zucker rats were studied by positron emission tomography to quantify perfusion and glucose uptake during FC and after AH in the liver, myocardium, skeletal muscle and subcutaneous adipose tissue. Progressively higher glucose uptake rates were observed from lean to obese (p < 0.05) and to diabetic rats (p < 0.05) in all tissues during both FC and AH. In FC, they were increased of 7-18 times in obese rats and 11-30 times in diabetic rats versus controls. Tissue glucose uptake was increased by over 10-fold during AH in controls; this response was severely blunted in diseased groups. AH tended to stimulate organ perfusion in control rats. Tissue glucose uptake was a function of intrinsic clearance and glycaemia (mass action) in healthy animals, but the latter component was lost in diseased animals. Differences in perfusion did not account for those in glucose uptake.

CONCLUSIONS

Each organ participates actively in the regulation of its glucose uptake, which is dependent on intrinsic tissue substrate extraction and extrinsic blood glucose delivery, but not on perfusion, and it is potently stimulated by AH. Obese and diabetic rats had an elevated organ glucose uptake but a blunted response to acute glucose intake.

Insulin-induced hypoglycemia increases hepatic sensitivity to glucagon in dogs

In individuals with type 1 diabetes, hypoglycemia is a common consequence of overinsulinization. Under conditions of insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production. In contrast, during euglycemia, insulin potently inhibits glucagon's effect on the liver. The first aim of the present study was to determine whether low blood sugar augments glucagon's ability to increase glucose production. Using a conscious catheterized dog model, we found that hypoglycemia increased glucagon's ability to overcome the inhibitory effect of insulin on hepatic glucose production by almost 3-fold, an effect exclusively attributable to marked enhancement of the effect of glucagon on net glycogen breakdown. To investigate the molecular mechanism by which this effect comes about, we analyzed hepatic biopsies from the same animals, and found that hypoglycemia resulted in a decrease in insulin signaling. Furthermore, hypoglycemia and glucagon had an additive effect on the activation of AMPK, which was associated with altered activity of the enzymes of glycogen metabolism.

Effects of insulin on the metabolic control of hepatic gluconeogenesis in vivo

OBJECTIVE

Insulin represses the expression of gluconeogenic genes at the mRNA level, but the hormone appears to have only weak inhibitory effects in vivo. The aims of this study were 1) to determine the maximal physiologic effect of insulin, 2) to determine the relative importance of its effects on gluconeogenic regulatory sites, and 3) to correlate those changes with alterations at the cellular level.

RESEARCH DESIGN AND METHODS

Conscious 60-h fasted canines were studied at three insulin levels (near basal, 4x, or 16x) during a 5-h euglycemic clamp. Pancreatic hormones were controlled using somatostatin with portal insulin and glucagon infusions. Glucose metabolism was assessed using the arteriovenous difference technique, and molecular signals were assessed.

RESULTS

Insulin reduced gluconeogenic flux to glucose-6-phosphate (G6P) but only at the near-maximal physiological level (16x basal). The effect was modest compared with its inhibitory effect on net hepatic glycogenolysis, occurred within 30 min, and was associated with a marked decrease in hepatic fat oxidation, increased liver fructose 2,6-bisphosphate level, and reductions in lactate, glycerol, and amino acid extraction. No further diminution in gluconeogenic flux to G6P occurred over the remaining 4.5 h of the study, despite a marked decrease in PEPCK content, suggesting poor control strength for this enzyme in gluconeogenic regulation in canines.

CONCLUSIONS

Gluconeogenic flux can be rapidly inhibited by high insulin levels in canines. Initially decreased hepatic lactate extraction is important, and later reduced gluconeogenic precursor availability plays a role. Changes in PEPCK appear to have little or no acute effect on gluconeogenic flux.

Cloning and expression of canine glucagon receptor and its use to evaluate glucagon receptor antagonists in vitro and in vivo

