Early phase insulin infusion and muscarinic blockade in obese and lean subjects

Mapping and targeted viral activation of pancreatic nerves in mice reveal their roles in the regulation of glucose metabolism

A lack of comprehensive mapping of ganglionic inputs into the pancreas and of technology for the modulation of the activity of specific pancreatic nerves has hindered the study of how they regulate metabolic processes. Here we show that the pancreas-innervating neurons in sympathetic, parasympathetic and sensory ganglia can be mapped in detail by using tissue clearing and retrograde tracing (the tracing of neural connections from the synapse to the cell body), and that genetic payloads can be delivered via intrapancreatic injection to target sites in efferent pancreatic nerves in live mice through optimized adeno-associated viruses and neural-tissue-specific promoters. We also show that, in male mice, the targeted activation of parasympathetic cholinergic intrapancreatic ganglia and neurons doubled plasma-insulin levels and improved glucose tolerance, and that tolerance was impaired by stimulating pancreas-projecting sympathetic neurons. The ability to map the peripheral ganglia innervating the pancreas and to deliver transgenes to specific pancreas-projecting neurons will facilitate the examination of ganglionic inputs and the study of the roles of pancreatic efferent innervation in glucose metabolism.

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

Optical Clearing and 3D Analysis Optimized for Mouse and Human Pancreata

The pancreas is a heavily innervated organ, but pancreatic innervation can be challenging to comprehensively assess using conventional histological methods. However, recent advances in whole-mount tissue clearing and 3D rendering techniques have allowed detailed reconstructions of pancreatic innervation. Optical clearing is used to enhance tissue transparency and reduce light scattering, thus eliminating the need to section the tissue. Here, we describe a modified version of the optical tissue clearing protocol iDISCO+ (immunolabeling-enabled three-dimensional imaging of solvent-cleared organs) optimized for pancreatic innervation and endocrine markers. The protocol takes 13-19 days, depending on tissue size. In addition, we include protocols for imaging using light sheet and confocal microscopes and for 3D segmentation of pancreatic innervation and endocrine cells using Imaris.

Association of Serum Amylase Activity and the Copy Number Variation of AMY1/2A/2B with Metabolic Syndrome in Chinese Adults

PURPOSE

Low serum amylase activity and copy number (CN) variation (CNV) of the salivary amylase gene (AMY1) are reportedly associated with obesity and abnormal glucose metabolism; however, this association remains controversial. We aimed to clarify the relationship between serum amylase activity and the CNV of AMY1/2A/2B with the occurrence of metabolic syndrome (MetS) in Chinese adults.

PATIENTS AND METHODS

Anthropometry, metabolic risk factors, and serum amylase activity were assessed in 560 subjects (260 MetS patients; 300 healthy controls). AMY1/2A/2B CNs were evaluated using the highly sensitive droplet digital PCR.

RESULTS

The serum total, pancreatic, and salivary amylase activity, but not the AMY1/2A/2B CNs, was significantly lower in MetS patients than that in the control subjects. Patients <45 y had a lower AMY1 CN, compared to that in healthy controls. Low serum amylase activity was significantly associated with high MetS prevalence (p < 0.001). In the receiver operating characteristic curve analysis, serum amylase activity was a significant diagnostic indicator for MetS. The diagnostic value of total amylase was second only to that of γ-glutamyl transpeptidase; it was higher than that of alanine aminotransferase and uric acid.

CONCLUSION

Low serum amylase activity was significantly associated with increased risk of MetS in Chinese adults. Therefore, amylase could be a potential biomarker for predicting MetS.

Diet-induced obesity in young mice: Consequences on the pancreatic intrinsic nervous system control of insulin secretion

INTRODUCTION

Obesity has become a pandaemic even in children. We aimed to investigate the impact of obesity in youth on later pancreatic intrinsic nervous system (PINS) phenotype and control of insulin secretion.

METHODS

Young mice (5-week-old, T0 group) were fed either a normal diet (ND group) or a Western diet (WD group) for 12 weeks. Pancreas nervous system density, PINS phenotype and pancreas anatomy were analysed by immunohistochemistry at T0 and in adulthood (ND and WD groups). Insulin secretion was also studied in these 3 groups using a new model of ex vivo pancreatic culture, where PINS was stimulated by nicotinic and nitrergic agonists with and without antagonists. Insulin was assayed in supernatants by ELISA.

RESULTS

Pancreas nervous system density decreased with age in ND (P < .01) but not in WD mice (P = .08). Western diet decreased the PINS nitrergic component as compared to normal diet (P < .01) but it did not modify its cholinergic component (P = .50). Nicotinic PINS stimulation induced greater insulin secretion in ND compared to WD mice (P < .001) whereas nitrergic stimulation significantly decreased insulin secretion in ND mice (P < .001) and tended to increase insulin secretion in WD mice (P = .08). Endocrine pancreas anatomy was not modified by the Western diet as compared to the normal diet (P = .93).

