Дисертації з теми "Incretin hormones"

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第18366号
人博第679号
新制||人||163(附属図書館)
25||人博||679(吉田南総合図書館)
31224
京都大学大学院人間・環境学研究科共生人間学専攻
(主査)教授 林 達也, 教授 森谷 敏夫, 教授 石原 昭彦, 教授 津田 謹輔
学位規則第4条第1項該当

Incretins GLP-1 and GIP are glucose regulating peptides released from L cells and K cells in the intestine in response to food intake. They act on pancreatic β-cells to potentiate glucose-stimulated insulin release. However, incretins also play a role in regulating body weight: GLP-1 promotes weight loss by acting as a satiety signal, whereas GIP is associated with weight gain by promoting adiposity. The association between incretins and obesity/diabetes highlights the need to identify physiological signals that control their release; however this has been hampered because of our lack of a handle on these cells. Here, we report that using tissues from transgenic mice with L cell-specific or K cell-specific expression of a fluorescent protein, these rare cell populations can be highly purified and changes in their total number and/or gene expression profile (e.g. in obesity) can be monitored by FACS and quantitative RT-PCR. We have developed a cell culture system to maintain L cells and K cells in vitro. This system is suitable to characterize these cells by electrophysiology, calcium imaging, hormone secretion studies and these cells are conducive to adenoviral transfection. Affymetrix based comparative global gene expression profiles indicated that L cells from the small intestine share more commonality to K cells than to L cells in the colon. Furthermore, we have identified a number of candidate sensors for dietary fatty acids, amino acids or glucose as well as signalling pathways preferentially expressed in L cells and/or K cells. A better understanding of these genes and pathways may help us to determine how the levels of these important hormones can be modulated in patients with type 2 diabetes and obesity.

INTRODUÇÃO: O diabetes mellitus tipo 2 (DM2) é uma doença correlacionada com a obesidade mórbida. O paciente obeso apresenta efeito incretínico suprimido e consequente desbalanço da homeostase glicêmica. Diversos estudos evidenciam a melhora do DM2 após a confecção da Gastroplastia com derivação intestinal em Y de Roux (GDYR). Os mecanismos de controle da glicemia podem ser de longo e curto prazo. Os mecanismos de ação precoce estão ligados à restrição calorica, melhora da resistência insulínica, da função da célula beta pancreática e retorno do efeito incretínico pelo aumento do GLP1 e GIP, porém os dados são conflitantes. MÉTODOS: Onze pacientes obesos graves diabéticos foram submetidos à GDYR com confecção de gastrostomia no remanescente gástrico após perda de peso inicial de 10%. Os pacientes foram submetidos à coleta de entero-hormônios, perfil glicêmico e Teste de Tolerância Oral à glicose (TTOG) no pré-operatório em curva temporal que foi comparado ao pós-operatório por Via Oral e por Via da Gastrostomia em até 7 dias após o procedimento. RESULTADOS: A média da idade foi 46,09±7,08 anos. No pré-operatorio, o peso médio foi 120,97±17,02 quilogramas, altura 1,67±0,11 metros, IMC médio 44,06±6,59 kg/m2, glicemia de jejum média 194,55±62,45 mg/dl e hemoglobina glicada 8,74±1,64%. Em 77,7% dos pacientes, houve remissão precoce do DM2 no pós-operatório avaliado pelo TTOG. Também foi observada queda significante da glicemia, insulinemia e do HOMA-IR independente da via administrada. Ocorreu aumento significativo do GLP1 e redução do GIP pela Via Oral pós-operatória. A Grelina não apresentou alterações. CONCLUSÃO: Evidenciou-se redução da glicemia e da resistência periférica nos primeiros dias de pós-operatório da GDYR, independente da via de passagem do alimento. A alteração no efeito incretínico (aumento do GLP1 e redução do GIP) só foi observada na Via Oral pós-operatória
INTRODUCTION: Type 2 diabetes mellitus (DM2) is a disease correlated with morbidly obesity. The obese patient has a suppressed incretin effect and consequent inbalance of glycemic homeostasis. Several studies have shown an improvement in DM2 after Gastroplasty with Roux-en-Y gastric bypass (RYGB). The mechanisms of glycemic control may be long-term and shortterm. The mechanisms of early action are linked to caloric restriction, improvement of insulin resistance, pancreatic beta cell function and return of the incretin effect through the increase of GLP1 and GIP, but the data are conflicting. METHODS: Eleven diabetic obese patients underwent RYGB with gastrostomy in gastric remnant after initial 10% weight loss. Patients were submitted to assessment of enterohormones, glycemic profile and Oral Glucose Tolerance Test (OGTT) in the preoperative period in a time curve that was compared to the postoperative period by Oral Via and Gastrostomy Via up to 7 days after the procedure .RESULTS: The mean age of the group was 46.09 ± 7.08 years. In the preoperative the mean weight was 120.97 ± 17.02 kilograms, height of 1.67 ± 0.11 meters, mean BMI of 44.06 ± 6, 59 kg/m2, mean fasting blood glucose of 194.55 ± 62.45 mg/dl and glycated hemoglobin 8.74 ± 1.64%. In 77.7% of the patients there was remission of DM2 in postoperative evaluated by the OGTT. Significant decrease in glycemia, insulinemia and HOMA-IR was also observed, regardless of the route of administration. There was a significant increase in GLP1 and reduction of GIP by the postoperative oral route. Ghrelin did not change. CONCLUSION: A reduction in glycemia and peripheral insulinal resistance was observed in early postoperative days of RYGB, independent of the food route. The change in incretin effect (increase of GLP1 and reduction of GIP) was only observed in the postoperative oral route

