Melanie Scott

Insulin's direct effects on the liver dominate the control of hepatic glucose production

Insulin inhibits glucose production through both direct and indirect effects on the liver; however, considerable controversy exists regarding the relative importance of these effects. The first aim of this study was to determine which of these processes dominates the acute control of hepatic glucose production (HGP). Somatostatin and portal vein infusions of insulin and glucagon were used to clamp the pancreatic hormones at basal levels in the nondiabetic dog. After a basal sampling period, insulin infusion was switched from the portal vein to a peripheral vein. As a result, the arterial insulin level doubled and the hepatic sinusoidal insulin level was reduced by half. While the arterial plasma FFA level and net hepatic FFA uptake fell by 40-50%, net hepatic glucose output increased more than 2-fold and remained elevated compared with that in the control group. The second aim of this study was to determine the effect of a 4-fold rise in head insulin on HGP during peripheral hyperinsulinemia and hepatic insulin deficiency. Sensitivity of the liver was not enhanced by increased insulin delivery to the head. Thus, this study demonstrates that the direct effects of insulin dominate the acute regulation of HGP in the normal dog.

Chronic consumption of a high fat/high fructose diet renders the liver incapable of net hepatic glucose uptake

The objective of this study was to assess the response of a large animal model to high dietary fat and fructose (HFFD). Three different metabolic assessments were performed during 13 wk of feeding an HFFD (n = 10) or chow control (CTR, n = 4) diet: oral glucose tolerance tests (OGTTs; baseline, 4 and 8 wk), hyperinsulinemic-euglycemic clamps (HIEGs; baseline and 10 wk) and hyperinsulinemic-hyperglycemic clamps (HIHGs, 13 wk). The ΔAUC for glucose during the OGTTs more than doubled after 4 and 8 wk of HFFD feeding, and the average glucose infusion rate required to maintain euglycemia during the HIEG clamps decreased by ≈30% after 10 wk of HFFD feeding. These changes did not occur in the CTR group. The HIHG clamps included experimental periods 1 (P1, 0-90 min) and 2 (P2, 90-180 min). During P1, somatostatin, basal intraportal glucagon, 4 × basal intraportal insulin, and peripheral glucose (to double the hepatic glucose load) were infused; during P2, glucose was also infused intraportally (4.0 mg·kg(-1)·min(-1)). Net hepatic glucose uptake during P1 and P2 was -0.4 ± 0.1 [output] and 0.2 ± 0.8 mg·kg(-1)·min(-1) in the HFFD group, respectively, and 1.8 ± 0.8 and 3.5 ± 1.0 mg·kg(-1)·min(-1) in the CTR group, respectively (P < 0.05 vs. HFFD during P1 and P2). Glycogen synthesis through the direct pathway was 0.5 ± 0.2 and 1.5 ± 0.4 mg·kg(-1)·min(-1) in the HFFD and CTR groups, respectively (P < 0.05 vs. HFFD). In conclusion, chronic consumption of an HFFD diminished the sensitivity of the liver to hormonal and glycemic cues and resulted in a marked impairment in NHGU and glycogen synthesis.

Inhibition of Dipeptidyl Peptidase-4 by Vildagliptin during Glucagon-like Peptide-1 Infusion Increases Liver Glucose Uptake in the Conscious Dog

OBJECTIVE—This study investigated the acute effects of treatment with vildagliptin on dipeptidyl peptidase-4 (DPP-4) activity, glucagon-like peptide 1 (GLP-1) concentration, pancreatic hormone levels, and glucose metabolism. The primary aims were to determine the effects of DPP-4 inhibition on GLP-1 clearance and on hepatic glucose uptake. RESEARCH DESIGN AND METHODS—Fasted conscious dogs were studied in the presence (n = 6) or absence (control, n = 6) of oral vildagliptin (1 mg/kg). In both groups, GLP-1 was infused into the portal vein (1 pmol · kg−1 · min−1) for 240 min. During the same time, glucose was delivered into the portal vein at 4 mg · kg−1 · min−1 and into a peripheral vein at a variable rate to maintain the arterial plasma glucose level at 160 mg/dl. RESULTS—Vildagliptin fully inhibited DPP-4 over the 4-h experimental period. GLP-1 concentrations were increased in the vildagliptin-treated group (50 ± 3 vs. 85 ± 7 pmol/l in the portal vein in control and vildagliptin-treated dogs, respectively; P < 0.05) as a result of a 40% decrease in GLP-1 clearance (38 ± 5 and 22 ± 2 ml · kg−1 · min−1, respectively; P < 0.05). Although hepatic insulin and glucagon levels were not significantly altered, there was a tendency for plasma insulin to be greater (hepatic levels were 73 ± 10 vs. 88 ± 15 μU/ml, respectively). During vildagliptin treatment, net hepatic glucose uptake was threefold greater than in the control group. This effect was greater than that predicted by the change in insulin. CONCLUSIONS—Vildagliptin fully inhibited DPP-4 activity, reduced GLP-1 clearance by 40%, and increased hepatic glucose disposal by means beyond the effects of GLP-1 on insulin and glucagon secretion.

