Differences in glucagon-like peptide-1 and GIP responses following sucrose ingestion

Augmented GLP-1 Secretion as Seen After Gastric Bypass May Be Obtained by Delaying Carbohydrate Digestion

CONTEXT

Exaggerated postprandial glucagon-like peptide-1 (GLP-1) secretion seems important for weight loss and diabetes remission after Roux-en-Y gastric bypass (RYGB) and may result from carbohydrate absorption in the distal small intestine.

OBJECTIVE

To investigate distal [GLP-1; peptide YY (PYY)] and proximal [glucose-dependent insulinotropic polypeptide (GIP)] gut hormone secretion in response to carbohydrates hydrolyzed at different rates. We hypothesized that slow digestion restricts proximal absorption, facilitating distal delivery of carbohydrates and thereby enhanced GLP-1 secretion in unoperated individuals, whereas this may not apply after RYGB.

DESIGN

Single-blinded, randomized, crossover study.

SETTING

Hvidovre Hospital, Hvidovre, Denmark.

PARTICIPANTS

Ten RYGB-operated patients and 10 unoperated matched subjects.

INTERVENTIONS

Four separate days with ingestion of different carbohydrate loads, either rapidly/proximally digested (glucose plus fructose; sucrose) or slowly/distally digested (isomaltulose; sucrose plus acarbose).

MAIN OUTCOME MEASURES

GLP-1 secretion (area under the curve above baseline). Secondary outcomes included PYY and GIP.

RESULTS

Isomaltulose enhanced secretion of GLP-1 nearly threefold (P = 0.02) and PYY ninefold (P = 0.08) compared with sucrose in unoperated subjects but had a modest effect after RYGB. Acarbose failed to increase sucrose induced GLP-1 secretion in unoperated subjects and diminished the responses by 50% after RYGB (P = 0.03). In both groups, GIP secretion was reduced by isomaltulose and even more so by sucrose plus acarbose when compared with sucrose intake.

CONCLUSIONS

GLP-1 secretion depends on the rate of carbohydrate digestion, but in a different manner after RYGB. Enhanced GLP-1 secretion is central after RYGB, but it may also be obtained in unoperated individuals by delaying hydrolysis of carbohydrates, pushing their digestion and absorption distally in the small intestine.

Selective sodium-dependent glucose transporter 1 inhibitors block glucose absorption and impair glucose-dependent insulinotropic peptide release

GSK-1614235 and KGA-2727 are potent, selective inhibitors of the SGLT1 sodium-dependent glucose transporter. Nonclinical (KGA-2727) and clinical (GSK-1614235) trials assessed translation of SGLT1 inhibitor effects from rats to normal human physiology. In rats, KGA-2727 (0.1 mg/kg) or vehicle was given before oral administration of 3-O-methyl-α-d-glucopyranose (3-O-methylglucose, 3-OMG) containing 3-[3H]OMG tracer. Tracer absorption and distribution were assessed from plasma, urine, and fecal samples. SGLT1 inhibition reduced urine 3-OMG recovery and increased fecal excretion. SGLT1 inhibitor effects on plasma glucose, insulin, gastric inhibitory peptide (GIP), and glucagon-like peptide-1 (GLP-1) concentrations were also measured during a standard meal. Incremental glucose, insulin, and GIP concentrations were decreased, indicating downregulation of β-cell and K cell secretion. Minimal effects were observed in the secretion of the L cell product, GLP-1. With the use of a three-way, crossover design, 12 healthy human subjects received placebo or 20 mg GSK-1614235 immediately before or after a meal. Five minutes into the meal, 3-OMG was ingested. Postmeal dosing had little impact, yet premeal dosing delayed and reduced 3-OMG absorption, with an AUC0-10 of 231±31 vs. 446±31 μg·h(-1)·ml(-1), for placebo. Recovery of tracer in urine was 1.2±0.7 g for premeal dosing and 2.2±0.1 g for placebo. Incremental concentrations of insulin, C-peptide, and GIP were reduced for 2 h with premeal GSK-1614235. Total GLP-1 concentrations were significantly increased, and a trend for increased peptide YY (PYY) was noted. SGLT1 inhibitors block intestinal glucose absorption and reduce GIP secretion in rats and humans, suggesting SGLT1 glucose transport is critical for GIP release. Conversely, GLP-1 and PYY secretion are enhanced by SGLT1 inhibition in humans.

Efficacy of α-glucosidase inhibitors combined with dipeptidyl-peptidase-4 inhibitor (alogliptin) for glucose fluctuation in patients with type 2 diabetes mellitus by continuous glucose monitoring

AIMS/INTRODUCTION

The combination therapy of dipeptidyl-peptidase (DPP)-4 inhibitor and α-glucosidase inhibitors (α-GIs) is highly effective in suppressing postprandial hyperglycemia. The aim of the present study was to compare the effects of voglibose and miglitol on glucose fluctuation, when used in combination with DPP-4 inhibitor by using continuous glucose monitoring (CGM).

