Mary Courtney Moore

Comparison of the time courses of insulin and the portal signal on hepatic glucose and glycogen metabolism in the conscious dog.

To investigate the temporal response of the liver to insulin and portal glucose delivery, somatostatin was infused into four groups of 42-h-fasted, conscious dogs (n = 6/group), basal insulin and glucagon were replaced intraportally, and hyperglycemia was created via a peripheral glucose infusion for 90 min (period 1). This was followed by a 240-min experimental period (period 2) in which hyperglycemia was matched to period 1 and either no changes were made (CON), a fourfold rise in insulin was created (INS), a portion of the glucose (22.4 mumol.kg-1.min-1) was infused via the portal vein (Po), or a fourfold rise in insulin was created in combination with portal glucose infusion (INSPo). Arterial insulin levels were similar in all groups during period 1 (approximately 45 pM) and were 45 +/- 9, 154 +/- 20, 43 +/- 7, and 128 +/- 14 pM during period 2 in CON, INS, Po, and INSPo, respectively. The hepatic glucose load was similar between periods and among groups (approximately 278 mumol.kg-1.min-1). Net hepatic glucose output was similar among groups during period 1 (approximately 0.1 mumol.kg-1.min-1) and did not change significantly in CON during period 2. In INS net hepatic glucose uptake (NHGU; mumol.kg-1.min-1) was -3.8 +/- 3.3 at 15 min of period 2 and did not reach a maximum (-15.9 +/- 6.6) until 90 min. In contrast, NHGU reached a maximum of -13.0 +/- 3.7 in Po after only 15 min of period 2. In INSPo, NHGU reached a maximum (-23.6 +/- 3.5) at 60 min. Liver glycogen accumulation during period 2 was 21 +/- 10, 84 +/- 17, 65 +/- 16, and 134 +/- 17 mumol/gram in CON, INS, Po, and INSPo, respectively. The increment (period 1 to period 2) in the active form of liver glycogen synthase was 0.7 +/- 0.4, 6.5 +/- 1.2, 2.8 +/- 1.0, and 8.5 +/- 1.3% in CON, INS, Po, and INSPo, respectively. Thus, in contrast to insulin, the portal signal rapidly activates NHGU. In addition, the portal signal independent of a rise in insulin, can cause glycogen accumulation in the liver.

Sources of carbon for hepatic glycogen synthesis in the conscious dog.

To identify the source(s) of carbon for the indirect pathway of hepatic glycogen synthesis, we studied nine 42-h fasted conscious dogs given a continuous intraduodenal infusion of glucose, labeled with [1-13C]glucose and [3-3H]glucose, at 8 mg.kg-1.min-1 for 240 min. Glycogen formation by the direct pathway was measured by 13C-NMR. Net hepatic balances of glucose, gluconeogenic amino acids, lactate, and glycerol were determined using the arteriovenous difference technique. During the steady-state period (the final hour of the infusion), 81% of the glucose infused was absorbed as glucose. Net gut output of lactate and alanine accounted for 5% and 3% of the glucose infused, respectively. The cumulative net hepatic uptakes were: glucose, 15.5 +/- 3.8 g; gluconeogenic amino acids, 32.2 +/- 2.2 mmol (2.9 +/- 0.2 g of glucose equivalents); and glycerol, 6.1 +/- 0.9 mmol (0.6 +/- 0.1 g of glucose equivalents). The liver produced a net of 29.2 +/- 9.6 mmol of lactate (2.6 +/- 0.8 g of glucose equivalents). Net hepatic glycogen synthesis totaled 9.3 +/- 2.5 g (1.8 +/- 0.4 g/100 g liver), with the direct pathway being responsible for 57 +/- 10%. Thus, net hepatic glucose uptake was sufficient to account for all glycogen formed by both the direct and indirect pathways. Total net hepatic uptake of gluconeogenic precursors (gluconeogenic amino acids, glycerol, and lactate) was able to account for only 20% of net glycogen synthesis by the indirect pathway. In a net sense, our data are consistent with an intrahepatic origin for most of the three-carbon precursors used for indirect glycogen synthesis.

