Hepatic vagal amino acid sensors modulate amino acid induced insulin and glucagon secretion in the rat

A Primary Role for α-Cells as Amino Acid Sensors

Glucagon and its partner insulin are dually linked in both their secretion from islet cells and their action in the liver. Glucagon signaling increases hepatic glucose output, and hyperglucagonemia is partly responsible for the hyperglycemia in diabetes, making glucagon an attractive target for therapeutic intervention. Interrupting glucagon signaling lowers blood glucose but also results in hyperglucagonemia and α-cell hyperplasia. Investigation of the mechanism for α-cell proliferation led to the description of a conserved liver-α-cell axis where glucagon is a critical regulator of amino acid homeostasis. In return, amino acids regulate α-cell function and proliferation. New evidence suggests that dysfunction of the axis in humans may result in the hyperglucagonemia observed in diabetes. This discussion outlines important but often overlooked roles for glucagon that extend beyond glycemia and supports a new role for α-cells as amino acid sensors.

Hormonal modulation of hepatic cAMP prevents estradiol 17β-D-glucuronide-induced cholestasis in perfused rat liver

BACKGROUND

Estradiol-17β-D-glucuronide (E17G) induces cholestasis in vivo, endocytic internalization of the canalicular transporters multidrug resistance-associated protein 2 (Abcc2) and bile salt export pump (Abcb11) being a key pathomechanism. Cyclic AMP (cAMP) prevents cholestasis by targeting these transporters back to the canalicular membrane. In hepatocyte couplets, glucagon and salbutamol, both of which increase cAMP, prevented E17G action by stimulating the trafficking of these transporters by different mechanisms, namely: glucagon activates a protein kinase A-dependent pathway, whereas salbutamol activates an exchange-protein activated by cAMP (Epac)-mediated, microtubule-dependent pathway.

METHODS

The present study evaluated whether glucagon and salbutamol prevent E17G-induced cholestasis in a more physiological model, i.e., the perfused rat liver (PRL). Additionally, the preventive effect of in vivo alanine administration, which induces pancreatic glucagon secretion, was evaluated.

RESULTS

In PRLs, glucagon and salbutamol prevented E17G-induced decrease in both bile flow and the secretory activity of Abcc2 and Abcb11. Salbutamol prevention fully depended on microtubule integrity. On the other hand, glucagon prevention was microtubule-independent only at early time periods after E17G administration, but it was ultimately affected by the microtubule disrupter colchicine. Cholestasis was associated with endocytic internalization of Abcb11 and Abcc2, the intracellular carriers being partially colocalized with the endosomal marker Rab11a. This effect was completely prevented by salbutamol, whereas some transporter-containing vesicles remained colocalized with Rab11a after glucagon treatment. In vivo, alanine administration increased hepatic cAMP and accelerated the recovery of bile flow and Abcb11/Abcc2 transport function after E17G administration. The initial recovery afforded by alanine was microtubule-independent, but microtubule integrity was required to sustain this protective effect.

CONCLUSION

We conclude that modulation of cAMP levels either by direct administration of cAMP modulators or by physiological manipulations leadings to hormone-mediated increase of cAMP levels (alanine administration), prevents estrogen-induced cholestasis in models with preserved liver architecture, through mechanisms similar to those arisen from in vitro studies.

Branched-chain amino acids enhance the cognitive recovery of patients with severe traumatic brain injury

OBJECTIVE

To investigate whether supplementation with branched-chain amino acids (BCAAs) in patients with severe traumatic brain injury (TBI) improves recovery of cognition and influences plasma concentrations of tyrosine and tryptophan, which are precursors of, respectively, catecholamine and serotonin neurotransmitters in the brain.

DESIGN

Forty patients with TBI were randomly assigned to 15 days of intravenous BCAA supplementation (19.6g/d) (n=20) or an isonitrogenous placebo (n=20).

SETTING

Tertiary care rehabilitation setting in Italy.

PARTICIPANTS

Forty men (mean age, 32+/-15 y) with TBI and 20 healthy subjects (controls) matched for age, sex, and sedentary lifestyle.

INTERVENTION

Supplementation with BCAAs.

MAIN OUTCOME MEASURES

Disability Rating Scale (DRS) and plasma concentrations of BCAAs, tyrosine, and tryptophan.

RESULTS

Fifteen days after admission to the rehabilitation department, the DRS score had improved significantly in both the placebo group (P<.05 vs baseline) and in the BCAA-supplemented group (P<.01 vs baseline). The difference between the 2 groups was significant (P<.004). Plasma tyrosine concentration improved in the group given BCAA supplementation, and tryptophan concentration increased in patients receiving placebo.

CONCLUSIONS

Supplemental BCAAs enhance the retrieval of DRS without causing negative effects on tyrosine and tryptophan concentration.

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.