MATERIALS AND METHODS

In a randomized cross-over study, 16 patients with type 2 diabetes who presented with postprandial hyperglycemia despite alogliptin (25 mg) were treated with voglibose (0.9 mg) or miglitol (150 mg). We measured standard deviation (SD); mean amplitude of glycemic excursions (MAGE), and mean, minimum and maximum glucose measured by CGM during three phases (alogliptin monotherapy, dual therapy of alogliptin and voglibose, and dual therapy of alogliptin and miglitol). The primary outcome measure was SD between α-GIs.

RESULTS

SD was significantly improved by the addition of either voglibose (18.9 ± 10.1) or miglitol (19.6 ± 8.2) to alogliptin monotherapy (36.2 ± 8.7). MAGE improved significantly with the addition of either voglibose (57.5 ± 26.1, P < 0.01) or miglitol (64.6 ± 26.2, P < 0.01) to alogliptin monotherapy (101.5 ± 21.5). There was no significant difference in glucose fluctuation between α-GIs. There were no differences between two groups in mean (132.6 ± 21.4 and 138.7 ± 25.4) and maximum (184.3 ± 48.7 and 191.9 ± 38.3). The minimum glucose under alogliptin plus voglibose (94.9 ± 20.2) was significantly lower than that under alogliptin and miglitol (105.3 ± 21.0).

CONCLUSIONS

Glucose fluctuation was improved by the addition of voglibose or miglitol to alogliptin. Glucose fluctuations and postprandial hyperglycemia were similar between α-GIs. This trial was registered with the University Hospital Medical Information Network (no. UMIN R000010028).

Comparison of glycemic variability in patients with type 2 diabetes given sitagliptin or voglibose: a continuous glucose monitoring-based pilot study

OBJECTIVE

This study compared glycemic variability in patients with type 2 diabetes given sitagliptin or voglibose.

PATIENTS AND METHODS

Seventeen type 2 diabetes patients were given sitagliptin 50 mg/day or voglibose 0.9 mg/day for 2 months and were hospitalized for a 4-day evaluation by continuous glucose monitoring (CGM). On discharge, they were crossed over to the other regimen for 2 months of treatment/4 days of evaluation. The CGM data were used to compare each parameter for glycemic variability.

RESULTS

The average glucose levels with sitagliptin and voglibose were significantly different at 138.6 and 152.6 mg/dL for 24 h (P=0.014) and 147.2 and 160.9 mg/dL for during daytime (P=0.050), respectively. The patients' glucose levels with sitagliptin and voglibose were significantly different at 125.3 and 139.7 mg/dL before breakfast (P=0.015) and 112.7 and 131.4 mg/dL before lunch (P=0.049), respectively. The time from before meal to postprandial peak glucose levels was significantly longer after dinner with voglibose than with sitagliptin (91.5 and 122.3 min, respectively; P=0.012). All of the slopes of glucose elevation were significantly lower with voglibose after each meal, with that after breakfast, lunch, and dinner being 1.16 and 0.86 mg/dL/min (P=0.031), 0.70 and 0.45 mg/dL/min (P=0.048), and 1.06 and 0.73 mg/dL/min (P=0.028), respectively.

CONCLUSIONS

This CGM-based pilot study revealed that sitagliptin significantly lowered 24-h and daytime mean glucose levels and glucose levels before breakfast and lunch compared with voglibose, whereas the time from before dinner to peak postprandial glucose levels was significantly longer, and the slope of postprandial elevation of glucose level was significantly lower after each meal, with voglibose compared with sitagliptin.

Chylomicron formation and secretion is required for lipid-stimulated release of incretins GLP-1 and GIP

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretins produced in the intestine that play a central role in glucose metabolism and insulin secretion. Circulating concentrations of GLP-1 and GIP are low and can be difficult to assay in rodents. These studies utilized the novel intestinal lymph fistula model we have established to investigate the mechanism of lipid-stimulated incretin secretion. Peak concentrations of GLP-1 and GIP following an enteral lipid stimulus (Liposyn) were significantly higher in intestinal lymph than portal venous plasma. To determine whether lipid-stimulated incretin secretion was related to chylomicron formation Pluronic L-81 (L-81), a surfactant inhibiting chylomicron synthesis, was given concurrently with Liposyn. The presence of L-81 almost completely abolished the increase in lymph triglyceride seen with Liposyn alone (P < 0.001). Inhibition of chylomicron formation with L-81 reduced GLP-1 secretion into lymph compared to Liposyn stimulation alone (P = 0.034). The effect of L-81 relative to Liposyn alone had an even greater effect on GIP secretion, which was completely abolished (P = 0.004). These findings of a dramatic effect of L-81 on lymph levels of GLP-1 and GIP support a strong link between intestinal lipid absorption and incretin secretion. The relative difference in the effect of L-81 on the two incretins provides further support that nutrient-stimulation of GIP and GLP-1 is via distinct mechanisms.