CONCLUSIONS

Early Western diet induced neuronal density and phenotype changes in PINS that might be involved in the pancreas insulin secretion dysfunctions associated with obesity.

Effects of Sucralose Ingestion versus Sucralose Taste on Metabolic Responses to an Oral Glucose Tolerance Test in Participants with Normal Weight and Obesity: A Randomized Crossover Trial

Here, we tested the hypothesis that sucralose differentially affects metabolic responses to labeled oral glucose tolerance tests (OGTTs) in participants with normal weight and obesity. Participants (10 with normal weight and 11 with obesity) without diabetes underwent three dual-tracer OGTTs preceded, in a randomized order, by consuming sucralose or water, or by tasting and expectorating sucralose (e.g., sham-fed; sweetness control). Indices of β-cell function and insulin sensitivity (S_I) were estimated using oral minimal models of glucose, insulin, and C-peptide kinetics. Compared with water, sucralose ingested (but not sham-fed) resulted in a 30 ± 10% increased glucose area under the curve in both weight groups. In contrast, the insulin response to sucralose ingestion differed depending on the presence of obesity: decreased within 20–40 min of the OGTT in normal-weight participants but increased within 90–120 min in participants with obesity. Sham-fed sucralose similarly decreased insulin concentrations within 60 min of the OGTT in both weight groups. Sucralose ingested (but not sham-fed) increased S_I in normal-weight participants by 52 ± 20% but did not affect S_I in participants with obesity. Sucralose did not affect glucose rates of appearance or β-cell function in either weight group. Our data underscore a physiological role for taste perception in postprandial glucose responses, suggesting sweeteners should be consumed in moderation.

Role of vagal activation in postprandial glucose metabolism after gastric bypass in individuals with and without hypoglycaemia

Patients who have undergone gastric bypass surgery (GB) have enhanced postprandial hyperinsulinaemia and a greater incretin effect is apparent. In the present study, we sought to determine the effect of vagal activation, a neural component of the enteroinsular axis, on postprandial glucose metabolism in patients with and without hypoglycaemia after GB. Seven patients with documented post-GB hypoglycaemia, seven asymptomatic patients without hypoglycaemia post-GB, and 10 weight-matched non-surgical controls with normal glucose tolerance were recruited. Blood glucose, and islet hormone and incretin secretion were compared during mixed meal tolerance tests (MMTs) with and without prior sham-feeding on two separate days. Sham feeding preceding the MMT caused a more rapid increase in prandial blood glucose levels but lowered overall glycaemia in all three groups (P < 0.05). Sham feeding had a similar effect to increase early (P < 0.05), but not overall, meal-induced insulin secretion in the three groups. Prandial glucagon concentrations were significantly greater in the GB groups, and sham feeding accentuated this response (P < 0.05). The effect of vagal activation on prandial glucose and islet-cell function is preserved in patients who have undergone GB, in those both with and without hypoglycaemia.

Modeling the effect of the cephalic phase of insulin secretion on glucose metabolism

The nervous system has a significant impact in glucose homeostasis and endocrine pancreatic secretion in humans, especially during the cephalic phase of insulin release (CPIR); that is, before a meal is absorbed. However, the underlying mechanisms of this neural-pancreatic interaction are not well understood and therefore often neglected, despite their significance to achieving an optimal glucose control. As a result, the dynamics of insulin release from the pancreas are currently described by mathematical models that reproduce the behavior of the β cells using exclusively glucose levels and other hormones as inputs. To bridge this gap, we have combined, for the first time, metabolic and neural mathematical models in a unified system to reproduce to a great extent the ideal glucoregulation observed in healthy subjects. Our results satisfactorily replicate the CPIR and its impact during the post-absorptive phase. Furthermore, the proposed model gives insight into the physiological interaction between the brain and the pancreas in healthy people and suggests the potential of considering the neural information for restoring glucose control in people with diabetes. Graphical Abstract (a) Physiological scenario. Diagram of the biological interaction among the most important organs involved in glucose control during meal intake. (b) Scheme of the unified bio-inspired neural-metabolic model. Each of the boxes represents one subsystem of the model. The pink shades boxes depicts the novel subsystems introduced to the current metabolic models (grey shaded boxes). Insulin-related action and mass fluxes (solid black lines) and glucose-related action and mass flux (dotted black lines) are depicted to show the relationship among the blocks. I(t), Ic(t), G(t) and SI related to plasma insulin, plasma cephalic insulin, plasma glucose and insulin sensitivity, respectively.

