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

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.

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.

Comparison of insulins detemir and glargine: effects on glucose disposal, hepatic glucose release and the central nervous system

AIMS

The effects of insulins detemir (Det) and glargine (Glar) on endogenous glucose production (EGP) and net hepatic glucose output (NHGO) were compared.

METHODS

Arteriovenous difference and tracer ([3-(3) H]glucose) techniques were employed during a two-step hyperinsulinemic euglycaemic clamp in conscious dogs (6 groups, n = 5-6/group). After equilibration and basal sampling (0-120 min), somatostatin was infused and basal glucagon was replaced intraportally. Det or Glar was infused via portal vein (Po), peripheral vein (IV), or bilateral carotid and vertebral arteries (H) at 0.1 and 0.3 mU/kg/min (low Insulin; Glar vs. Det, respectively, 120-420 min) and 4× the low insulin rate (high insulin; 420-540 min).

RESULTS

NHGO and EGP were suppressed and glucose R(d) and infusion rate were stimulated similarly by Det and Glar at both Low and high insulin with each infusion route. Non-esterified fatty acid (NEFA) concentrations during low insulin were 202 ± 37 versus 323 ± 75 µM in DetPo and GlarPo (p < 0.05) and 125 ± 39 versus 263 ± 48 µM in DetIV and GlarIV, respectively (p < 0.05). In DetH versus GlarH, pAkt/Akt (1.7 ± 0.2 vs. 1.0 ± 0.2) and pSTAT3/STAT3 (1.4 ± 0.2 vs. 1.0 ± 0.1) were significantly increased in the liver but not in the hypothalamus.

CONCLUSIONS

Det and Glar have similar net effects on acute regulation of hepatic glucose metabolism in vivo regardless of delivery route. Portal and IV detemir delivery reduces circulating NEFA to a greater extent than glargine, and head detemir infusion enhances molecular signalling in the liver. These findings indicate a need for further examination of Det's central and hepatic effects.

Hepatic portal venous delivery of a nitric oxide synthase inhibitor enhances net hepatic glucose uptake

Hepatic portal venous infusion of nitric oxide synthase (NOS) inhibitors causes muscle insulin resistance, but the effects on hepatic glucose disposition are unknown. Conscious dogs underwent a hyperinsulinemic (4-fold basal) hyperglycemic (hepatic glucose load 2-fold basal) clamp, with assessment of liver metabolism by arteriovenous difference methods. After 90 min (P1), dogs were divided into two groups: control (receiving intraportal saline infusion; n = 8) and LN [receiving N(G)-nitro-L-arginine methyl ester (L-NAME), a nonspecific NOS inhibitor; n = 11] intraportally at 0.3 mg x kg(-1) x min(-1) for 90 min (P2). During the final 60 min of study (P3), L-NAME was discontinued, and five LN dogs received the NO donor SIN-1 intraportally at 6 mug x kg(-1) x min(-1) while six received saline (LN/SIN-1 and LN/SAL, respectively). Net hepatic fractional glucose extraction (NHFE) in control dogs was 0.034 +/- 0.016, 0.039 +/- 0.015, and 0.056 +/- 0.019 during P1, P2, and P3, respectively. NHFE in LN was 0.045 +/- 0.009 and 0.111 +/- 0.007 during P1 and P2, respectively (P < 0.05 vs. control during P2), and 0.087 +/- 0.009 and 0.122 +/- 0.016 (P < 0.05) during P3 in LN/SIN-1 and LN/SAL, respectively. During P2, arterial glucose was 204 +/- 5 vs. 138 +/- 11 mg/dl (P < 0.05) in LN vs. control to compensate for L-NAME's effect on blood flow. Therefore, another group (LNlow; n = 4) was studied in the same manner as LN/SAL, except that arterial glucose was clamped at the same concentrations as in control. NHFE in LNlow was 0.052 +/- 0.008, 0.093 +/- 0.023, and 0.122 +/- 0.021 during P1, P2, and P3, respectively (P < 0.05 vs. control during P2 and P3), with no significant difference in glucose infusion rates. Thus, NOS inhibition enhanced NHFE, an effect partially reversed by SIN-1.