Glucagon-like peptide-1 regulation of carbohydrate intake is differentially affected by obesogenic diets

The incretin hormone glucagon-like peptide-1 (GLP-1) has been implicated in the regulation of appetite by acting as an anorexigenic gut-brain signal. The postprandial release of GLP-1 can be blunted in obese humans and animals. However, it remains unknown whether obesogenic diets with varying fat and carbohydrate content may differentially influence the effectiveness of GLP-1 feedback. To investigate this, male Sprague-Dawley rats were fed a standard (low fat) chow diet, or one of two high-energy diets varying in fat content (45 or 60 kcal%) for 28 weeks. Intake of sucrose and fructose solutions, two commonly added sugars in the Western diet, was then tested in nondeprived rats following administration of the GLP-1 receptor agonist, Exendin-4 (0, 0.5, 1, 2, 3 µg/kg; s.c.). Exendin-4 dose-dependently reduced short (2 h) sucrose and fructose intake. This effect was significantly attenuated in rats fed more dietary fat, despite both diets resulting in obesity. These findings demonstrate that intake of carbohydrates when offered as treats can be regulated by GLP-1 and suggests that dietary fat consumption, rather than extra calories or obesity, may lead to impaired GLP-1 feedback to curb carbohydrate intake. Future studies are warranted to investigate the relevance of these observations to humans and to elucidate the underlying mechanisms.

Secretion of glucagon-like peptide-1 (GLP-1) in type 2 diabetes: what is up, what is down?

The incretin hormones gastric inhibitory polypeptide and especially glucagon-like peptide (GLP) have an important physiological function in augmenting postprandial insulin secretion. Since GLP-1 may play a role in the pathophysiology and treatment of type 2 diabetes, assessment of meal-related GLP-1 secretory responses in type 2 diabetic patients vs healthy individuals is of great interest. A common view states that GLP-1 secretion in patients with type 2 diabetes is deficient and that this applies to a lesser degree in individuals with impaired glucose tolerance. Such a deficiency is the rationale for replacing endogenous incretins with GLP-1 receptor agonists or re-normalising active GLP-1 concentrations with dipeptidyl peptidase-4 inhibitors. This review summarises the literature on this topic, including a meta-analysis of published studies on GLP-1 secretion in individuals with and without diabetes after oral glucose and mixed meals. Our analysis does not support the contention of a generalised defect in nutrient-related GLP-1 secretory responses in type 2 diabetes patients. Rather, factors are identified that may determine individual incretin secretory responses and explain some of the variations in published findings of group differences in GLP-1 responses to nutrient intake.

Differential responses of the incretin hormones GIP and GLP-1 to increasing doses of dietary carbohydrate but not dietary protein in lean rats

Previous studies have shown that oral ingestion of nutrients stimulates secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1); however, it is unclear whether there is a dose-dependent response between the amount of nutrient ingested and the secretion of the hormones in vivo. Using our lymph fistula rat model, we previously demonstrated that both GIP and GLP-1 responded dose dependently to increasing amounts of infused dietary lipid and that the GLP-1-secreting cells were more sensitive to changes in intestinal lipid content. In the present study, we investigated the dose-dependent relationships between incretin secretion and the two remaining macronutrients, carbohydrate and protein. To accomplish this objective, the major mesenteric lymphatic duct of male Sprague-Dawley rats was cannulated. Each animal received a single bolus (3 ml) of saline, dextrin, whey protein, or casein hydrolysate (0.275, 0.55, 1.1, 2.2, 4.4 kcal) via a surgically inserted duodenal or ileal feeding tube. Lymph was continuously collected for 3 h and analyzed for GIP and GLP-1 content. Both GIP and GLP-1 outputs responded dose dependently to increasing amounts of dietary carbohydrate but not protein. Additionally, we found that the GIP-secreting cells were more sensitive than the GLP-1-secreting cells to changes in intestinal carbohydrate content.