Abstract

Background and aim:

Diabetes mellitus (DM) is a chronic and progressive illness that affects all type of populations and ages. According to World health organization (WHO) by 2030 it will be 366 million people effected world wild. Many new drugs are Glucagon-like peptide-1 (GLP-1) based therapy for treatment of type 2diabetes. GLP-1 is released from the intestinal L-cells, and is a potent stimulator of glucose-dependent insulin secretion. The aim of this study was to investigate the effect of GLP-1 and its stable analogs on cell proliferation of GLP-1 secreting GLUTag cells.

Material and methods:

GluTag cells were incubated for 48h in DMEM medium containing (0.5% fetal bovine serum and 100 IU/ml penicillin and 100 μg/ml streptomycin and 3mM glucose concentration) in the present or absence of the agents. DNA synthesis was measured using 3H- thymidine incorporation and Ki67 antigen staining. Western blot were performed to investigate the present of GLP-1 receptor in GLUTag cells.

Results/conclusions:

These results suggest that GLP-1 regulates proliferation of the GLP-1-secreting cell through a feedback mechanism via its receptor. Since serum GLP-1 levels are decreased in type 2 diabetic patients, the effect of GLP-1 on the GLP-1-secreting cell proliferation suggested here provides a novel beneficial long-term effect of the incretin-based drugs in clinical practice i.e. through increase of the GLP-1-secreting cell mass, augmenting the incretin effect. In addition, the feedback mechanism action of GLP-1 reveals a new insight in regulation manner of the L-cell proliferation.

GLP-1(7-36) increased cell proliferation in GLUTag cells, an effect which was blocked by the GLP-1 receptor antagonist exendin(9-39). The GLP-1 receptor was expressed in GluTag cells.

