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Journal of Clinical Endocrinology & Metabolism , doi:10.1210/jc.2005-2220
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The Journal of Clinical Endocrinology & Metabolism Vol. 91, No. 5 1916-1923
Copyright © 2006 by The Endocrine Society

Effects of Intravenous Glucagon-Like Peptide-1 on Gastric Emptying and Intragastric Distribution in Healthy Subjects: Relationships with Postprandial Glycemic and Insulinemic Responses

Tanya J. Little, Amelia N. Pilichiewicz, Antonietta Russo, Liza Phillips, Karen L. Jones, Michael A. Nauck, Judith Wishart, Michael Horowitz and Christine Feinle-Bisset

Department of Medicine (T.J.L., A.N.P., A.R., L.P., K.L.J., J.W., M.H., C.F.-B.), University of Adelaide, Royal Adelaide Hospital, Adelaide, South Australia 5000, Australia; and Diabeteszentrum (M.A.N.), 37431 Bad Lauterberg, Germany

Address all correspondence to: Christine Feinle-Bisset, Ph.D., National Health and Medical Research Council Senior Research Fellow, Department of Medicine, Royal Adelaide Hospital, North Terrace, Adelaide, South Australia 5000, Australia. E-mail: christine.feinle{at}adelaide.edu.au.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: The inhibitory action of glucagon-like peptide-1 (GLP-1) on gastric emptying (GE) is likely to be important in mediating its effects on postprandial glycemia, appetite, and gastrointestinal symptoms.

Objective: The objective of the study was to evaluate the effects of "low" and "high" doses of iv GLP-1 on GE, intragastric meal distribution, glycemia, insulinemia, and appetite.

Design: Ten healthy males were studied on 3 d. GE of a solid (ground beef)/liquid (glucose) meal, blood glucose, plasma insulin, glucagon and glucose-dependent insulinotropic peptide, appetite perceptions, and gastrointestinal symptoms were evaluated during iv infusion of: 1) GLP-1 at 0.3 pmol·kg–1·min–1 (GLP-1 0.3); 2) GLP-1 at 0.9 pmol·kg–1·min–1 (GLP-1 0.9); and 3) 0.9% saline.

Results: GLP-1 0.3 and 0.9 slowed GE of solid (intragastric retention at t = 100 min; saline: 28 ± 5%; GLP-1 0.3: 53 ± 6%; GLP-1 0.9: 58 ± 7%; P < 0.001) and liquid (time for 50% of the liquid to empty, saline: 28 ± 2 min; GLP-1 0.3: 42 ± 7 min; GLP-1 0.9: 50 ± 9 min; P < 0.001). Both doses of GLP-1 induced gastroparesis in about half the cohort and increased meal retention in the distal stomach (P < 0.05). GLP-1 attenuated the rises in glucose, insulin, and glucose-dependent insulinotropic peptide (P < 0.05). There was an inverse relationship between blood glucose at t = 15 min and the time for 50% of the liquid to empty (r = –0.70, P < 0.001).

Conclusions: In healthy subjects exogenous GLP-1 increases meal retention in the distal stomach and, even when administered in a "low" dose, frequently induces "gastroparesis," and the effects of GLP-1 on postprandial glycemia are predictable on the basis of its effect on GE, supporting the concept that GE is a major target mechanism for the clinical use of incretin mimetics.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
EXOGENOUS ADMINISTRATION OF glucagon-like peptide-1 (GLP-1) has diverse effects on gastrointestinal function in humans (1, 2, 3, 4). Intravenous GLP-1 slows gastric emptying (GE) (2, 5), and this is associated with relaxation of the proximal stomach (5, 6), inhibition of antral and duodenal motility (7), and stimulation of pyloric pressures (7). GLP-1 also appears to be a physiological modulator of gastric motility (1). Exogenous GLP-1 stimulates insulin and suppresses glucagon secretion, these effects being glucose dependent (8), and probably suppresses energy intake (4, 9). These actions of GLP-1 have stimulated interest in the potential for its use (10) and that of specific GLP-1 analogs and GLP-1 receptor agonists (incretin mimetics) to improve glycemic control in type 2 diabetes (11) and induce weight loss in obesity (4). Whereas there is increasing evidence that this approach is effective, the mechanisms by which incretin mimetics improve glycemia and suppress energy intake remain poorly defined.

