Important role of the hepatic vagus nerve in glucose uptake and production by the liver

We examined the role of the hepatic vagus nerve in hepatic and peripheral glucose metabolism. To assess endogenous glucose production (EGP), hepatic uptake of first-pass glucose infused intraportally (HGU), and the metabolic clearance rate of glucose (MCR), rats were subjected to hepatic vagotomy (HV, n = 7) or sham operation (SH, n = 8), after 10 days, they were then subjected to a euglycemic-hyperinsulinemic clamp together with a portal glucose load in the 24-hour fasting state. Metabolic parameters were determined by the dual-tracer method using stable isotopes. During the experiment, [6,6-2H2]glucose was continuously infused into the peripheral vein. To maintain euglycemia (4.5 mmol/L), insulin (54 pmol x kg(-1) x min(-1)) and glucose were infused peripherally after the 90-minute tracer equilibration and 30-minute basal periods, and glucose containing 5% enriched [U-13C]glucose was infused intraportally (50 micromol x kg(-1) x min(-1)) for 120 minutes (clamp period). EGP was significantly higher in HV rats versus SH rats during the basal period (64.3 +/- 7.6 v 43.6 +/- 5.3 micromol x kg(-1) x min(-1), P < .005)) and was comparable to EGP in SH rats during the clamp period (9.3 +/- 21.5 v 1.1 +/- 11.7 micromol x kg(-1) x min(-1)). HGU was reduced in HV rats compared with SH rats during portal glucose infusion (5.9 +/- 2.4 v 10.1 +/- 3.2 micromol x kg(-1) x min(-1)). The MCR in HV rats was significantly higher than in SH rats in the basal period (11.0 +/- 2.0 v 7.9 +/- 0.8 mL x kg(-1) x min(-1), P < .01)) and was comparable to the MCR in SH rats during the clamp period (41.9 +/- 10.0 and 36.6 +/- 5.7 mL x kg(-1) x min(-1)). We conclude that innervation of the hepatic vagus nerve is important for the regulation of hepatic glucose production in the postabsorptive state and HGU in the postprandial state.     

Brain-derived neurotrophic factor ameliorates hepatic insulin resistance in Zucker fatty rats

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophins, has been reported to ameliorate hyperglycemia in obese diabetic animal models. To elucidate the mechanism of BDNF on glucose metabolism, we determined the glucose turnover under basal and euglycemic hyperinsulinemic (insulin infusion rate, 54 pmol. kg(-1). min(-1)) clamp conditions in obese insulin-resistant rats, male Zucker fatty rats, which had been acutely administered a subcutaneous injection of BDNF (20 mg/kg) (n = 9, BDNF) or vehicle (n = 8, vehicle). Under the basal condition, acute administration of BDNF did not affect the blood glucose level, plasma insulin level, rate of glucose disappearance (Rd), and endogenous glucose production (EGP). Under the clamp condition, the glucose infusion rate (GIR) was significantly higher in BDNF than in vehicle (mean +/- SD, 61.4 +/- 19.1 v 41.4 +/- 4.9 micromol. kg(-1). min(-1), P <.05). There was no significant difference in Rd and EGP between the 2 groups under the clamp condition, but the insulin-mediated suppression ratio of endogenous glucose production in BDNF was significantly greater than in vehicle (48.9 +/- 22.2 v 22.4% +/- 20.6%, P <.05). In BDNF, mRNA expressions of hepatic phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) were comparable to those of vehicle, while hepatic glucokinase (GK) mRNA expression was significantly higher (1.57 +/- 0.33 v 1.03 +/- 0.17, P <.05). We conclude that BDNF mainly improves hepatic insulin resistance in obese insulin-resistant rats, probably by affecting the hepatic GK flux. Copyright 2003, Elsevier Science (USA). All rights reserved.     

Relationship between hepatic/visceral fat and hepatic insulin resistance in nondiabetic and type 2 diabetic subjects

BACKGROUND AND AIMS: Abdominal fat accumulation (visceral/hepatic) has been associated with hepatic insulin resistance (IR) in obesity and type 2 diabetes (T2DM). We examined the relationship between visceral/hepatic fat accumulation and hepatic IR/accelerated gluconeogenesis (GNG). METHODS: In 14 normal glucose tolerant (NGT) (body mass index [BMI] = 25 +/- 1 kg/m(2)) and 43 T2DM (24 nonobese, BMI = 26 +/- 1; 19 obese, BMI = 32 +/- 1 kg/m(2)) subjects, we measured endogenous (hepatic) glucose production (3-(3)H-glucose) and GNG ((2)H(2)O) in the basal state and during 240 pmol/m(2)/min euglycemic-hyperinsulinemic clamp, and liver (LF) subcutaneous (SAT)/visceral (VAT) fat content by magnetic resonance spectroscopy/magnetic resonance imaging. RESULTS: LF was increased in lean T2DM compared with lean NGT (18% +/- 3% vs 9% +/- 2%, P < .03), but was similar in lean T2DM and obese T2DM (18% +/- 3% vs 22% +/- 3%; P = NS). Both VAT and SAT increased progressively from lean NGT to lean T2DM to obese T2DM. T2DM had increased basal endogenous glucose production (EGP) (NGT, 15.1 +/- 0.5; lean T2DM, 16.3 +/- 0.4; obese T2DM, 17.2 +/- 0.6 micromol/min/kg(ffm); P = .02) and basal GNG flux (NGT, 8.6 +/- 0.4; lean T2DM, 9.6 +/- 0.4; obese T2DM, 11.1 +/- 0.6 micromol/min/kg(ffm); P = .02). Basal hepatic IR index (EGP x fasting plasma insulin) was increased in T2DM (NGT, 816 +/- 54; lean T2DM, 1252 +/- 164; obese T2DM, 1810 +/- 210; P = .007). In T2DM, after accounting for age, sex, and BMI, both LF and VAT, but not SAT, were correlated significantly (P < .05) with basal hepatic IR and residual EGP during insulin clamp. Basal percentage of GNG and GNG flux were correlated positively with VAT (P < .05), but not with LF. LF, but not VAT, was correlated with fasting insulin, insulin-stimulated glucose disposal, and impaired FFA suppression by insulin (all P < .05). CONCLUSIONS: Abdominal adiposity significantly affects both lipid (FFA) and glucose metabolism. Excess VAT primarily increases GNG flux. Both VAT and LF are associated with hepatic IR.     

