Hepatic glucose-sensitive unit regulation of glucose-induced insulin secretion in rats

Hepatic interoception in health and disease

The liver is a large organ with crucial functions in metabolism and immune defense, as well as blood homeostasis and detoxification, and it is clearly in bidirectional communication with the brain and rest of the body via both neural and humoral pathways. A host of neural sensory mechanisms have been proposed, but in contrast to the gut-brain axis, details for both the exact site and molecular signaling steps of their peripheral transduction mechanisms are generally lacking. Similarly, knowledge about function-specific sensory and motor components of both vagal and spinal access pathways to the hepatic parenchyma is missing. Lack of progress largely owes to controversies regarding selectivity of vagal access pathways and extent of hepatocyte innervation. In contrast, there is considerable evidence for glucose sensors in the wall of the hepatic portal vein and their importance for glucose handling by the liver and the brain and the systemic response to hypoglycemia. As liver diseases are on the rise globally, and there are intriguing associations between liver diseases and mental illnesses, it will be important to further dissect and identify both neural and humoral pathways that mediate hepatocyte-specific signals to relevant brain areas. The question of whether and how sensations from the liver contribute to interoceptive self-awareness has not yet been explored.

Use of a bio-electronic device comprising of targeted dual neuromodulation of the hepatic and celiac vagal branches demonstrated enhanced glycemic control in a type 2 diabetic rat model as well as in an Alloxan treated swine model

Background

There is an unmet need for new type 2 diabetes treatments providing improved efficacy, durability and customized to improve patient’s compliance. Bio-electronic neuromodulation of Vagus nerve branches innervating organs that regulate plasma glucose, may be a method for treating type 2 diabetes. The pancreas has been shown to release insulin during Vagus stimulation. The hepatic vagal branch, innervating the liver, has been shown to decrease glucose release and decrease insulin resistance following ligation. However, standalone stimulation of the Vagus nerve has shown mixed results and Vagus nerve ligation has undesirable effects. Little is known; however, of the effect on plasma glucose with combined neuromodulation consisting of stimulation of the celiac branch innervating the pancreas with simultaneous high frequency alternating current (HFAC) blockade of the hepatic branch. This study tested the effects of this approach on increasing glycemic control in rat a model of type 2 diabetes and Alloxan treated swine.

Materials and methods

Zucker obese (fatty) male rats (ZDF fa/fa) were used as a model of type 2 diabetes as well as glucose intolerant Alloxan treated swine. In ZDF rat experiments glycemic control was accessed with an intravenous glucose tolerance test during HFAC-induced hepatic branch block with concurrent celiac stimulation (HFAC + stimulation). In swine experiments glycemic control was accessed by an oral glucose tolerance test during HFAC + stimulation. Insulin measurements were taken prior to and following swine experiments giving insight into beta cell exhaustion. Histopathology was conducted to determine safety of HFAC + stimulation on Vagal branches.

Results

Zucker rats demonstrated a significant improvement to an intravenous glucose tolerance test during HFAC + stimulation compared to sham. There was no significant difference from sham compared to hepatic vagotomy or celiac stimulation. In Alloxan treated swine, when subjected to HFAC + stimulation, there was a significant improvement in glycemic control as measured by an improvement on oral glucose tolerance tests and a decrease in fasting plasma glucose. Insulin responses were similar prior to and following HFAC + stimulation experiments. Histopathology demonstrated healthy swine Vagus nerves.

Conclusion

Electrical blockade of the hepatic Vagus branch with simultaneous stimulation of the celiac Vagus branch may be a novel, adjustable and localized approach for a treatment of type 2 diabetes.