Magnitude of Negative Arterial-Portal Glucose Gradient Alters Net Hepatic Glucose Balance in Conscious Dogs

To examine the relationship between the magnitude of the negative arterial-portal glucose gradient and net hepatic glucose uptake, two groups of 42-h fasted, conscious dogs were infused with somatostatin, to suppress endogenous insulin and glucagon secretion, and the hormones were replaced intraportally to create hyperinsulinemia (3- to 4-fold basal) and basal glucagon levels. The hepatic glucose load to the liver was doubled and different negative arterial-portal glucose gradients were established by altering the ratio between portal and peripheral vein glucose infusions. In protocol 1 (n = 6) net hepatic glucose uptake was 42.2 ± 6.7, 35.0 ± 3.9, and 33.3 ± 4.4 μmol · kg−1 · min−1 at arterial-portal plasma glucose gradients of −4.1 ± 0.9, −1.8 ± 0.4, and −0.8 ± 0.1 mM, respectively. In protocol 2 (n = 6) net hepatic glucose uptake was 26.1 ± 2.8 and 12.2 ± 1.7 μmol · kg−1 · min−1 at arterial-portal plasma glucose gradients of −0.9 ± 0.2 and −0.4 ± 0.1 mM, respectively. No changes in the hepatic insulin or glucose loads were evident within a given protocol. Although net hepatic glucose uptake was lower in protocol 2 when compared with protocol 1 (26.1 ± 2.8 vs. 33.3 ± 4.4 μmol · kg−1 · min−1) in the presence of a similar arterial-portal plasma glucose gradient (−0.9 vs. −0.8 mM) the difference could be attributed to the hepatic glucose load being lower in protocol 2 (i.e., hepatic fractional glucose extraction was not significantly different) primarily as a result of lower hepatic blood flow. In conclusion, in the presence of fixed hepatic glucose and insulin loads, the magnitude of the negative arterial-portal glucose gradient can modify net hepatic glucose uptake in vivo.

Novel PEGylated Basal Insulin LY2605541 Has a Preferential Hepatic Effect on Glucose Metabolism

The impact of the novel basal insulin LY2605541 (LY) on hepatic and nonhepatic glucose uptake (non-HGU) was evaluated. Conscious dogs underwent euglycemic clamps with tracer and hepatic balance measurements. Clamp period infusions were peripheral venous regular insulin (0.1 nmol ⋅ kg−1 ⋅ h−1 [control], n = 6) or LY (bolus [nmol/kg], continuous [nmol ⋅ kg−1 ⋅ h−1]: 0.5, 0.5 [n = 6]; 0.375, 0.375 [n = 5]; 0.25, 0.25 [n = 4]), somatostatin, and glucose, as well as intraportal glucagon (basal). During the clamp, the dogs switched from net hepatic glucose output to uptake (rates reached 2.1 ± 1.2, 0.9 ± 2.1, 8.6 ± 2.3, and 6.0 ± 1.1 µmol ⋅ kg−1 ⋅ min−1 within 5 h in control, LY0.25, LY0.375, and LY0.5, respectively). Non-HGU in LY increased less than in control; the ratio of change from basal in non-HGU to change in net hepatic glucose balance, calculated when glucose infusion rates (GIRs) were ~20 µmol ⋅ kg-1 ⋅ min−1 in all groups, was higher in control (1.17 ± 0.38) versus LY0.25 (0.39 ± 0.33), LY0.375 (−0.01 ± 0.13), and LY0.5 (−0.09 ± 0.07). Likewise, the change from baseline in glucose Rd-to-Ra ratio was greatest in control (1.4 ± 0.3 vs. 0.6 ± 0.4, 0.5 ± 0.2, and 0.6 ± 0.2 in LY0.25, LY0.375, and LY0.5, respectively). In contrast to exogenously administered human insulin, LY demonstrated preferential hepatic effects, similar to endogenously secreted insulin. Therefore, the analog might reduce complications associated with current insulin therapy.

Regulation of hepatic and peripheral glucose disposal

Precise regulation of hepatic and peripheral glucose uptake is essential to preserve glucose homeostasis. The liver extracts approximately 1/3 of an oral glucose load, skeletal muscle extracts approximately 1/3, and other tissues, particularly the central nervous system and the formed elements of the blood, take up the balance. The load of glucose reaching the liver, the insulin concentration, and the route of glucose delivery (the hepatic portal or a peripheral vein) are key determinants of the rate of net hepatic glucose uptake. Glucose uptake by muscle requires three steps: delivery of glucose from the blood to the muscle, transport of glucose across the muscle membrane, and phosphorylation of glucose, processes affected by glycaemia and insulinaemia. Exercise stimulates insulin-dependent and -independent muscle glucose uptake, as well as the livers ability to take up glucose.

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.