Effect of 11β-hydroxysteroid dehydrogenase-1 inhibition on hepatic glucose metabolism in the conscious dog

Inactive cortisone is converted to active cortisol within the liver by 11 beta-hydroxysteroid dehydrogenase-1 (11 beta-HSD1), and impaired regulation of this process may be related to increased hepatic glucose production (HGP) in individuals with type 2 diabetes. The primary aim of this study was to investigate the effect of acute 11 beta-HSD1 inhibition on HGP and fat metabolism during insulin deficiency. Sixteen conscious, 42-h-fasted, lean, healthy dogs were studied. Somatostatin was infused to create insulin deficiency, and the animals were treated with a specific 11 beta-HSD1 inhibitor (compound 531) or placebo for 5 h. 11 beta-HSD1 inhibition completely suppressed hepatic cortisol production, and this attenuated the increase in HGP that occurred during insulin deficiency. PEPCK and glucose-6-phosphatase expression were decreased when 11 beta-HSD1 was inhibited, but gluconeogenic flux was unchanged, implying an effect on glycogenolysis. Since inhibition of hepatic cortisol production reduces HGP during insulin deficiency, 11 beta-HSD1 is a potential therapeutic target for the treatment of excess glucose production that occurs in diabetes.

Inhibition of glycogenolysis enhances gluconeogenic precursor uptake by the liver of conscious dogs

We investigated the effect of inhibiting glycogenolysis on gluconeogenesis in 18-h-fasted conscious dogs with the use of intragastric administration of BAY R 3401, a glycogen phosphorylase inhibitor. Isotopic ([3-3H]glucose and [U-14C]alanine) and arteriovenous difference methods were used to assess glucose metabolism. Each study consisted of a 100-min equilibration, a 40-min control, and two 90-min test periods. Endogenous insulin and glucagon secretions were inhibited with somatostatin (0.8 microgram.kg-1.min-1), and the two hormones were replaced intraportally (insulin: 0.25 mU.kg-1.min-1; glucagon: 0.6 ng.kg-1.min-1). Drug (10 mg/kg) or placebo was given after the control period. Insulin and glucagon were kept at basal levels in the first test period, after which glucagon infusion was increased to 2.4 ng.kg-1.min-1; BAY R 3401 decreased tracer-determined endogenous glucose production [rate of glucose production (Ra): 14 +/- 1 to 7 +/- 1 mumol.kg-1.min-1] and net hepatic glucose output (11 +/- 1 to 3 +/- 2 mumol.kg-1.min-1) during test 1. It increased the net hepatic uptake of gluconeogenic substrates from 9.0 +/- 2.0 to 11.6 +/- 0.6 mumol.kg-1.min-1. Basal glycogenolysis was decreased by drug (9.1 +/- 0.7 to 1.5 +/- 0.2 mumol glucosyl U.kg-1.min-1). Placebo had no effect on Ra or the uptake of gluconeogenic precursors by the liver. The rise in glucagon increased Ra by 22 +/- 3 and by 8 +/- 2 mumol.kg-1.min-1 (at 10 min) in placebo and drug, respectively. The rise in glucagon caused little change in the net hepatic uptake (mumol.kg-1.min-1) of gluconeogenic substrates in placebo (8.2 +/- 0.6 to 9.0 +/- 1.0) but increased it markedly (11.6 +/- 0.6 to 15.4 +/- 1.0) in drug. Glucagon increased glycogenolysis by 22.1 +/- 2.5 and by 7.8 +/- 1.6 mumol.kg-1.min-1 in placebo and drug, respectively. The amount of glycogen (mumol glucosyl U/kg) synthesized from gluconeogenic carbon was four times higher in drug (48.6 +/- 9.7) than in placebo (11.3 +/- 1.7). We conclude that BAY R 3401 caused a marked reduction in basal and glucagon-stimulated glycogenolysis. As a result of these changes, there was an increase in the net hepatic uptake of gluconeogenic precursors and in glycogen synthesis.

Insulin Delivery Into the Peripheral Circulation: A Key Contributor to Hypoglycemia in Type 1 Diabetes.