Reduced Liver-Specific PGC1a Increases Susceptibility for Short-Term Diet-Induced Weight Gain in Male Mice

The central integration of peripheral neural signals is one mechanism by which systemic energy homeostasis is regulated. Previously, increased acute food intake following the chemical reduction of hepatic fatty acid oxidation and ATP levels was prevented by common hepatic branch vagotomy (HBV). However, possible offsite actions of the chemical compounds confound the precise role of liver energy metabolism. Herein, we used a hepatocyte PGC1a heterozygous (LPGC1a) mouse model, with associated reductions in mitochondrial fatty acid oxidation and respiratory capacity, to assess the role of liver energy metabolism in systemic energy homeostasis. LPGC1a male, but not female, mice had a 70% greater high-fat/high-sucrose (HFHS) diet-induced weight gain compared to wildtype (WT) mice (p < 0.05). The greater weight gain was associated with altered feeding behavior and lower activity energy expenditure during the HFHS diet in LPGC1a males. WT and LPGC1a mice underwent sham surgery or HBV to assess whether vagal signaling was involved in the HFHS-induced weight gain of male LPGC1a mice. HBV increased HFHS-induced weight gain (85%, p < 0.05) in male WT mice, but not LPGC1a mice. These data demonstrate a sex-specific role of reduced liver energy metabolism in acute diet-induced weight gain, and the need for a more nuanced assessment of the role of vagal signaling in short-term diet-induced weight gain.

Hepatocyte membrane potential regulates serum insulin and insulin sensitivity by altering hepatic GABA release

Hepatic lipid accumulation in obesity correlates with the severity of hyperinsulinemia and systemic insulin resistance. Obesity-induced hepatocellular lipid accumulation results in hepatocyte depolarization. We have established that hepatocyte depolarization depresses hepatic afferent vagal nerve firing, increases GABA release from liver slices, and causes hyperinsulinemia. Preventing hepatic GABA release or eliminating the ability of the liver to communicate to the hepatic vagal nerve ameliorates the hyperinsulinemia and insulin resistance associated with diet-induced obesity. In people with obesity, hepatic expression of GABA transporters is associated with glucose infusion and disposal rates during a hyperinsulinemic euglycemic clamp. Single-nucleotide polymorphisms in hepatic GABA re-uptake transporters are associated with an increased incidence of type 2 diabetes mellitus. Herein, we identify GABA as a neuro-hepatokine that is dysregulated in obesity and whose release can be manipulated to mute or exacerbate the glucoregulatory dysfunction common to obesity.

The Medullary Targets of Neurally Conveyed Sensory Information from the Rat Hepatic Portal and Superior Mesenteric Veins

Vagal and spinal sensory endings in the wall of the hepatic portal and superior mesenteric veins (PMV) provide the brain with chemosensory information important for energy balance and other functions. To determine their medullary neuronal targets, we injected the transsynaptic anterograde viral tracer HSV-1 H129-772 (H129) into the PMV wall or left nodose ganglion (LNG) of male rats, followed by immunohistochemistry (IHC) and high-resolution imaging. We also determined the chemical phenotype of H129-infected neurons, and potential vagal and spinal axon terminal appositions in the dorsal motor nucleus of the vagus (DMX) and the nucleus of the solitary tract (NTS). PMV wall injections generated H129-infected neurons in both nodose ganglia and in thoracic dorsal root ganglia (DRGs). In the medulla, cholinergic preganglionic parasympathetic neurons in the DMX were virtually the only targets of chemosensory information from the PMV wall. H129-infected terminal appositions were identified on H129-infected somata and dendrites in the DMX, and on H129-infected DMX dendrites that extend into the NTS. Sensory transmission via vagal and possibly spinal routes from the PMV wall therefore reaches DMX neurons via axo-somatic appositions in the DMX and axo-dendritic appositions in the NTS. However, the dearth of H129-infected NTS neurons indicates that sensory information from the PMV wall terminates on DMX neurons without engaging NTS neurons. These previously underappreciated direct sensory routes into the DMX enable a vago-vagal and possibly spino-vagal reflexes that can directly influence visceral function.