Intraportal administration of neuropeptide Y and hepatic glucose metabolism

We examined whether intraportal delivery of neuropeptide Y (NPY) affects glucose metabolism in 42-h-fasted conscious dogs using arteriovenous difference methodology. The experimental period was divided into three subperiods (P1, P2, and P3). During all subperiods, the dogs received infusions of somatostatin, intraportal insulin (threefold basal), intraportal glucagon (basal), and peripheral intravenous glucose to increase the hepatic glucose load twofold basal. Following P1, in the NPY group (n = 7), NPY was infused intraportally at 0.2 and 5.1 pmol.kg(-1).min(-1) during P2 and P3, respectively. The control group (n = 7) received intraportal saline infusion without NPY. There were no significant changes in hepatic blood flow in NPY vs. control. The lower infusion rate of NPY (P2) did not enhance net hepatic glucose uptake. During P3, the increment in net hepatic glucose uptake (compared with P1) was 4 +/- 1 and 10 +/- 2 micromol.kg(-1).min(-1) in control and NPY, respectively (P < 0.05). The increment in net hepatic fractional glucose extraction during P3 was 0.015 +/- 0.005 and 0.039 +/- 0.008 in control and NPY, respectively (P < 0.05). Net hepatic carbon retention was enhanced in NPY vs. control (22 +/- 2 vs. 14 +/- 2 micromol.kg(-1).min(-1), P < 0.05). There were no significant differences between groups in the total glucose infusion rate. Thus, intraportal NPY stimulates net hepatic glucose uptake without significantly altering whole body glucose disposal in dogs.

Role of the hepatic sympathetic nerves in the regulation of net hepatic glucose uptake and the mediation of the portal glucose signal

Portal glucose delivery enhances net hepatic glucose uptake (NHGU) relative to peripheral glucose delivery. We hypothesize that the sympathetic nervous system normally restrains NHGU, and portal glucose delivery relieves the inhibition. Two groups of 42-h-fasted conscious dogs were studied using arteriovenous difference techniques. Denervated dogs (DEN; n=10) underwent selective sympathetic denervation by cutting the nerves at the celiac nerve bundle near the common hepatic artery; control dogs (CON; n=10) underwent a sham procedure. After a 140-min basal period, somatostatin was given along with basal intraportal infusions of insulin and glucagon. Glucose was infused peripherally to double the hepatic glucose load (HGL) for 90 min (P1). In P2, glucose was infused intraportally (3-4 mg.kg(-1).min(-1)), and the peripheral glucose infusion was reduced to maintain the HGL for 90 min. This was followed by 90 min (P3) in which portal glucose infusion was terminated and peripheral glucose infusion was increased to maintain the HGL. P1 and P3 were averaged as the peripheral glucose infusion period (PE). The average HGLs (mg.kg(-1).min(-1)) in CON and DEN were 55+/-3 and 54+/-4 in the peripheral periods and 55+/-3 and 55+/-4 in P2, respectively. The arterial insulin and glucagon levels remained basal in both groups. NHGU (mg.kg(-1).min(-1)) in CON averaged 1.7+/-0.3 during PE and increased to 2.9+/-0.3 during P2. NHGU (mg.kg(-1).min(-1)) was greater in DEN than CON (P<0.05) during PE (2.9+/-0.4) and failed to increase significantly (3.2+/-0.2) during P2 (not significant vs. CON). Selective sympathetic denervation increased NHGU during hyperglycemia but significantly blunted the response to portal glucose delivery.