The inhibitory effect of exogenous GLP-1 on GE has been demonstrated in healthy (2, 5), type 2 (3), and obese (12) subjects and has implications for an understanding of its potential therapeutic efficacy as well as upper gastrointestinal adverse effects (13). It is not known whether GLP-1 has the capacity to markedly slow GE of a mixed (solid/liquid) meal when administered at low doses. This is of importance, given the high prevalence of delayed GE in type 2 patients (14). It is known that there are fundamental differences in the regulatory mechanisms underlying gastric emptying of solids and liquids (14) and that the effects of pharmacological agents on gastric emptying may be dependent on meal composition (15). Exogenous GLP-1 at 0.3 pmol·kg–1·min–1 iv ("low dose") has been reported to result in physiological, whereas 0.9 pmol·kg–1·min–1 ("high dose") results in supraphysiological plasma concentrations (6). Infusion of GLP-1 in a dose of 1.2 pmol·kg–1·min–1 profoundly delays GE of a drink in type 2 patients (16). While lower doses of GLP-1 slow GE in healthy (2) and type 2 (3) subjects, the magnitude of this effect is uncertain, particularly because results were not compared with a control range. Although scintigraphy is the "gold standard" method (17, 18, 19), the majority of studies relating to the effects of GLP-1 on GE have used less than optimal techniques including dye dilution (2), radioisotopic breath tests (3), and measurement of paracetamol absorption (20); the effects of GLP-1 on GE of discrete solid and liquid meal components and intragastric meal distribution have not been evaluated.

The rate of entry of carbohydrate into the small intestine is a critical determinant of postprandial glycemia in both healthy subjects and type 2 diabetes (21, 22, 23, 24). Whereas exogenous GLP-1 may stimulate fasting insulin secretion, postprandial insulin levels are reduced, rather than increased, in healthy subjects (2) and type 2 patients (3). Reversal of the inhibitory effect of exogenous GLP-1 on GE by the gastrokinetic drug, erythromycin, attenuates its glucose-lowering effect (25). Hence, slowing of GE is likely to be the dominant mechanism by which exogenous GLP-1 improves postprandial glycemia, and GLP-1 may not be a "physiological" incretin (2). It is not known whether there is a direct relationship between postprandial blood glucose and insulin concentrations and the slowing of GE induced by GLP-1.

The effects of GLP-1 on GE and intragastric meal distribution have implications for an understanding of its capacity to induce gastrointestinal symptoms (13) and suppress energy intake (4). In healthy subjects the perception of postprandial fullness (26, 27, 28, 29) and energy intake (26) is more closely related to the content of the distal, than the total, stomach. There is no information about the relationship between the effects of GLP-1 on gastrointestinal symptoms and perceptions of appetite with those on GE and intragastric meal distribution.

The aims of this study were to determine the effects of "low" (0.3 pmol·kg–1·min–1) and "high" (0.9 pmol·kg–1·min–1) doses of GLP-1 on GE, intragastric distribution, glycemia and insulinemia, and appetite perceptions after ingestion of a solid/liquid meal. Carbohydrate was included only in the liquid component of the meal so that the relationship between glycemic responses and GE could be evaluated precisely.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Ten healthy males [mean age 27 ± 2 (range 19–44) yr, with body mass index 22 ± 0.8 (range 19–25) kg/m2] were studied. Subjects did not have any gastrointestinal disease or symptoms or any other illness. No subject was taking any medication known to affect gastrointestinal motility or appetite or consumed more than 10 cigarettes or more than 20 g of alcohol per day. The study protocol was approved by the Royal Adelaide Hospital Research Ethics Committee. All subjects provided informed, written consent before their enrollment.

Study design

Each subject was studied on three occasions, separated by 3–10 d. The effects of 150 min iv infusions of GLP-1 at 0.3 pmol·kg–1·min–1 (GLP-1 0.3), GLP-1 at 0.9 pmol·kg–1·min–1 (GLP-1 0.9) (Merck Biosciences, Läufelfingen, Switzerland), and 0.9% isotonic saline on GE, intragastric meal distribution, blood glucose, plasma insulin, glucagon and glucose-dependent insulinotropic peptide (GIP) concentrations, appetite perceptions, and gastrointestinal symptoms were evaluated in double-blind, randomized fashion.