Effect of portal glucose sensing on incretin hormone secretion in a canine model

It is unknown whether activation of hepato-portal vein (PV) glucose sensors plays a role in incretin hormone amplification of oral glucose-stimulated insulin secretion (GSIS). In previous studies, PV glucose infusion increased GSIS through unknown mechanisms, perhaps neural stimulation of pancreatic β-cells and/or stimulation of gut incretin hormone release. Thus, there could be a difference in the incretin effect when comparing GSIS with portal rather than leg vein (LV) glucose infusion. Plasma insulin and incretin hormones were studied in six overnight-fasted dogs. An oral glucose tolerance test (OGTT) was administered, and then 1 and 2 wk later the arterial plasma glucose profile from the OGTT was mimicked by infusing glucose into either the PV or a LV. The arterial glucose levels were nearly identical between groups (AUCs within 1% of each other). Oral glucose administration increased arterial GLP-1 and GIP levels by more than sixfold, whereas they were not elevated by PV or LV glucose infusion. Oral glucose delivery was associated with only a small incretin effect (arterial insulin and C-peptide were 21 ± 23 and 24 ± 17% greater, respectively, during the 1st hour with oral compared with PV glucose and 14 ± 37 and 13 ± 35% greater, respectively, in oral versus LV; PV versus LV responses were not significantly different from each other). Thus, following an OGTT incretin hormone release did not depend on activation of PV glucose sensors, and the insulin response was not greater with PV compared with LV glucose infusion in the dog. The small incretin effect points to species peculiarities, which is perhaps related to diet.

Ingestion of a moderate high-sucrose diet results in glucose intolerance with reduced liver glucokinase activity and impaired glucagon-like peptide-1 secretion

Aims/Introduction:  Excessive intake of sucrose can cause severe health issues, such as diabetes mellitus. In animal studies, consumption of a high‐sucrose diet (SUC) has been shown to cause obesity, insulin resistance and glucose intolerance. However, several in vivo experiments have been carried out using diets with much higher sucrose contents (50–70% of the total calories) than are typically ingested by humans. In the present study, we examined the effects of a moderate SUC on glucose metabolism and the underlying mechanism.

Materials and Methods:  C57BL/6J mice received a SUC (38.5% sucrose), a high‐starch diet (ST) or a control diet for 5 weeks. We assessed glucose tolerance, incretin secretion and liver glucose metabolism.

Results:  An oral glucose tolerance test (OGTT) showed that plasma glucose levels in the early phase were significantly higher in SUC‐fed mice than in ST‐fed or control mice, with no change in plasma insulin levels at any stage. SUC‐fed mice showed a significant improvement in insulin sensitivity. Glucagon‐like peptide‐1 (GLP‐1) secretion 15 min after oral glucose administration was significantly lower in SUC‐fed mice than in ST‐fed or control mice. Hepatic glucokinase (GCK) activity was significantly reduced in SUC‐fed mice. During the OGTT, the accumulation of glycogen in the liver was suppressed in SUC‐fed mice in a time‐dependent manner.

Conclusions:  These results indicate that mice that consume a moderate SUC show glucose intolerance with a reduction in hepatic GCK activity and impairment in GLP‐1 secretion. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00208.x, 2012)

Efficacy of celiac branch preservation in Roux-en-y reconstruction after laparoscopy-assisted distal gastrectomy

BACKGROUND

The present study investigated the efficacy of preserving the celiac branch of the vagus nerve after laparoscopy-assisted distal gastrectomy (LADG) with Roux-en-Y (R-Y) reconstruction.

METHODS

Between January 2004 and September 2008, a total of 159 consecutive patients who underwent LADG for gastric carcinoma were classified into groups according to preservation of the celiac branch of the vagus nerve-preservation group (P-LADG; n = 70) and the resection group (R-LADG; n = 89). The parameters analyzed included patient and tumor characteristics, operative details, postoperative outcomes, and nutritional state. The endoscopic findings of the gastric remnant and lower esophagus were evaluated at 12 months postoperatively.