Using the lymph fistula rat model to study incretin secretion

The past several decades have witnessed a flourish of interest in the field of incretin biology. The importance of the two incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), in health and disease is becoming more apparent as the prevalence of type 2 diabetes and other metabolic disorders escalates. Rodent models have become indispensable in the study of the physiological function of GIP and GLP-1; however, investigators have run into several roadblocks when untangling the regulation of incretin secretion in these systems. The low circulating levels of the incretin hormones combined with sensitivity of the currently available assays require substantial amounts of blood to be removed from an animal if the hormones are to be analyzed over a period of time. Because of these limitations, continuous monitoring of GIP and GLP-1 secretion becomes difficult. A more effective means of studying incretin secretion in small animal models is therefore desirable. This chapter evaluates the use of the lymph fistula rat as a model to study the secretion of incretins. Lymph fistula models, in a variety of animals, have been used for decades to study the absorption and transport of lipid and lipophilic compounds; however, only recently has the value of this model been appreciated as a tool to explore incretin secretion.

Stimulation of incretin secretion by dietary lipid: is it dose dependent?

After the ingestion of nutrients, secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) by the enteroendocrine cells increases rapidly. Previous studies have shown that oral ingestion of fat stimulates secretion of both incretins; however, it is unclear whether there is a dose-dependent relationship between the amount of lipid ingested and the secretion of the hormones in vivo. Recently, we found a higher concentration of the incretin hormones in intestinal lymph than in peripheral or portal plasma. We therefore used the lymph fistula rat model to test for a dose-dependent relationship between the secretion of GIP and GLP-1 and dietary lipid. Under isoflurane anesthesia, the major mesenteric lymphatic duct of male Sprague-Dawley rats was cannulated. Each animal received a single, intraduodenal bolus of saline or varying amounts of the fat emulsion Liposyn II (0.275, 0.55, 1.1, 2.2, and 4.4 kcal). Lymph was continuously collected for 3 h and analyzed for triglyceride, GIP, and GLP-1 content. In response to increasing lipid calories, secretion of triglyceride, GIP, and GLP-1 into lymph increased dose dependently. Interestingly, the response to changes in intraluminal lipid content was greater in GLP-1- than in GIP-secreting cells. The different sensitivities of the two cell types to changes in intestinal lipid support the concept that separate mechanisms may underlie lipid-induced GIP and GLP-1 secretion. Furthermore, we speculate that the increased sensitivity of GLP-1 to intestinal lipid content reflects the hormone's role in the ileal brake reflex. As lipid reaches the distal portion of the gut, GLP-1 is secreted in a dose-dependent manner to reduce intestinal motility and enhance proximal fat absorption.

Chronic administration of voglibose, an alpha-glucosidase inhibitor, increases active glucagon-like peptide-1 levels by increasing its secretion and decreasing dipeptidyl peptidase-4 activity in ob/ob mice

Administration of an alpha-glucosidase inhibitor, voglibose, increases the secretion of glucagon-like peptide (GLP)-1, a key modulator of pancreatic islet hormone secretion and glucose homeostasis. In the present study, novel mechanisms by which voglibose increases active GLP-1 circulation were evaluated. Voglibose (0.001 and 0.005%) was administered in the diet to ob/ob mice for 1 day or 3 to 4 weeks to determine effects on incretin profiles and plasma activity of dipeptidyl peptidase-4 (DPP-4), an enzyme responsible for GLP-1 degradation. Voglibose showed no direct inhibitory effect against DPP-4 in vitro (DPP-4 inhibitor alogliptin, IC(50) < 10 nM). Likewise, 1-day treatment with voglibose did not change plasma DPP-4 activity; however, it increased plasma active GLP-1 by 1.6- to 3.4-fold. After chronic treatment, voglibose stimulated GLP-1 secretion, as evidenced by the 1.3- to 1.5-fold increase in plasma active plus inactive amidated GLP-1 levels. Plasma DPP-4 activity was decreased unexpectedly by 40 to 51%, resulting from reduced plasma DPP-4 concentrations in voglibose-treated mice. Voglibose increased GLP-1 content by 1.5- to 1.6-fold and 1.4- to 1.6-fold in the lower intestine and colon, respectively. The increased GLP-1 content in the colon was associated with elevated expression of gut glucagon gene. Chronic treatment with voglibose resulted in 1.9- to 4.1-fold increase in active GLP-1 circulation, which was higher than 1-day treatment. A similar treatment with pioglitazone (0.03%), an insulin sensitizer, did not affect plasma DPP-4 activity or GLP-1 levels. These results suggest that increased GLP-1 secretion, decreased DPP-4 activity, and increased gut GLP-1 content may have contributed to increased active GLP-1 circulation after chronic treatment with voglibose in a glucose control-independent manner in ob/ob mice.

