Interaction of insulin and prior exercise in control of hepatic metabolism of a glucose load

To determine if prior exercise enhances insulin-stimulated extraction of glucose by the liver, chronically catheterized dogs were submitted to 150 min of treadmill exercise or rest. After exercise or rest, dogs received portal glucose (18 micro mol x kg(-1) x min(-1)), peripheral somatostatin, and basal portal glucagon infusions from t = 0 to 150 min. A peripheral glucose infusion was used to clamp arterial blood glucose at 8.3 mmol/l. Insulin was infused into the portal vein to create either basal levels or mild hyperinsulinemia. Prior exercise did not increase whole-body glucose disposal in the presence of basal insulin (25.5 +/- 1.5 vs. 20.3 +/- 1.7 micro mol x kg(-1) x min(-1)), but resulted in a marked enhancement in the presence of elevated insulin (97.2 +/- 15.1 vs. 64.4 +/- 7.4 micro mol x kg(-1) x min(-1)). Prior exercise also increased net hepatic glucose uptake in the presence of both basal insulin (7.5 +/- 1.2 vs. 2.9 +/- 2.4 micro mol x kg(-1) x min(-1)) and elevated insulin (22.0 +/- 3.5 vs. 11.5 +/- 1.8 micro mol x kg(-1) x min(-1)). Likewise, net hepatic glucose fractional extraction was increased by prior exercise with both basal insulin (0.04 +/- 0.01 vs. 0.01 +/- 0.01 micro mol x kg(-1) x min(-1)) and elevated insulin (0.10 +/- 0.01 vs. 0.05 +/- 0.01). Hepatic glycogen synthesis was increased by elevated insulin, but was not enhanced by prior exercise. Although the increase in glucose extraction after exercise could be ascribed to increased insulin action, the increase in hepatic glycogen synthesis was independent of it.     

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside causes acute hepatic insulin resistance in vivo

The infusion of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) causes a rise in tissue concentrations of the AMP analog 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranotide (ZMP), which mimics an elevation of cellular AMP levels. The purpose of this work was to determine the effect of raising hepatic ZMP levels on hepatic insulin action in vivo. Dogs had sampling and infusion catheters as well as flow probes implanted 16 days before an experiment. After an 18-h fast, blood glucose was 82 +/- 1 mg/dl and basal net hepatic glucose output 1.5 +/- 0.2 mg . kg(-1) . min(-1). Dogs received portal venous glucose (3.2 mg . kg(-1) . min(-1)), peripheral venous somatostatin, and basal portal venous glucagon infusions from -90 to 60 min. Physiological hyperinsulinemia was established with a portal insulin infusion (1.2 mU . kg(-1) . min(-1)). Peripheral venous glucose infusion was used to clamp arterial blood glucose at 150 mg/dl. Starting at t = 0 min, dogs received portal venous AICAR infusions of 0, 1, or 2 mg . kg(-1) . min(-1). Net hepatic glucose uptake was 2.4 +/- 0.5 mg . kg(-1) . min(-1) (mean of all groups) before t = 0 min. In the absence of AICAR, net hepatic glucose uptake was 1.9 +/- 0.4 mg . kg(-1) . min(-1) at t = 60 min. The lower-dose AICAR infusion caused a complete suppression of net hepatic glucose uptake (-1.0 +/- 1.7 mg . kg(-1) . min(-1) at t = 60 min). The higher AICAR dose resulted in a profound shift in hepatic glucose balance from net uptake to a marked net output (-6.1 +/- 1.9 mg . kg(-1) . min(-1) at t = 60 min), even in the face of hyperglycemia and hyperinsulinemia. These data show that elevations in hepatic ZMP concentrations, induced by portal venous AICAR infusion, cause acute hepatic insulin resistance. These findings have important implications for the targeting of AMP kinase for the treatment of insulin resistance, using AMP analogs.     

Portal venous 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion overcomes hyperinsulinemic suppression of endogenous glucose output