Mediation of the hepatic effects of growth hormone by its lipolytic activity.

The aim of the study was to investigate the acute effect of GH per se, independent from its lipolytic activity, on glucose and lipid oxidation and glucose turnover in seven healthy subjects. Five tests lasting 360 min were performed. Each test consisted of a 4-h equilibration period followed by a euglycemic hyperinsulinemic (25 mU/kg x h) clamp lasting 2 h. In test 1 (control experiment) saline was infused, leaving GH and FFA at basal levels. In tests 2, 3, and 4, GH was infused (80 ng/kg x min) to increase GH levels. Whereas in test 2 FFA levels were free to increase due to GH lipolytic activity, in test 3 FFA elevation was prevented by using an antilipolytic compound (Acipimox) that allowed evaluation of the effect of GH at low FFA levels. In test 4 (GH+Acipimox+heparin) GH infusion was associated with the administration of Acipimox and heparin to maintain FFA at the basal level to evaluate the effect of GH per se independent from GH lipolytic activity. In test 5 Acipimox and a variable heparin infusion were given to evaluate possible effects of Acipimox other than the inhibition of lipolysis. During the euglycemic hyperinsulinemic clamp in the presence of high GH and FFA levels (test 2), glucose oxidation was significantly lower and lipid oxidation was significantly higher than in tests 1, 3, 4, and 5. During the same period, hepatic glucose production was completely suppressed in the control study (test 1; 94%) and in test 5 (99.6%), whereas it was significantly less inhibited (65%, 74%, and 73%) when GH was administered in tests 2, 3, and 4. In conclusion, these results suggest that GH directly mediates the reduction of insulin's effect on the liver. In addition, the effect of GH on glucose and lipid oxidation is not direct, but is mediated by its lipolytic activity.     

Insulin-mediated hepatic glucose uptake is impaired in type 2 diabetes: evidence for a relationship with glycemic control

Impaired hepatic glucose uptake (HGU) has been implicated in the development of hyperglycemia in type 2 diabetes; the relative impact of plasma glucose and insulin levels on this process remains controversial. We compared the effects of euglycemic hyperinsulinemia on HGU, skeletal muscle glucose uptake, and hepatic influx rate-constant (H-Ki) in 38 diet-treated diabetic patients and 22 nondiabetic controls, using positron emission tomography with (18)F-fluorodeoxyglucose and the insulin clamp technique. Control subjects were divided into two subgroups: one including older, heavier, insulin-resistant controls (whole-body glucose uptake, M = 21.4 +/- 5.4 micromol x min(-1) x kg(-1)) to match characteristics of diabetic patients (M = 20.4 +/- 9.9); the other including younger, leaner, insulin-sensitive controls (M = 48.2 +/- 9.9, P < 0.01). Skeletal muscle glucose uptake showed a similar group distribution as the M value. Insulin clearance rates were lower, whereas glycosylated hemoglobin and clamp plasma insulin levels were higher in diabetic patients than in controls. HGU and H-Ki were similar in the two nondiabetic subgroups and lower in diabetic patients than in controls (1.9 +/- 0.5 vs. 2.3 +/- 0.7 micromol x min(-1) x 100 ml(-1), and 0.37 +/- 0.09 vs. 0.44 +/- 0.14 ml x min(-1) x 100 ml(-1), P < or = 0.01). In the whole dataset, H-Ki was inversely related to fasting plasma glucose (correlation coefficient = -0.40, P = 0.0018). In diabetic subjects, H-Ki was reciprocally related to glycosylated hemoglobin (correlation coefficient = -0.36, P = 0.029). We conclude that insulin-mediated HGU is impaired, in type 2 diabetes, in some proportion to the degree of glycemic control.     

Comparison of the priming effects of pulsatile and continuous insulin delivery on insulin action in man