Hypoglycemia limits optimal glycemic control in type 1 diabetes mellitus (T1DM), making novel strategies to mitigate it desirable. We hypothesized that portal (Po) vein insulin delivery would lessen hypoglycemia. In the conscious dog, insulin was infused into the hepatic Po vein or a peripheral (Pe) vein at a rate four times of basal. In protocol 1, a full counterregulatory response was allowed, whereas in protocol 2, glucagon was fixed at basal, mimicking the diminished α-cell response to hypoglycemia seen in T1DM. In protocol 1, glucose fell faster with Pe insulin than with Po insulin, reaching 56 ± 3 vs. 70 ± 6 mg/dL (P = 0.04) at 60 min. The change in area under the curve (ΔAUC) for glucagon was similar between Pe and Po, but the peak occurred earlier in Pe. The ΔAUC for epinephrine was greater with Pe than with Po (67 ± 17 vs. 36 ± 14 ng/mL/180 min). In protocol 2, glucose also fell more rapidly than in protocol 1 and fell faster in Pe than in Po, reaching 41 ± 3 vs. 67 ± 2 mg/dL (P < 0.01) by 60 min. Without a rise in glucagon, the epinephrine responses were much larger (ΔAUC of 204 ± 22 for Pe vs. 96 ± 29 ng/mL/180 min for Po). In summary, Pe insulin delivery exacerbates hypoglycemia, particularly in the presence of a diminished glucagon response. Po vein insulin delivery, or strategies that mimic it (i.e., liver-preferential insulin analogs), should therefore lessen hypoglycemia.

Changes in Glucose and Fat Metabolism in Response to the Administration of a Hepato-Preferential Insulin Analog

Endogenous insulin secretion exposes the liver to three times higher insulin concentrations than the rest of the body. Because subcutaneous insulin delivery eliminates this gradient and is associated with metabolic abnormalities, functionally restoring the physiologic gradient may provide therapeutic benefits. The effects of recombinant human insulin (HI) delivered intraportally or peripherally were compared with an acylated insulin model compound (insulin-327) in dogs. During somatostatin and basal portal vein glucagon infusion, insulin was infused portally (PoHI; 1.8 pmol/kg/min; n = 7) or peripherally (PeHI; 1.8 pmol/kg/min; n = 8) and insulin-327 (Pe327; 7.2 pmol/kg/min; n = 5) was infused peripherally. Euglycemia was maintained by glucose infusion. While the effects on liver glucose metabolism were greatest in the PoHI and Pe327 groups, nonhepatic glucose uptake increased most in the PeHI group. Suppression of lipolysis was greater during PeHI than PoHI and was delayed in Pe327 infusion. Thus small increments in portal vein insulin have major consequences on the liver, with little effect on nonhepatic glucose metabolism, whereas insulin delivered peripherally cannot act on the liver without also affecting nonhepatic tissues. Pe327 functionally restored the physiologic portal–arterial gradient and thereby produced hepato-preferential effects.

Hepatic glycogen can regulate hypoglycemic counterregulation via a liver-brain axis

Liver glycogen is important for the counterregulation of hypoglycemia and is reduced in individuals with type 1 diabetes (T1D). Here, we examined the effect of varying hepatic glycogen content on the counterregulatory response to low blood sugar in dogs. During the first 4 hours of each study, hepatic glycogen was increased by augmenting hepatic glucose uptake using hyperglycemia and a low-dose intraportal fructose infusion. After hepatic glycogen levels were increased, animals underwent a 2-hour control period with no fructose infusion followed by a 2-hour hyperinsulinemic/hypoglycemic clamp. Compared with control treatment, fructose infusion caused a large increase in liver glycogen that markedly elevated the response of epinephrine and glucagon to a given hypoglycemia and increased net hepatic glucose output (NHGO). Moreover, prior denervation of the liver abolished the improved counterregulatory responses that resulted from increased liver glycogen content. When hepatic glycogen content was lowered, glucagon and NHGO responses to insulin-induced hypoglycemia were reduced. We conclude that there is a liver-brain counterregulatory axis that is responsive to liver glycogen content. It remains to be determined whether the risk of iatrogenic hypoglycemia in T1D humans could be lessened by targeting metabolic pathway(s) associated with hepatic glycogen repletion.

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.

Inhalation of human insulin is associated with improved insulin action compared with subcutaneous injection and endogenous secretion in dogs

This study compared the effects of endogenous (portal) insulin secretion versus peripheral insulin administration with subcutaneous or inhaled human insulin [INH; Exubera, insulin human (rDNA origin) inhalation powder] on glucose disposal in fasted dogs. In the control group, glucose was infused into the portal vein (Endo; n = 6). In two other groups, glucose was infused portally, whereas insulin was administered peripherally by inhalation (n = 13) or s.c. injection (n = 6) with somatostatin and basal glucagon. In the Endo group, over the first 3 h, the arterial insulin concentration was twice that of the peripheral groups, whereas hepatic sinusoidal insulin levels were half as much. Although net hepatic glucose uptake was greatest in the Endo group, the peripheral groups demonstrated larger increases in nonhepatic glucose uptake so that total glucose disposal was greater in the latter groups. Compared with s.c. insulin action, glucose excursions were smaller and shorter, and insulin action was at least twice as great after INH. Thus, at the glucose dose and insulin levels chosen, peripheral insulin delivery was associated with greater whole-body glucose disposal than endogenous (portal) insulin secretion. INH administration resulted in increased insulin sensitivity in nonhepatic but not in hepatic tissues compared with s.c. delivery.