Contribution of parasympathetic muscarinic augmentation of insulin secretion to olanzapine-induced hyperinsulinemia

Atypical antipsychotic drugs have been associated with the development of obesity and diabetes. In particular, olanzapine can induce peripheral insulin resistance and compensatory hyperinsulinemia independent of weight gain or psychiatric disease. To determine if this compensatory increase in insulin is mediated by parasympathetic muscarinic stimulation, we randomized 15 healthy subjects 2:1 to receive double-blind olanzapine or placebo for 9 days under diet- and activity-controlled inpatient conditions. Before and after 7 days of study drug administration, subjects underwent frequently sampled intravenous glucose tolerance tests with either saline or atropine infused on subsequent days to assess insulin secretion and hepatic insulin extraction in the absence or presence of muscarinic blockade. We found that olanzapine led to an increase in the acute insulin response to glucose, which was not seen with placebo, and was attenuated in the olanzapine group by atropine. Deconvolution of C-peptide data confirmed an increase in insulin secretion with olanzapine, which was blocked by atropine, with a modest reduction in hepatic insulin extraction with olanzapine. These results support the contribution of muscarinic augmentation of insulin secretion to olanzapine-induced hyperinsulinemia, and provide a mechanism for the compensatory hyperinsulinemia that normally serves to prevent deterioration of glucose tolerance under conditions of metabolic challenge.

Nutrition and taste and smell dysfunction

Food selection plays a pivotal role in maintaining adequate nutrient intake, thus elucidating drivers of food choice is a meaningful strategy to maintain health and manage disease. Taste and smell are key determinants of food choice and warrant careful consideration. In this review, we first discuss how sensory stimulation influences food selection and metabolism. We then review the evidence regarding the relationship between taste and smell dysfunction and food preferences and selection, with attention given to contexts of certain chronic diseases. We conclude with brief recommendations for the management of chemosensory disorders. While sensory abilities influence food selection, the effect of taste and smell dysfunction on long-term consumption patterns and health status must be considered in light of environment, exposure, and culture.

Cholinergic signaling mediates the effects of xenin-25 on secretion of pancreatic polypeptide but not insulin or glucagon in humans with impaired glucose tolerance

We previously demonstrated that infusion of an intestinal peptide called xenin-25 (Xen) amplifies the effects of glucose-dependent insulinotropic polypeptide (GIP) on insulin secretion rates (ISRs) and plasma glucagon levels in humans. However, these effects of Xen, but not GIP, were blunted in humans with type 2 diabetes. Thus, Xen rather than GIP signaling to islets fails early during development of type 2 diabetes. The current crossover study determines if cholinergic signaling relays the effects of Xen on insulin and glucagon release in humans as in mice. Fasted subjects with impaired glucose tolerance were studied. On eight separate occasions, each person underwent a single graded glucose infusion- two each with infusion of albumin, Xen, GIP, and GIP plus Xen. Each infusate was administered ± atropine. Heart rate and plasma glucose, insulin, C-peptide, glucagon, and pancreatic polypeptide (PP) levels were measured. ISRs were calculated from C-peptide levels. All peptides profoundly increased PP responses. From 0 to 40 min, peptide(s) infusions had little effect on plasma glucose concentrations. However, GIP, but not Xen, rapidly and transiently increased ISRs and glucagon levels. Both responses were further amplified when Xen was co-administered with GIP. From 40 to 240 min, glucose levels and ISRs continually increased while glucagon concentrations declined, regardless of infusate. Atropine increased resting heart rate and blocked all PP responses but did not affect ISRs or plasma glucagon levels during any of the peptide infusions. Thus, cholinergic signaling mediates the effects of Xen on insulin and glucagon release in mice but not humans.

The cephalic phase insulin response to nutritive and low-calorie sweeteners in solid and beverage form

The purpose of the study was to examine the role of the cephalic phase insulin response (CPIR) following exposure to nutritive and low-calorie sweeteners in solid and beverage form in overweight and obese adults. In addition, the role of learning on the CPIR to nutritive and low-calorie sweetener exposure was tested. Sixty-four overweight and obese adults (age: 18-50years, BMI: 24-37kg/m2, body fat percentage>25% for men and >32% for women) were sham-fed (at 2-minute intervals for 14min) a randomly assigned test load comprised of a nutritive (sucrose) or low-calorie sweetener (sucralose) in beverage or solid form in phase 1 of the study. A 2-3ml blood sample was collected before and 2, 6, 10, 14, 61, 91 and 121min after oral exposure for serum insulin and glucose analysis. During phase 2, participants underwent a 2-week training period to facilitate associative learning between the sensory properties of test loads and their post-ingestive effects. In phase 3, participants were retested for their cephalic phase responses as in phase 1. Participants were classified as responders if they demonstrated a positive insulin response (rise of serum insulin above baseline i.e. Δ insulin) 2min post-stimulus in phase 1. Among responders exposed to the same sweetener in Phases 1 and 3, the proportion of participants that displayed a rise of insulin with oral exposure to sucralose was significantly greater when the stimulus was in the solid form compared to the beverage form. Sucralose and sucrose exposure elicited similarly significant increases in serum insulin 2min after exposure and significant decreases after 2min in responders in both food forms. The solid food form elicited greater CPIR over 2, 6 and 10min than the beverage form. There was no effect of learning on insulin responses after training. The results indicate the presence of a significant CPIR in a subset of individuals with overweight or obesity after oral exposure to sucralose, especially when present in solid food form. Future studies must confirm the reliability of this response.