AMP-activated protein kinase (AMPK) plays a key role in regulating metabolism, serving as a metabolic master switch. The aim of this study was to assess whether increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-beta-D-ribosyl-5-monophosphate, in the liver would create a metabolic response consistent with an increase in whole-body metabolic need. Dogs had sampling (artery, portal vein, hepatic vein) and infusion (vena cava, portal vein) catheters and flow probes (hepatic artery, portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and hyperinsulinemic-euglycemic or -hypoglycemic clamp periods (0-150 min). At t = 0 min, somatostatin was infused and glucagon was replaced in the portal vein at basal rates. An intraportal hyperinsulinemic (2 mU . kg(-1) . min(-1)) infusion was also initiated at this time. Glucose was clamped at hypoglycemic or euglycemic levels in the presence (H-AIC, n = 6; E-AIC, n = 6) or absence (H-SAL, n = 6; E-SAL, n = 6) of a portal venous 5-aminoimidazole-4-carboxamide-ribofuranoside (AICAR) infusion (1 mg . kg(-1) . min(-1)) initiated at t = 60 min. In the presence of intraportal saline, glucose was infused into the vena cava to match glucose levels seen with intraportal AICAR. Glucagon remained fixed at basal levels, whereas insulin rose similarly in all groups. Glucose fell to 50 +/- 2 mg/dl by t = 60 min in hypoglycemic groups and remained at 105 +/- 3 mg/dl in euglycemic groups. Endogenous glucose production (R(a)) was similarly suppressed among groups in the presence of euglycemia or hypoglycemia before t = 60 min and remained suppressed in the H-SAL and E-SAL groups. However, intraportal AICAR infusion stimulated R(a) to increase by 2.5 +/- 1.0 and 3.4 +/- 0.4 mg . kg(-1) . min(-1) in the E-AIC and H-AIC groups, respectively. Arteriovenous measurement of net hepatic glucose output showed similar results. AICAR stimulated hepatic glycogen to decrease by 5 +/- 3 and 19 +/- 5 mg/g tissue (P < 0.05) in the presence of euglycemia and hypoglycemia, respectively. AICAR significantly increased net hepatic lactate output in the presence of hypoglycemia. Thus, intraportal AICAR infusion caused marked stimulation of both hepatic glucose output and net hepatic glycogenolysis, even in the presence of high levels of physiological insulin. This stimulation of glucose output by AICAR was equally marked in the presence of both euglycemia and hypoglycemia. However, hypoglycemia amplified the net hepatic glycogenolytic response to AICAR by approximately fourfold.     

Direct and indirect effects of insulin on glucose uptake and storage by the liver

Studies were conducted in conscious 42-h-fasted dogs to determine how much of insulin's effect on hepatic glucose uptake arises from its direct hepatic action versus its indirect (extrahepatic) action. Each experiment consisted of equilibration, basal, and experimental periods. During the latter, somatostatin, basal intraportal glucagon, portal glucose (21.3 micromol x kg(-1) x min(-1)), and peripheral glucose (to double the hepatic glucose load) were infused. During the experimental period, insulin was infused intraportally at a basal rate (BI, n = 6), at a fourfold basal rate (PoI, n = 6), or via a peripheral vein to create a selective increase in the arterial insulin level similar to that in PoI (PeI, n = 6). Arterial and hepatic sinusoidal insulin levels (in picomoles per liter) during the experimental period were 31 +/- 5 and 113 +/- 15 in BI, 97 +/- 11 and 394 +/- 66 in PoI, and 111 +/- 13 and 96 +/- 9 in PeI, respectively. Net hepatic glucose uptake (NHGU) averaged 7.0 +/- 1.1 micromol x kg(-1) x min(-1), 15.7 +/- 2.7 micromol x kg(-1) x min(-1) (P < 0.05 vs. BI), and 12.0 +/- 2.4 micromol x kg(-1) x min(-1) (P < 0.05 vs. BI) in BI, PoI, and PeI, respectively. Net hepatic carbon retention was 4.4 +/- 1.2 micromol glucose equivalents. kg(-1) x min(-1), 12.3 +/- 2.5 micromol glucose equivalents x kg(-1) x min(-1) (P < 0.05 vs. BI, P < 0.05 vs. PeI), and 7.1 +/- 1.0 micromol glucose equivalents x kg(-1) x min(-1) (P < 0.05 vs. BI) in BI, PoI, and PeI, respectively. Both direct and indirect insulin actions increase NHGU, but the rise in hepatic sinusoidal insulin appears critical for efficient storage of glucose as hepatic glycogen.     

Exercise-induced changes in insulin and glucagon are not required for enhanced hepatic glucose uptake after exercise but influence the fate of glucose within the liver

To test whether pancreatic hormonal changes that occur during exercise are necessary for the postexercise enhancement of insulin-stimulated net hepatic glucose uptake, chronically catheterized dogs were exercised on a treadmill or rested for 150 min. At the onset of exercise, somatostatin was infused into a peripheral vein, and insulin and glucagon were infused in the portal vein to maintain basal levels (EX-Basal) or simulate the response to exercise (EX-Sim). Glucose was infused as needed to maintain euglycemia during exercise. After exercise or rest, somatostatin infusion was continued in exercised dogs and initiated in dogs that had remained sedentary. In addition, basal glucagon, glucose, and insulin were infused in the portal vein for 150 min to create a hyperinsulinemic-hyperglycemic clamp in EX-Basal, EX-Sim, and sedentary dogs. Steady-state measurements were made during the final 50 min of the clamp. During exercise, net hepatic glucose output (mg x kg(-1) x min(-1)) rose in EX-Sim (7.6 +/- 2.8) but not EX-Basal (1.9 +/- 0.3) dogs. During the hyperinsulinemic-hyperglycemic clamp that followed either exercise or rest, net hepatic glucose uptake (mg x kg(-1) x min(-1)) was elevated in both EX-Basal (4.0 +/- 0.7) and EX-Sim (4.6 +/- 0.5) dogs compared with sedentary dogs (2.0 +/- 0.3). Despite this elevation in net hepatic glucose uptake after exercise, glucose incorporation into hepatic glycogen, determined using [3-3H]glucose, was not different in EX-Basal and sedentary dogs, but was 50 +/- 30% greater in EX-Sim dogs. Exercise-induced changes in insulin and glucagon, and consequent glycogen depletion, are not required for the increase in net hepatic glucose uptake after exercise but result in a greater fraction of the glucose consumed by the liver being directed to glycogen.     