Glucose homeostasis is maintained by the combined actions of insulin and glucagon. Hyperglucagonemia and/or elevation of glucagon/insulin ratio have been reported in diabetic patients and in animal models of diabetes. Therefore, antagonizing glucagon receptor function has long been considered a useful approach to lower hyperglycemia. Dogs serve as an excellent model for studying glycemic control and various aspects of glucagon biology in vivo; however, the amino acid sequence of the dog glucagon receptor has not been reported. To better understand the pharmacology of the dog glucagon receptor and to characterize glucagon receptor antagonists, we cloned a cDNA corresponding to the glucagon receptor from dog liver RNA. The dog glucagon receptor shares a significant (>75%) homology at both nucleotide and amino acid levels with the glucagon receptor from human, monkey, mouse, and rat. The protein is highly conserved among all species in areas corresponding to the 7 trans-membrane domains. However, it shows significant divergence at the carboxy terminus such that the receptor from dog has the longest cytoplasmic tail among all species examined. When expressed in chinese hamster ovary cells, the dog glucagon receptor bound [125I]Glucagon with a K(d) of 477+/-106 pM. Glucagon stimulated the rise of intracellular cAMP levels in these cells with an EC(50) of 9.6+/-1.7 nM and such effects could be blocked by known peptidyl and non-peptidyl small molecule antagonists. In addition we show that a small molecule glucagon receptor antagonist with significant activity in cell based assays also blocked the ability of glucagon to induce elevation in blood glucose in beagle dogs. These data demonstrate that the cloned cDNA encodes a functional dog glucagon receptor. The availability of the dog cDNA will facilitate the understanding of glucagon pharmacology and aid in the characterization of novel glucagon antagonists that may serve as anti-hyperglycemic treatment for type 2 diabetes mellitus.

The effect of an acute elevation of NEFA concentrations on glucagon-stimulated hepatic glucose output

To determine the effect of nonesterified fatty acids (NEFA) on glucagon action, glucagon was infused intraportally (1.65 ng.min(-1).kg(-1)) for 3 h into 18-h-fasted, pancreatic-clamped conscious dogs in the presence [NEFA + glucagon (GGN)] or absence (GGN) of peripheral Intralipid plus heparin infusion. Additionally, hyperglycemic (HG), hyperglycemic-hyperlipidemic (NEFA + HG), and glycerol plus glucagon (GLYC + GGN) controls were studied. Arterial plasma glucagon concentrations rose equally in GGN, NEFA + GGN, and GLYC + GGN but remained basal in hyperglycemic controls. Peripheral infusions of Intralipid and heparin increased arterial plasma NEFA concentrations equally in NEFA + GGN and NEFA + HG and did not change in other protocols. After 15 min, glucagon infusion resulted in a rapid, brief increase in net hepatic glycogenolysis (NHGLY, mg.min(-1).kg(-1)) of approximately 6.0 in GGN and GLYC + GGN but only increased by 3.8 +/- 1.3 in NEFA + GGN. Thus increases in NHGLY, and consequently net hepatic glucose output (NHGO), were blunted by 40%, with no difference between the groups in the last 2.5 h of the study. NHGO and NHGLY did not significantly change in HG and NEFA + HG. Net hepatic gluconeogenic flux did not change in GGN, GLYC + GGN, or HG. However, Intralipid and heparin infusion resulted in similar increases in net hepatic gluconeogenic flux in NEFA + GGN and NEFA + HG. Thus elevated NEFA limit the initial increase in glucagon-stimulated HGO by blunting glycogenolysis, without having any effect on the gluconeogenic or glycogenolytic contributions or NHGO thereafter.

The regulation of glucose effectiveness: how glucose modulates its own production

PURPOSE OF REVIEW

'Glucose effectiveness' refers to the ability of glucose per se to suppress endogenous glucose production and stimulate glucose uptake. In addition to the inhibitory effects of insulin on endogenous glucose production, rising glucose levels have important direct effects on glucose homeostasis. The loss of glucose effectiveness in type 2 diabetes mellitus contributes importantly to hyperglycemia in those individuals. Given the rapidly increasing incidence and serious complications of type 2 diabetes mellitus, understanding the regulation of glucose effectiveness has great potential therapeutic benefits.

RECENT FINDINGS

The loss of this important regulation appears to be secondary to the chronic 'diabetic milieu' in type 2 diabetes mellitus, which includes elevated plasma glucose and free fatty acid levels. Glucose effectiveness is completely restored by normalizing plasma free fatty acid levels. Increased free fatty acid availability stimulates gluconeogenesis and alters flux through key hepatic enzymes. It is likely that at least part of this regulation is through central pathways. In addition, hormones that may exert important effects on hepatic glucose effectiveness include cortisol, insulin and glucagon-like peptide 1. The effectiveness of glucose to stimulate glucose uptake is impaired by elevated free fatty acid levels and may be enhanced by glucagon-like peptide 1.

SUMMARY

The regulation of glucose effectiveness involves a complex interplay of hormonal and metabolic factors, with free fatty acid and glucoregulatory hormones playing key roles. The loss of this regulation in type 2 diabetes mellitus contributes importantly to hyperglycemia, and may largely be caused by increased free fatty acid levels.