Cephalic phase of insulin secretion in response to a meal is unrelated to family history of type 2 diabetes

The pre-absorptive cephalic phase of insulin secretion is elicited during the first ten min of a meal and before glucose levels rise. Its importance for insulin release during the post-absorptive phase has been well documented in animals but its presence or importance in man has become increasingly controversial. We here examined the presence of an early cephalic phase of insulin release in 31 well matched individuals without (n = 15) or with (n = 16) a known family history of type 2 diabetes (first-degree relatives; FDR). We also examined the potential differences in individuals with or without impaired fasting (IFG) and impaired glucose tolerance (IGT). We here demonstrate that a cephalic phase of insulin secretion was present in all individuals examined and without any differences between control persons and FDR or IFG/IGT. However, the overall importance of the cephalic phase is conjectural since it was unrelated to the subsequent post-absorptive insulin release or glucose tolerance. One of the best predictors of the incremental cephalic phase of insulin release was fasting insulin level and, thus, a relation to degree of insulin sensitivity is likely. In conclusion, an early pre-absorptive and cephalic phase of insulin release is robustly present in man. However, we could not document any relation to family history of Type 2 diabetes nor to the post-absorptive phase and, thus, confirm its importance for subsequent degree of insulin release or glucose tolerance.

Ghrelin: A link between memory and ingestive behavior

Feeding is a highly complex behavior that is influenced by learned associations between external and internal cues. The type of excessive feeding behavior contributing to obesity onset and metabolic deficit may be based, in part, on conditioned appetitive and ingestive behaviors that occur in response to environmental and/or interoceptive cues associated with palatable food. Therefore, there is a critical need to understand the neurobiology underlying learned aspects of feeding behavior. The stomach-derived "hunger" hormone, ghrelin, stimulates appetite and food intake and may function as an important biological substrate linking mnemonic processes with feeding control. The current review highlights data supporting a role for ghrelin in mediating the cognitive and neurobiological mechanisms that underlie conditioned feeding behavior. We discuss the role of learning and memory on food intake control (with a particular focus on hippocampal-dependent memory processes) and provide an overview of conditioned cephalic endocrine responses. A neurobiological framework is provided through which conditioned cephalic ghrelin secretion signals in neurons in the hippocampus, which then engage orexigenic neural circuitry in the lateral hypothalamus to express learned feeding behavior.

Cephalic phase secretion of insulin and other enteropancreatic hormones in humans

Enteropancreatic hormone secretion is thought to include a cephalic phase, but the evidence in humans is ambiguous. We studied vagally induced gut hormone responses with and without muscarinic blockade in 10 glucose-clamped healthy men (age: 24.5 ± 0.6 yr, means ± SE; body mass index: 24.0 ± 0.5 kg/m(2); HbA1c: 5.1 ± 0.1%/31.4 ± 0.5 mmol/mol). Cephalic activation was elicited by modified sham feeding (MSF, aka "chew and spit") with or without atropine (1 mg bolus 45 min before MSF + 80 ng·kg(-1)·min(-1) for 2 h). To mimic incipient prandial glucose excursions, glucose levels were clamped at 6 mmol/l on all days. The meal stimulus for the MSF consisted of an appetizing breakfast. Participants (9/10) also had a 6 mmol/l glucose clamp without MSF. Pancreatic polypeptide (PP) levels rose from 6.3 ± 1.1 to 19.9 ± 6.8 pmol/l (means ± SE) in response to MSF and atropine lowered basal PP levels and abolished the MSF response. Neither insulin, C-peptide, glucose-dependent insulinotropic polypeptide (GIP), nor glucagon-like peptide-1 (GLP-1) levels changed in response to MSF or atropine. Glucagon and ghrelin levels were markedly attenuated by atropine prior to and during the clamp: at t = 105 min on the atropine (ATR) + clamp (CLA) + MSF compared with the saline (SAL) + CLA and SAL + CLA + MSF days; baseline-subtracted glucagon levels were -10.7 ± 1.1 vs. -4.0 ± 1.1 and -4.7 ± 1.9 pmol/l (means ± SE), P < 0.0001, respectively; corresponding baseline-subtracted ghrelin levels were 303 ± 36 vs. 39 ± 38 and 3.7 ± 21 pg/ml (means ± SE), P < 0.0001. Glucagon and ghrelin levels were unaffected by MSF. Despite adequate PP responses, a cephalic phase response was absent for insulin, glucagon, GLP-1, GIP, and ghrelin.