Using the lymph fistula rat model to study the potentiation of GIP secretion by the ingestion of fat and glucose

Glucose-dependent insulinotropic polypeptide (GIP) is an important incretin produced in the K cells of the intestine and secreted into the circulating blood following ingestion of carbohydrate- and fat-containing meals. GIP contributes to the regulation of postprandial insulin secretion and is essential for normal glucose tolerance. We have established a method of assaying GIP in response to nutrients using the intestinal lymph fistula model. Administration of Ensure, a mixed-nutrient liquid meal, stimulated a significant increase in intestinal lymphatic GIP levels that were approximately threefold those of portal plasma. Following the meal, lymph GIP peaked at 60 min (P < 0.001) and remained elevated for 4 h. Intraduodenal infusions of isocaloric and isovolumetric lipid emulsions or glucose polymer induced lymph GIP concentrations that were four and seven times the basal levels, respectively. The combination of glucose plus lipid caused an even greater increase of lymph GIP than either nutrient alone. In summary, these findings demonstrated that intestinal lymph contains high concentrations of GIP that respond to both enteral carbohydrate and fat absorption. The change in lymphatic GIP concentration is greater than the change observed in the portal blood. These studies allow the detection of GIP levels at which they exert their local physiological actions. The combination of glucose and lipid has a potentiating effect in the stimulation of GIP secretion. We conclude from these studies that the lymph fistula rat is a novel approach to study in vivo GIP secretion in response to nutrient feeding in conscious rats.

The regulation of the lymphatic secretion of glucagon-like peptide-1 (GLP-1) by intestinal absorption of fat and carbohydrate

Glucagon-like peptide-1 (GLP-1) is an important incretin produced in the L cells of the intestine. It is essential in the regulation of insulin secretion and glucose homeostasis. Systemic GLP-1 concentrations are typically low in rodents, so it can be difficult to assay physiological levels or detect changes in response to nutrients. We have established a method of assaying GLP-1 in response to nutrients using the intestinal lymph fistula model. Intraduodenal infusion of Intralipid (4.43 kcal/3 ml) induced a significant increase of lymphatic GLP-1 concentration compared with saline control at the peak of 30 min. (P < 0.001). Isocaloric and isovolumetric treatment with dextrin, a glucose polymer, also caused a significant fourfold increase in peak concentration at 60 min (P = 0.001). These findings indicate that intestinal lymph contains high concentrations of postprandial GLP-1. Second, they reveal that GLP-1 secretion into lymph occurs in response to both enteral carbohydrate and fat, but the response to dextrin occurs later than to Intralipid with peak times at 60 and 30 min, respectively. Third, the combination of Intralipid plus dextrin demonstrated an additive effect in the stimulation of GLP-1 with peak at 30 min. These results indicate that assessment of levels in lymph is a novel and powerful means of studying the secretion of GLP-1 and potentially other gastrointestinal hormones in vivo. Furthermore, the lymph fistula rat model provides insight into the gut hormone concentrations to which the neurons and cells in the lamina propria of the gut are likely exposed.

The incretins: a link between nutrients and well-being

The glucoincretins, glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), are intestinal peptides secreted in response to glucose or lipid intake. Data on isolated intestinal tissues, dietary treatments and knockout mice strongly suggest that GIP and GLP-1 secretion requires glucose and lipid metabolism by intestinal cells. However, incretin secretion can also be induced by non-digestible carbohydrates and involves the autonomic nervous system and endocrine factors such as GIP itself and cholecystokinin. The classical pharmacological approach and the recent use of knockout mice for the incretin receptors have shown that a remarkable feature of incretins is the ability to stimulate insulin secretion in the presence of hyperglycaemia only, hence avoiding any hypoglycaemic episode. This important role is the basis of ongoing clinical trials using GLP-1 analogues. Since most of the data concern GLP-1, we will focus on this incretin. In addition, GLP-1 is involved in glucose sensing by the autonomic nervous system of the hepato-portal vein controlling muscle glucose utilization and indirectly insulin secretion. GLP-1 has been shown to decrease glucagon secretion, food intake and gastric emptying, preventing excessive hyperglycaemia and overfeeding. Another remarkable feature of GLP-1 is its secretion by the brain. Recently, elegant data showed that cerebral GLP-1 is involved in cognition and memory. Experiments using knockout mice suggest that the lack of the GIP receptor prevents diet-induced obesity. Consequently, macronutrients controlling intestinal glucose and lipid metabolism would control incretin secretion and would consequently be beneficial for health. The control of incretin secretion represents a major goal for new therapeutic as well as nutrition strategies for treating and/or reducing the risk of hyperglycaemic syndromes, excessive body weight and thus improvement of well-being.

