5-Aminoimidazole-4-carboxamide-1-β-d-ribofuranoside renders glucose output by the liver of the dog insensitive to a pharmacological increment in insulin

This study aimed to test whether stimulation of net hepatic glucose output (NHGO) by increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-β-d-ribosyl-5-monophosphate, can be suppressed by pharmacological insulin levels. 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 study. Protocols consisted of equilibration (−130 to −30 min), basal (−30 to 0 min), and hyperinsulinemic-euglycemic (0–150 min) periods. At time ( t) = 0 min, somatostatin was infused, and basal glucagon was replaced via the portal vein. Insulin was infused in the portal vein at either 2 (INS2) or 5 (INS5) mU·kg−1·min−1. At t = 60 min, 1 mg·kg−1·min−1portal venous 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) infusion was initiated. Arterial insulin rose ∼9- and ∼27-fold in INS2 and INS5, respectively. Glucagon, catecholamines, and cortisol did not change throughout the study. NHGO was completely suppressed before t = 60 min. Intraportal AICAR stimulated NHGO by 1.9 ± 0.5 and 2.0 ± 0.5 mg·kg−1·min−1in INS2 and INS5, respectively. AICAR stimulated tracer-determined endogenous glucose production similarly in both groups. Intraportal AICAR infusion significantly increased hepatic acetyl-CoA carboxylase (ACC, Ser79) phosphorylation in INS2. Hepatic ACC (Ser79) phosphorylation, however, was not increased in INS5. Thus intraportal AICAR infusion renders hepatic glucose output insensitive to pharmacological insulin. The effectiveness of AICAR in countering the suppressive effect of pharmacological insulin on NHGO occurs even though AICAR-stimulated ACC phosphorylation is completely blocked.

Hepatic glucose autoregulation: responses to small, non-insulin-induced changes in arterial glucose

The purpose of this study was to determine whether the sedentary dog is able to autoregulate glucose production (Ra) in response to non-insulin-induced changes (<20 mg/dl) in arterial glucose. Dogs had catheters implanted >16 days before study. Protocols consisted of basal (−30 to 0 min) and bilateral renal arterial phloridzin infusion (0–180 min) periods. Somatostatin was infused, and glucagon and insulin were replaced to basal levels. In one protocol (Phl ± Glc), glucose was allowed to fall from t = 0–90 min. This was followed by a period when glucose was infused to restore euglycemia (90–150 min) and a period when glucose was allowed to fall again (150–180 min). In a second protocol (EC), glucose was infused to compensate for the renal glucose loss due to phloridzin and maintain euglycemia from t = 0–180 min. Arterial insulin, glucagon, cortisol, and catecholamines remained at basal in both protocols. In Phl ± Glc, glucose fell by ∼20 mg/dl by t = 90 min with phloridzin infusion. Radid not change from basal in Phl ± Glc despite the fall in glucose for the first 90 min. Rawas significantly suppressed with restoration of euglycemia from t = 90–150 min ( P < 0.05) and returned to basal when glucose was allowed to fall from t = 150–180 min. Radid not change from basal in EC. In conclusion, the liver autoregulates Rain response to small changes in glucose independently of changes in pancreatic hormones at rest. However, the liver of the resting dog is more sensitive to a small increment, rather than decrement, in arterial glucose.

Effect of hepatic denervation on peripheral insulin sensitivity in conscious dogs

We tested the hypothesis that the loss of hepatic nerves decreases peripheral insulin sensitivity. Surgical hepatic denervation (DN) was performed in 22 dogs ∼16 days before study; 7 dogs (Sham-Sal) had a sham procedure. A euglycemic hyperinsulinemic (1 mU · kg−1 · min−1; arterial insulin 35 ± 1 μU/ml in all dogs) clamp was performed in conscious dogs. From 0 to 90 min of the clamp, all dogs received the same treatment; then the DN dogs were divided into three groups. From 90 to 180 min, DN-PeA ( n = 7) and DN-PoA ( n = 7) groups received acetylcholine 2.5 μg · kg−1 · min−1 via peripheral or portal vein, respectively, and DN-Sal ( n= 8) received no acetylcholine. During 150–180 min, the Sham-Sal, DN-Sal, DN-PeA, and DN-PoA groups exhibited glucose infusion rates of 12.4 ± 0.8, 9.3 ± 0.8 ( P < 0.05 vs. Sham-Sal), 9.1 ± 0.1 ( P < 0.05 vs. Sham-Sal), and 12.7 ± 1.6 mg · kg−1 · min−1; nonhepatic glucose uptakes of 11.5 ± 0.9, 8.9 ± 0.7 ( P < 0.05 vs. Sham-Sal), 8.6 ± 0.9 ( P < 0.05 vs. Sham-Sal), and 11.9 ± 1.7 mg · kg−1 · min−1; net hindlimb glucose uptakes of 18.4 ± 2.1, 13.7 ± 1.1 ( P< 0.05 vs. Sham-Sal), 17.5 ± 1.9, and 16.7 ± 3.2 mg/min; and glucose utilization rates of 14.4 ± 1.4, 10.4 ± 0.8 ( P < 0.05 vs. Sham-Sal), 9.8 ± 0.9 ( P< 0.05 vs. Sham-Sal), and 13.6 ± 1.8 mg · kg−1 · min−1, respectively. DN caused peripheral insulin resistance, and intraportal but not peripheral acetylcholine restored insulin sensitivity.