Keywords:

Incretin hormone, GLP-1, GLP-1 receptor, Exendin-4, Diabetes

Cette thèse présente les adaptations spécifiques des hibernants ‘food-storing’ qui s’alimentent au cours de l’hibernation, et les conséquences de la qualité du régime alimentaire sur leur cycle annuel. Tandis que les espèces ‘fat-storing’ jeûnent pendant toute l’hibernation, les ‘food-storing’ alternent jeûnes courts et réalimentations. L’adiponectine stimulerait la lipolyse pendant l’hibernation contribuant ainsi à la cétogenèse. Le maintien d’un système digestif fonctionnel conduisant à la sécrétion d’incrétines, permet l’absorption optimale de nutriments lors des courtes euthermies inter-torpeurs. Une absorption accrue de glucose en particulier permettrait de restaurer la glycémie et les réserves de glycogène. Par ailleurs, un régime appauvri en protéines et enrichi en lipides induit un engraissement augmenté en période pré-hibernatoire provoquant une moindre utilisation de la torpeur donc une perte de masse accrue lors de l’hibernation, et une baisse du succès reproducteur
This thesis presents the specific adaptations of food-storing hibernators that feed during hibernation, and the impact of diet quality on their annual cycle. In contrast to the fat-Storing species which fast during hibernation, the food-storing presents metabolic responses to an alternation of short fasting phases and hyperphagia. These responses involve one hand use of fat reserves during hibernation contributing to ketogenesis, which would be induced by adiponectin. On the other hand, maintaining a functional digestive system leading to the secretion of incretins, permits optimal nutrient absorption in the short inter-torpor euthermia. Increased glucose uptake in particular would restore body reserves to spare. Moreover, a lean protein diet enriched in fat and induces increased in body mass in pre-hibernation period causing reduced use of torpor thus an increased loss of mass during hibernation, and decreased reproductive success