Gastric bypass alters both glucose-dependent and glucose-independent regulation of islet hormone secretion

OBJECTIVE

Roux-en-Y gastric bypass surgery (GB) is characterized by accentuated but short-lived postprandial elevations of blood glucose and insulin. This profile has been attributed to effects of relative hyperglycemia to directly stimulate β-cells and an augmented incretin effect. An additional glucose-independent stimulation of insulin secretion in GB subjects was hypothesized.

METHODS

Fifteen subjects with prior GB, six matched obese non surgical controls, and seven lean individuals were recruited. Islet hormones were measured before and after meal ingestion during hyperinsulinemic hypoglycemic clamps to minimize the direct effects of glycemia and glucose-dependent gastrointestinal hormones on insulin secretion.

RESULTS

The GB subjects had less suppression of fasting β-cell secretion during the insulin clamp compared to controls. In addition, meal-induced insulin secretion increased in the GB subjects but not controls during fixed sub-basal glycemia. In contrast, the glucagon responses to hypoglycemia and meal ingestion were lower in the GB subjects than controls.

CONCLUSIONS

Among subjects with GB, the response of insulin and glucagon secretion to decreasing blood glucose is blunted, but meal-induced insulin secretion is stimulated even at fixed systemic sub-basal glycemia. These findings indicate that, following GB, islet hormone secretion is altered as a result of factors beyond circulatory glucose levels.

Antipsychotic-induced insulin resistance and postprandial hormonal dysregulation independent of weight gain or psychiatric disease

Atypical antipsychotic (AAP) medications that have revolutionized the treatment of mental illness have become stigmatized by metabolic side effects, including obesity and diabetes. It remains controversial whether the defects are treatment induced or disease related. Although the mechanisms underlying these metabolic defects are not understood, it is assumed that the initiating pathophysiology is weight gain, secondary to centrally mediated increases in appetite. To determine if the AAPs have detrimental metabolic effects independent of weight gain or psychiatric disease, we administered olanzapine, aripiprazole, or placebo for 9 days to healthy subjects (n = 10, each group) under controlled in-patient conditions while maintaining activity levels. Prior to and after the interventions, we conducted a meal challenge and a euglycemic-hyperinsulinemic clamp to evaluate insulin sensitivity and glucose disposal. We found that olanzapine, an AAP highly associated with weight gain, causes significant elevations in postprandial insulin, glucagon-like peptide 1 (GLP-1), and glucagon coincident with insulin resistance compared with placebo. Aripiprazole, an AAP considered metabolically sparing, induces insulin resistance but has no effect on postprandial hormones. Importantly, the metabolic changes occur in the absence of weight gain, increases in food intake and hunger, or psychiatric disease, suggesting that AAPs exert direct effects on tissues independent of mechanisms regulating eating behavior.

How neural mediation of anticipatory and compensatory insulin release helps us tolerate food

Learned anticipatory and compensatory responses allow the animal and human to maintain metabolic homeostasis during periods of nutritional challenges, either acutely within each meal or chronically during periods of overnutrition. This paper discusses the role of neurally-mediated anticipatory responses in humans and their role in glucoregulation, focusing on cephalic phase insulin and pancreatic polypeptide release as well as compensatory insulin release during the etiology of insulin resistance. The necessary stimuli required to elicit CPIR and vagal activation are discussed and the role of CPIR and vagal efferent activation in intra-meal metabolic homeostasis and during chronic nutritional challenges are reviewed.

Palmitic acid acutely inhibits acetylcholine- but not GLP-1-stimulated insulin secretion in mouse pancreatic islets

Fatty acids, acetylcholine, and GLP-1 enhance insulin secretion in a glucose-dependent manner. However, the interplay between glucose, fatty acids, and the neuroendocrine regulators of insulin secretion is not well understood. Therefore, we studied the acute effects of PA (alone or in combination with glucose, acetylcholine, or GLP-1) on isolated cultured mouse islets. Two different sets of experiments were designed. In one, a fixed concentration of 0.5 mM of PA bound to 0.15 mM BSA was used; in the other, a PA ramp from 0 to 0.5 mM was applied at a fixed albumin concentration of 0.15 mM so that the molar PA/BSA ratio changed within the physiological range. At a fixed concentration of 0.5 mM, PA markedly inhibited acetylcholine-stimulated insulin release, the rise of intracellular Ca(2+), and enhancement of cAMP production but did not influence the effects of GLP-1 on these parameters of islet cell function. 2-ADB, an IP(3) receptor inhibitor, reduced the effect of acetylcholine on insulin secretion and reversed the effect of PA on acetylcholine-stimulated insulin release. Islet perfusion for 35-40 min with 0.5 mM PA significantly reduced the calcium storage capacity of ER measured by the thapsigargin-induced Ca(2+) release. Oxygen consumption due to low but not high glucose was reduced by PA. When a PA ramp from 0 to 0.5 mM was applied in the presence of 8 mM glucose, PA at concentrations as low as 50 microM significantly augmented glucose-stimulated insulin release and markedly reduced acetylcholine's effects on hormone secretion. We thus demonstrate that PA acutely reduces the total oxygen consumption response to glucose, glucose-dependent acetylcholine stimulation of insulin release, Ca(2+), and cAMP metabolism, whereas GLP-1's actions on these parameters remain unaffected or potentiated. We speculate that acute emptying of the ER calcium by PA results in decreased glucose stimulation of respiration and acetylcholine potentiation of insulin secretion.