Insulin secretion-independent effects of GLP-1 on canine liver glucose metabolism do not involve portal vein GLP-1 receptors

Whether glucagon-like peptide (GLP)-1 requires the hepatic portal vein to elicit its insulin secretion-independent effects on glucose disposal in vivo was assessed in conscious dogs using tracer and arteriovenous difference techniques. In study 1, six conscious overnight-fasted dogs underwent oral glucose tolerance testing (OGTT) to determine target GLP-1 concentrations during clamp studies. Peak arterial and portal values during OGTT ranged from 23 to 65 pM and from 46 to 113 pM, respectively. In study 2, we conducted hyperinsulinemic-hyperglycemic clamp experiments consisting of three periods (P1, P2, and P3) during which somatostatin, glucagon, insulin and glucose were infused. The control group received saline, the PePe group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally, the PePo group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then intraportally (P3), and the PeHa group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then through the hepatic artery (P3) to increase the hepatic GLP-1 load to the same extent as in P3 in the PePo group (n = 8 dogs/group). Arterial GLP-1 levels increased similarly in all groups during P2 ( approximately 50 pM), whereas portal GLP-1 levels were significantly increased (2-fold) in the PePo vs. PePe and PeHa groups during P3. During P2, net hepatic glucose uptake (NHGU) increased slightly but not significantly (vs. P1) in all groups. During P3, GLP-1 increased NHGU in the PePo and PeHa groups more than in the control and PePe groups (change of 10.8 +/- 1.3 and 10.6 +/- 1.0 vs. 5.7 +/- 1.0 and 5.4 +/- 0.8 micromol.kg(-1).min(-1), respectively, P < 0.05). In conclusion, physiological GLP-1 levels increase glucose disposal in the liver, and this effect does not involve GLP-1 receptors located in the portal vein.

Portal 5-hydroxytryptophan infusion enhances glucose disposal in conscious dogs

Intraportal serotonin infusion enhances net hepatic glucose uptake (NHGU) during glucose infusion but blunts nonhepatic glucose uptake and can cause gastrointestinal discomfort and diarrhea at high doses. Whether the serotonin precursor 5-hydroxytryptophan (5-HTP) could enhance NHGU without gastrointestinal side effects during glucose infusion was examined in conscious 42-h-fasted dogs, using arteriovenous difference and tracer ([3-3H]glucose) techniques. Experiments consisted of equilibration (-120 to -30 min), basal (-30 to 0 min), and experimental (EXP; 0-270 min) periods. During EXP, somatostatin, fourfold basal intraportal insulin, basal intraportal glucagon, and peripheral glucose (to double the hepatic glucose load) were infused. In one group of dogs (HTP, n = 6), saline was infused intraportally from 0 to 90 min (P1), and 5-HTP was infused intraportally at 10, 20, and 40 microg x kg(-1) x min(-1) from 90 to 150 (P2), 150 to 210 (P3), and 210 to 270 (P4) min, respectively. In the other group (SAL, n = 7), saline was infused intraportally from 0 to 270 min. NHGU in SAL was 14.8 +/- 1.9, 18.5 +/- 2.3, 16.3 +/- 1.4, and 19.7 +/- 1.6 micromol x kg(-1) x min(-1) in P1-P4, whereas NHGU in 5-HTP averaged 16.4 +/- 2.6, 18.5 +/- 1.4, 20.8 +/- 2.0, and 27.6 +/- 2.6 micromol x kg(-1) x min(-1) (P < 0.05 vs. SAL). Nonhepatic glucose uptake (micromol x kg(-1) x min(-1)) in SAL was 30.2 +/- 4.3, 36.8 +/- 5.8, 44.3 +/- 5.8, and 54.6 +/- 11.8 during P1-P4, respectively, whereas in HTP the corresponding values were 26.3 +/- 6.8, 44.9 +/- 10.1, 47.5 +/- 11.7, and 51.4 +/- 13.2 (not significant between groups). Intraportal 5-HTP enhances NHGU without significantly altering nonhepatic glucose uptake or causing gastrointestinal side effects, raising the possibility that a related agent might have a role in reducing postprandial hyperglycemia.