This thesis focuses on the inter-dependent relationships between gastric emptying, incretin hormones (GIP and GLP-1) and postprandial glycaemic and insulinaemic responses in health and type 2 diabetes. Key themes relate to: 1) Evaluation of the relationships of ‘early’ and ‘late’ glycaemic responses with gastric emptying in subjects with normal glucose tolerance, impaired glucose tolerance and type 2 diabetes. 2) Evaluation of the relationships of the insulin secretory response and the oral disposition index with gastric emptying in subjects with normal glucose tolerance. 3) Utilisation of intraduodenal glucose infusions to assess the impact of gastric emptying on: a) the incretin effect, gastrointestinal glucose disposal and the glucagon response, b) the oral disposition index, c) the secretion of incretin hormone secretory pattern in Caucasian compared with Han Chinese subjects and d) postprandial blood pressure and heart rate in type 2 diabetes. Gastric emptying, which regulates the entry of nutrients into the small intestine, is a major determinant of the ‘initial’ glycaemic response (i.e. at 30 min) following an oral glucose tolerance test in health, as well as in type 2 diabetes such that if gastric emptying is more rapid, the initial blood glucose levels are greater. However, the relationships of the 60 min blood glucose (a known predictor of type 2 diabetes) and 120 min blood glucose (used for diagnosis) with gastric emptying during oral glucose tolerance tests have not been studied. My study explored the relationships of 30 min, 60 min and 120 min blood glucose with gastric emptying (measured scintigraphically – the ‘gold standard’ method) in participants with normal glucose tolerance, impaired glucose tolerance and type 2 diabetes. The relationship between the insulin secretory response (calculated as the ratio of change in insulin at 30 min to the change in glucose at 30 represented as ΔI₀₋₃₀ / ΔG₀₋₃₀) and insulin sensitivity (calculated as the reciprocal of fasting insulin and represented as 1/fasting insulin) during an oral glucose tolerance test is hyperbolic in subjects with normal glucose tolerance such that their product, referred to as the ‘oral disposition index’ (ΔI₀₋₃₀ / ΔG₀₋₃₀ X 1/fasting insulin) is always constant. This implies that as long as the pancreatic beta cells are able to compensate adequately (by up-regulating insulin secretion) for any reduction in insulin sensitivity, the oral disposition index remains constant so that the individual has a ‘normal glucose tolerance’. It is, therefore, the failure to compensate fully for the reduction in insulin sensitivity (resulting in a lower oral disposition index) that leads to development of impaired glucose tolerance and type 2 diabetes. Oral disposition index is widely used as a predictor of type 2 diabetes. While the relationship of the early glycaemic response (ΔG₀₋₃₀) with gastric emptying has been characterised, the relationships of the early insulin response (ΔI₀₋₃₀ / ΔG₀₋₃₀) and the oral disposition index with gastric emptying are uncertain. My study explored these relationships in subjects with normal glucose tolerance. There is a wide inter-individual, but relatively little intra-individual variation in the overall rate of gastric emptying (between 1-4 kcal/min in health); this range is even wider in diabetes as a substantial proportion of patients have gastroparesis (i.e. delayed gastric emptying) while in some gastric emptying is abnormally accelerated. This has profound implications for control of glycaemia in diabetes, as even minor variations in the rate of entry of nutrients into the small intestine may be associated with substantial changes in postprandial glycaemic and insulinaemic responses. The incretin hormones, GIP