Cephalic phase pancreatic polypeptide responses to liquid and solid stimuli in humans

The hormone, pancreatic polypeptide (PP) is postulated to be involved in body weight regulation. PP release is dependent on vagal activation and is a marker of vagal efferent activity. Because vagal activity plays a role in glucose homeostasis, elucidating the conditions of activation has important implications for nutrient metabolism. In humans, modified sham-feeding is known to elicit vagally-mediated hormonal responses. We present results of 3 studies in which healthy human subjects tasted various stimuli including sweet and salty liquids, unflavored and flavored gum and mixed nutrient foods flavored with either sweet or salt and rendered palatable or unpalatable. We examined the effects of these stimuli on PP levels relative to fasting. We found that liquids flavored with either glucose or salt, did not elicit an increase in PP levels greater than fasting. Similarly, chewing gum, whether unflavored or flavored with a non-nutritive sweetener or the sweetener paired with a mint flavor, did not significantly increase PP levels. In contrast, when subjects tasted mixed nutrient foods, these reliably elicited increases in PP levels at 4 min post-stimulus (sweet palatable, p<0.002; sweet unpalatable, p<0.001; salty, palatable, p<0.05, salty unpalatable, p<0.05). The magnitude of release was influenced by the flavor, i.e. a sweet palatable stimulus (320.1+/-93.7 pg/ml/30 min) elicited the greatest increase in PP compared with a salty palatable stimulus (142.4+/-88.7 pg/ml/30 min; p<0.05). These data suggest that liquids and chewing gum do not provide adequate stimulation for vagal efferent activation in humans and that mixed nutrient foods are the optimal stimuli.

Visceral nerves: vagal and sympathetic innervation

The autonomic nervous system is the primary neural mediator of physiological responses to internal and external stimuli. It is composed of 2 branches: the sympathetic nervous system, which mediates catabolic responses, and the parasympathetic nervous system, composed of the vagus nerve, which regulates anabolic responses. As the vagus nerve innervates most tissues involved in nutrient metabolism, including the stomach, pancreas, and liver, activation of vagal efferent activity has the potential to influence how nutrients are absorbed and metabolized. Vagal efferent activity is initially activated at the onset of food intake by receptors in the oropharyngeal cavity and then during food intake postprandially. Vagal efferent innervation of the pancreas contributes to early-phase insulin release as well as to optimizing postprandial insulin release. In the absence of vagal activation, which occurs when glucose is administered intragastrically, postprandial glucose levels are higher and insulin levels blunted compared with when there is activation of oropharyngeal receptors by food. An induction of vagal efferent activity also occurs during chronic pancreatic B-cell challenge with 48-hour glucose infusions. Under these conditions, the compensatory increase in insulin secretion is partially mediated by an increase in vagal efferent activity. In conclusion, the vagus nerve, part of the parasympathetic nervous system, plays a critical role in the regulation of blood glucose levels and is an often overlooked factor contributing to glucose homeostasis.

Cholinergic regulation of ghrelin and peptide YY release may be impaired in obesity

OBJECTIVE

Ghrelin and peptide YY (PYY) are both hormones derived from the gastrointestinal tract involved in appetite regulation. The cholinergic part of the vagal nerve is involved in the regulation of glucose and insulin. The aim of this study was to examine the effects of the cholinergic antagonist atropine on ghrelin, PYY, glucose, and insulin under basal conditions and after meal ingestion in lean and obese subjects.

RESEARCH DESIGN AND METHODS

Eight lean and eight obese subjects were included in a randomized, double-blind, placebo-controlled crossover study with 4 study days in randomized order (atropine/placebo +/- breakfast). Plasma ghrelin, PYY, insulin, and glucose were measured. Hunger and satiety feelings were rated on a 10-cm visual analog scale.

RESULTS

In lean individuals, atropine led to a decrease in ghrelin concentrations comparable and nonadditive with breakfast ingestion and a significant decrease in both basal and meal-induced PYY concentrations. In obese subjects, atropine did not significantly change ghrelin or PYY concentrations, whereas it induced a comparable increase in heart rate and meal-induced glucose concentrations in the two study groups. Only lean, not obese, subjects experienced sustained feelings of satiety after breakfast.