Insulin is normally secreted in man in regular pulses every 5 to 15 minutes. Disordered pulsation has been demonstrated in several insulin-resistant states and it is unclear whether this represents a primary beta-cell defect contributing to impairment of peripheral insulin action or rather is a consequence of insulin resistance. Basal or near basal insulin administration by pulsatile infusion augments hypoglycemic effect and improves insulin-mediated glucose uptake compared with insulin by continuous infusion. To date no study has examined whether normal basal insulin pulsatility is required to preserve subsequent insulin sensitivity during hyperinsulinemia. We studied the effect of overnight pulsatile versus continuous basal insulin on a subsequent hyperinsulinemic euglycemic clamp. Nineteen normal volunteers (male:female ratio, 17:2; mean age +/- SEM, 26.1 +/- 2.3 years) were studied on 2 occasions each. Endogenous insulin secretion was inhibited by octreotide (0.43 microg kg(-1). h(-1)) and replaced overnight at 5.4 mU kg(-1). h(-1) either by continuous infusion or in 2-minute pulses every 13 minutes (n = 10) or every 7 minutes (n = 9). Glucagon was replaced at physiological concentration by continuous infusion (30 ng. kg(-1). h(-1)). Venous plasma glucose overnight was not significantly different between the pulsatile and continuous protocols. After discontinuing the overnight insulin infusion, insulin action was assessed during a hyperinsulinemic euglycemic clamp (1 mU kg(-1). h(-1)). Glucose infusion rates at steady-state during the hyperinsulinemic clamp were similar between continuous and both frequencies of pulsatile infusion (continuous 44.6 +/- 4.3 micromol. kg(-1). min(-1) v 13-minute pulsatile 41.7 +/- 5.9 micromol. kg(-1). min(-1), P =.27; continuous 34.6 +/- 2.5 micromol. kg(-1) min(-1) v 7-minute pulsatile 41.4 +/- 3.2 micromol. kg(-1). min(-1), P =.08). We conclude that overnight pulsatile compared with continuous insulin administration has no different effect on subsequent peripheral insulin-mediated glucose uptake. A priming effect cannot therefore explain the previously demonstrated association between endogenous insulin pulse frequency and peripheral insulin action.     

Effects of recombinant human growth hormone on hepatic lipid and carbohydrate metabolism in HIV-infected patients with fat accumulation

We recently reported that treatment with a pharmacologic dose of recombinant human growth hormone (GH) resulted in a significant loss of body fat and gain in lean tissue in HIV-infected patients with syndromes of fat accumulation. However, insulin-mediated glucose disposal decreased transiently after one month of GH therapy. The present paper focuses on the changes of hepatic carbohydrate and fat metabolism associated with GH treatment in the same subjects. We assessed hepatic insulin sensitivity under both fasting and hyperinsulinemic-euglycemic clamp conditions prior to and after one and six months of GH treatment (3 mg/day) in five patients using stable isotope tracer techniques. Indirect calorimetry, and measurements of lipid concentrations. Fasting endogenous glucose production (EGP) increased significantly at one month (12.0 +/- 0.7 to 14.9 +/- 0.9 micromol/kg/min, P < 0.03), and the increase was sustained at six months of GH treatment (14.0 +/- 1.1 micromol/kg/min, NS). This increase in EGP was driven in part by increased glucogenesis (GNG) (3.5 +/- 0.9 to 5.2 +/- 0.9 and 5.8 +/-1.2 micromol/kg/min, n = 4, P < 0.01 and P < 0.01 at one and six months, respectively); small changes in hepatic glycogenolysis also contributed. Sustained increases in lipolysis and progressive decreases in hepatic fractional de novo lipogenesis (DNL) and triglyceride concentrations occurred with GH treatment. These changes were accompanied by an improved lipid profile with a significant increase in HDL cholesterol and significant decreases in total and LDL cholesterol and triglyceride levels, the latter consistent with the decrease in hepatic DNL. During a hyperinsulinemic-euglycemic glucose clamp, EGP and GNG were markedly suppressed compared to the corresponding time points under fasting conditions, albeit less so when measured after one month of GH treatment. Thus, in HIV-infected patients with abnormal fat distribution, pharmacologic doses of GH improved the overall lipid profile, but worsened glucose homeostasis under both fasting and hyperinsulinemic conditions. The combined implications of these positive and negative metabolic effects for cardiovascular disease risk remain unknown.     

Effect of hepatic glucose production on acute insulin resistance induced by lipid-infusion in awake rats

AIM: To explore the influence of hepatic glucose production on acute insulin resistance induced by a lipid infusion in awake rats. METHODS: A hyperinsulinaemic-euglycaemic clamp was established in awake chronically catheterized rats. Two groups of rats were studied either with a 4-h intraarterial infusion of lipid/heparin or saline. Insulin-mediated peripheral and hepatic glucose metabolism was assessed by hyperinsulinaemic-euglycaemic clamp combined with [3-^3H]-glucose infusion. RESULTS: During hyperinsulinaemic-euglycaemic clamp,there was a significant increase in plasma free fatty acid (FFA, from 741.9&#177;50.6 to 2346.44&#177;238.5μmol/L, P&lt;0.01) in lipid-infused group. The glucose infusion rates (GIR) in the lipid infusion rats, compared to control rats, were significantly reduced (200-240 min average: lipid infusion; 12.64&#177;1.5 vs control; 34.04&#177;1.6 mg/kg.min, P&lt;0.01), declining to - 35% of the corresponding control values during the last time of the clamp (240min: lipid infusion; 12.04&#177;1.9 vs control; 34.74&#177;1.7 mg/kg&#183;min, P&lt;0.0001). At the end of clamp study,the hepatic glucose production (HGP) in control rats was significantly suppressed (88%) from 19.04&#177;4.5 (basal) to 2.34&#177;0.9 mg/kg.min (P&lt;0.01). The suppressive effect of insulin on HGP was significantly blunted in the lipid-infused rats (200-240 min: from 18.74&#177;3.0 to 23.24&#177;3.1 mg/kg.min (P&lt;0.05). The rate of glucose disappearance (GRd) was a slight decrease in the lipid-infused rats compared with controls during the clamp.CONCLUSION: These data suggest that lipid infusion could induces suppression of hepatic glucose production, impairs the abilities of insulin to suppress lipolysis and mediate glucose utilization in peripheral tissue. Therefore, we conclude that lipid-infusion induces an acute insulin resistance in vivo.     