and GLP-1, located in the gut and stimulated by exposure of nutrients to the intestine, play a major role in postprandial glucose metabolism accounting for up to 50% of the post-meal insulin response in health. The incretin hormones are responsible for the so-called ‘incretin effect’ – the amplified insulin secretory response following oral, compared with intravenous, glucose. The incretin effect is known to be attenuated in type 2 diabetes. The impact of gut in glucose disposal can also be described by the so-called ‘gastrointestinal glucose disposal’ (GIGD). GIGD, the amount of glucose required by intravenous infusion to ‘copy’ the glucose excursions after the oral load, was calculated as follows: if 25g intravenous glucose is required to copy a 75g oral glucose load, the GIGD amounts to 100 × (75 – 25)/75 = 66%. GIGD is also reduced in type 2 diabetes. Intraduodenal glucose infusions (via a naso-duodenal catheter) bypass the pylorus and allow glucose to be delivered directly into the small intestine at a pre-determined rate. This model has been employed to study the impact of gastric emptying on postprandial glycaemic and insulinaemic excursions. The outcome of these studies, in which glucose was infused at variable rates within the ‘physiological’ range of gastric emptying i.e. 1,2,3 and 4 kcal/min, indicate that the relationship between the rise in glycaemia and the rate of small intestinal glucose exposure is non-linear. While the glycaemic response was significantly greater in response to 2 kcal/min intraduodenal infusion than 1 kcal/min, increasing the infusion rate further (i.e. 3 and 4 kcal/min) only resulted in minimal, if any, further increase in blood glucose. Glucagon, the hormone produced by the α cells of pancreas, is suppressed following ingestion of glucose in health and thus an important determinant of postprandial blood glucose response, although glucagon suppression is impaired in type 2 diabetes. While GLP-1 suppresses glucagon, GIP does not and may, in fact, modestly elevate it. The effect of gastric emptying on glucagon responses in health and type 2 diabetes is not known. My study looked at the impact of variable duodenal glucose load on the incretin effect and GIGD as well as on glucagon responses in health as well as in type 2 diabetes.There is evidence that East Asians secrete less insulin than Caucasians following oral glucose suggesting that impaired insulin secretion is fundamental to the pathogenesis of type 2 diabetes. However, information about the secretory patterns of GIP and GLP-1, dependent on duodenal glucose load in East Asians, is limited. My study evaluated the glycaemic, insulinaemic and incretin hormone response to a duodenal glucose load in healthy Han Chinese men compared with healthy Caucasian men. Postprandial hypotension (PPH), defined as a fall in systolic blood pressure ≥20mmHg after a meal, occurs frequently in diabetes and its management remains sub-optimal. As well as influencing postprandial glycaemia, gastric emptying also affects the postprandial hypotensive response in ‘healthy’ older subjects and type 2 patients, such that when GE is relatively more rapid, the magnitude of fall in systolic blood pressure is greater. In healthy older subjects, when gastric distension – which may influence blood pressure – is ‘bypassed’ by infusing glucose directly into the duodenum, the fall in systolic blood pressure is greater in response to 2 and 3 kcal/min than 1 kcal/min. It is not known whether duodenal glucose delivery influences blood pressure in type 2 patients. My study evaluated the effects of variations in the intraduodenal glucose load on blood pressure and heart rate in type 2 patients.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Medicine, 2016.