Protocol

Subjects attended the Department of Nuclear Medicine, PET and Bone Densitometry, at 0845 h after an overnight fast from 2200 h from solids and liquids. An iv cannula was inserted into each forearm for the collection of blood samples and administration of iv infusions, respectively. Subjects were then seated with their back upright against a {gamma}-camera. The study protocol is summarized in Fig. 1Go. A baseline blood sample was taken (t = –45 min) and a visual analog scale questionnaire (VAS), assessing appetite perceptions and gastrointestinal symptoms, completed. After 15 min (i.e. t = –30 min), an iv infusion of GLP-1 0.3, GLP-1 0.9, or saline was commenced and maintained for 150 min (i.e. until t = 120 min). Twenty-five minutes later (t = –5 min), subjects completed a VAS and then consumed a meal over 5 min. The meal consisted of a 100-g ground beef patty (270 kcal, 25 g protein, 21 g fat) labeled with 15 MBq 99mTc-sulfur colloid chicken liver, followed by 150 ml of 10% dextrose labeled with 4 MBq 67Ga-EDTA (17). The time of meal completion was defined as t = 0 min. GE and intragastric distribution were then measured for 2 h (i.e. t = 0–120 min). Blood samples were collected for determination of blood glucose at t = –45, –30, –15, 0, 15, 30, 45, 60, 75, 90, and 120 min. Plasma insulin, GIP, and GLP-1 were measured on the blood samples obtained at t = –30, 0, 30, 60, 90, and 120 min and plasma glucagon at t = –30, 0, 30, and 60 min; VASs were completed at 15-min intervals.


Figure 1
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  		FIG. 1. Outline of the study protocol. At t = –30 min, an iv infusion of GLP-1 at 0.3 or 0.9 pmol·kg–1·min–1 or 0.9% saline was commenced and maintained for 150 min. At t = –5 min, subjects consumed a radioisotopically labeled mixed solid/liquid meal (100 g minced beef patty, 150 ml 10% dextrose). GE and intragastric meal distribution were measured between t = 0 and 120 min. Blood samples for analysis of blood glucose, plasma insulin, glucagon, GIP, and GLP-1 were collected and VAS questionnaires for the evaluation of appetite and gastrointestinal symptoms administered at each of the time points indicated.

 
Outcome measures

Gastric emptying. Radioisotopic data were acquired in 1-min frames for the first hour and in 3-min frames for the remaining 60 min. Data were corrected for subject movement, radionuclide decay, and {gamma}-ray attenuation (18). Regions-of-interest were drawn for total, proximal, and distal gastric regions and GE curves, expressed as percent retention over time, derived (18, 30). For both the solid and liquid meal components, the amount remaining in the total, proximal, and distal stomach between t = 0–120 min was derived at 15-min intervals. The lag phase for solid and liquid was determined as the time period between meal completion and the appearance of radioactivity in the proximal small intestine (18). The amount of solid remaining in the stomach at t = 100 min and the time for 50% of the liquid to empty (T50) were calculated (18). GE was classified as delayed when the solid percent retention at t = 100 min was more than 61% and/or the liquid T50 was more than 31 min, based on an established normal range (17).

Blood glucose and plasma insulin, glucagon, GIP, and GLP-1 concentrations

Venous blood glucose concentrations (millimoles per liter) were determined immediately using a glucose meter (Medisense Precision QLD, Abbott Laboratories, Bedford, MA). For measurement of plasma insulin, glucagon, GIP, and GLP-1 concentrations, blood samples (20 ml) were collected into ice-chilled EDTA-treated tubes containing 400 kIU aprotinin per milliliter blood (Trasylol; Bayer Australia Ltd., Pymble, Australia). Plasma was separated by centrifugation at 3200 rpm for 15 min at 4 C within 30 min of collection and stored at –70 C until assayed.