Effect of fast duration on disposition of an intraduodenal glucose load in the conscious dog

The effects of prior fast duration (18 h, n = 8; 42 h, n = 8) on the glycemic and tissue-specific responses to an intraduodenal glucose load were studied in chronically catheterized conscious dogs. [3-3H]glucose was infused throughout the study. After basal measurements, glucose spiked with [U-14C]glucose was infused for 150 min intraduodenally. Arterial insulin and glucagon were similar in the two groups. Arterial glucose (mg/dl) rose ∼70% more during glucose infusion after 42 h than after an 18-h fast. The net hepatic glucose balance (mg ⋅ kg−1⋅ min−1) was similar in the two groups (basal: 1.8 ± 0.2 and 2.0 ± 0.3; glucose infusion: −2.2 ± 0.5 and −2.2 ± 0.7). The intrahepatic fate of glucose was 79% glycogen, 13% oxidized, and 8% lactate release after a 42-h fast; it was 23% glycogen, 21% oxidized, and 56% lactate release after an 18-h fast. Net hindlimb glucose uptake was similar between groups. The appearance of intraduodenal glucose during glucose infusion (mg/kg) was 900 ± 50 and 1,120 ± 40 after 18- and 42-h fasts ( P < 0.01). Conclusion: glucose administration after prolonged fasting induces higher circulating glucose than a shorter fast (increased appearance of intraduodenal glucose); liver and hindlimb glucose uptakes and the hormonal response, however, are unchanged; finally, an intrahepatic redistribution of carbons favors glycogen deposition.

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 effect of diet asynchrony on portal-drained viscera metabolism

Synchronising the rate of rumen breakdown and availability of dietary energy and nitrogenous components can improve the capture of rumen degradable nitrogen and improve the amount and efficiency of rumen microbial protein synthesis. It is not clear what influence rumen synchrony has on nutrient use by the portal-drained viscera (PDV). This experiment has examined the impact of feeding two diets formulated to be asynchronous or synchronous with respect to the potential hourly supply of energy and nitrogen to the microbial fraction of the rumen on portal blood flow, net flux of ammonia and urea across the PDV and arterial insulin concentrations.

Paradoxical insulin-induced increase in gluconeogenesis in response to prolonged hypoglycemia in conscious dogs

The aim of this study was to determine the effects of differing insulin concentrations on the gluconeogenic response to equivalent prolonged hypoglycemia. Insulin was infused intraportally, for 3 h, into normal 18-h fasted conscious dogs at 2 (lower, n = 6) or 8 mU.kg-1.min-1 (high, n = 7) on separate occasions. This resulted in steady-state arterial insulin levels of 80 +/- 8 and 610 +/- 55 microU/ml, respectively. Glucose was infused during high dose to maintain the hypoglycemic plateau (50 +/- 1 mg/dl) equivalent to lower. Epinephrine (806 +/- 180 vs. 2,589 +/- 260 pg/ml), norepinephrine (303 +/- 55 vs. 535 +/- 60 pg/ml), cortisol (5.8 +/- 1.2 vs. 12.1 +/- 1.5 micrograms/dl), and pancreatic polypeptide (598 +/- 250 vs. 1,198 +/- 150 pg/ml) were all increased (P < 0.05) in the presence of high-dose insulin. Net hepatic glucose production increased significantly from 2.2 +/- 0.3 to 3.8 +/- 0.5 mg.kg-1.min-1 (P < 0.05) during high-dose infusion but remained at basal levels (2.3 +/- 0.4 mg.kg-1.min-1) during lower-dose insulin. During the 3rd h of hypoglycemia, gluconeogenesis accounted for between 42 and 100% of glucose production during high-dose infusion but only 22-52% during lower-dose insulin. Intrahepatic gluconeogenic efficiency, however, increased similarly during both protocols. Lipolysis, as indicated by arterial blood glycerol levels, increased by a greater amount during high- compared with lower-dose insulin infusion. Six hyperinsulinemic euglycemic control experiments (2 or 8 mU.kg-1.min-1, n = 3 in each) provided baseline data. Gluconeogenesis remained similar to basal levels, but lipolysis was significantly suppressed during both series of hyperinsulinemic euglycemic studies. In summary, these data suggest that 1) the important counterregulatory processes of gluconeogenesis and lipolysis can be significantly increased during prolonged hypoglycemia despite an eightfold increase in circulating insulin levels and 2) the amplified gluconeogenic rate present during the hypoglycemic high-dose insulin infusions was caused by enhanced substrate delivery to the liver rather than an increase in intrahepatic gluconeogenic efficiency.

Endothelin-1 infusion reduces splanchnic glucose production in humans

Two groups of six healthy subjects received an intravenous endothelin-1 (ET-1) infusion (4 pmol.kg-1.min-1 for 20 min) in the basal state. Blood was drawn from catheters in an artery (n = 12), a hepatic vein (n = 12), and a renal vein (n = 6) for determinations of blood flows and substrate exchanges. During the ET-1 infusion, splanchnic and renal blood flows were reduced by approximately 50 (P < 0.01) and 25% (P < 0.001), respectively. Arterial glucose concentration and splanchnic glucose production fell by approximately 4 (P < 0.01) and 55% (P < 0.001), respectively. The latter was still 30% below basal level 3 h after the infusion (P < 0.001). Arterial glycerol increased by 64% (P < 0.01), whereas arterial lactate was unchanged. Splanchnic uptakes of lactate and glycerol were unchanged. Arterial insulin and glucagon showed transient falls with a maximal drop of approximately 35% (P < 0.001) during the infusion. In conclusion, ET-1 infusion causes reduced splanchnic glucose production due to reduced glycogen-derived glucose release. The latter could partly be connected with the transient fall in arterial glucagon, but the prolonged suppressive effect on splanchnic glycogenolysis seems to be linked with other ET-1-related factors. We propose that the underlying mechanism to the transient falls in both arterial glucagon and insulin might be coupled to the ET-1-arginine-NO system.