Mechanisms to augment the cellular function and mass of beta-cells may be effective means of treating type 2 diabetes. Important in the physiological control of beta-cell function and nutrient disposal are factors released from gut enteroendocrine cells during nutrient digestion. In enteroendocrine L-cells, post-translational processing of proglucagon gives rise to a number of proglucagon-derived peptides. One such peptide, glucagon-like peptide-1 (GLP-1), acts via its own receptor (GLP-1R) to stimulate beta-cell insulin secretion, proliferation and survival. Another, oxyntomodulin (OXM), weakly activates the GLP-1R and inhibits food intake in a GLP-1R-dependent manner in rodents, which led us to hypothesize that OXM modulates GLP-1R-dependent glucoregulation. While OXM did not mimic the inhibitory effect of GLP-1 on gastric emptying in mice, OXM stimulated insulin secretion, beta-cell survival and improved glucose tolerance in a GLP-1R-dependent manner. In a similar manner to GLP-1, glucose-dependent insulinotropic polypeptide (GIP), secreted from enteroendocrine K-cells, physiologically stimulates insulin secretion via a distinct GIP receptor (GIPR) in beta-cells. Beyond the beta-cell, GIP and GLP-1 appear to exert divergent actions for the control of glucose homeostasis. Moreover, I illustrate that physiological and pharmacological GLP-1R signalling may be comparatively more important for the preservation of beta-cell mass and glucose homeostasis in murine streptozotocin-induced diabetes. Lastly, studies in rodents and humans have showed that metformin increases circulating levels of GLP-1, leading us to hypothesize that GIP and GLP-1 may be involved in the glucoregulatory effects of metformin. Interestingly, transcripts for the Glp1r and Gipr were significantly increased within islets of metformin-treated mice, and metformin treatment enhanced the sensitivity of cultured beta-cells to GIP and GLP-1. In summary, these studies illustrate mechanisms by which enteroendocrine peptides compare and contrast with respect to beta-cell survival and function and the control of glucose homeostasis.

Indiana University-Purdue University Indianapolis (IUPUI)
Glucagon-like peptide 1 (GLP-1) receptor agonists are a class of incretin based therapeutics which aid in blood glucose management in Type II diabetes mellitus (T2DM). Recent studies have demonstrated direct cardiovascular benefits conferred by these agents including protection in ischemia and heart failure. Despite these observations, human clinical trials fail to support improvements in cardiovascular outcomes independent of glucose lowering effects in the T2DM populations. Prior data from our lab demonstrate that obesity impairs GLP-1 associated increases in myocardial glucose uptake. However, the reasons for this impairment/resistance to cardiac effects of GLP-1 in the setting of obesity remain ill defined. This investigation tested the hypothesis that underlying differences in the cardiac proteome and microRNA (miR) transcriptome could contribute to distinct cardiac responses to ischemia and activation of GLP-1 signaling in the setting of obesity. To identify whether obesity modulated cardiac functional responses to GLP 1 related drugs, we first examined the effects of obesity on cardiac function, miR transcriptome, and proteome in response to short duration ischemia-reperfusion (I/R). We observed divergent physiologic responses (e.g. increased diastolic volume and systolic pressure in lean, decreased diastolic volumes in obese) to regional I/R in obese vs lean hearts that were associated with significant molecular changes as detected by protein mass spectrometry and miR microarray. Molecular changes were related to myocardial calcium handling (SERCA2a, histidine-rich Ca2+ binding protein), myocardial structure and function (titin), and miRs relating to cardiac metabolism, hypertrophy, and cell death, including miR-15, miR-30, miR-199a, miR-214. Importantly, these effects were modified differently by GLP-1 agonism in lean vs obese swine. Additional studies investigated the functional effects of 30 days of treatment with the GLP-1 analogue liraglutide on a model of slowly-developing, unrelieved coronary ischemia. Liraglutide failed to reduce infarct size or collagen deposition. However, analysis of left ventricular pressure-volume relationships support that liraglutide improved diastolic relaxation/filling, load-dependent indices of cardiac function, and cardiac efficiency in response to sympathetic stimulation in obese swine. Taken together, these findings support that miR and proteomic differences underlie distinct changes in functional cardiac responses to I/R and pharmacologic activation of GLP-1 signaling in the setting of obesity.