Plasma insulin (milliunits per liter) was measured by ELISA (Diagnostics Systems Laboratories Inc., Webster, TX). The sensitivity of the assay was 0.26 mU/liter, the intraassay and interassay coefficients of variation (CVs) were 2.6 and 6.2%, respectively (31). Plasma glucagon (picograms per milliliter) was measured by the Central Sydney Laboratory Service (Royal Prince Alfred Hospital, Sydney, New South Wales, Australia) using a RIA (Diagnostic Products Corp. Pty. Ltd., Los Angeles, CA). The minimum detectable limit was 13 pg/ml, and both the intraassay and interassay CVs were 15.7%. Plasma GIP was measured by RIA (23). The minimum detectable limit was 2 pmol/liter, and both the intraassay and interassay CVs were 15%. Total plasma GLP-1 (picomoles per liter) was measured by RIA (23). The intraassay CV was 17% and the interassay CV was 18%, with a sensitivity of 1.5 pmol/liter.

Appetite perceptions and gastrointestinal symptoms

Appetite perceptions (hunger and fullness) and gastrointestinal symptoms (nausea and bloating) were assessed using VAS. Each VAS evaluated a sensation on a 100-mm horizontal line, in which 0 mm represented sensation not felt at all and 100-mm sensation felt the greatest; subjects placed a vertical mark on the line.

Statistical analysis

GE, absolute blood glucose, plasma insulin, glucagon, GIP, and GLP-1 concentrations and VAS data were analyzed by repeated-measures ANOVA with time and treatment as factors. For plasma insulin, one subject was excluded due to high basal values. Areas under the curve (AUCs) were calculated (using the trapezoidal rule) for the magnitude of the rise in blood glucose and plasma insulin between t = 0–60 min and t = 0–120 min. The T50 of liquid and the percent retention of solid at t = 100 min were analyzed by one-way ANOVA. Post hoc paired comparisons, corrected for multiple comparisons by Bonferroni’s correction, were performed if ANOVAs revealed significant effects. Correlations, corrected for repeated measures, were determined among: 1) the solid percent retention at t = 100 min and the liquid T50; 2) the change in blood glucose at t = 15 min, the AUCs for the change in blood glucose and change in plasma insulin from t = 0–60 min and t = 0–120 min with the liquid T50; and 3) appetite perceptions and gastrointestinal symptoms with the amount of solid (milliliters) and liquid (milliliters) in the total, proximal, and distal stomach for pooled data (overall group) and for each treatment. Statistical significance was accepted at P < 0.05, and data are presented as means ± SEM.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The study protocol was tolerated well and no adverse effects were reported.

Gastric emptying: solid

Total stomach. There was no difference in the lag phase among the three treatments. The percent retention at t = 100 min was greater with GLP-1 0.3 and 0.9, compared with saline (P < 0.001, saline: 29 ± 5%, GLP-1 0.3: 53 ± 6%, GLP-1 0.9: 58 ± 7%), with no difference between the two doses of GLP-1. During saline, GE was within the normal range in all subjects; GE was delayed, as assessed by the percent retention at t = 100 min, with GLP-1 0.3 in five subjects and GLP-1 0.9 in four subjects. There was a treatment*time interaction for the amount of solid remaining in the total stomach (P < 0.001) (Fig. 2AGo). The intragastric retention of solid was greater during GLP-1 0.3 and 0.9, compared with saline between t = 45 and 120 min (P < 0.01), with no difference between the two doses of GLP-1. At t = 120 min intragastric retention was slightly greater for GLP-1 0.9 than GLP-1 0.3 (30.1 ± 5.4 vs. 22.1 ± 5.6%, P < 0.05).


Figure 2
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  		FIG. 2. GE curves for the solid and liquid components of the meal for the total, proximal, and distal stomach during iv infusion of GLP-1 at 0.3 and 0.9 pmol·kg–1·min–1 or 0.9% saline. Data are means ± SEM, n = 10. *, GLP-1 0.3 and 0.9 vs. saline: P < 0.05; #, GLP-1 0.9 vs. GLP-1 0.3: P < 0.05.

 
Proximal stomach. There was no difference in the amount of solid remaining in the proximal stomach during treatment with GLP-1 0.3 or 0.9, compared with saline (Fig. 2BGo).

Distal stomach. More of the solid was retained in the distal stomach during treatment with GLP-1 0.3 between t = 60 and 120 min and GLP-1 0.9 between t = 75 and 120 min (P < 0.001, for both), compared with saline, with no difference between the two doses of GLP-1 (Fig. 2CGo).