Sustained reduction in plasma free fatty acid concentration improves insulin action without altering plasma adipocytokine levels in subjects with strong family history of type 2 diabetes

To investigate the effect of a sustained (7-d) decrease in plasma free fatty acid (FFA) concentration in individuals genetically predisposed to develop type 2 diabetes mellitus (T2DM), we studied the effect of acipimox, a potent inhibitor of lipolysis, on insulin action and adipocytokine concentrations in eight normal glucose-tolerant subjects (aged 40 +/- 4 yr, body mass index 26.5 +/- 0.8 kg/m(2)) with at least two first-degree relatives with T2DM. Subjects received an oral glucose tolerance test (OGTT) and 120 min euglycemic insulin clamp (80 mU/m(2).min) with 3-[(3)H] glucose to quantitate rates of insulin-mediated whole-body glucose disposal (Rd) and endogenous (primarily hepatic) glucose production (EGP) before and after acipimox, 250 mg every 6 h for 7 d. Acipimox significantly reduced fasting plasma FFA (515 +/- 64 to 285 +/- 58 microm, P < 0.05) and mean plasma FFA during the OGTT (263 +/- 32 to 151 +/- 25 microm, P < 0.05); insulin-mediated suppression of plasma FFA concentration during the insulin clamp also was enhanced (162 +/- 18 to 120 +/- 15 microm, P < 0.10). Following acipimox, fasting plasma glucose (5.1 +/- 0.1 vs. 5.2 +/- 0.1 mm) did not change, whereas mean plasma glucose during the OGTT decreased (7.6 +/- 0.5 to 6.9 +/- 0.5 mm, P < 0.01) without change in mean plasma insulin concentration (402 +/- 90 to 444 +/- 102 pmol/liter). After acipimox Rd increased from 5.6 +/- 0.5 to 6.8 +/- 0.5 mg/kg.min (P < 0.01) due to an increase in insulin-stimulated nonoxidative glucose disposal (2.5 +/- 0.4 to 3.5 +/- 0.4 mg/kg.min, P < 0.05). The increment in Rd correlated closely with the decrement in fasting plasma FFA concentration (r = -0.80, P < 0.02). Basal EGP did not change after acipimox (1.9 +/- 0.1 vs. 2.0 +/- 0.1 mg/kg.min), but insulin-mediated suppression of EGP improved (0.22 +/- 0.09 to 0.01 +/- 0.01 mg/kg.min, P < 0.05). EGP during the insulin clamp correlated positively with the fasting plasma FFA concentration (r = 0.49, P = 0.06) and the mean plasma FFA concentration during the insulin clamp (r = 0.52, P < 0.05). Plasma adiponectin (7.1 +/- 1.0 to 7.2 +/- 1.1 microg/ml), resistin (4.0 +/- 0.3 to 3.8 +/- 0.3 ng/ml), IL-6 (1.4 +/- 0.3 to 1.6 +/- 0.4 pg/ml), and TNFalpha (2.3 +/- 0.3 to 2.4 +/- 0.3 pg/ml) did not change after acipimox treatment.We concluded that sustained reduction in plasma FFA concentration in subjects with a strong family history of T2DM increases peripheral (muscle) and hepatic insulin sensitivity without increasing adiponectin levels or altering the secretion of other adipocytokines by the adipocyte. These results suggest that lipotoxicity already is well established in individuals who are genetically predisposed to develop T2DM and that drugs that cause a sustained reduction in the elevated plasma FFA concentration may represent an effective modality for the prevention of T2DM in high-risk, genetically predisposed, normal glucose-tolerant individuals despite the lack of an effect on adipocytokine concentrations.     

Increased insulin clearance in peroxisome proliferator-activated receptor gamma2 Pro12Ala

The Pro12Ala polymorphism of the peroxisome proliferator-activated receptor (PPARgamma(2)) is associated with reduced risk for type 2 diabetes. Although increased insulin sensitivity of glucose disposal and lipolysis has been reported, the exact mechanism by which the risk reduction is conferred is not clear. Because the conclusion of greater insulin sensitivity hinged upon lower insulin levels in some studies, it is possible that more efficient insulin clearance is involved. We therefore estimated insulin clearance during a euglycemic hyperinsulinemic clamp (insulin infusion rate divided by steady-state insulin concentration, 229 normal glucose tolerant [NGT] subjects), an oral glucose tolerance test (OGTT) (mean C-peptide divided by mean insulin concentrations, 406 NGT, 54 impaired glucose tolerant or mildly diabetic subjects), and a hyperglycemic clamp (120 minutes, 10 mmol/L, C-peptide divided by insulin in the steady-state, 56 NGT subjects). In the carriers of the Ala allele (prevalence approximately 24%), insulin clearance in all 3 protocols was significantly greater ( approximately 10%), than in controls. While the results from the euglycemic clamp reflect both hepatic and peripheral insulin clearance, those from the OGTT and the hyperglycemic clamp reflect mainly hepatic insulin extraction. Free fatty acids (FFA) during the steady state of the euglycemic hyperinsulinemic clamp were significantly lower in carriers of the Ala allele (26 +/- 5 micromol/L) than in controls (46 +/- 3 micromol/L, P =.02). In conclusion, the Pro12Ala polymorphism is associated with increased insulin clearance. This could be the result of reduced FFA delivery, which has been shown to improve hepatic insulin removal and sensitivity. Because PPARgamma(2) is mainly expressed in adipose tissue, one of the main regulatory effects of the polymorphism may well be the more efficient suppression of (possibly intra-abdominal) lipolysis.     