Route-dependent effect of nutritional support on liver glucose uptake

The liver is a major site of glucose disposal during chronic (5 day) total parenteral (TPN) and enteral (TEN) nutrition. Net hepatic glucose uptake (NHGU) is dependent on the route of delivery when only glucose is delivered acutely; however, the hepatic response to chronic TPN and TEN is very similar. We aimed to determine whether the route of nutrient delivery altered the acute (first 8 h) response of the liver and whether chronic enteral delivery of glucose alone could augment the adaptive response to TPN. Chronically catheterized conscious dogs received either TPN or TEN containing glucose, Intralipid, and Travasol for either 8 h or 5 days. Another group received TPN for 5 days, but approximately 50% of the glucose in the nutrition was given via the enteral route (TPN+EG). Hepatic metabolism was assessed with tracer and arteriovenous difference techniques. In the presence of similar arterial plasma glucose levels (approximately 6 mM), NHGU and net hepatic lactate release increased approximately twofold between 8 h and 5 days in TPN and TEN. NHGU (26 +/- 1 vs. 23 +/- 3 micromol.kg(-1).min(-1)) and net hepatic lactate release (44 +/- 1 vs. 34 +/- 6 micromol.kg(-1).min(-1)) in TPN+EG were similar to results for TPN, despite lower insulin levels (96 +/- 6 vs. 58 +/- 16 pM, TPN vs. TPN+EG). TEN does not acutely enhance NHGU or disposition above that seen with TPN. However, partial delivery of enteral glucose is effective in decreasing the insulin requirement during chronic TPN.

Inhalation of insulin (Exubera) is associated with augmented disposal of portally infused glucose in dogs

The results of the present study, using the conscious beagle dog, demonstrate that inhaled insulin (INH; Exubera) provides better glycemic control during an intraportal glucose load than identical insulin levels induced by insulin (Humulin) infusion into the inferior vena cava (IVC). In the INH group (n = 13), portal glucose infusion caused arterial plasma glucose to rise transiently (152 +/- 9 mg/dl), before it returned to baseline (65 min) for the next 2 h. Net hepatic glucose uptake was minimal, whereas nonhepatic uptake rose to 12.5 +/- 0.5 mg x kg(-1) x min(-1) (65 min). In the IVC group (n = 9), arterial glucose rose rapidly (172 +/- 6 mg/dl) and transiently fell to 135 +/- 13 mg/dl (65 min) before returning to 165 +/- 15 mg/dl (125 min). Plasma glucose excursions and hepatic glucose uptake were much greater in the IVC group, whereas nonhepatic uptake was markedly less (8.6 +/- 0.9 mg x kg(-1) x min(-1); 65 min). Insulin kinetics and areas under the curve were identical in both groups. These data suggest that inhalation of Exubera results in a unique action on nonhepatic glucose clearance.

Interaction of a selective serotonin reuptake inhibitor with insulin in the control of hepatic glucose uptake in conscious dogs

Whether hyperinsulinemia is required for stimulation of net hepatic glucose uptake (NHGU) by a selective serotonin reuptake inhibitor (SSRI) was examined in four groups of conscious 42-h-fasted dogs, using arteriovenous difference and tracer ([3-3H]glucose) techniques. Experiments consisted of equilibration (-120 to -30 min), basal (-30 to 0 min), and experimental periods (Exp; 0-240 min). During Exp, somatostatin, intraportal insulin [at basal (Ins groups) or 4-fold basal rates (INS groups)], basal intraportal glucagon, and peripheral glucose (to double hepatic glucose load) were infused. In the Fluv-Ins (n = 7) and Fluv-INS groups (n = 6), saline was infused intraportally from 0 to 90 min (P1), and fluvoxamine was infused intraportally at 2 microg x kg(-1) x min(-1) from 90 to 240 min (P2). Sal-Ins (n = 9) and Sal-INS (n = 8) received intraportal saline in P1 and P2. NHGU during P2 was 8.4 +/- 1.4 and 6.9 +/- 2.3 micromol x kg(-1) x min(-1) in Sal-Ins and Fluv-Ins, respectively (not significant), and 13.3 +/- 2.2 and 20.9 +/- 3.1 micromol x kg(-1) x min(-1) (P < 0.05) in Sal-INS and Fluv-INS. Unidirectional (tracer-determined) hepatic glucose uptake was twofold greater (P < 0.05) in Fluv-INS than Sal-INS. Net hepatic carbon retention during P2 was significantly greater in Fluv-INS than Sal-INS (18.5 +/- 2.7 vs. 12.2 +/- 1.9 micromol x kg(-1) x min(-1)). Nonhepatic glucose uptake was reduced in Fluv-INS vs. Sal-INS (20.0 +/- 1.3 vs. 38.4 +/- 5.4 micromol x kg(-1) x min(-1), P < 0.05). Intraportal fluvoxamine enhanced NHGU and net hepatic carbon retention in the presence of hyperinsulinemia but not euinsulinemia, suggesting that hepatocyte-targeted SSRIs may reduce postprandial hyperglycemia.