Rapid reversal of the effects of the portal signal under hyperinsulinemic conditions in the conscious dog

Experiments were performed on two groups of 42-h-fasted conscious dogs ( n = 6/group). Somatostatin was given peripherally with insulin (4-fold basal) and glucagon (basal) intraportally. In the first experimental period, glucose was infused peripherally to double the hepatic glucose load (HGL) in both groups. In the second experimental period, glucose (21.8 μmol ⋅ kg-1 ⋅ min-1) was infused intraportally and the peripheral glucose infusion rate (PeGIR) was reduced to maintain the precreating HGL in the portal signal (PO) group, whereas saline was given intraportally in the control (CON) group and PeGIR was not changed. In the third period, the portal glucose infusion was stopped in the PO group and PeGIR was increased to sustain HGL. PeGIR was continued in the CON group. The glucose loads to the liver did not differ in the CON and PO groups. Net hepatic glucose uptake was 9.6 ± 2.5, 11.6 ± 2.6, and 15.5 ± 3.2 vs. 10.8 ± 1.8, 23.7 ± 3.0, and 15.5 ± 1.1 μmol ⋅ kg-1 ⋅ min-1, and nonhepatic glucose uptake (non-HGU) was 29.8 ± 1.1, 40.1 ± 4.5, and 49.5 ± 4.0 vs. 26.6 ± 4.3, 23.2 ± 4.0, and 40.4 ± 3.1 μmol ⋅ kg-1 ⋅ min-1in the CON and PO groups during the three periods, respectively. Cessation of the portal signal shifted NHGU and non-HGU to rates similar to those evident in the CON group within 10 min. These results indicate that even under hyperinsulinemic conditions the effects of the portal signal on hepatic and peripheral glucose uptake are rapidly reversible.Experiments were performed on two groups of 42-h-fasted conscious dogs (n = 6/group). Somatostatin was given peripherally with insulin (4-fold basal) and glucagon (basal) intraportally. In the first experimental period, glucose was infused peripherally to double the hepatic glucose load (HGL) in both groups. In the second experimental period, glucose (21.8 micromol. kg-1. min-1) was infused intraportally and the peripheral glucose infusion rate (PeGIR) was reduced to maintain the precreating HGL in the portal signal (PO) group, whereas saline was given intraportally in the control (CON) group and PeGIR was not changed. In the third period, the portal glucose infusion was stopped in the PO group and PeGIR was increased to sustain HGL. PeGIR was continued in the CON group. The glucose loads to the liver did not differ in the CON and PO groups. Net hepatic glucose uptake was 9.6 +/- 2.5, 11.6 +/- 2.6, and 15.5 +/- 3.2 vs. 10.8 +/- 1.8, 23.7 +/- 3.0, and 15.5 +/- 1.1 micromol. kg-1. min-1, and nonhepatic glucose uptake (non-HGU) was 29.8 +/- 1.1, 40.1 +/- 4.5, and 49.5 +/- 4.0 vs. 26.6 +/- 4.3, 23.2 +/- 4.0, and 40.4 +/- 3.1 micromol. kg-1. min-1 in the CON and PO groups during the three periods, respectively. Cessation of the portal signal shifted NHGU and non-HGU to rates similar to those evident in the CON group within 10 min. These results indicate that even under hyperinsulinemic conditions the effects of the portal signal on hepatic and peripheral glucose uptake are rapidly reversible.

Hepatic glucose uptake rapidly decreases after removal of the portal signal in conscious dogs