Effect of pioglitazone on abdominal fat distribution and insulin sensitivity in type 2 diabetic patients

We examined the effect of pioglitazone on abdominal fat distribution to elucidate the mechanisms via which pioglitazone improves insulin resistance in patients with type 2 diabetes mellitus. Thirteen type 2 diabetic patients (nine men and four women; age, 52 +/- 3 yr; body mass index, 29.0 +/- 1.1 kg/m(2)), who were being treated with a stable dose of sulfonylurea (n = 7) or with diet alone (n = 6), received pioglitazone (45 mg/d) for 16 wk. Before and after pioglitazone treatment, subjects underwent a 75-g oral glucose tolerance test (OGTT) and two-step euglycemic insulin clamp (insulin infusion rates, 40 and 160 mU/m(2).min) with [(3)H]glucose. Abdominal fat distribution was evaluated using magnetic resonance imaging at L4-5. After 16 wk of pioglitazone treatment, fasting plasma glucose (179 +/- 10 to 140 +/- 10 mg/dl; P < 0.01), mean plasma glucose during OGTT (295 +/- 13 to 233 +/- 14 mg/dl; P < 0.01), and hemoglobin A(1c) (8.6 +/- 0.4% to 7.2 +/- 0.5%; P < 0.01) decreased without a change in fasting or post-OGTT insulin levels. Fasting plasma FFA (674 +/- 38 to 569 +/- 31 microEq/liter; P < 0.05) and mean plasma FFA (539 +/- 20 to 396 +/- 29 microEq/liter; P < 0.01) during OGTT decreased after pioglitazone. In the postabsorptive state, hepatic insulin resistance [basal endogenous glucose production (EGP) x basal plasma insulin concentration] decreased from 41 +/- 7 to 25 +/- 3 mg/kg fat-free mass (FFM).min x microU/ml; P < 0.05) and suppression of EGP during the first insulin clamp step (1.1 +/- 0.1 to 0.6 +/- 0.2 mg/kg FFM.min; P < 0.05) improved after pioglitazone treatment. The total body glucose MCR during the first and second insulin clamp steps increased after pioglitazone treatment [first MCR, 3.5 +/- 0.5 to 4.4 +/- 0.4 ml/kg FFM.min (P < 0.05); second MCR, 8.7 +/- 1.0 to 11.3 +/- 1.1 ml/kg FFM(.)min (P < 0.01)]. The improvement in hepatic and peripheral tissue insulin sensitivity occurred despite increases in body weight (82 +/- 4 to 85 +/- 4 kg; P < 0.05) and fat mass (27 +/- 2 to 30 +/- 3 kg; P < 0.05). After pioglitazone treatment, sc fat area at L4-5 (301 +/- 44 to 342 +/- 44 cm(2); P < 0.01) increased, whereas visceral fat area at L4-5 (144 +/- 13 to 131 +/- 16 cm(2); P < 0.05) and the ratio of visceral to sc fat (0.59 +/- 0.08 to 0.44 +/- 0.06; P < 0.01) decreased. In the postabsorptive state hepatic insulin resistance (basal EGP x basal immunoreactive insulin) correlated positively with visceral fat area (r = 0.55; P < 0.01). The glucose MCRs during the first (r = -0.45; P < 0.05) and second (r = -0.44; P < 0.05) insulin clamp steps were negatively correlated with the visceral fat area. These results demonstrate that a shift of fat distribution from visceral to sc adipose depots after pioglitazone treatment is associated with improvements in hepatic and peripheral tissue sensitivity to insulin.     

Ghrelin enhances glucose-induced insulin secretion in scheduled meal-fed sheep

The purpose of this study was to investigate the effects of physiologic levels of ghrelin on insulin secretion and insulin sensitivity (glucose disposal) in scheduled fed-sheep, using the hyperglycemic clamp and hyperinsulinemic euglycemic clamp respectively. Twelve castrated Suffolk rams (69.8 +/- 0.6 kg) were conditioned to be fed alfalfa hay cubes (2% of body weight) once a day. Three hours after the feeding, synthetic ovine ghrelin was intravenously administered to the animals at a rate of 0.025 and 0.05 mug/kg body weight (BW) per min for 3 h. Concomitantly, the hyperglycemic clamp or the hyperinsulinemic euglycemic clamp was carried out. In the hyperglycemic clamp, a target glucose concentration was clamped at 100 mg/100 ml above the initial level. In the hyperinsulinemic euglycemic clamp, insulin was intravenously administered to the animals for 3 h at a rate of 2 mU/kg BW per min. Basal glucose concentrations (44+/- 1 mg/dl) were maintained by variably infusing 100 mg/dl glucose solution. In both clamps, plasma ghrelin concentrations were dose-dependently elevated and maintained at a constant level within the physiologic range. Ghrelin infusions induced a significant (ANOVA; P < 0.01) increase in plasma GH concentrations. In the hyperglycemic clamp, plasma insulin levels were increased by glucose infusion and were significantly (P < 0.05) greater in ghrelin-infused animals. In the hyperinsulinemic euglycemic clamp, glucose infusion rate, an index of insulin sensitivity, was not affected by ghrelin infusion. In conclusion, the present study has demonstrated for the first time that ghrelin enhances glucose-induced insulin secretion in the ruminant animal.     