This thesis presents a series of clinical research studies focusing on postprandial blood pressure (BP), glycaemic and incretin hormone responses in healthy ageing. The studies address the underlying pathophysiology, natural history, and approaches to management, of postprandial hypotension (PPH), longitudinal changes in and relationship between postprandial plasma levels of the incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). PPH, usually defined as a fall in systolic BP of greater than, or equal to, 20 mmHg within 2 h of a meal occurs frequently, particularly in older people and those with type 2 diabetes (T2D). PPH increases the incidence of falls, syncope, cerebrovascular disease, angina, and has been associated with a higher risk of cardiovascular mortality even when it is asymptomatic. After a meal, there is a substantial increase in splanchnic blood flow, leading to a reduction of blood volume returning to the heart. PPH occurs when compensatory responses are inadequate to maintain BP. Multiple factors are involved in the pathophysiology of PPH, including small intestinal nutrient delivery, changes in autonomic function, the release of gastrointestinal hormones and changes in splanchnic blood flow. Chapter 1, 2 and 3 are structured as narrative reviews which aim to provide a comprehensive background to the studies. Chapter 1 summarises ‘up-to-date’ knowledge relating to PPH, including the prevalence, clinical relevance, pathophysiology and approaches to management. Chapter 2 is a brief review of the physiology of gastric emptying, which is pivotal to the pathophysiology of PPH, while Chapter 3 focusses on the interrelated relationship between gastric emptying and glycaemia. In healthy older subjects and patients with T2D, there is a correlation between the magnitude of fall in BP induced by glucose with the rate of gastric emptying of glucose, so that relatively more rapid gastric emptying is associated with a greater fall in BP. Cross-sectional studies indicate that healthy ageing is associated with a modest slowing of gastric emptying, however, there is limited information about longitudinal changes in gastric emptying in a healthy, ageing population and no studies which have evaluated the natural history of the fall in BP induced by glucose with ageing. In the study described in Chapter 4, longitudinal changes in the BP response to, and gastric emptying of, glucose were evaluated in 33 healthy older people at an initial study and after 5.8 ± 0.1yr. BP, heart rate (HR) and gastric emptying (using a stable isotope breath test technique) were assessed concurrently after participants consumed a 300mL drink containing 75g glucose and 150mg C13-acetate. PPH is under-recognised, but common. Following health concerns about excessive consumption of sugar, there has been an increasing trend to use low- or non-nutritive sweeteners as an alternative. Due to the lack of literature in this area, a systematic review described in Chapter 5 was conducted to identify important gaps in information relevant to the effects of different types of sweeteners on postprandial BP. While all macronutrients reduce BP comparably, the hypotensive responses to fat and protein occur slightly later than the response to glucose in healthy older people, probably reflecting the more prolonged time for digestion. Moreover, xylose, a poorly absorbed pentose sugar, empties from the stomach at a comparable rate to glucose, but has no effect on BP in healthy older subjects, as is also the case for fructose. The effect of artificial sweeteners, such as sucralose, on postprandial BP, was unknown. In the study described in Chapter 6, the effects of intraduodenal (ID) infusion of sucralose and glucose versus saline, on BP and HR, superior mesenteric artery (SMA) blood flow and blood glucose, were assessed in healthy older individuals. Current management of PPH is suboptimal. Acarbose is known to attenuate the fall in systolic BP induced by oral sucrose in healthy older adults, associated with slowing of gastric emptying and enhanced release of GLP-1. Gastric distension with water at a volume as low as 300 mL mitigates the fall in BP in response to ID glucose. In the study described in Chapter 7, the effects of gastric distension and acarbose, either alone or in combination, on BP, glycaemia and SMA flow after oral sucrose were assessed in healthy older people. A whey protein/guar preload has been shown to reduce postprandial glycaemia in T2D, an effect suggested to be mediated by slowing of gastric emptying. However, the latter has only been assessed using a stable isotope breath test technique, which cannot discriminate between slowing of gastric emptying and a delay in small intestinal absorption. This preload also has potential for use in the management of postprandial hypotension. In the study reported in Chapter 8, the effects of a guar/whey protein preload on gastric emptying (using scintigraphy), glucose absorption, glycaemic/insulinaemic and BP responses to an oral glucose load, were evaluated in healthy older people. The rate of gastric emptying is a major determinant of the glycaemic response to carbohydratecontaining meals in healthy subjects, as well as individuals with T2D. Gastric emptying also influences the release of incretin hormones, GLP-1 and GIP, which impact postprandial glycaemic excursions. It is not known whether baseline and/or nutrient-stimulated GLP-1 or GIP levels are predictable within an individual or affected by ageing. The study described in Chapter 9 re-evaluated a cohort of healthy older subjects after an interval of ~ 5.9 years and determined changes in fasting and glucose-stimulated plasma GLP-1 and GIP concentrations and their relationships with gastric emptying. The incretin hormones, GLP-1 and GIP, are secreted following intestinal macronutrient exposure - GIP primarily from the proximal small intestine and GLP-1 from the more distal small intestine and colon. Their relative importance to the incretin effect in health has been contentious, although recent studies employing a specific GIP antagonist now indicate that GIP has the dominant role. It is uncertain whether there is a relationship between GIP and GLP-1 secretion. The study described in Chapter 10 evaluates the relationship between GIP and GLP-1 responses to a 75g oral glucose load in older individuals with either normal (NGT) or impaired glucose tolerance (IGT).
Thesis (Ph.D.) -- University of Adelaide, School of Medicine, 2019