The aim of this study was to assess the decay of the effect of the portal signal on net hepatic glucose uptake (NHGU). Experiments were performed on five 42-h-fasted conscious dogs. After the 40-min basal period, somatostatin was given peripherally along with insulin (1.8 pmol ⋅ kg-1 ⋅ min-1) and glucagon (0.65 ng ⋅ kg-1 ⋅ min-1) intraportally. In the first experimental period (Pe-GLU-1; 90 min), glucose was infused into a peripheral vein to double the glucose load to the liver (HGL). In the second experimental period (Po-GLU; 90 min), glucose (20.1 μmol ⋅ kg-1 ⋅ min-1) was infused intraportally and the peripheral glucose infusion was reduced to maintain the same HGL. In the third period (Pe-GLU-2; 120 min), the portal glucose infusion was stopped and the peripheral glucose infusion was increased to again sustain HGL. Arterial insulin levels (42 ± 3, 47 ± 3, 43 ± 3 pmol/l) were basal and similar in the Pe-GLU-1, Po-GLU, and Pe-GLU-2 periods, respectively. Arterial glucagon levels were also basal and similar (51 ± 3, 49 ± 2, 46 ± 2 ng/l) in the three experimental periods. The glucose loads to the liver were 251 ± 11, 274 ± 14, and 276 ± 12 μmol ⋅ kg-1 ⋅ min-1, respectively. NHGU was 6.3 ± 2.4, 19.1 ± 2.8, and 9.2 ± 1.2 μmol ⋅ kg-1 ⋅ min-1, and nonhepatic glucose uptake (non-HGU) was 23.6 ± 3.0, 5.3 ± 1.8, and 25.5 ± 3.7 μmol ⋅ kg-1 ⋅ min-1in the three periods, respectively. Cessation of the portal signal for only 10 min shifted NHGU and non-HGU to 9.4 ± 2.2 and 25.0 ± 2.8 μmol ⋅ kg-1 ⋅ min-1, respectively; thus the effect of the portal signal was rapidly reversed both at the liver and peripheral tissues.The aim of this study was to assess the decay of the effect of the portal signal on net hepatic glucose uptake (NHGU). Experiments were performed on five 42-h-fasted conscious dogs. After the 40-min basal period, somatostatin was given peripherally along with insulin (1.8 pmol. kg-1. min-1) and glucagon (0.65 ng. kg-1. min-1) intraportally. In the first experimental period (Pe-GLU-1; 90 min), glucose was infused into a peripheral vein to double the glucose load to the liver (HGL). In the second experimental period (Po-GLU; 90 min), glucose (20.1 micromol. kg-1. min-1) was infused intraportally and the peripheral glucose infusion was reduced to maintain the same HGL. In the third period (Pe-GLU-2; 120 min), the portal glucose infusion was stopped and the peripheral glucose infusion was increased to again sustain HGL. Arterial insulin levels (42 +/- 3, 47 +/- 3, 43 +/- 3 pmol/l) were basal and similar in the Pe-GLU-1, Po-GLU, and Pe-GLU-2 periods, respectively. Arterial glucagon levels were also basal and similar (51 +/- 3, 49 +/- 2, 46 +/- 2 ng/l) in the three experimental periods. The glucose loads to the liver were 251 +/- 11, 274 +/- 14, and 276 +/- 12 micromol. kg-1. min-1, respectively. NHGU was 6.3 +/- 2.4, 19.1 +/- 2.8, and 9.2 +/- 1.2 micromol. kg-1. min-1, and nonhepatic glucose uptake (non-HGU) was 23.6 +/- 3.0, 5.3 +/- 1.8, and 25.5 +/- 3.7 micromol. kg-1. min-1 in the three periods, respectively. Cessation of the portal signal for only 10 min shifted NHGU and non-HGU to 9.4 +/- 2.2 and 25.0 +/- 2.8 micromol. kg-1. min-1, respectively; thus the effect of the portal signal was rapidly reversed both at the liver and peripheral tissues.

Insulin action on the liver in vivo

Insulin has a potent inhibitory effect on hepatic glucose production by direct action at hepatic receptors. The hormone also inhibits glucose production by suppressing both lipolysis in the fat cell and secretion of glucagon by the alpha-cell. Neural sensing of insulin levels appears to participate in control of hepatic glucose production in rodents, but a role for brain insulin sensing has not been documented in dogs or humans. The primary effect of insulin on the liver is its direct action.

Insulin is required for the liver to respond to intraportal glucose delivery in the conscious dog

To determine whether insulin is essential for the augmented hepatic glucose uptake observed in the presence of intraportal glucose delivery, SRIF was used to induce acute insulin deficiency in 5 conscious dogs, and glucose was infused into the portal vein or a peripheral vein in two sequential, randomized periods. Insulin and C-peptide levels were below the limits of detection after SRIF infusion, and the load of glucose presented to the liver was approximately doubled and equivalent during the portal and peripheral periods. Net hepatic glucose output was 2.9 ± 0.9 and 2.1 ± 1.1 μmol · kg−1 · min−1 during portal and peripheral glucose delivery, respectively. In an additional set of protocols, pancreatectomized dogs were used to investigate the effects of prolonged insulin deficiency (n = 5) and acute insulin replacement (n = 4) on the hepatic response to intraportal glucose delivery. In the prolonged insulin deficiency protocol, SRIF was used to lower glucagon and thereby reduce circulating glucose levels, and glucose was infused into the portal or peripheral circulations in two sequential, randomized periods. As with acute insulin deficiency, net hepatic glucose output was still evident and similar (3.6 ± 1.1 and 4.2 ± 1.3 μmol · kg−1 · min−1) during portal and peripheral glucose delivery, respectively. When the pancreatectomized dogs were restudied using a similar protocol, but one in which insulin was replaced (4X-basal), and the glucose load to the liver was matched to that which occurred in the prolonged insulin deficiency protocol, net hepatic glucose uptake was 23.6 ± 6.1 μmol · kg−1 · min−1 during portal glucose delivery but only 10.3 ± 3.5 μmol · kg−1 · min−1 during peripheral glucose delivery. These results suggest that the induction of net hepatic glucose uptake and the augmented hepatic response to intraportal glucose delivery require the presence of insulin.