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog.

Intraportal infusion of small amounts of fructose markedly augmented net hepatic glucose uptake (NHGU) during hyperglycemic hyperinsulinemia in conscious dogs. In this study, we examined whether the inclusion of catalytic amounts of fructose with a glucose load reduces postprandial hyperglycemia and the pancreatic beta-cell response to a glucose load in conscious 42-h-fasted dogs. Each study consisted of an equilibration (-140 to -40 min), control (-40 to 0 min), and test period (0-240 min). During the latter period, glucose (44.4 micromol x kg(-1) x min(-1)) was continuously given intraduodenally with (2.22 micromol x kg(-1) x min(-1)) or without fructose. The glucose appearance rate in portal vein blood was not significantly different with or without the inclusion of fructose (41.3 +/- 2.7 vs. 37.3 +/- 8.3 micromol x kg(-1) x min(-1), respectively). In response to glucose infusion without the inclusion of fructose, the net hepatic glucose balance switched from output to uptake (from 10 +/- 2 to 11 +/- 4 micromol x kg(-1) x min(-1)) by 30 min and averaged 17 +/- 6 micromol x kg(-1) x min(-1). The fractional extraction of glucose by the liver during the infusion period was 7 +/- 2%. Net glycogen deposition was 2.44 mmol glucose equivalent/kg body wt; 49% of deposited glycogen was synthesized via the direct pathway. Net hepatic lactate production was 1.4 mmol/kg body wt. Arterial blood glucose rose from 4.1 +/- 0.2 to 7.3 +/- 0.4 mmol/l, and arterial plasma insulin rose from 42 +/- 6 to 258 +/- 66 pmol/l at 30 min, after which they decreased to 7.0 +/- 0.5 mmol/l and 198 +/- 66 pmol/l, respectively. Arterial plasma glucagon decreased from 54 +/- 7 to 32 +/- 3 ng/l. In response to intraduodenal glucose infusion in the presence of fructose, net hepatic glucose balance switched from 9 +/- 1 micromol x kg(-1) x min(-1) output to 12 +/- 3 and 28 +/- 5 micromol x kg(-1) x min(-1) uptake by 15 and 30 min, respectively. The average NHGU (28 +/- 5 micromol x kg(-1) x min(-1)) and fractional extraction during infusion period (12 +/- 2%), net glycogen deposition (3.68 mmol glucose equivalent/kg body wt), net hepatic lactate production (3.27 mmol/kg), and glycogen synthesis via the direct pathway (68%) were significantly higher (P < 0.05) compared to that in the absence of fructose. The increases in arterial blood glucose (from 4.4 +/- 0.1 to 6.4 +/- 0.2 mmol/l at 30 min) and arterial plasma insulin (from 48 +/- 6 to 126 +/- 30 pmol/l at 30 min) were significantly smaller (P < 0.05). In summary, the inclusion of small amounts of fructose with a glucose load augmented NHGU, increased hepatic glycogen synthesis via the direct pathway, and augmented hepatic glycolysis. As a result, postprandial hyperglycemia and insulin release by the pancreatic beta-cell were reduced. In conclusion, catalytic amounts of fructose have the ability to improve glucose tolerance.     

Portal vein afferents are critical for the sympathoadrenal response to hypoglycemia

We sought to elucidate the role of the portal vein afferents in the sympathetic response to hypoglycemia. Laparotomy was performed on 27 male Wistar rats. Portal veins were painted with either 90% phenol (denervation group [PDN]) or 0.9% saline solution (sham-operated group [SHAM]). Rats were chronically cannulated in the carotid artery (sampling), jugular vein (infusion), and portal vein (infusion). After a recovery period of 5 days, animals were exposed to a hyperinsulinemic-hypoglycemic clamp, with glucose infused either portally (POR) or peripherally (PER). In all animals, systemic hypoglycemia (2.48+/-0.09 mmol/l) was induced via jugular vein insulin infusion (50 mU x kg(-1) x min(-1)). Arterial plasma catecholamines were assessed at basal (-30 and 0 min) and during sustained hypoglycemia (60, 75, 90, and 105 min). By design, portal vein glucose concentrations were significantly elevated during POR versus PER (4.4+/-0.14 vs. 2.5+/-0.07 mmol/l; P<0.01, respectively) for both PDN and SHAM. There were no significant differences in arterial glucose or insulin concentration between the four experimental conditions at any point in time. When portal glycemia and systemic glycemia fell concomitantly (SHAM-PER), epinephrine increased 12-fold above basal (3.75+/-0.34 and 44.56+/-6.1 nmol/l; P<0.001). However, maintenance of portal normoglycemia (SHAM-POR) caused a 50% suppression of the epinephrine response, despite cerebral hypoglycemia (22.2+/-3.1 nmol/l, P<0.001). Portal denervation resulted in a significant blunting of the sympathoadrenal response to whole-body hypoglycemia (PDN-PER 27.6+/-3.8 nmol/l vs. SHAM-PER; P<0.002). In contrast to the sham experiments, there was no further suppression in arterial epinephrine concentrations observed during PDN-POR versus PDN-PER (P = 0.8). These findings indicate that portal vein afferent innervation is critical for hypoglycemic detection and normal sympathoadrenal counterregulation.     