Regulation of glucose homeostasis in humans with denervated livers

The liver plays a major role in regulating glucose metabolism, and since its function is influenced by sympathetic/ parasympathetic innervation, we used liver graft as a model of denervation to study the role of CNS in modulating hepatic glucose metabolism in humans. 22 liver transplant subjects were randomly studied by means of the hyperglycemic/ hyperinsulinemic (study 1), hyperglycemic/isoinsulinemic (study 2), euglycemic/hyperinsulinemic (study 3) as well as insulin-induced hypoglycemic (study 4) clamp, combined with bolus-continuous infusion of [3-3H]glucose and indirect calorimetry to determine the effect of different glycemic/insulinemic levels on endogenous glucose production and on peripheral glucose uptake. In addition, postabsorptive glucose homeostasis was cross-sectionally related to the transplant age (range = 40 d-35 mo) in 4 subgroups of patients 2, 6, 15, and 28 mo after transplantation. 22 subjects with chronic uveitis (CU) undergoing a similar immunosuppressive therapy and 35 normal healthy subjects served as controls. The results showed that successful transplantation was associated with fasting glucose concentration and endogenous glucose production in the lower physiological range within a few weeks after transplantation, and this pattern was maintained throughout the 28-mo follow-up period. Fasting glucose (4. 55+/-0.06 vs. 4.75+/-0.06 mM; P = 0.038) and endogenous glucose production (11.3+/-0.4 vs. 12.9+/-0.5 micromol/[kg.min]; P = 0.029) were lower when compared to CU and normal patients. At different combinations of glycemic/insulinemic levels, liver transplant (LTx) patients showed a comparable inhibition of endogenous glucose production. In contrast, in hypoglycemia, after a temporary fall endogenous glucose production rose to values comparable to those of the basal condition in CU and normal subjects (83+/-5 and 92+/-5% of basal), but it did not in LTx subjects (66+/-7%; P < 0.05 vs. CU and normal subjects). Fasting insulin and C-peptide levels were increased up to 6 mo after transplantation, indicating insulin resistance partially induced by prednisone. In addition, greater C-peptide but similar insulin levels during the hyperglycemic clamp (study 1) suggested an increased hepatic insulin clearance in LTx as compared to normal subjects. Fasting glucagon concentration was higher 6 mo after transplantation and thereafter. During euglycemia/hyperinsulinemia (study 3), the insulin-induced glucagon suppression detectable in CU and normal subjects was lacking in LTx subjects; furthermore, the counterregulatory response during hypoglycemia was blunted. In summary, liver transplant subjects have normal postabsorptive glucose metabolism, and glucose and insulin challenge elicit normal response at both hepatic and peripheral sites. Nevertheless, (a) minimal alteration of endogenous glucose production, (b) increased concentration of insulin and glucagon, and (c) defective counterregulation during hypoglycemia may reflect an alteration of the liver-CNS-islet circuit which is due to denervation of the transplanted graft.

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.

Innervation of the liver: morphology and function

Although it has been known for many years that the liver receives a nerve supply, it is only with the advent of immunohistochemistry that this innervation has been analysed in depth. It is now appreciated not only that many different nerve types are present, but also that there are significant differences between species, especially in the degree of parenchymal innervation. This has stimulated more detailed investigation of the innervation of the human liver in both health and disease. At the same time, functional studies have been underlining the important roles that these nerves play in processes as diverse as osmoreception and liver regeneration. This article briefly reviews current understanding of the morphology and functions of the hepatic nerve supply.

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.

Meal patterns and plasma liver enzymes and metabolites after total liver denervations

The liver by way of afferent nerves has been suggested to be a controller of food intake. However, long term meal pattern analysis of chow intake by rats has revealed that total liver denervation (TLD) did not affect the patterns. Nevertheless, these studies have been criticized because they may have missed initial subtle difference in meal patterns that were corrected later by redundant mechanisms. TLD also can alter physiological regulator mechanisms that could secondarily affect feeding behavior. Furthermore, the TLD procedure itself can produce pathologic changes if extreme care is not taken; if pathology does occur it could subsequently affect food intake measurements. In Experiment 1, male rats were given TLD or sham operations (SHAM) at the beginning of the light phase and food withheld until the onset of the dark period. Using a computer operated system, postsurgery onset on feeding, meal size, duration, and frequency were found to be comparable between the groups from the first meal onward. In Experiment 2, 21 days after TLD or SHAM feeding patterns were recorded; all meal parameters were similar between the groups. The rats were then killed and plasma analyzed for liver enzymes (a reflection of liver damage), nutrients and metabolites: no differences were found between the groups. The data suggests that TLD doesn't affect normal regulation of chow intake from the rats' first postsurgery meal onwards. The second study showed that the TLD technique, in our hands, did not produce an adverse effect on normal liver function.

Effect of inhibition of gluconeogenesis on arginine-induced insulin secretion

It is well known that several amino acids, such as arginine, are potent stimuli for insulin and glucagon secretion from the pancreas. Recently, vagal arginine sensors, which modulate arginine-induced pancreatic hormone secretion, have been reported to exist in the liver. The present investigation was designed to evaluate the role played by gluconeogenesis in this hepatic influence. To this end, we studied the effects of an intraperitoneal injection of 3-mercaptopicolinic acid (3-MPA), a gluconeogenic inhibitor, on the pancreatic hormonal response induced by intraperitoneal administration of arginine (1 g/kg body mass) to hepatic vagotomized and sham vagotomized rats. Fifteen min following the injection of arginine, the increases in glucose and insulin concentrations were significantly lower in rats with an inhibited gluconeogenesis than in rats with an intact capacity for gluconeogenesis. There were no effects of the hepatic vagotomy on the arginine-induced hormonal responses either with or without the 3-MPA injection. The results suggest that gluconeogenesis is implicated in the hepatic modulation of arginine-induced pancreatic hormone secretion.