RESULTS

In regard to postoperative complications, no significant differences were found between groups. With R-LADG, 14 patients suffered from dumping syndrome (15.7%), compared with only 2 patients with P-LADG (2.9%; P = .007). The amount of meal consumption compared with the preoperative value and the rate of weight reduction at 12 months postoperatively did not differ significantly between groups. Endoscopic findings showed significantly more residue with P-LADG (34.3%) than with R-LADG (16.9%; P = .011).

CONCLUSION

The celiac branch is useful in regulating gastrointestinal motility by maintaining postoperative physiologic function. Celiac branch preservation seems to represent a feasible and beneficial method for LADG.

Ghrelin infused into the portal vein inhibits glucose-stimulated insulin secretion in Wistar rats

Although accumulating evidence has shown crucial roles of ghrelin and insulin in food intake and energy metabolism, the exact relationship between these hormones remains unclear. In this study, we determined the in vivo effect of ghrelin on insulin secretion. We demonstrated that ghrelin inhibited the glucose-stimulated release of insulin when infused into the portal vein of Wistar rats. However, ghrelin infusion into the femoral vein did not induce such an inhibitory effect. Hepatic vagotomy or coinfusion with atropine methyl bromide diminished the inhibitory effect of ghrelin on glucose-stimulated insulin secretion. In conclusion, ghrelin exerts an inhibitory effect on glucose-stimulated insulin secretion via the hepatic portal system and the vagus nerve. The decrease in ghrelin level after a meal is important for the occurrence of the incretin effect in rats.

Effect of distal subtotal gastrectomy with preservation of the celiac branch of the vagus nerve to gastrointestinal function: an experimental study in conscious dogs

OBJECTIVE

To evaluate the effects of distal subtotal gastrectomy with preservation of the celiac branch of the vagus nerve on gastrointestinal function.

SUMMARY BACKGROUND DATA

The operative procedure of distal subtotal gastrectomy with preservation of the celiac branch of the vagus nerve is now in the spotlight in Japan with the goal of finding a function-preserving surgical technique. However, there has been no analysis of the effect of this type of surgery on gastrointestinal function. In this article, we describe the results of a fundamental experiment on distal subtotal gastrectomy with preservation of the celiac branch of the vagus nerve.

METHODS

Twenty conscious dogs were divided into 2 groups, each subdivided into 2 groups of 5: a normal intact dog group (NG) divided into 2 groups, with preservation (PNG) and resection (RNG; these dogs were truncally vagotomized including transaction of the celiac branch) of the celiac branch, and a gastrectomy dog group (GG) divided into 2 groups, with preservation (PGG) and resection (RGG) of the celiac branch. The motility of the dogs was recorded using strain gauge force transducers. The effects of the preservation of the celiac branch of the vagus nerve on gastrointestinal motility, gastric emptying, and pancreatic insulin release were evaluated.

RESULTS

The motility index of gastrointestinal motility with preservation of the celiac branch was higher than the motility index with resection of the celiac branch in fasted and fed of NG and GG. In gastric emptying, significant differences were found between the PNG and RNG but not between the PGG and RGG. In the fasted state for 80 minutes of the PNG and PGG, the serum insulin concentration reached a peak during the early phase III at 20 minutes in the gastric body and the antrum.

CONCLUSIONS

This study has shown that it is effective to preserve the celiac branch of the vagus nerve for gastroduodenal motility, gastric emptying, and pancreatic insulin release after a gastrectomy.

Capsaicin-sensitive vagal afferents modulate posttranscriptional regulation of the rat Na+/glucose cotransporter SGLT1

INTRODUCTION

the intestinal Na(+)/glucose cotransporter (SGLT1) displays rapid anticipatory diurnal rhythms in mRNA and protein expression. The vagus nerve has been implicated in the entrainment of some transporters. We aimed to clarify the influence of the vagus nerve on the diurnal entrainment pathway for SGLT1 and examine the role of vagal afferent fibers.