Insulin sensitively controls the glucagon response to mild hypoglycemia in the dog

In the present study, we examined how the arterial insulin level alters the alpha-cell response to a fall in plasma glucose in the conscious overnight fasted dog. Each study consisted of an equilibration (-140 to -40 min), a control (-40 to 0 min), and a test period (0 to 180 min), during which BAY R 3401 (10 mg/kg), a glycogen phosphorylase inhibitor, was administered orally to decrease glucose output in each of four groups (n = 5). In group 1, saline was infused. In group 2, insulin was infused peripherally (3.6 pmol. kg(- 1). min(-1)), and the arterial plasma glucose level was clamped to the level seen in group 1. In group 3, saline was infused, and euglycemia was maintained. In group 4, insulin (3.6 pmol. kg(-1). min(-1)) was given, and euglycemia was maintained by glucose infusion. In group 1, drug administration decreased the arterial plasma glucose level (mmol/l) from 5.8 +/- 0.2 (basal) to 5.2 +/- 0.3 and 4.4 +/- 0.3 by 30 and 90 min, respectively (P < 0.01). Arterial plasma insulin levels (pmol/l) and the hepatic portal-arterial difference in plasma insulin (pmol/l) decreased (P < 0.01) from 78 +/- 18 and 90 +/- 24 to 24 +/- 6 and 12 +/- 6 over the first 30 min of the test period. The arterial glucagon levels (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) rose from 43 +/- 5 and 5 +/- 2 to 51 +/- 5 and 10 +/- 5 by 30 min (P < 0.05) and to 79 +/- 16 and 31 +/- 15 (P < 0.05) by 90 min, respectively. In group 2, in response to insulin infusion, arterial insulin (pmol/l) was elevated from 48 +/- 6 to 132 +/- 6 to an average of 156 +/- 6. The hepatic portal-arterial difference in plasma insulin was eliminated, indicating a complete inhibition of endogenous insulin release. The arterial glucagon level (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) did not rise significantly (40 +/- 5 and 7 +/- 4 at basal, 44 +/- 4 and 9 +/- 4 at 90 min, and 44 +/- 8 and 15 +/- 7 at 180 min). In group 3, when euglycemia was maintained, the insulin and glucagon levels and the hepatic portal-arterial difference remained constant. In group 4, the arterial plasma glucose level remained basal (5.9 +/- 1.1 mmol/l) throughout, whereas insulin infusion increased the arterial insulin level to an average of 138 +/- 6 pmol/l. The hepatic portal-arterial difference in plasma insulin was again eliminated. Arterial glucagon level (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) did not change significantly (43 +/- 2 and 9 +/- 2 at basal, 39 +/- 3 and 9 +/- 2 at 90 min, and 37 +/- 3 and 7 +/- 2 at 180 min). Thus, a difference of approximately 120 pmol/l in arterial insulin completely abolished the response of the alpha-cell to mild hypoglycemia.     

Importance of hepatic glucoreceptors in sympathoadrenal response to hypoglycemia.

To ascertain whether hepatic glucoreceptors are important to hypoglycemic counterregulation, a localized euglycemic clamp was employed across the liver during general hypoglycemia. Dogs were infused peripherally with insulin (18-21 pmol.kg-1.min-1) for 150 min to induce systemic hypoglycemia. During the liver-clamp (LC) protocol, glucose was infused via the portal vein to maintain euglycemia at the liver. In control experiments, i.e., matched infusion (MI), glucose was infused peripherally at a rate determined to yield similar arterial glycemia levels in the two protocols. Arterial glucose concentrations were not different between protocols during the final hour of insulin infusion (3.26 +/- 0.21 and 3.25 +/- 0.21 mM during LC and MI, respectively; P = 0.91). Calculated hepatic glucose concentrations during the same period were significantly higher for LC (5.22 +/- 0.23 mM) than for MI (3.25 +/- 0.21 mM). During MI, both epinephrine and norepinephrine rose significantly from basal values of 562 +/- 87 pM and 1.21 +/- 0.19 nM to plateaus of 3691 +/- 1097 pM (P = 0.0001) and 2.38 +/- 0.35 nM (P = 0.0002), respectively. However, during LC, the elevation in epinephrine was suppressed by 42 +/- 8% (P = 0.015) relative to MI. Six of seven animals demonstrated a suppression in the norepinephrine response, averaging 32 +/- 13% (NS, P = 0.068). The glucagon response to hypoglycemia was unaffected by the level of hepatic glycemia. Hepatic hypoglycemia is essential to produce the full sympathoadrenal response to insulin-induced hypoglycemia.     