Plasma resistin concentration, hepatic fat content, and hepatic and peripheral insulin resistance in pioglitazone-treated type II diabetic patients

OBJECTIVES: To study the effect of pioglitazone (PIO) on plasma resistin concentration, endogenous glucose production (EGP), and hepatic fat content (HFC) in patients with type II diabetes (T2DM). SUBJECTS: A total of 13 T2DM patients (age=51+/-2 y, BMI=29.7+/-1.1 kg/m(2), HbA(1c)=8.0+/-0.5%). METHODS: HFC (magnetic resonance spectroscopy) and basal plasma resistin concentration were quantitated before and after PIO treatment (45 mg/day) for 16 weeks. Subjects received a 3 h euglycemic insulin (100 mU/m(2)/min) clamp with 3-[(3)H] glucose to determine rates of EGP and tissue glucose disappearance (Rd) before and after PIO. RESULTS: PIO reduced fasting plasma glucose (10.3+/-0.7 to 7.6+/-0.6 mmol/l, P<0.001) and HbA(1c) (8.0+/-0.4 to 6.8+/-0.3%, P<0.001) despite increased body weight (83.2+/-3.4 to 86.3+/-3.4 kg, P<0.001). PIO improved Rd (4.9+/-0.4 to 6.6+/-0.5 mg/kg/min, P<0.005) and reduced EGP (0.22+/-0.04 to 0.06+/-0.02 mg/kg/min, P<0.01) during the insulin clamp. Following PIO, HFC decreased from 21.1+/-3.5 to 11.2+/-2.1% (P<0.005), and plasma resistin decreased from 5.3+/-0.6 to 3.5+/-0.3 ng/ml (P<0.01). Plasma resistin concentration correlated positively with HFC before (r=0.58, P<0.05) and after (r=0.55, P<0.05) PIO treatment. Taken collectively, plasma resistin concentration, before and after PIO treatment, correlated positively with hepatic fat content (r=0.66, P<0.001) and EGP during the insulin clamp (r=0.41, P<0.05). However, the plasma resistin concentration did not correlate with whole body glucose disposal (Rd) during the insulin clamp either before (r=-0.18, P=NS) or after (r=-0.13, P=NS) PIO treatment. CONCLUSIONS: PIO treatment in T2DM causes a significant decrease in plasma resistin concentration. The decrease in plasma resistin is positively correlated with the decrease in hepatic fat content and improvement in hepatic insulin sensitivity.     

Glucose and lipid metabolism after liver transplantation in inbred rats: consequences of hepatic denervation

The liver plays a central role in glucose and lipid homeostasis. Because liver transplantation severs the hepatic nerves which influence this function, we hypothesized that insulin resistance and hyperlipidemia develop after liver transplantation, thus increasing the atherosclerotic risk. Therefore, we studied inbred rats 8 months after orthotopic liver transplantation (Tx, n = 39) or laparotomy (sham, n = 37) by either oral glucose tolerance test (Tx, n = 13; sham, n = 8), meal tolerance test (Tx, n = 9; sham, n = 13), or euglycemic hyperinsulinemic clamp with tritiated glucose infusion (Tx, n = 17; sham, n = 16). We found that liver transplantation significantly increased basal hepatic glucose production (HGP) in the clamp study by 20% (37.3 +/- 2.2 vs 31.0 +/- 2.1 micromol kg -1 .min -1 , P < .05) and fasting plasma low-density lipoprotein (LDL) cholesterol by 36% (0.79 +/- 0.06 vs 0.58 +/- 0.05 mmol/L, P < .05). However, it did not affect HGP, total glucose uptake, metabolic clearance rate of insulin, and suppression of plasma nonesterified fatty acids, which were all normal in response to rising plasma insulin concentrations in the dose-response clamp studies. The oral glucose tolerance test and meal tolerance test also showed normal glucose and nonesterified fatty acids homeostasis with adequate pancreatic insulin secretion and hepatic insulin clearance after liver transplantation. The only consequences of liver transplantation are increased basal HGP and plasma LDL cholesterol, which may be caused by persistent vagal denervation of the liver. Although insulin resistance is absent, elevated plasma LDL cholesterol increases the atherosclerotic risk.     

Effect of a 2-day very low-energy diet on skeletal muscle insulin sensitivity in obese type 2 diabetic patients on insulin therapy

This study investigates the molecular mechanisms underlying the blood glucose-lowering effect of a 2-day very low-energy diet (VLED, 1883 kJ/d) in 12 obese (body mass index, 36.3 +/- 1.0 kg/m2 [mean +/- SEM]) type 2 diabetic (HbA(1C) 7.3% +/- 0.4%) patients simultaneously taken off all glucose-lowering therapy, including insulin. Endogenous glucose production (EGP) and glucose disposal ([6,6-2H2]-glucose) were measured before and after the VLED in basal and hyperinsulinemic (40 mU/m2 per minute) euglycemic conditions. Insulin signaling and expression of GLUT-4, FAT/CD36, and triglycerides were assessed in muscle biopsies, obtained before the clamp and after 30 minutes of hyperinsulinemia. Fasting plasma glucose decreased from 11.3 +/- 1.3 to 10.3 +/- 1.0 mmol/L because of a decreased basal EGP (14.2 +/- 1.0 to 11.9 +/- 0.7 micromol/kg per minute, P = .009). Insulin-stimulated glucose disposal did not change. No diet effect was found on the expression of the insulin receptor and insulin receptor substrate-1 or on phosphatidylinositol 3'-kinase activity, or on FAT/CD36 expression pattern, GLUT-4 translocation, or triglyceride distribution in either the basal or insulin-stimulated situation. Unexpectedly, basal PKB/Akt phosphorylation on T308 and S473 increased after the diet, at equal protein expression. In conclusion, a 2-day VLED lowers fasting plasma glucose via a decreased basal EGP without an effect on glucose disposal. Accordingly, no changes in activation of phosphatidylinositol 3'-kinase, triglyceride distribution, FAT/CD36 expression, and GLUT-4 translocation were found in skeletal muscle biopsies.     