Liquid

Total stomach. There was no difference in the lag phase among the three treatments. The T50 was longer with GLP-1 0.3 and 0.9, compared with saline (P < 0.01, saline: 28 ± 2 min, GLP-1 0.3: 42 ± 7 min, GLP-1 0.9: 50 ± 9 min), with no difference between the two doses of GLP-1. During saline GE was normal in all subjects; GE was delayed, as assessed by the T50, with GLP-1 0.3 in six and GLP-1 0.9 in seven subjects. There was a treatment*time interaction for the amount of liquid remaining in the total stomach (P < 0.001) (Fig. 2DGo). The intragastric retention of liquid was greater during GLP-1 0.3 and 0.9 between t = 15 and 120 min, compared with saline (P < 0.05), with the effect of GLP-1 0.9 being greater than that of GLP-1 0.3 between t = 30 and 120 min (P < 0.01). At t = 120 min, intragastric retention was greater with GLP-1 0.9 than GLP-1 0.3 (15.4 ± 4.4 vs. 7.0 ± 3.0%, P < 0.05).

Proximal stomach. GLP-1 0.3 and 0.9 increased the amount of liquid remaining in the proximal stomach between t = 15 and 75 min, compared with saline (P < 0.05), with the effect of GLP-1 0.9 being greater than that of GLP-1 0.3 between t = 15 and 60 min (P < 0.05) (Fig. 2EGo).

Distal stomach. GLP-1 0.3 and 0.9 increased the amount of liquid in the distal stomach, compared with saline (P < 0.05), with no difference between the two doses of GLP-1 (Fig. 2FGo).

Relationship between solid and liquid GE

There was a relationship between solid and liquid GE [i.e. solid percent retention at t = 100 min vs. liquid T50 (min)] in the overall group (r = 0.66, P < 0.001) and with GLP-1 0.3 (r = 0.81, P < 0.01), but not with GLP-1 0.9 or saline.

Blood glucose and plasma hormone responses

Blood glucose. There was no difference in baseline blood glucose among the 3 study days. There was a treatment*time interaction for blood glucose concentrations (P = 0.001) (Fig. 3Go). GLP-1 0.3 (P < 0.01) and 0.9 (P < 0.05) slightly decreased fasting blood glucose concentrations at t = –15 min, compared with saline (saline: 5.5 ± 0.2 mmol/liter, GLP-1 0.3: 5.1 ± 0.2 mmol/liter, GLP-1 0.9: 5.0 ± 0.1 mmol/liter). Whereas glucose concentrations increased with all treatments (P < 0.05), GLP-1 0.3 attenuated the postprandial rise between t = 15 and 45 min and at t = 105 min and GLP-1 0.9 from t = 15 to 30 min, compared with saline (P < 0.05 for all), with the effect of GLP-1 0.9 being greater than GLP-1 0.3 at t = 30 min (P < 0.01). The AUC for the change in blood glucose between t = 0 and 60 min was less (P < 0.05), with GLP-1 0.3 and 0.9, compared with saline (P < 0.05), with no difference between the two doses of GLP-1. Peak blood glucose was greater for saline (7.9 ± 0.2 mmol/liter) than GLP-1 0.3 (6.7 ± 0.3 mmol/liter) and GLP-1 0.9 (6.3 ± 0.2 mmol/liter). There was a trend for the AUC for the change in blood glucose between t = 0 and 120 min to be less during infusion of GLP-1 0.3 (P = 0.08) but not GLP-1 0.9 (P = 0.87), compared with saline.


Figure 3
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  		FIG. 3. Blood glucose (A), plasma insulin (B), and plasma GIP (C) concentrations during iv infusion of GLP-1, at 0.3 and 0.9 pmol·kg–1·min–1, or 0.9% saline. Data are means ± SEM, n = 10. *, GLP-1 0.3 and 0.9 vs. saline: P < 0.05; #, GLP-1 0.9 vs. GLP-1 0.3: P < 0.05.