Exercise-induced fall in insulin and increase in fat metabolism during prolonged muscular work

The role of the exercise-induced fall in insulin in fat metabolism was studied in dogs during 150 min of treadmill exercise alone (controls) or with insulin clamped at basal levels by an intraportal infusion to prevent the normal fall in insulin concentration (ICs). To counteract the suppressive effect of insulin on glucagon release, glucagon was supplemented by an intraportal infusion in ICs. In all dogs, catheters were placed in a carotid artery and in the portal and hepatic veins for sampling and in the vena cava and the splenic vein for infusion purposes. Glucose levels were clamped in ICs to recreate the glycemic response evident in controls. In controls, insulin fell by 7 +/- 1 microU/ml but was unchanged from basal levels in ICs (0 +/- 2 microU/ml). Glucagon, norepinephrine, epinephrine, and cortisol rose similarly in controls and ICs. Arterial free-fatty acid (FFA) levels rose by 644 +/- 126 mu eq/L in controls but did not increase in ICs (-12 +/- 148 mu eq/L). Arterial glycerol levels rose by 337 +/- 43 and 183 +/- 19 microM in controls and ICs. Hepatic FFA delivery and fractional extraction increased by 17 +/- 3 and 0.06 +/- 0.02 mumol.kg-1.min-1, respectively, in controls. In ICs, hepatic FFA delivery increased by only 1 +/- 2 mumol.kg-1.min-1, whereas hepatic fractional extraction fell slightly (-0.03 +/- 0.03). Consequently, net hepatic FFA uptake rose by 4.8 +/- 1.5 mumol.kg-1.min-1 in controls but decreased slightly in ICs (-0.5 +/- 1.1 mumol.kg-1.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulsatile insulin secretion dictates systemic insulin delivery by regulating hepatic insulin extraction in humans

In health, insulin is secreted in discrete pulses into the portal vein, and the regulation of the rate of insulin secretion is accomplished by modulation of insulin pulse mass. Several lines of evidence suggest that the pattern of insulin delivery by the pancreas determines hepatic insulin clearance. In previous large animal studies, the amplitude of insulin pulses was related to the extent of insulin clearance. In humans (and in large animals), the amplitude of insulin oscillations is approximately 100-fold higher in the portal vein than in the systemic circulation, despite only a fivefold dilution, implying preferential hepatic extraction of insulin pulses. In the present study, by direct hepatic vein sampling in healthy humans, we sought to establish the extent of first-pass hepatic insulin extraction and to determine whether the pattern of insulin secretion (insulin pulse mass and amplitude) dictates the hepatic insulin clearance and thereby delivery of insulin to extrahepatic insulin-responsive tissues. Five nondiabetic subjects (two men and three women, mean age 32 years [range 25-39], BMI 24.9 kg/m(2) [21.2-27.1]) participated. Insulin and C-peptide delivery from the splanchnic bed was measured in basal overnight-fasted state and during a glucose infusion of 2 mg . kg(-1) . min(-1) by simultaneous sampling from the hepatic vein and an arterialized vein along with direct estimation of splanchnic blood flow. Fractional insulin extraction was calculated from the difference between the C-peptide and insulin delivery rates from the liver. The time patterns of insulin concentrations and hepatic insulin clearance were analyzed by deconvolution and Cluster analysis, respectively. Cross-correlation analysis was used to relate C-peptide secretion and insulin clearance. Glucose infusion increased peripheral glucose concentrations from 5.4 +/- 0.1 to 6.4 +/- 0.4 mmol/l (P < 0.05). Likewise, insulin and C-peptide concentrations increased during glucose infusion (P < 0.05). Hepatic insulin clearance increased with glucose infusion (1.06 +/- 0.18 vs. 2.55 +/- 0.38 pmol . kg(-1) . min(-1); P < 0.01), but fractional hepatic insulin clearance was stable (78.2 +/- 4.4 vs. 84 0. +/- 3.9%, respectively; P = 0.18). Insulin secretory-burst mass rose during glucose infusion (P < 0.05), whereas the interburst interval remained unchanged (4.4 +/- 0.2 vs. 4.5 +/- 0.3 min; P = 0.36). Cluster analysis identified an oscillatory pattern in insulin clearance, with peaks occurring approximately every 5 min. Cross-correlation analysis between prehepatic C-peptide secretion and hepatic insulin clearance demonstrated a significant positive association without detectable (<1 min) time lag. Insulin secretory-burst mass strongly predicted insulin clearance (r = 0.81, P = 0.0043). In conclusion, in humans, approximately 80% of insulin is extracted during the first liver passage. The liver rapidly responds to fluctuations in insulin secretion, preferentially extracting insulin delivered in pulses. The mass (and therefore amplitude) of insulin pulses traversing the liver is the predominant determinant of hepatic insulin clearance. Therefore, through this means, the pulse mass of insulin release dictates both hepatic (directly) as well as extra-hepatic (indirectly) insulin delivery. These findings emphasize the dual role of the liver and pancreas and their relationship mediated through magnitude of insulin pulse mass in regulating the quantity and pattern of systemic insulin delivery.     