The effect of CP 68,722, a thiozolidinedione derivative, on insulin sensitivity in lean and obese Zucker rats

The effect of a new drug (CP 68,722, Pfizer) on parameters of insulin sensitivity in an established insulin-resistant animal model was examined. Rates of hepatic glucose production (HGP) and peripheral glucose uptake in obese Zucker (fa/fa) rats treated with a 10-day course of the medication using a two-step (2 and 10 mU/kg/min) euglycemic hyperinsulinemic clamp technique were measured. In addition, changes in substrate concentrations after drug treatment were examined. Basal HGP rates were similar in the lean versus the obese animals (37 +/- 3 v 39 +/- 3 mumol/kg/min); however, the obese animals had impaired insulin-induced suppression of HGP at both 2 mU/kg/min (36 +/- 3 v 23 +/- 4 mumol/kg/min) and 10 mU/kg/min (18 +/- 5 v 2 +/- 1 mumol/kg/min). Insulin stimulation of glucose disposal was also defective in the obese animals (37 +/- 2 v 88 +/- 7 mumol/kg/min at 2 mU/kg/min and 98 +/- 9 v 219 +/- 18 mumol/kg/min at 10 mU/kg/min). In addition, obese animals had elevated free fatty acid (FFA) and ketone levels, both of which were resistant to insulin-induced suppression. After drug treatment, few alterations in glucose or lipid metabolism were found in the lean animals. In the obese animals, insulin suppression of HGP was normalized during the higher insulin infusion rate (0 v 18 +/- 5 mumol/kg/min at 10 mU/kg/min), and peripheral glucose disposal was enhanced at both steps of the insulin clamp (54 +/- 4 v 37 +/- 2 mumol/kg/min at 2 mU/kg/min and 134 +/- 12 v 98 +/- 9 mumol/kg/min at 10 mU/kg/min).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of lipolysis during acute GH exposure increases insulin sensitivity in previously untreated GH-deficient adults

OBJECTIVE: Previous studies evaluating the lipolytic effect of GH have in general been performed in subjects on chronic GH therapy. In this study we assessed the lipolytic effect of GH in previously untreated patients and examined whether the negative effect of enhanced lipolysis on glucose metabolism could be counteracted by acute antilipolysis achieved with acipimox. METHODS: Ten GH-deficient (GHD) adults participated in four experiments each, during which they received in a double-blind manner: placebo (A); GH (0.88+/-0.13 mg) (B); GH+acipimox 250 mg b.i.d. (C); and acipimox b.i.d. (no GH) (D), where GH was given the night before a 2 h euglycemic, hyperinsulinemic clamp combined with infusion of [3-(3)H]glucose and indirect calorimetry. RESULTS: GH increased basal free fatty acid (FFA) levels by 74% (P=0.0051) and insulin levels by 93% (P=0.0051). This resulted in a non-significant decrease in insulin-stimulated glucose uptakes (16.61+/-8.03 vs 12.74+/-5.50 micromol/kg per min (s.d.), P=0.07 for A vs B). The rates of insulin-stimulated glucose uptake correlated negatively with the FFA concentrations (r=-0.638, P<0.0001). However, acipimox caused a significant improvement in insulin-stimulated glucose uptake in the GH-treated patients (17.35+/-5.65 vs 12.74+/-5.50 micromol/kg per min, P=0.012 for C vs B). The acipimox-induced enhancement of insulin-stimulated glucose uptake was mainly due to an enhanced rate of glucose oxidation (8.32+/-3.00 vs 5.88+/-2.39 micromol/kg per min, P=0.07 for C vs B). The enhanced rates of glucose oxidation induced by acipimox correlated negatively with the rate of lipid oxidation in GH-treated subjects both in basal (r=-0.867, P=0.0093) and during insulin-stimulated (r=-0.927, P=0.0054) conditions. GH did not significantly impair non-oxidative glucose metabolism (6.86+/-5.22 vs 8.67+/-6.65 micromol/kg per min, P=NS for B vs A). The fasting rate of endogenous glucose production was unaffected by GH and acipimox administration (10.99+/-1.98 vs 11.73+/-2.38 micromol/kg per min, P=NS for B vs A and 11.55+/-2.7 vs 10.99+/-1.98 micromol/kg per min, P=NS for C vs B). On the other hand, acipimox alone improved glucose uptake in the untreated GHD patients (24.14+/-8.74 vs 16.61+/-8.03 micromol/kg per min, P=0.0077 for D vs A) and this was again due to enhanced fasting (7.90+/-2.68 vs 5.16+/-2.28 micromol/kg per min, P=0.01 for D vs A) and insulin-stimulated (9.78+/-3.68 vs 7.95+/-2.64 micromol/kg per min, P=0.07 for D vs A) glucose oxidation. CONCLUSION: The study of acute administration of GH to previously untreated GHD patients provides compelling evidence that (i) GH-induced insulin resistance is mainly due to induction of lipolysis by GH; and (ii) inhibition of lipolysis can prevent the deterioration of insulin sensitivity. The question remains whether GH replacement therapy should, at least at the beginning of therapy, be combined with means to prevent an excessive stimulation of lipolysis by GH.     

Impaired splanchnic and peripheral glucose uptake in liver cirrhosis.