CONCLUSIONS

The impaired cholinergic regulation of the postprandial drop in ghrelin concentrations and rise in PYY concentrations might be part of the deregulated food intake in obese subjects.

Anticipatory physiological regulation in feeding biology: cephalic phase responses

Anticipatory physiological regulation is an adaptive strategy that enables animals to respond faster to physiologic and metabolic challenges. The cephalic phase responses are anticipatory responses that prepare animals to digest, absorb, and metabolize nutrients. They enable the sensory aspects of the food to interact with the metabolic state of the animal to influence feeding behavior. The anticipatory digestive secretions and metabolic adjustments in response to food cues are key adaptations that affect digestive and metabolic efficiency and aid in controlling the resulting elevation of metabolic fuels in the blood. Cephalic phase responses enable digestion, metabolism, and appetite to be regulated in a coordinated fashion. These responses have significant effects on meal size. For example, if the cephalic phase insulin response is blocked the result is poor glucose control and smaller meals. Cephalic phase responses also are linked to motivation to feed, and may play a more direct role in regulating meal size beyond the permissive one of ameliorating negative consequences of feeding. For example, the orexigenic peptide ghrelin appears to display a cephalic phase response, rising before expected meal times. This anticipatory ghrelin response increases appetite; interestingly it also enhances fat absorption, linking appetite with digestion and metabolism.

48-h glucose infusion in humans: effect on hormonal responses, hunger and food intake

Experimentally-induced hyperglycemia by prolonged glucose infusion allows investigation of the effects of sustained stimulation of the pancreatic beta-cell on insulin secretion and sensitivity. Hormonal responses to a meal following prolonged glucose infusions have not been investigated. To determine if a 48-h glucose infusion alters hormonal responses to a test meal as well as food intake and hunger in normal weight individuals, 16 subjects (8 men, 8 women, age 18-30 years, mean BMI=21.7+/-1.6 kg/m2) were infused for 48 h with either saline (50 ml/h) or 15% glucose (200 mg/m2/min). Subjects ingested a 600 kcal mixed nutrient meal 3 h after infusion termination. Blood samples were taken during the 48 h and for 4 h following food ingestion. The 48-h glucose infusion elicited a metabolic profile of a glucose intolerant obese subjects, with increased plasma glucose, insulin and leptin (all P<0.01) and increased HOMA-IR (P<0.001). During meal ingestion, early insulin secretion was increased (P<0.05) but post-prandial glucose (P<0.01) and insulin (P<0.01) excursions were lower following the glucose infusion. Post-prandial plasma triglyceride concentrations were increased after glucose compared with saline. Food intake and hunger ratings were not different between the two conditions. Plasma leptin levels were inversely correlated with hunger (P<0.03) in both conditions and with food intake (P<0.003) during the glucose condition only. Thus, a 48-h glucose infusion does not impair post-prandial hormonal responses, alter food intake or hunger in normal weight subjects. The glucose-induced increases in plasma leptin result in a stronger inverse relationship between plasma leptin and hunger as well as food intake. These data are the first to demonstrate a relationship between leptin and hunger in normal weight, non-calorically restricted human subjects.

{beta}-Cell function following human islet transplantation for type 1 diabetes

Islet transplantation can provide metabolic stability for patients with type 1 diabetes; however, more than one donor pancreas is usually required to achieve insulin independence. To evaluate possible mechanistic defects underlying impaired graft function, we studied five subjects at 3 months and four subjects at 12 months following intraportal islet transplantation who had received comparable islet equivalents per kilogram (12,601 +/- 1,732 vs. 14,384 +/- 2,379, respectively). C-peptide responses, as measures of beta-cell function, were significantly impaired in both transplant groups when compared with healthy control subjects (P < 0.05) after intravenous glucose (0.3 g/kg), an orally consumed meal (600 kcal), and intravenous arginine (5 g), with the greatest impairment to intravenous glucose and a greater impairment seen in the 12-month compared with the 3-month transplant group. A glucose-potentiated arginine test, performed only in insulin-independent transplant subjects (n = 5), demonstrated significant impairments in the glucose-potentiation slope (P < 0.05) and the maximal response to arginine (AR(max); P < 0.05), a measure of beta-cell secretory capacity. Because AR(max) provides an estimate of the functional beta-cell mass, these results suggest that a low engrafted beta-cell mass may account for the functional defects observed after islet transplantation.