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.     

Hypoglycemic detection does not occur in the hepatic artery or liver: findings consistent with a portal vein glucosensor locus

Our laboratory has previously demonstrated that hypoglycemic detection occurs in the portal vein, not the liver. To ascertain whether hypoglycemic detection may also occur in the hepatic artery, normoglycemia was established across the liver via a localized hepatic artery glucose infusion. Male mongrel dogs (n = 7) were infused with insulin (5.0 mU x kg(-1) x min(-1)) via the jugular vein to induce systemic hypoglycemia. Animals participated in two hyperinsulinemic-hypoglycemic clamp experiments distinguished by the site of glucose infusion. During the liver irrigation protocol, glucose was infused via the hepatic artery (HA protocol) to maintain liver normoglycemia as systemic glucose concentrations were systematically lowered over 260 min (nadir = 2.2 +/- 0.01 mmol/l). During control experiments, glucose was infused peripherally (PER protocol) to control reductions in blood glucose. Arterial glucose concentrations were not significantly different at any time between the two protocols (P = 0.73). Hepatic artery and liver glucose concentrations were significantly elevated in the HA versus PER protocol throughout the duration of the progressive hyperinsulinemic-hypoglycemic clamp. During the PER protocol, epinephrine and norepinephrine concentrations increased significantly above basal values (0.53 +/- 0.06 and 0.85 +/- 0.2 nmol/l, respectively) to plateaus of 4.4 +/- 0.86 (P = 0.0001) and 3.6 +/- 0.69 nmol/l (P = 0.001), respectively. There were no significant differences between the two protocols in the epinephrine (P = 0.81) and the norepinephrine (P = 0.68) response to hypoglycemia. The current findings indicate that glucosensors important to hypoglycemic detection do not reside in the hepatic artery. Furthermore, these data confirm our previous findings that glucosensors important to hypoglycemic detection are not present in the liver, but are in fact localized to the portal vein.     

Alpha- and beta-cell responses to small changes in plasma glucose in the conscious dog

The responses of the pancreatic alpha- and beta-cells to small changes in glucose were examined in overnight-fasted conscious dogs. Each study consisted of an equilibration (-140 to -40 min), a control (-40 to 0 min), and a test period (0 to 180 min), during which BAY R3401 (10 mg/kg), a glycogen phosphorylase inhibitor, was administered orally, either alone to create mild hypoglycemia or with peripheral glucose infusion to maintain euglycemia or create mild hyperglycemia. Drug administration in the hypoglycemic group decreased net hepatic glucose output (NHGO) from 8.9 +/- 1.7 (basal) to 6.0 +/- 1.7 and 5.8 +/- 1.0 pmol x kg(-1) x min(-1) by 30 and 90 min. As a result, the arterial plasma glucose level decreased from 5.8 +/- 0.2 (basal) to 5.2 +/- 0.3 and 4.4 +/- 0.3 mmol/l by 30 and 90 min, respectively (P < 0.01). Arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin decreased (P < 0.01) from 78 +/- 18 and 90 +/- 24 to 24 +/- 6 and 12 +/- 12 pmol/l over the first 30 min of the test period and decreased to 18 +/- 6 and 0 pmol/l by 90 min, respectively. The arterial glucagon levels and the hepatic portal-arterial difference in plasma glucagon increased from 43 +/- 5 and 4 +/- 2 to 51 +/- 5 and 10 +/- 5 ng/l by 30 min (P < 0.05) and to 79 +/- 16 and 31 +/- 15 ng/l by 90 min (P < 0.05), respectively. In euglycemic dogs, the arterial plasma glucose level remained at 5.9 +/- 0.1 mmol/l, and the NHGO decreased from 10 +/- 0.6 to -3.3 +/- 0.6 pmol x kg(-1) x min(-1) (180 min). The insulin and glucagon levels and the hepatic portal-arterial differences remained constant. In hyperglycemic dogs, the arterial plasma glucose level increased from 5.9 +/- 0.2 to 6.2 +/- 0.2 mmol/l by 30 min, and the NHGO decreased from 10 +/- 1.7 to 0 pmol x kg(-1) x min(-1) by 30 min. The arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin increased from 60 +/- 18 and 78 +/- 24 to 126 +/- 30 and 192 +/- 42 pmol/l by 30 min, after which they averaged 138 +/- 24 and 282 +/- 30 pmol/l, respectively. The arterial plasma glucagon levels and the hepatic portal-arterial difference in plasma glucagon decreased slightly from 41 +/- 7 and 4 +/- 3 to 34 +/- 7 and 3 +/- 2 ng/l during the test period. These data show that the alpha- and beta-cells of the pancreas respond as a coupled unit to very small decreases in the plasma glucose level.     