METHODS

male Sprague-Dawley rats were randomized to three groups, total subdiaphragmatic vagotomy, selective deafferentation of the vagus with capsaicin, or sham laparotomy. Postoperatively, animals were maintained in a 12-h light-dark cycle with food access limited to night. On the ninth postoperative day, animals were euthanized to harvest jejunal mucosa at 6-h intervals starting at 10 AM. Whole cell SGLT1 protein was measured by semiquantitative densitometry of immunoblots. Sglt1 and regulatory subunit RS1 mRNA was assessed by quantitative PCR. Fluorogold tracer technique was used to confirm adequacy of the vagotomy.

RESULTS

the diurnal rhythm in intestinal SGLT1, with a 5.3-fold increase in Sglt1 mRNA at 4 PM, was preserved in both vagotomy and capsaicin groups. However, the rhythmicity in SGLT1 protein expression (2.3-fold peak at 10 PM; P = 0.041) was abolished following either total vagotomy or deafferentation. Lack of change in RS1 mRNA suggests this is independent of the RS1 regulatory pathway.

CONCLUSION

SGLT1 transcription is independent of the vagus. However, dissociation of the protein rhythm from the underlying mRNA signal by vagotomy suggests the vagus may be involved in posttranscriptional regulation of SGLT1 in an RS1 independent pathway. Disruption following afferent ablation by capsaicin suggests this limb is specifically necessary.

Intraportally delivered GLP-1, in the presence of hyperglycemia induced via peripheral glucose infusion, does not change whole body glucose utilization

After a meal, glucagon-like peptide-1 (GLP-1) and glucose levels are significantly greater in the hepatic portal vein than in the artery. We have previously reported that, in the presence of intraportal glucose delivery, a physiological increase of GLP-1 in the hepatic portal vein increases nonhepatic glucose uptake via a mechanism independent of changes in pancreatic hormone secretion. The aim of the present study was to determine whether intraportal glucose delivery is required to observe this effect. Experiments consisted of a 40-min basal period, followed by a 240-min experimental period, during which conscious 42-h fasted dogs received glucose peripherally to maintain arterial plasma glucose levels at approximately 160 mg/dl. In addition, either saline (n = 6) or GLP-1 (1 pmol.kg(-1).min(-1); GLP-1, n = 6) was administered intraportally during the experimental period. As in the previous study, the presence of GLP-1 did not alter pancreatic hormone levels; however, in the present study, intraportal GLP-1 infusion did not result in an increase in whole body glucose utilization. This is despite the fact that arterial and hepatic portal vein GLP-1 levels were maintained at the same level as the previous study. Therefore, a physiological elevation of GLP-1 in the hepatic portal vein does not increase whole body glucose uptake when hyperglycemia is induced by peripheral glucose infusion. This indicates that a physiological increase in GLP-1 augments glucose utilization only when GLP-1 and glucose gradients conditions mimic the postprandial state.

Mechanism of rapid-phase insulin response to elevation of portal glucose concentration

The hepatoportal region is important for glucose sensing; however, the relationship between the hepatoportal glucose-sensing system and the postprandial rapid phase of the insulin response has been unclear. We examined whether a rapid-phase insulin response to low amounts of intraportal glucose infusion would occur, compared that with the response to intrajugular glucose infusion in conscious rats, and assessed whether this sensing system was associated with autonomic nerve activity. The increases in plasma glucose concentration did not differ between the two infusions at 3 min, but the rapid-phase insulin response was detected only in the intraportal infusion. A sharp and rapid insulin response was observed at 3 min after intraportal infusion of a small amount of glucose but not after intrajugular infusion. Furthermore, this insulin response was also induced by intraportal fructose infusion but not by nonmetabolizable sugars. The rapid-phase insulin response at 3 min during intraportal infusion did not differ between rats that had undergone hepatic vagotomy or chemical sympathectomy with 6-hydroxydopamine compared with control rats, but this response disappeared in rats that had undergone chemical vagotomy with atropine. We conclude that the elevation of glucose concentration in the hepatoportal region induced afferent signals from undetectable sensors and that these signals stimulate pancreas to induce the rapid-phase insulin response via cholinergic nerve action.