 
Plasma insulin

There was a treatment*time interaction for plasma insulin (P < 0.05) (Fig. 3BGo). Plasma insulin concentrations at t = –30 min (saline: 12.5 ± 3.7 mU/liter; GLP-1 0.3: 11.9 ± 3.0 mU/liter; GLP-1 0.9: 11.4 ± 2.7 mU/liter) and t = 0 min (saline: 13.3 ± 4.0 mU/liter; GLP-1 0.3: 12.8 ± 2.7 mU/liter; GLP-1 0.9: 13.4 ± 3.1 mU/liter) were not different. Postprandial plasma insulin concentrations were less with GLP-1 0.3 and 0.9 at t = 30 min (P < 0.05), with the effect of GLP-1 0.9 being greater than that of GLP-1 0.3 (P < 0.05). The AUC for the change in plasma insulin between t = 0 and 60 min was less (P < 0.05) with GLP-1 0.9, compared with GLP-1 0.3 and saline (P < 0.01). There was a trend for the AUC for the change in plasma insulin between t = 0 and 120 min to be less during infusion of GLP-1 0.9 (P = 0.11) but not GLP-1 0.3 (P = 0.30), compared with saline.

Glucagon

There was no effect of treatment, or time, on fasting (saline: 82.5 ± 4.4 pg/ml; GLP-1 0.3: 75.7 ± 6.9 pg/ml; GLP-1 0.9: 84.5 ± 6.3 pg/ml) or mean postprandial (saline: 80.4 ± 7.0 pg/ml; GLP-1 0.3: 83.9 ± 6.1 pg/ml; GLP-1 0.9: 91.2 ± 7.8 pg/ml) plasma glucagon concentrations.

GIP

There was no effect of treatment on fasting GIP concentrations. Whereas GIP increased after the meal with all treatments (P < 0.001), the rise at t = 60 min was less with GLP-1 0.3 (P < 0.01) and tended to be less with GLP-1 0.9 (P = 0.09), compared with saline (Fig. 3CGo).

GLP-1

In response to iv GLP-1, plasma concentrations increased to reach steady-state levels after approximately 30 min, which were greater with GLP-1 0.9 than GLP-1 0.3 (P < 0.001) (Fig. 4Go). Peak GLP-1 concentrations during infusion of GLP-1 0.3 and 0.9 were 25.3 ± 5.3 and 43.8 ± 8.4 pmol/liter, respectively (P < 0.001). There was no relationship between GE of solid or liquid with the AUC of plasma GLP-1 from t = 0 to 120 min (data not shown).


Figure 4
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  		FIG. 4. Plasma GLP-1 concentrations during iv infusion of GLP-1, at 0.3 and 0.9 pmol·kg–1·min–1, or 0.9% saline. Data are means ± SEM, n = 10. *, GLP-1 0.3 and 0.9 vs. saline: P < 0.05; #, GLP-1 0.9 vs. GLP-1 0.3: P < 0.05.

 
Relationship among GE, blood glucose, and plasma insulin

There was an inverse relationship between the magnitude of the postprandial rise in blood glucose and GE (T50) of liquid (i.e. the carbohydrate component of the meal). The increase in blood glucose between t = 0 and 15 min was related to the T50 during saline (r = –0.73, P < 0.01), GLP-1 0.3 (r = –0.63, P < 0.05), and GLP-1 0.9 (r = –0.69, P < 0.05) infusions as well as overall (r = –0.70, P < 0.001) (Fig. 5AGo). The AUC for the change in blood glucose from t = 0 to 60 min was also related to the liquid T50 (r = –0.46, P < 0.01) in the overall group. There was no significant relationship between the AUC for the change in blood glucose from t = 0 to 120 min with the liquid T50.


Figure 5
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  		FIG. 5. Relationships among the increase from baseline (t = 0 min) in blood glucose at t = 15 min and GE (T50) (A), incremental plasma insulin (AUC of the change in plasma insulin from t = 0 to 60 min) and GE (T50) (B), and incremental plasma insulin (AUC of the change in plasma insulin from t = 0 to 60 min) and incremental blood glucose (AUC of the change in blood glucose from t = 0 to 60 min) (C).