Portal infusion of a selective serotonin reuptake inhibitor enhances hepatic glucose disposal in conscious dogs

Intraportal delivery of serotonin enhanced net hepatic glucose uptake (NHGU) during a hyperinsulinemic hyperglycemic clamp, but serotonin elevated catecholamines and can cause gastrointestinal distress. We hypothesized that the selective serotonin reuptake inhibitor (SSRI) fluvoxamine would enhance NHGU without side effects. Arteriovenous difference and tracer ([3-(3)H]glucose) techniques were used in conscious 42-h-fasted dogs. Experiments consisted of equilibration (-120 to -30 min), basal (-30 to 0 min), and experimental (EXP; 0-270 min) periods. During EXP, somatostatin, fourfold basal intraportal insulin, basal intraportal glucagon, and peripheral glucose (to double the hepatic glucose load) were infused. Saline (SAL) was infused intraportally during 0-90 min (P1), and fluvoxamine was infused intraportally at 0.5, 1, and 2 mug.kg(-1).min(-1) from 90 to 150 (P2), 150 to 210 (P3), and 210 to 270 (P4) min, respectively, in the FLUV group (n = 8). The SAL group (n = 9) received intraportal saline during 0-270 min. NHGU in SAL was 13.9 +/- 1.7 and 17.0 +/- 2.0 mumol.kg(-1).min(-1) in P3-P4, respectively, while NHGU in FLUV averaged 19.7 +/- 2.8 and 26.6 +/- 3.0 mumol.kg(-1).min(-1) (P < 0.05 vs. SAL). Net hepatic carbon retention was greater (P < 0.05) in FLUV than in SAL (17.6 +/- 2.6 vs. 13.9 +/- 2.7 and 23.8 +/- 3.0 vs. 14.4 +/- 3.3 mumol.kg(-1).min(-1) in P3-P4, respectively), and final hepatic glycogen concentrations were 50% greater in FLUV (P < 0.005). Nonhepatic glucose uptake was greater in SAL than in FLUV at 270 min (P < 0.05). Catecholamine concentrations remained basal, and the animals evidenced no distress. Thus fluvoxamine enhanced NHGU and hepatic carbon storage without raising circulating serotonin concentrations or causing stress, suggesting that hepatic-targeted SSRIs might be effective in reducing postprandial hyperglycemia in individuals with diabetes or impaired glucose tolerance.

Insulin-independent effects of GLP-1 on canine liver glucose metabolism: duration of infusion and involvement of hepatoportal region

Whether glucagon-like peptide-1 (GLP-1) has insulin-independent effects on glucose disposal in vivo was assessed in conscious dogs by use of tracer and arteriovenous difference techniques. After a basal period, each experiment consisted of three periods (P1, P2, P3) during which somatostatin, glucagon, insulin, and glucose were infused. The control group (C) received saline in P1, P2, and P3, the PePe group received saline in P1 and GLP-1 (7.5 pmol.kg(-1).min(-1)) peripherally (Pe; iv) in P2 and P3, and the PePo group received saline in P1 and GLP-1 peripherally (iv) (P2) and then into the portal vein (Po; P3). Glucose and insulin concentrations increased to two- and fourfold basal, respectively, and glucagon remained basal. GLP-1 levels increased similarly in the PePe and PePo groups during P2 ( approximately 200 pM), whereas portal GLP-1 levels were significantly increased (3-fold) in PePo vs. PePe during P3. In all groups, net hepatic glucose uptake (NHGU) occurred during P1. During P2, NHGU increased slightly but not significantly in all groups. During P3, NHGU increased in PePe and PePo groups to a greater extent than in C, but no significant effect of the route of infusion of GLP-1 was demonstrated (16.61 +/- 2.91 and 14.67 +/- 2.09 vs. 4.22 +/- 1.57 micromol.kg(-1).min(-1), respectively).