Glucagon-like peptide-1 response to acarbose in elderly type 2 diabetic subjects

The anti-hyperglycemic effect of alpha-glucosidase inhibitors (AGI) is partly attributed to their ability to stimulate the secretion of glucagon-like peptide-1 (GLP-1), a gut hormone with insulin stimulating capability. To determine if this mechanism of action contributes significantly to the therapeutic efficacy of AGI in the elderly, 10 type 2 diabetic subjects over the age of 65 years were given a standardized test meal with or without 25, 50, or 100 mg acarbose. The serum glucose, insulin, triglycerides and GLP-1 levels were measured at baseline and at 1 and 2 h postprandially. The anti-hyperglycemic effect of acarbose was maximal at 25-mg dose under these experimental conditions. Serum postprandial insulin and triglycerides levels were not significantly altered with acarbose treatment. The postprandial serum GLP-1 levels rose significantly only in two subjects and only during treatment with 100-mg acarbose. There were no significant correlations between serum GLP-1 and serum glucose or insulin levels. It is concluded that in most elderly type 2 diabetic subjects, maximal anti-hyperglycemic effects can be achieved with relatively small doses of acarbose and that GLP-1 is unlikely to contribute to the clinical efficacy of this agent in this subgroup of subjects.

Therapeutic potential of CPT I inhibitors: cardiac gene transcription as a target

Inhibitors of carnitine palmitoyl-transferase I (CPT I), the key enzyme for the transport of long-chain acyl-coenzyme A (acyl-CoA) compounds into mitochondria, have been developed as agents for treating diabetes mellitus Type 2. Findings that the CPT I inhibitor, etomoxir, has effects on overloaded heart muscle, which are associated with an improved function, were unexpected and can be attributed to selective changes in the dysregulated gene expression of hypertrophied cardiomyocytes. Also, the first clinical trial with etomoxir in patients with heart failure showed that etomoxir improved the clinical status and several parameters of heart function. In view of the action of etomoxir on gene expression, putative molecular mechanisms involved in an increased expression of SERCA2, the Ca(2+) pump of sarcoplasmic reticulum (SR) and alpha-myosin heavy chain (MHC) of failing overloaded heart muscle are described. The first 225 bp of human, rabbit, rat and mouse SERCA2 promoter sequence have high identity. Various cis-regularory elements are also given for the promoter of the rat cardiac alpha-MHC gene. It is hypothesised that etomoxir increases glucose-phosphate intermediates resulting in activation of signalling pathway(s) mediated by phosphatases. Regarding the possible direct action of etomoxir on peroxisome proliferator activated receptor alpha (PPAR-alpha) activation, it could upregulate the expression of various enzymes that participate in beta-oxidation, thereby modulating some effects of CPT 1 inhibition. Any development of alternative drugs requires a better understanding of the signal pathways involved in the altered gene expression. In particular, signals need to be identified which are altered in overloaded hearts and can selectively be re-activated by etomoxir.

Pathophysiologic mechanisms of postprandial hyperglycemia

The role of postprandial hyperglycemia in the etiology of diabetes-related complications and outcomes, although still being elucidated, is greater than previously thought. Acute glucose elevations after meal ingestion are associated with a variety of glucose-mediated tissue defects-oxidative stress, glycation, and advanced glycation end product formation-which have far-reaching structural and functional consequences for virtually every human organ system. Lowering glycosylated hemoglobin to levels that prevent or delay these complications can be achieved only by reducing both postprandial and fasting plasma glucose levels. The alpha-glucosidase inhibitors (acarbose, voglibose, miglitol) have been effective in delaying the digestion and absorption of carbohydrates, thus diminishing the postprandial surge in blood glucose levels without loss of calories. However, greater emphasis needs to be placed on the measurement of postprandial glycemia, so that readings can be used to guide treatment.

Drug therapy of postprandial hyperglycaemia

It is widely accepted that the most challenging goal in the management of patients with diabetes mellitus is to achieve blood glucose levels as close to normal as possible. In general, normalising postprandial blood glucose levels is more difficult than normalising fasting hyperglycaemia. In addition, some epidemiological studies suggest that postprandial hyperglycaemia (PPHG) or hyperinsulinaemia are independent risk factors for the development of macrovascular complications of diabetes mellitus. Recently, several drugs with differing pharmacodynamic profiles have been developed which target PPHG. These include insulin lispro, amylin analogues, alpha-glucosidase inhibitors and meglitinide analogues. Insulin lispro has a more rapid onset of action and shorter duration of efficacy compared with regular human insulin. In clinical trials, the use of insulin lispro was associated with improved control of PPHG and a reduced incidence of hypoglycaemic episodes. Repaglinide, a meglitinide analogue, is a short-acting insulinotropic agent which. when given before meals, stimulates endogenous insulin secretions and lowers postprandial hyperglycaemic excursions. Both insulin lispro and repaglinide are associated with postprandial hyperinsulinaemia. In contrast, amylin analogues reduce PPHG by slowing gastric emptying and delivery of nutrients to the absorbing surface of the gut. Alpha-Glucosidase inhibitors such as acarbose, miglitol and voglibose also reduce PPHG primarily by interfering with the carbohydrate-digesting enzymes and delaying glucose absorption. With the availability of agents which preferentially reduce postprandial blood glucose excursions, it is now possible to achieve glycaemic goals in a larger proportion of individuals with diabetes mellitus.