 
There were significant relationships between the magnitude of the postprandial rise in plasma insulin with both GE of liquid (T50) and blood glucose. The AUC for the change in plasma insulin from t = 0 to 60 min was inversely related to the T50 (r = –0.42, P < 0.05) (Fig. 5BGo) and directly related to the AUC for the change in blood glucose from t = 0 to 60 min during GLP-1 0.3 (r = 0.70, P < 0.05) as well as overall (r = 0.50, P < 0.01) but not during GLP-1 0.9 or saline (Fig. 5CGo). There was no significant relationship between the AUC for the change in plasma insulin from t = 0 to 120 min with the liquid T50 (r = 0.31, P = 0.12).

Appetite perceptions and gastrointestinal symptoms

There was no effect of treatment on appetite, i.e. hunger and fullness, or gastrointestinal symptoms, i.e. nausea and bloating (Fig. 6Go). There was an effect of time on hunger and fullness (P < 0.05). Hunger increased before (P < 0.05) and decreased (P < 0.05) after the meal. Fullness did not change before meal ingestion; after the meal fullness increased (P < 0.05) and then progressively decreased over time. There was no effect of meal ingestion on nausea or bloating.


Figure 6
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  		FIG. 6. Perceptions of hunger (A), fullness (B), nausea (C), and bloating (D) during infusion of GLP-1, at 0.3 and 0.9 pmol·kg–1·min–1, or 0.9% saline. Data are means ± SEM, n = 10.

 
Relationship between appetite perceptions and gastrointestinal symptoms with intragastric meal distribution

There was no significant relationship among hunger, fullness, or nausea with the amount of solid or liquid remaining in the total, proximal, or distal stomach. At t = 105 min, there was a relationship between bloating and the amount of solid and liquid in the total (r = 0.46, P < 0.01 and r = 0.40, P < 0.05, respectively) and distal stomach (r = 0.41, P < 0.05 and r = 0.43, P < 0.05, respectively). There was no relationship between bloating and the amount of solid or liquid in the proximal stomach.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Our study provides novel insights into the effects of exogenous GLP-1 on GE, which have relevance to an understanding of its glycemic and appetite-suppressant properties. In particular: 1) the magnitude of the slowing of solid and liquid GE by GLP-1 was sufficient to induce "gastroparesis" in a substantial proportion of subjects, even when administered in a "low" dose, and was associated with increased meal retention in the distal stomach; and 2) the suppressive effect of GLP-1 on postprandial glycemic and insulinemic responses could be predicted by the rate of GE of carbohydrate. It is implicit that our observations also have implications for the use of incretin mimetics, such as liraglutide and exenatide (32).

The magnitude of the observed slowing of GE by exogenous GLP-1 was substantial: gastroparesis was induced in more than 50% of the subjects by "low"-dose GLP-1. While there was a significant relationship between solid and liquid emptying, this was not strong, as noted previously (18), presumably reflecting differences in the underlying motor mechanisms (33). The prevalence of abnormally delayed GE in longstanding type 2 diabetes is 30–50% (14), and in such patients further slowing of GE has the potential to induce, or exacerbate, gastrointestinal symptoms as well as affect oral drug absorption. Accordingly, the acute and chronic effects of GLP-1 on GE in type 2 patients, with and without gastroparesis, warrant investigation. The inhibitory effect of GLP-1 on GE may be mediated by vagal mechanisms (10, 34, 35), and it is possible that the effect of GLP-1 to slow GE will be attenuated in diabetic patients who have autonomic neuropathy (35).

The reduction in fasting blood glucose concentrations by GLP-1 is well documented (2), as is the attenuation of the postprandial rise (2, 25). Because we, and others (25), have failed to observe a stimulatory effect of GLP-1 on fasting plasma insulin concentrations in healthy subjects studied during euglycemia, it appears unlikely that the reduction in preprandial glycemia is accounted for by an insulinotropic effect of GLP-1. An inhibitory effect of exogenous GLP-1 on fasting and postprandial plasma glucagon concentrations, although not evident in the current study, has been well established (2).