The effect of vagal cooling on canine hepatic glucose metabolism in the presence of hyperglycemia of peripheral origin

We examined the role of vagus nerves in the transmission of the portal glucose signal in conscious dogs. At time 0, somatostatin infusion was started along with intraportal insulin and glucagon at 4-fold basal and basal rates, respectively. Glucose was infused via a peripheral vein to create hyperglycemia ( approximately 2 fold basal). At t = 90, hollow coils around the vagus nerves were perfused with -10 degrees C or 37 degrees C solution in the vagally cooled (COOL) and sham-cooled (SHAM) groups, respectively (n = 6 per group). Effectiveness of vagal blockade was demonstrated by increase in heart rate during perfusion in the COOL vs SHAM groups (183 +/- 3 vs 102 +/- 5 beats per minute, respectively) and by prolapse of the third eyelid in the COOL group. Arterial plasma insulin (22 +/- 2 and 24 +/- 3 micro U/mL) and glucagon (37 +/- 5 and 40 +/- 4 pg/mL) concentrations did not change significantly between the first experimental period and the coil perfusion period in either the SHAM or COOL group, respectively. The hepatic glucose load throughout the entire experiment was 46 +/- 1 and 50 +/- 2 mg . kg(-1) . min(-1) in the SHAM and COOL groups, respectively. Net hepatic glucose uptake (NHGU) did not differ in the SHAM and COOL groups before (2.2 +/- 0.5 and 2.9 +/- 0.8 mg . kg(-1) . min(-1), respectively) or during the cooling period (3.0 +/- 0.5 and 3.4 +/- 0.6 mg . kg(-1) . min(-1), respectively). Likewise, net hepatic glucose fractional extraction and nonhepatic glucose uptake and clearance were not different between groups during coil perfusion. Interruption of vagal signaling in the presence of hyperinsulinemia and hyperglycemia resulting from peripheral glucose infusion did not affect NHGU, further supporting our previous suggestion that vagal input to the liver is not a primary determinant of NHGU.

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.

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 liver's ability to take up glucose.

Intraportal glucose infusion and pancreatic islet blood flow in anesthetized rats

The aim of the study was to evaluate whether a selective increase in portal vein blood glucose concentration can affect pancreatic islet blood flow. Anesthetized rats were infused (0.1 ml/min for 3 min) directly into the portal vein with saline, glucose, or 3-O-methylglucose. The infused dose of glucose (1 mg. kg body wt(-1). min(-1)) was chosen so that the systemic blood glucose concentration was unaffected. Intraportal infusion of D-glucose increased insulin release and islet blood flow; the osmotic control substance 3-O-methylglucose had no such effect. A bilateral vagotomy performed 20 min before the infusions potentiated the islet blood flow response and also induced an increase in whole pancreatic blood flow, whereas the insulin response was abolished. Administration of atropine to vagotomized animals did not change the blood flow responses to intraportal glucose infusions. When the vagotomy was combined with a denervation of the hepatic artery, there was no stimulation of islet blood flow or insulin release after intraportal glucose infusion. We conclude that a selective increase in portal vein blood glucose concentration may participate in the islet blood flow increase in response to hyperglycemia. This effect is probably mediated via periarterial nerves and not through the vagus nerve. Furthermore, this blood flow increase can be dissociated from changes in insulin release.

Importance of the hepatic arterial glucose level in generation of the portal signal in conscious dogs