Incretin hormone expression in the gut of diabetic mice and rats

To elucidate the question of whether production of the insulinotropic gut hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) is altered by a diabetic metabolic state, their intestinal expression pattern was evaluated. Two rodent models for diabetes mellitus were used, non-obese diabetic (NOD) mice as a model for insulin-dependent diabetes and Zucker diabetic fatty (ZDF) rats for non-insulin-dependent diabetes mellitus (NIDDM). Expression of both incretin hormones followed typical patterns, which were similar in both animals and unaltered by the diabetic state. The GIP gene was greatly expressed in the duodenum, jejunum, and ileum, with a continuous decrease from the upper to lower intestines. This pattern was observed in both NOD mice and ZDF rats regardless of the diabetic state. This expression data was corroborated by radioimmunoassay (RIA) analysis of the gene product GIP. Expression of the proglucagon gene encoding GLP-1 had an opposite appearance. The highest expression was seen in the large bowel and the ileum. RIA analysis of the gene product GLP-1 mirrored these data. Although the distribution pattern was similar in both animal models, in contrast to diabetic NOD mice, a regulated expression was found in diabetic ZDF rats. Compared with lean nondiabetic controls, fatty hyperglycemic animals showed an increased expression of the proglucagon gene in the colon and a concomitant reduction in the small intestine. This was mirrored by the GLP-1 content of the colon and ileum. Overall, basal GLP-1 plasma levels were increased in ZDF rats (17.0 +/- 2.8 pmol) compared with lean Zucker rats (12.4 +/- 1.8 pmol). In conclusion, incretin hormone expression (GIP and GLP-1) follows specific patterns throughout the gut and is unaltered by the diabetic state. In ZDF rats, regulation of proglucagon expression occurs mainly in the large intestine.

Improvement of insulin sensitivity and dyslipidemia with a new alpha-glucosidase inhibitor, voglibose, in nondiabetic hyperinsulinemic subjects

This study was undertaken to investigate the effect of voglibose, a new alpha-glucosidase inhibitor, on glucose and lipid metabolism in nondiabetic hyperinsulinemic subjects. Sixteen nondiabetic subjects with hyperinsulinemia participated in the study. They were divided into two groups of eight subjects with normal (NGT) and impaired (IGT) glucose tolerance. A meal tolerance test and a 75-g oral glucose tolerance test (OGTT) were performed at the beginning (baseline phase) and end (treatment phase) of the 12-week treatment. Serum lipid levels were measured every 4 weeks throughout the treatment phase and follow-up phase (8 weeks). All patients received 1 0.2-mg tablet of voglibose before each test meal (3 tablets per day). We also measured insulin sensitivity using a steady-state plasma glucose (SSPG) method in eight normotensive hyperinsulinemic subjects and in eight age- and body mass index (BMI)-matched control subjects before and after the drug treatment. Voglibose significantly decreased the responses of plasma glucose and insulin on the meal tolerance test. The area under the curve for 2-hour insulin during the 75-g OGTT decreased after treatment, whereas that for 2-hour glucose did not change before and after treatment. SSPG was reduced after treatment, indicating improvement of insulin sensitivity. Moreover, treatment with voglibose resulted in a significant decline of triglyceride level and an elevation of high-density lipoprotein (HDL) cholesterol and apolipoprotein A-1. These values returned to near-baseline levels after the drug was discontinued. Consequently, we conclude that this agent not only has a direct hypoglycemic effect through decreased absorption of carbohydrate, but also a hypoinsulinemic and hypolipidemic effect via improved insulin sensitivity.

Pharmacokinetic-pharmacodynamic relationships of Acarbose

Acarbose represents a new pharmacological approach to achieving the metabolic benefits of a slower carbohydrate absorption in diabetes, by acting as a potent, competitive inhibitor of intestinal alpha-glucosidases. Acarbose molecules attach to the carbohydrate binding sites of alpha-glucosidases, with an affinity constant that is much higher than that of the normal substrate. Because of the reversible nature of the inhibitor-enzyme interaction, the conversion of oligosaccharides to monosaccharides is only delayed rather than completely blocked. Acarbose has the structural features of a tetrasaccharide and does not cross the enterocytes after ingestion. Thus, its pharmacokinetic properties are well suited to the pharmacological action directed exclusively towards the intestinal glucosidases. The most important clinical consequence of the delayed carbohydrate digestion caused by acarbose is the attenuation of postprandial increases in blood glucose levels. Other effects have also been described: a decreased beta-pancreatic response to meals, and influences on gut hormone secretion and plasma lipid levels. Gastrointestinal discomfort is frequently reported as an adverse effect of acarbose administration, but incidence usually decreases with time. The suitability of acarbose for improving glucose homeostasis as an adjunct to dietary control or to administration of sulphonylureas or insulin has been extensively studied in patients both with type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus. Acarbose can be used as first-line therapy in patients with type 2 diabetes which is poorly controlled by diet alone. Moreover, the lack of bodyweight gain or hypoglycaemic effects reported during acarbose treatment may be advantageous for obese or elderly patients. Finally, the reduction in fluctuations of glucose levels throughout the day may help to control type 1 diabetes in patients with 'brittle diabetes'. Long term prospective studies are still needed to confirm these indications and the usefulness of acarbose in conditions other than diabetes, notably reactive hypoglycaemia and dumping syndrome.