Interaction between insulin and glucose-delivery route in regulation of net hepatic glucose uptake in conscious dogs

In the presence of fixed basal levels of insulin, the route of intravenous glucose delivery (protal vs. peripheral) determines whether net hepatic glucose uptake (NHGU) occurs. Our aims were to determine if the route of intravenous glucose delivery also plays a role in regulating NHGU in the presence of hyperinsulinemia and to determine if length of fast (18 vs. 36 h) influences regulation of NHGU. Five conscious dogs fasted 18 h were given somatostatin and replacement insulin (245 +/- 34 microU.kg-1.min-1) and glucagon (0.65 ng.kg-1.min-1) infusions intraportally. After a 40-min control period, the insulin infusion rate was increased fourfold, and glucose was infused for 3 h. Glucose was given either through a peripheral vein or the portal vein for 90 min to double the glucose load reaching the liver. The order of infusions was randomized. NHGU was measured with the arterial - venous difference technique. Insulin and glucagon levels were 12 +/- 2, 35 +/- 6, and 36 +/- 5 microU/ml and 55 +/- 12, 61 +/- 13, and 59 +/- 7 pg/ml during the control, peripheral, and portal infusions, respectively. The glucose infusion rate, the load of glucose reaching the liver, and the arterial-portal plasma glucose gradient were 0, 9.58 +/- 2.28, and 10.44 +/- 2.94 mg.kg-1.min-1; 29.4 +/- 3.6, 56.8 +/- 3.4, and 56.8 +/- 2.8 mg.kg-1.min-1; and 2 +/- 1, 5 +/- 1, and -51 +/- 15 mg/dl during the same periods.(ABSTRACT TRUNCATED AT 250 WORDS)     

The measurement of hepatic circulation before and after orthotopic liver transplantation in dog--by using transit-time blood flow meter and H2 clearance method

Orthotopic liver transplantation was successfully carried out in 40 mongrel dogs, in which hepatic circulation was investigated before and after grafting. Blood flows in hepatic artery, portal vein and intrahepatic inferior vena cava were measured by using transit-time ultrasonic blood flow meter and regional tissue blood flow was determined by hydrogen gas clearance method. Before transplantation the mean blood flows were 234 +/- 95mg/min in portal vein, 118 +/- 76ml/min in hepatic artery and 291 +/- 103ml/min in inferior vena cava in 40 recipients. The blood flow ratio of portal vein and hepatic artery was 2.9 +/- 2.2. The mean regional blood flow of the liver was 63 +/- 24ml/min/100g. After transplantation, the mean blood flows decreased to 189 +/- 86ml/min in portal vein, 77 +/- 51ml/min in hepatic artery and 179 +/- 111ml/min in inferior vena cava and the regional tissue blood flow was 57 +/- 25ml/min/100g. Hepatic arterial flow decreased by 37 percent after transplantation, however, portal venous flow decreased by 24 percent and the regional blood flow decreased by 9 percent after transplantation of the liver. These data suggested that the microcirculation of the liver was slightly disturbed after liver transplantation in dog, which was in part due to the decreased blood flows of the hepatic artery and portal vein.     

Intraoperative estimation of liver blood flow in man.

The intraoperative measurement of the afferent circulation of the liver, namely the hepatic artery flow and portal venous flow was carried out upon 14 anaesthetized patients having carcinoma of the splanchnic area, mainly in the head of the pancreas, by means of transit time ultrasonic volume flowmeter. The hepatic artery flow, portal venous flow and total hepatic flow were 0.377 +/- 0.10; 0.614 +/- 0.21; 0.992 +/- 0.276 l/min, respectively. The ratio of hepatic arterial flow to portal venous flow was 0.66 +/- 0.259. There was a sharp, significant increase in hepatic arterial flow (29.8 +/- 6.1%, p < 0.01) after the temporary occlusion of portal vein, while the temporary occlusion of hepatic artery did not have any significant effect on portal venous circulation. The interaction between hepatic arterial flow and portal venous flow is a much disputed question, but according to the presented data here, it is unquestionable, that the decrease of portal venous flow immediately results a significant increase in hepatic artery circulation.     