The aim of this study was to determine whether the elimination of the hepatic arterial-portal (A-P) venous glucose gradient would alter the effects of portal glucose delivery on hepatic or peripheral glucose uptake. Three groups of 42-h-fasted conscious dogs (n = 7/group) were studied. After a 40-min basal period, somatostatin was infused peripherally along with intraportal insulin (7.2 pmol x kg(-1) x min(-1)) and glucagon (0.65 ng x kg(-1) x min(-1)). In test period 1 (90 min), glucose was infused into a peripheral vein to double the hepatic glucose load (HGL) in all groups. In test period 2 (90 min) of the control group (CONT), saline was infused intraportally; in the other two groups, glucose was infused intraportally (22.2 micromol x kg(-1) x min(-1)). In the second group (PD), saline was simultaneously infused into the hepatic artery; in the third group (PD+HAD), glucose was infused into the hepatic artery to eliminate the negative hepatic A-P glucose gradient. HGL was twofold basal in each test period. Net hepatic glucose uptake (NHGU) was 10.1 +/- 2.2 and 12.8 +/- 2.1 vs. 11.5 +/- 1.6 and 23.8 +/- 3.3* vs. 9.0 +/- 2.4 and 13.8 +/- 4.2 micromol x kg(-1) x min(-1) in the two periods of CONT, PD, and PD+HAD, respectively (* P < 0.05 vs. same test period in PD and PD+HAD). NHGU was 28.9 +/- 1.2 and 39.5 +/- 4.3 vs. 26.3 +/- 3.7 and 24.5 +/- 3.7* vs. 36.1 +/- 3.8 and 53.3 +/- 8.5 micromol x kg(-1) x min(-1) in the first and second periods of CONT, PD, and PD+HAD, respectively (* P < 0.05 vs. same test period in PD and PD+HAD). Thus the increment in NHGU and decrement in extrahepatic glucose uptake caused by the portal signal were significantly reduced by hepatic arterial glucose infusion. These results suggest that the hepatic arterial glucose level plays an important role in generation of the effect of portal glucose delivery on glucose uptake by liver and muscle.

Lack of effects of an acute hepatic vagotomy on insulin and catecholamine responses in rats following exercise

The purpose of the present investigation was to evaluate the effects of an acute hepatic vagotomy on hormonal responses to hyperglycemic and hypoglycemic challenges in rats previously submitted to an exercise protocol. Two experiments were conducted. In a first experiment, 8-week trained (TR) and untrained (UNTR) rats, subdivided into acutely hepatic vagotomized (HV) and sham-operated (SHM) groups, were submitted to an intraperitoneal glucose tolerance test (0.5 g/kg) under anesthesia. Training was associated with a tendency (P = 0.07) for blood glucose levels to be less elevated (at time point 10 min), and with a significant (P < 0.01) lower glucose/insulin ratio following the glucose injection. The HV did not have any effects on these responses. In a second experiment, non-exercised rats and a group of rats submitted to an acute bout of exercise (treadmill, 60 min, 26 m/min, 5% slope) 24 h before the experiment, each one of these two groups being subdivided into acutely HV and SHM groups, were submitted to an insulin-induced hypoglycemia protocol, under anesthesia. Blood glucose concentrations were decreased significantly (P < 0.01) to approximately 40 mg/dl in all groups 60 and 80 min after the insulin injection. Plasma adrenaline and noradrenaline levels were increased significantly (P < 0.01) in all groups. The catecholamine increase was not influenced by the HV or the acute exercise bout. The present results do not indicate an implication of the hepatic vagus nerve on hormonal responses to hyper and hypoglycemia following exercise.

Section of the vagal celiac branch in man reduces glucagon-stimulated insulin release

The purpose of this study was to determine whether section of the celiac branch of the vagus nerve in man affects the insulin response to intravenous glucagon injection. Patients who received a subtotal gastrectomy with lymph node dissection for gastric carcinoma were divided into two groups: the celiac-preserved group (n = 16) and the celiac-sectioned group (n = 13). The hepatic branches of the vagus were preserved in both groups. The glucagon test was performed twice in each patient during the operation; before and after manipulation of the celiac branch. Blood samples were collected just before and 6 min after the injection. No difference in the mean increases in blood glucose, insulin and C-peptide levels were seen between the two groups before the nerve manipulation. In the celiac-preserved group, the glucagon stimulated glucose-related C-peptide ratio (x 10(-3) was 0.5 +/- 0.7 before the nerve manipulation and 3.5 +/- 3.0 after it, a significant difference (p < 0.01). In the celiac-sectioned group, this increase was not observed, the ratio was 0.7 +/- 0.6 before the nerve manipulation and 0.8 +/- 3.4 after. These results indicate that the vagal celiac branch in man may also be involved in the control of pancreatic insulin release.