Galanin is a potent inhibitor of glucagon-like peptide-1 secretion from rat ileum

The insulinotropic glucagon-like peptide 1 (GLP-1) originates from the lower intestines. Surprisingly, food ingestion induces a rapid increase of GLP-1 plasma levels. Therefore, a complex regulation for postprandial GLP-1 secretion must exist, which cannot be solely explained by direct contact of nutrients in the gut lumen with the GLP-1-releasing L cells. This was addressed in the present study utilizing an isolated vascularly perfused rat ileum preparation. Cholinergic (methacholine) as well as peptidergic stimulation by glucose-dependent insulin-releasing polypeptide (synonym: gastric inhibitory polypeptide) (GIP) strongly enhanced GLP-1 secretion from the rat ileum. The stimulation of GLP-1 secretion by methacholine was abolished by addition of atropine and partly reduced by galanin. Galanin dose-dependently antagonized the stimulatory effect of GIP on GLP-1 release. Atropine was without effect. Furthermore, employing double immunohistochemistry labeling techniques galanin-immunoreactive nerves were detected in the vicinity of GLP-1-immunostained cells. Our data indicate that stimulatory and inhibitory mediators regulate GLP-1 secretion and that galanin is a likely inhibitor.

An alpha-glucosidase inhibitor, AO-128, retards carbohydrate absorption in rats and humans

The present study was designed to determine the possible significance of a therapeutic dose (0.2 mg) of AO-128 on carbohydrate absorption by measuring the breath hydrogen concentration, which is an index of the amount of unabsorbed carbohydrate in the large intestine. Post-prandial hyperglycemia is common among diabetic patients. AO-128, a potent alpha-glucosidase inhibitor, suppressed post-prandial hyperglycemia and hyperinsulinemia in healthy volunteers at a dose of 0.2 mg with each meal. These volunteers increased the breath hydrogen concentration in response to ingestion of non-absorbable lactulose, but decreased only slightly its concentration from the basal level after sucrose ingestion, indicating complete absorption. When AO-128 (0.2 mg) was given with sucrose, hydrogen production increased only slightly compared with placebo, suggesting that the inhibitory effect of AO-128 on sucrose absorption was minimal. Only 5 g of the 100 g of sucrose was not absorbed and this 5% reduction is too small to explain the observed inhibitory effect on the post-prandial rise in plasma glucose. Sucrose loading in rats (about 443 mg) sharply increased blood glucose and was accompanied by the rapid disappearance of sucrose from the upper small intestine. AO-128 (0.03 or 0.1 mg/kg) lessened the elevation of blood glucose after sucrose ingestion. The lower dose (0.03 mg/kg) retarded small intestinal absorption, but did not induce an influx of sucrose into the cecum and large intestine, while the higher dose (0.1 mg/kg) caused an increased influx of sucrose into the large bowel. These results indicated that AO-128 retards the absorption of carbohydrate and reduces post-prandial hyperglycemia.

Intestinal effects of alpha-glucosidase inhibitors: absorption of nutrients and enterohormonal changes

The present paper addresses the question how alpha-glucosidase inhibitors affect glucose homeostasis. To facilitate this already established data on the effects of induced malabsorption on gut hormones such as gastric inhibitory polypeptide (GIP) in connection with preliminary findings which deal with the new incretin hormone glucagon-like peptide 1 (7-36) amide (GLP-1) are discussed. To emphasize the possibly important impact of a regulated GLP-1 release in response to glucosidase inhibitor treatment we evaluate the recently introduced concept of 'glucose competence' of pancreatic beta-cells. The slowing of nutrient (i.e. glucose) absorption by therapeutic means (for example, acarbose) could supplement a new approach in the treatment of type 2 diabetics which would utilize the well-preserved insulinotropic activity of GLP-1 in these patients, its glucagon-lowering effect, and its possible inhibition of gastric emptying rates, the latter helping to reduce the requirement for rapid insulin secretory responses as is intended while using alpha-glucosidase inhibitor treatment.