A pilot study on early versus delayed hypertonic saline dextran resuscitation in a porcine model of near-lethal liver injury: early hemodynamic response and short-term survival

BACKGROUND: We studied the effects of early versus delayed fluid resuscitation on hemodynamic response and short-term survival in a porcine model of severe hepatic injury associated with hemorrhagic shock. MATERIALS AND METHODS: Eighteen anesthetized swine were randomized after standardized liver injury into two groups: early resuscitation (ER, n = 9) and delayed resuscitation (DR, n = 9). The ER and DR groups were resuscitated with hypertonic saline dextran (HSD) 20 min and 40 min after the injury, respectively. Mean arterial pressure (MAP), cardiac output (CO), and arterial blood gases were measured in addition to vascular blood flow rates in the aorta, hepatic artery and portal vein. The duration of follow-up was 100 min. RESULTS: MAP decreased from 112 +/- 4 to 23 +/- 2 mmHg (P < 0.05) during 20 min after the injury. Bolus infusion of HSD significantly elevated MAP, CO, and flow rates in the aorta, portal vein and common hepatic artery in both groups. Portal vein flow remained relatively high during the shock. Intra-abdominal bleeding (ER, 701 +/- 42 mL; DR 757 +/- 78 mL) and the mortality rate (ER 44%; DR 33%) did not differ between the groups 100 min after injury (P > 0.05). Aortic flow, portal vein flow, common hepatic artery flow, MAP, CO, PaO(2), PaCO(2), base deficit, pH, hemoglobin measurements, and the volume of blood shed into the intraperitoneal cavity did not affect survival in the Cox regression analysis. CONCLUSIONS: Early versus delayed fluid infusion with HSD resulted in a comparable hemodynamic response and survival 100 min after injury. No rebleeding was observed.     

Inhalation of insulin in dogs: assessment of insulin levels and comparison to subcutaneous injection

Pulmonary insulin delivery is being developed as a more acceptable alternative to conventional subcutaneous administration. In 15 healthy Beagle dogs (average weight 9.3 kg), we compared insulin distribution in arterial, deep venous, and hepatic portal circulation. Dogs received 0.36 units/kg s.c. regular human insulin (n = 6) or 1 mg (2.8 units/kg) or 2 mg (5.6 units/kg) dry-powder human inhaled insulin (n = 3 and 6, respectively). Postinhalation of inhaled insulin (1 or 2 mg), arterial insulin levels quickly rose to a maximum of 55 +/- 6 or 92 +/- 9 microU/ml, respectively, declining to typical fasting levels by 3 h. Portal levels were lower than arterial levels at both doses, while deep venous levels were intermediate to arterial and portal levels. In contrast, subcutaneous insulin was associated with a delayed and lower peak arterial concentration (55 +/- 8 microU/ml at 64 min), requiring 6 h to return to baseline. Peak portal levels for subcutaneous insulin were comparable to those for 1 mg and significantly less than those for 2 mg inhaled insulin, although portal area under the curve (AUC) was comparable for the subcutaneous and 2-mg groups. The highest insulin levels with subcutaneous administration were seen in the deep venous circulation. Interestingly, the amount of glucose required for maintaining euglycemia was highest with 2 mg inhaled insulin. We conclude that plasma insulin AUC for the arterial insulin level (muscle) and hepatic sinusoidal insulin level (liver) is comparable for 2 mg inhaled insulin and 0.36 units/kg subcutaneous insulin. In addition, arterial peak concentration following insulin inhalation is two times greater than subcutaneous injection; however, the insulin is present in the circulation for half the time.     

Systemic venous drainage of pancreas allografts as independent cause of hyperinsulinemia in type I diabetic recipients

To evaluate the metabolic consequences of pancreas transplantation with systemic venous drainage on beta-cell function, we examined insulin and C-peptide responses to glucose and arginine in type I (insulin-dependent) diabetic pancreas recipients (n = 30), nondiabetic kidney recipients (n = 8), and nondiabetic control subjects (n = 28). Basal insulin levels were 66 +/- 5 pM in control subjects, 204 +/- 18 pM in pancreas recipients (P less than 0.0001 vs. control), and 77 +/- 17 pM in kidney recipients. Acute insulin responses to glucose were 416 +/- 44 pM in control subjects, 763 +/- 91 pM in pancreas recipients (P less than 0.01 vs. control), and 589 +/- 113 pM in kidney recipients (NS vs. control). Basal and stimulated insulin levels in two pancreas recipients with portal venous drainage were normal. Integrated acute C-peptide responses were not statistically different (25.3 +/- 4.3 nM/min in pancreas recipients, 34.2 +/- 5.5 nM/min in kidney recipients, and 23.7 +/- 2.1 nM/min in control subjects). Similar insulin and C-peptide results were obtained with arginine stimulation, and both basal and glucose-stimulated insulin-C-peptide ratios in pancreas recipients were significantly greater than in control subjects. We conclude that recipients of pancreas allografts with systemic venous drainage have elevated basal and stimulated insulin levels and that these alterations are primarily due to alterations of first-pass hepatic insulin clearance, although insulin resistance secondary to immunosuppressive therapy (including prednisone) probably plays a contributing role. To avoid hyperinsulinemia and its possible long-term adverse consequences, transplantation of pancreas allografts into sites with portal rather than systemic venous drainage should be considered.