Parasympathetic blockade attenuates augmented pancreatic polypeptide but not insulin secretion in Pima Indians

There is evidence from animal models of obesity and type 2 diabetes that increased parasympathetic vagal input to the pancreas contributes to hyperinsulinemia. Compared with Caucasians, Pima Indians have a high risk of type 2 diabetes and exhibit marked hyperinsulinemia and elevated plasma levels of pancreatic polypeptide (PP), an islet hormone considered a surrogate marker of parasympathetic nervous system (PNS) drive to the pancreas. To test if hyperinsulinemia in Pima Indians is due to increased vagal input to the beta-cell, we examined the effect of PNS blockade in 17 Caucasian (aged 35 +/- 8 years, body fat 23 +/- 7% [mean +/- SD]) and 17 Pima Indian males (aged 28 +/- 8 years, body fat 29 +/- 5%) with normal glucose tolerance. Each participant underwent four consecutive standardized liquid meal tests (64% carbohydrate, 22% fat, and 14% protein) during which a primed infusion of atropine was administered for 120 min at the following doses: 0, 2.5, 5, and 10 micro g. kg fat-free mass (FFM)(-1). h(-1). Areas under the curve for early (AUC(0-30 min)) and total (AUC(0-120 min)) postprandial insulin and PP secretory responses were calculated. Early postprandial insulin and PP secretory responses were higher in Pima Indians compared with those of Caucasians (both P = 0.01). Secretion of insulin and PP was inhibited by atropine (both P < 0.001). Increasing doses of atropine attenuated the ethnic difference in PP (P = 0.01) but not in early insulin secretory responses (P = 0.6), an effect that was not due to differences in gastric emptying rate (acetaminophen test) and/or circulating glucose. Similar results were observed for total secretory responses. These results confirm that compared with Caucasians, Pima Indians have an exaggerated PNS drive to pancreatic F-cells that secrete PP. However, the hyperinsulinemia of this population does not appear to be due to increased vagal input to pancreatic beta-cells.

Insufficient islet compensation to insulin resistance vs. reduced glucose effectiveness in glucose-intolerant mice

This study evaluated the relative contribution of insulin-dependent mechanisms vs. mechanisms independent on dynamic insulin for glucose intolerance induced by high-fat diet. C57BL/6J mice underwent a frequently sampled intravenous glucose tolerance test (1 g/kg glucose) at 1 wk and 1, 3, and 10 mo after initiation of a high-fat diet (58% fat; control diet 11% fat) to measure glucose effectiveness (S(G)) and disposition index (DI), i.e., insulin sensitivity (S(I)) times early or total insulin secretion. Glucose disappearance (K(G)) and S(I) were reduced in high-fat-fed mice at all time points. Total (50 min) insulin secretion was sufficiently increased at all time points to compensate for the reduced S(I), as judged by normal DI(50) (min). In contrast, early (10 min) insulin secretion was not sufficiently increased; DI(10) (min) was reduced after 1, 3, and 10 mo. S(G) was reduced after 1 wk; the reduction persisted throughout the study period. Thus glucose intolerance induced by high-fat diet is, in early phases, solely explained by reduced glucose effectiveness, whereas insufficient early insulin secretion is of importance after long-term feeding.

The cephalic insulin response to meal ingestion in humans is dependent on both cholinergic and noncholinergic mechanisms and is important for postprandial glycemia

We studied the mechanisms and physiological relevance of the cephalic insulin response to meal ingestion in 12 healthy women (age 63 +/- 0.4 years; BMI 27.7 +/- 1.7 kg/m2). The ganglionic antagonist, trimethaphan, which impairs neurotransmission across parasympathetic and sympathetic autonomic ganglia, or atropine or saline was given intravenously during the first 15 min after ingestion of a standard meal (350 kcal). During saline infusion, insulin levels increased during the first 10 min after meal ingestion, whereas the first increase in glucose was evident at 15 min. The preabsorptive 10-min insulin response was reduced by 73 +/- 11% by trimethaphan (P = 0.009), accompanied by impaired reduction of glucose levels from 25 to 60 min after meal ingestion (deltaglucose = -1.27 +/- 0.5 [with saline] vs. 0.1 +/- 0.4 mmol/l [with trimethaphan]; P = 0.008). This reduction at 25-60 min in glucose levels correlated significantly to the 10-min insulin response (r = 0.65, P = 0.024). The 10-min insulin response to meal ingestion was also reduced by atropine, but only by 20 +/- 9% (P = 0.045), which was lower than the reduction with trimethaphan (P = 0.004). The preabsorptive insulin response was not accompanied by any increase in circulating levels of gastric inhibitory polypeptide (GIP) or glucagon-like peptide 1 (GLP-1). In conclusion, 1) the early preabsorptive insulin response to meal ingestion in humans can be largely attributed to autonomic activation mediated by noncholinergic and cholinergic mechanisms, 2) this cephalic insulin response is required for a normal postprandial glucose tolerance, and 3) GIP and GLP-1 do not contribute to the preabsorptive cephalic phase insulin response to meal ingestion.