The relative importance of the mechanisms by which exogenous GLP-1 reduces postprandial glycemia remains controversial, and there is ongoing debate as to whether GLP-1 has a physiological role as an incretin hormone (2, 25, 36). The postprandial improvement in glycemia occurred despite diminished insulin secretion, as noted previously (2), and recent observations indicate that the inhibitory effect of GLP-1 on GE represents the dominant mechanism mediating the improvement in postprandial glycemia (2, 25). Our novel observation that there is a direct relationship between postprandial glycemic and insulinemic responses with the slowing of GE of carbohydrate by GLP-1 adds to this. The decreased postprandial GIP response during infusion of GLP-1 (25), which probably reflects the slowing of GE resulting in reduced glucose-mediated GIP secretion from the proximal small intestine, may contribute to the reduction in insulinemia. In studies using the specific GLP-1 receptor antagonist, exendin (9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39), to examine the role of endogenous GLP-1 in postprandial glycemia in healthy humans (1, 37), GE was not quantified. Edwards et al. (37) reported that exendin (9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39) impaired the glycemic response to oral glucose, which was also associated with an increase in plasma insulin, and postulated that this reflected more rapid GE. The improvement in postprandial glycemia induced by GLP-1 may also potentially reflect an effect on glucose absorption, which has, to our knowledge, not been evaluated, although GLP-1 modulates small intestinal motility (36, 38).

Exogenous GLP-1 has been reported to suppress appetite and energy intake in a number of studies (4), and intragastric mechanisms are known to be important in the regulation of appetite and energy intake in humans (26). While proximal stomach distension increases the perception of fullness (39), antral distension may play a more important role (26, 28, 29). Our observation that the perception of bloating was related to the amount of the meal retained in the distal, but not the proximal, stomach is, accordingly, of interest. It should, however, be recognized that our study design was less than optimal to evaluate the effects of GLP-1 on appetite and gastrointestinal symptoms, and this was a secondary aim. Limitations include: 1) the relatively small number of subjects, compared with previous studies in which an inhibitory effect of GLP-1 on appetite was observed (4); 2) the choice of a test meal of small volume [given that it was designed to quantify GE in patients with and without gastroparesis (17)] so that postprandial changes in hunger and fullness were modest; and 3) the absence of gastrointestinal adverse effects.

In conclusion, the slowing of GE of a mixed solid/liquid meal by GLP-1 is a major determinant of its effects on postprandial glycemia and insulinemia in healthy subjects, and the magnitude of the slowing of GE, even by a low dose of GLP-1, is substantial and associated with a relative increase in the content of the distal stomach.


   Acknowledgments
 
The authors thank staff of the Department of Nuclear Medicine, PET and Bone Densitometry, Royal Adelaide Hospital, for providing radiopharmaceuticals and {gamma}-camera time.


   Footnotes
 
No reprints will be available.

This work was supported by the Australian-German Joint Research Cooperation Scheme from the University of Adelaide (to C.F.-B. and M.H.) and a Project Grant provided by the National Health and Medical Research Council of Australia (NHMRC). T.J.L. is supported by a Postgraduate Research Scholarship from the University of Adelaide, A.N.P. by a Dawes Postgraduate Research Scholarship from the Royal Adelaide Hospital, C.F.-B. by a Career Development Award from the NHMRC, and K.L.J. by a fellowship jointly awarded by Diabetes Australia and the NHMRC.

T.J.L., A.N.P., A.R., L.P., K.L.J., J.W., and C.F.-B. have nothing to declare. M.A.N. consults for Bayer Vital Pharma, Berlin Chemie/Menarini, ConjuChem, Eli-Lilly Pty. Ltd., Hoffman La Roche, Merck, MSD, Novartis Pharma, NovoNordisk, Pfizer, Probiodrug, Restoragen (BioNebraska), and Sanofi-Aventis Pharma, and has received grant support from Amylin Pharmaceuticals (2003), Sanofi-Aventis Pharma (2001), Bayer Vital Pharma (2002), Novartis Pharma (2002), NovoNordisk (2002), Probiodrug (2000), and Restoragen (BioNebraska) (2003). M.H. consults for Eli-Lilly Pty. Ltd. and Amylin Pharmaceuticals.

First Published Online February 21, 2006

Abbreviations: AUC, Area under the curve; CV, coefficient of variation; GE, gastric emptying; GIP, glucose-dependent insulinotropic peptide; GLP-1, glucagon-like peptide-1; T50, time for 50% of the liquid to empty; VAS, visual analog scale.

Received October 7, 2005.

Accepted February 15, 2006.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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