Nutritional status influences the insulin response produced by acute hepatic vagotomy

It has been established that the liver, through the afferent pathway of the vagus nerve, can influence insulin secretion. The purpose of the present study was to determine if this influence can be altered by different nutritional status aimed at inducing metabolic changes in the liver. This was carried out by comparing the insulin response 30 min after sectioning of the hepatic vagus branch in five experimental conditions: a normal (NCD) and a medium-fat (MFD) for 3 weeks, both with and without an overnight fast, and after an overloading liver glycogen protocol (normal diet). All experiments were conducted using anesthetized, adrenodemedullated rats. Blood was collected before and after (30 min) the hepatic vagotomy (HV) or a sham operation (SHM). As expected, liver glycogen levels were significantly (p < 0.01) lower in the fasted than in the fed condition, and were approximately 50% higher (p < 0.01) in the overloaded than in the normally fed condition. Basal insulin concentrations were also lower (p < 0.01) in the fasted compared to the fed groups, but were significantly (p < 0.01) increased by the medium-fat diet. Plasma glucose levels were significantly (p < 0.01) decreased by the overnight fast, but were not affected by the hepatic vagotomy. Plasma catecholamine concentrations were similar in all experimental conditions. Insulin concentrations were significantly (p < 0.05) increased by the HV, compared to SHM rats, in all experimental conditions (from 50% to 75%). The extent of this response was altered by the diet manipulations as the HV-induced insulin increase was greater (p < 0.01) in the MFD than in the NCD groups, whether fed or fasted. Furthermore, and contrary to our expectations, high hepatic glycogen contents did not reduce the insulin response to an acute hepatic vagotomy. These results indicate that the insulin increase induced by an acute HV is influenced by the prevailing metabolic conditions, and suggest that the hepatic vagus nerve exerts a constant inhibition on insulin secretion, independently of the hepatic glycogen content.

Effects of pyruvate and lactate on food intake in rat strains sensitive and resistant to dietary obesity

We have investigated the effects of peripherally administered pyruvate and lactate on the intake of high fat (HF) and low fat (LF) diets by a strain of rat either sensitive (Osborne-Mendel, OM) or resistant (SSB/Pl) to high fat-induced obesity. Both pyruvate and lactate inhibited the intake of HF and LF diets by OM rats and these effects were blocked by selective hepatic vagotomy. In contrast, in S5B/Pl rats fed an LF diet the responses to pyruvate and lactate were attenuated and were absent in S5B/Pl rats fed the HF diet. These data suggest that OM and S5B/Pl rats differ either in their metabolism of pyruvate and lactate or in their responses to these metabolites.

Effects of selective vagotomy on circadian rhythms of plasma glucose, insulin and food intake in control and ventromedial hypothalamic (VMH) lesioned rats

Effects of hepatic and celiac vagotomy on circadian rhythms of plasma glucose, insulin, and food intake were examined in sham-operated (control) and ventromedial hypothalamic (VMH) lesioned rats. Rats were acclimated to the condition with a 12-hour light-dark cycle for 1 week before surgery. One week after VMH lesions, control and VMH lesioned rats were divided into three groups: sham vagotomy, hepatic vagotomy, and celiac vagotomy. Three days after vagotomy, food intake was measured at 6-hour intervals. Seven days after vagotomy, plasma glucose and insulin were measured at the midpoint of each feeding period. In control rats, hepatic vagotomy destroyed circadian rhythms of plasma glucose and insulin probably due to removal of afferent function. In VMH lesioned rats, celiac vagotomy destroyed circadian rhythm of food intake due to the reduction of plasma insulin by removal of efferent function without affecting the loss of circadian rhythms of plasma glucose and insulin.