IN CONCLUSION

GLP-1 increased glucose disposal in the liver independently of insulin secretion; its full action required long-term infusion. The route of infusion did not modify the hepatic response.

Characterization of the portal signal during 24-h glucose delivery in unrestrained, conscious rats

To characterize the "portal signal" during physiological glucose delivery, liver glycogen was measured in unrestrained rats during portal (Po) and peripheral (Pe) constant-rate infusion, with minimal differences in hepatic glucose load (HGL) and portal insulin between the delivery routes. Hepatic blood flows were measured by Doppler flowmetry during open surgery. Changes in hepatic glucose, portal insulin, glucagon, lactate, and free fatty acid concentrations were generally similar in either delivery except for glucagon at 4 h. Hepatic glycogen, however, increased continuously in Po and was higher than Pe at 8 and 24 h, although it decreased to the level of Pe upon the removal of Po at 8 h. There was a near-linear relationship between hepatic glycogen and HGL in either delivery, with the slope being twice as high in Po and the intercepts converging to basal HGL. The hepatic response to Po did not alter upon 80% replacement by Pe. These results suggest that negative arterial-portal glucose gradients increase the rate of hepatic glycogen synthesis against the incremental HGL in an all-or-nothing mode.

Vagal cooling and concomitant portal norepinephrine infusion do not reduce net hepatic glucose uptake in conscious dogs

We examined the role of efferent neural signaling in regulation of net hepatic glucose uptake (NHGU) in two groups of conscious dogs with hollow perfusable coils around their vagus nerves, using tracer and arteriovenous difference techniques. Somatostatin, intraportal insulin and glucagon at fourfold basal and basal rates, and intraportal glucose at 3.8 mg.kg(-1).min(-1) were infused continuously. From 0 to 90 min [period 1 (P1)], the coils were perfused with a 37 degrees C solution. During period 2 [P2; 90-150 min in group 1 (n = 3); 90-180 min in group 2 (n = 6)], the coils were perfused with -15 degrees C solution to eliminate vagal signaling, and the coils were subsequently perfused with 37 degrees C solution during period 3 (P3). In addition, group 2 received an intraportal infusion of norepinephrine at 16 ng.kg(-1).min(-1) during P2. The effectiveness of vagal suppression was demonstrated by the increase in heart rate during P2 (111 +/- 17, 167 +/- 16, and 105 +/- 13 beats/min in group 1 and 71 +/- 6, 200 +/- 11, and 76 +/- 6 beats/min in group 2 during P1-P3, respectively) and by prolapse of the third eyelid during P2. Arterial plasma glucose, insulin, and glucagon concentrations; hepatic blood flow; and hepatic glucose load did not change significantly during P1-P3. NHGU during P1-P3 was 2.7 +/- 0.4, 4.1 +/- 0.6, and 4.0 +/- 1.2 mg.kg(-1).min(-1) in group 1 and 5.0 +/- 0.9, 5.6 +/- 0.7, and 6.1 +/- 0.9 mg.kg(-1).min(-1) in group 2 (not significant among periods). Interruption of vagal signaling with or without intraportal infusion of norepinephrine to augment sympathetic tone did not suppress NHGU during portal glucose delivery, suggesting the portal signal stimulates NHGU independently of vagal efferent flow.