BACKGROUND/AIM: Patients with liver cirrhosis are insulin-resistant and frequently glucose-intolerant. Although peripheral glucose uptake has been shown to be impaired in liver cirrhosis, little is known about the significance of splanchnic (hepatic) glucose uptake after oral glucose load. METHODS/RESULTS: We performed an oral glucose tolerance test and euglycemic hyperinsulinemic clamp with oral glucose load for eight patients with liver cirrhosis and eight patients with chronic active hepatitis. The patients with liver cirrhosis had higher plasma glucose levels 2 h after glucose load than those with chronic active hepatitis (228+/-22 mg/dl vs. 102+/-9 mg/dl, p<0.01). Using the euglycemic hyperinsulinemic clamp with oral glucose load, we simultaneously measured peripheral and splanchnic glucose uptake. Peripheral glucose uptake in liver cirrhosis was 6.1+/-0.7 mg x kg(-1) x min(-1), which was lower than that in healthy volunteers (10.5+/-0.9 mg x kg(-1) x min(-1), p<0.05) and in chronic active hepatitis (8.4+/-0.3 mg x kg(-1) x min(-1), p<0.05). Furthermore, splanchnic glucose uptake in liver cirrhosis was much lower (20.1+/-3.4%) than in healthy volunteers (36.0+/-4.0%, p<0.05) and in chronic active hepatitis (37.2+/-3.1%, p<0.05). CONCLUSION: These results suggest that glucose intolerance in patients with liver cirrhosis is caused by a defect of the glucose uptake of both splanchnic and peripheral tissues.     

A potential important role of skeletal muscle in human counterregulation of hypoglycemia

CONTEXT: During hypoglycemia, systemic glucose uptake (SGU) decreases and endogenous glucose release (EGR) increases. Skeletal muscle appears to be primarily responsible for the reduced SGU and may be important for the increased EGR by providing lactate for gluconeogenesis (GN). OBJECTIVE: The objective of the study was to test the hypothesis that reduced muscle glucose uptake and increased muscle lactate release both make major contributions to glucose counterregulation using systemic isotopic techniques in combination with forearm net balance measurements. SETTING: The study was conducted at the University of Giessen Clinical Research Center. PARTICIPANTS: Nine healthy volunteers participated in the study. Intervention: A 2-h hyperinsulinemic euglycemic clamp (blood glucose approximately 4.4 mm) was followed by a 90-min hypoglycemic clamp (blood glucose approximately 2.6 mm). RESULTS: Compared with the euglycemic clamp, SGU decreased (21.0 +/- 2.0 vs. 29.6 +/- 1.8 micromol.kg body weight(-1).min(-1); P < 0.001), whereas EGR (11.2 +/- 1.7 vs. 4.9 +/- 1.3 micromol.kg body weight(-1) .min(-1); P < 0.003), arterial lactate concentrations (1051 +/- 162 vs. 907 +/- 115 microm; P < 0.02), systemic lactate release (23.5 +/- 0.9 vs. 17.1 +/- 0.9 micromol.kg body weight(-1).min(-1); P < 0.001), and lactate GN (4.50 +/- 0.60 vs. 2.74 +/- 0.30 micromol.kg body weight(-1).min(-1); P < 0.02) increased during hypoglycemia; the proportion of lactate used for GN remained unchanged (38 +/- 4 vs. 32 +/- 3%; P = 0.27). Whole-body muscle glucose uptake decreased approximately 50% during hypoglycemia (6.4 +/- 1.9 vs. 13.6 +/- 2.9 micromol.kg body weight(-1).min(-1); P < 0.001), which accounted for approximately 85% of the reduction of SGU. Whole-body muscle lactate release increased 6.6 +/- 1.6 micromol.kg body weight(-1). min(-1) (P < 0.01), which could have accounted for all the increase in systemic lactate release and, considering the proportion of lactate used for GN, contributed 1.4 +/- 0.4 micromol.kg body weight(-1).min(-1) (approximately 25%) to the increase in EGR. CONCLUSIONS: Reduced muscle glucose uptake and increased muscle lactate release both make major contributions to glucose counterregulation in humans.     

Growth hormone response to GHRH + GHRP-6 in type 2 diabetes during euglycemic and hyperglycemic clamp

The aim of this study was to investigate the effect of two different glucose levels on GH response to the combined administration of GHRH+GHRP-6 in patients with type 2 diabetes. GH response to i.v. bolus of GHRH+GHRP-6 (100 mcg, each) was measured in 12 male patients with type 2 diabetes (mean age: 53.9+/-1.59 years; BMI: 25.58+/-0.39 kg/m(2); mean HbA(1c): 8.7+/-0.42%), during a euglycemic (mean glucose: 4.92+/-0.08 mmol) hyperinsulinemic clamp (insulin infusion rate of 100 mU/kg/h) and a hyperglycemic clamp (mean glucose: 12.19+/-0.11 mmol/l). There was no difference in basal GH levels between the hyperglycemic and euglycemic clamps (2.9+/-0.99 mU/l versus 1.48+/-0.44 mU/l; P>0.05). Peak GH response to GHRH+GHRP-6 during the hyperglycemic clamp was lower than in the englycemic clamp (112.45+/-14.45 mU/l versus 151.06+/-16.87 mU/l; P<0.05). Area under the GH curve was lower in the hyperglycemic than in the euglycemic clamp (6974.49+/-1001.95 mU/l/min versus 9560.75+/-1140.65 mU/l/min; P<0.05). It is concluded that hyperglycemia significantly reduces GH response to combined administration of GHRH+GHRP-6 in normal weight patients with type 2 diabetes. It is suggested that ambient glucose levels should be taken into account during interpretation of GH response to combined administration of GHRH+GHRP-6 in patients with type 2 diabetes.