This thesis focuses on gastric motor function in patients with longstanding diabetes, and the role of gastric emptying and gastrointestinal hormones in the regulation of glycaemia in health and patients with type 2 diabetes mellitus. Diabetes is a common chronic disorder worldwide, with the prevalence of type 2 diabetes escalating due to an increasingly sedentary lifestyle and rising rates of obesity. Diabetes is associated with micro- and macrovascular complications, particularly in the context of poor glycaemic control (1993, 1998). Another complication of type 1 and 2 diabetes is gastroparesis (Horowitz et al., 2001, Horowitz M, 1986, Horowitz et al., 1989, Horowitz et al., 1991) (delayed gastric emptying in diabetes) and there is limited information about the natural history and prognosis of this condition. While the prognosis of diabetic gastroparesis has been assumed to be poor, limited data in a small cohort followed for a mean period of 12 years suggest otherwise, with neither deterioration in the rate of gastric emptying (Jones et al., 2002) nor increased mortality due to the condition itself (Kong et al., 1999). The study reported in Chapter 3 evaluated the longitudinal progression of gastric emptying in patients with longstanding diabetes over a 25 year period to determine if there is a progressive slowing of gastric emptying or whether gastric emptying is relatively stable with a good prognosis from the outset, and to ascertain the potential impact of glycaemic control and/or autonomic function. The study concludes that gastric emptying and upper gastrointestinal symptoms are relatively stable over 25 years, while there was a deterioration in autonomic function and an improvement in glycaemic control. The study reported in Chapter 4 examined the prognosis of diabetic gastroparesis and its findings highlight that this condition is neither associated with a poor prognosis nor a higher rate of mortality. There is increasing recognition that glycated haemoglobin (HbA1c), which is a measure of overall glycaemic control, is influenced more by postprandial, rather than fasting, blood glucose levels in the majority of patients with type 2 diabetes. This makes intuitive sense, because the majority of one’s time is spent in a postprandial state, digesting the caloric load of the ingested meal, which in healthy subjects empties from the stomach in a tightly regulated process at a rate of 1-4kcal/minute (Khoo et al., 2009). Accordingly, good control of postprandial glucose excursions should be a priority for the treatment of diabetes. The rate of gastric emptying itself influences the magnitude of the initial rise in postprandial glycaemia in health as well as type 1 and 2 diabetes (Jones et al., 1996, Horowitz et al., 1993, Horowitz et al., 1986), whereby slower emptying is associated with diminished postprandial glucose excursions. The overall rate of gastric emptying is dependent on the integration of motor activity in each region of the stomach and slower gastric emptying is associated with suppression of antral and duodenal contractions, and stimulation of phasic and tonic pyloric pressures, with the latter acting as a brake to gastric outflow (Horowitz et al., 1994). When glucose is given by the oral/enteral route, the stimulation of insulin is markedly greater than with an isoglycaemic intravenous glucose infusion. This phenomenon is known as the ‘incretin effect’ and is mediated by the gastrointestinal hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), which are secreted from the small intestine in response to nutrients (Ma et al., 2009a). GLP-1 and GIP both stimulate insulin secretion from the pancreas in the setting of elevated blood glucose levels, and are responsible for ~70% of the postprandial insulin response in healthy humans (Horowitz and Nauck, 2006). GLP-1 analogues, such as exenatide, are now widely used in the management of type 2 diabetes, in whom the response to exogenous GIP is attenuated markedly (Holst and Gromada, 2004) but the insulin response to GLP-1 remains intact (Elahi et al., 1994). It appears that an important action of GLP-1 analogues in reducing postprandial glycaemia is by retardation of small intestinal motility modulating carbohydrate absorption (Linnebjerg et al., 2008, Little et al., 2006). An alternative to the use of exogenous GLP-1 analogues in the management of type 2 diabetes is to develop dietary strategies which stimulate endogenous GLP-1 release. Glutamine, which is widely used as a nutritional supplement, appears to be the most potent amino acid in inducing GLP-1 release (Reimann et al., 2004). It has been reported that 30g glutamine, given in 300mL water, stimulates GLP-1 release in both healthy subjects and patients with type 2 diabetes (Greenfield et al., 2009) and Samocha-Bonet et al (Samocha-Bonet et al., 2011) reported that 15g and 30g glutamine when given as a drink, before an oral glucose load in patients with type 2 diabetes, dose-dependently stimulate GLP-1 and diminish subsequent glycaemic excursion. However, the effect of glutamine on the rate of gastric emptying of glucose could potentially influence the observed effect on glycaemia as it is now appreciated that the rate of gastric emptying itself has a major influence on postprandial glucose levels in healthy subjects and patients with type 1 and 2 diabetes (Chang et al., 2010). The study reported in Chapter 5 examined the effects of intraduodenal glutamine on GLP-1, GIP and insulin release and the subsequent glycaemic response to an intraduodenal glucose load, in health and type 2 diabetes, of which the intraduodenal route of delivery of glutamine will bypass the stomach, thus, eliminating any influence of glutamine on the rate of gastric emptying of glucose. This study showed that intraduodenal glutamine has minimal effect on the glycaemic response to intraduodenal glucose, despite its ability to stimulate GLP-1, GIP and insulin release, and stimulate phasic pyloric contractions, suggesting that slowing of gastric emptying may play a major role for the glucose lowering effect seen with oral glutamine.
Thesis (M.Phil.) -- University of Adelaide, School of Medicine, 2012