Portal serotonin infusion and glucose disposal in conscious dogs

Whether serotonin (5-hydroxytryptamine [5-HT]) enhances net hepatic glucose uptake (NHGU) during glucose infusion was examined in conscious 42-h-fasted dogs, using arteriovenous difference and tracer ([3-(3)H]glucose) techniques. Experiments consisted of equilibration (-120 to -30 min), basal (-30 to 0 min), and experimental (0-390 min) periods. During the experimental period, somatostatin, fourfold basal intraportal insulin, basal intraportal glucagon, and peripheral glucose (to double the hepatic glucose load) were infused. In one group of dogs (SER; n = 8), saline was infused intraportally from 0 to 90 min (P1), and 5-HT was infused intraportally at 10, 20, and 40 microg.kg(-1).min(-1) from 90 to 150 (P2), 150 to 210 (P3), and 210 to 270 (P4) min, respectively. In the other group (SAL; n = 8), saline was infused intraportally from 0 to 270 min. NHGU in SAL was 12.4 +/- 2.3, 14.9 +/- 2.7, 13.4 +/- 2.1, and 15.1 +/- 1.8 micromol.kg(-1).min(-1) in P1 to P4, respectively, whereas NHGU in SER averaged 13.2 +/- 3.0, 16.4 +/- 2.4, 19.0 +/- 2.4 (P < 0.05 vs. SAL), and 22.0 +/- 2.9 micromol.kg(-1).min(-1) (P < 0.05 vs. SAL). Nonhepatic glucose uptake ( micromol.kg(-1).min(-1)) in SAL was 31.7 +/- 4.9, 43.9 +/- 5.1, 55.1 +/- 5.6, and 66.2 +/- 8.6 during P1 to P4, respectively, whereas in SER, the corresponding values were 26.1 +/- 5.7, 31.6 +/- 9.4, 35.1 +/- 7.6 (P < 0.05 vs. SAL), and 34.7 +/- 7.7 (P < 0.05 vs. SAL). Intraportal 5-HT enhances NHGU but blunts nonhepatic glucose uptake, raising the possibility that hepatic-targeted 5-HT or 5-HT receptor agonists might reduce postprandial hyperglycemia.

Effect of intraportal glucagon-like peptide-1 on glucose metabolism in conscious dogs

Arteriovenous difference and tracer ([3-(3)H]glucose) techniques were used in 42-h-fasted conscious dogs to identify any insulin-like effects of intraportally administered glucagon-like peptide 1-(7-36)amide (GLP-1). Each study consisted of an equilibration, a basal, and three 90-min test periods (P1, P2, and P3) during which somatostatin, intraportal insulin (3-fold basal) and glucagon (basal), and peripheral glucose were infused. Saline was infused intraportally in P1. During P2 and P3, GLP-1 was infused intraportally at 0.9 and 5.1 pmol. kg(-1). min(-1) in eight dogs, at 10 and 20 pmol. kg(-1). min(-1) in seven dogs, and at 0 pmol. kg(-1). min(-1) in eight dogs (control group). Net hepatic glucose uptake was significantly enhanced during GLP-1 infusion at 20 pmol. kg(-1). min(-1) [21.8 vs. 13.4 micromol. kg(-1). min(-1) (control), P < 0.05]. Glucose utilization was significantly increased during infusion at 10 and 20 pmol. kg(-1). min(-1) [87.3 +/- 8.3 and 105.3 +/- 12.8, respectively, vs. 62.2 +/- 5.3 and 74.7 +/- 7.4 micromol. kg(-1). min(-1) (control), P < 0.05]. The glucose infusion rate required to maintain hyperglycemia was increased (P < 0.05) during infusion of GLP-1 at 5.1, 10, and 20 pmol. kg(-1). min(-1) (22, 36, and 32%, respectively, greater than control). Nonhepatic glucose uptake increased significantly during delivery of GLP-1 at 5.1 and 10 pmol. kg(-1). min(-1) (25 and 46% greater than control) and tended (P = 0.1) to increase during GLP-1 infusion at 20 pmol. kg(-1). min(-1) (24% greater than control). Intraportal infusion of GLP-1 at high physiological and pharmacological rates increased glucose disposal primarily in nonhepatic tissues.