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Keywords:

  • 3D ultrasound;
  • gastric emptying;
  • l-NAME;
  • motility;
  • nitric oxide

Abstract

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References

Abstract  The aim of this study was to determine whether the nitric oxide (NO) synthase inhibitor, Ng-nitro-l-arginine-methyl-ester (l-NAME), reverses the effects of acute hyperglycaemia on gastric emptying and antropyloroduodenal (APD) motility. The study had a four-way randomized crossover (hyperglycaemia vs euglycaemia; l-NAME vs placebo) design in a clinical laboratory setting. Seven healthy volunteers [four males; age 30.3 ± 3.8 years; body mass index (BMI) 23.6 ± 1.2 kg m−2] were the study subjects. After positioning a transnasal manometry catheter across the pylorus, the blood glucose concentration was maintained at either 15 or 5 mmol L−1 using a glucose/insulin clamp. An intravenous infusion of l-NAME (180 μg kg−1 h−1) or placebo (0.9% saline) was commenced (T = −30 min) and continued for 150 min. At T = −2 min, subjects ingested a drink containing 50 g of glucose made up to 300 mL with water. Gastric emptying was measured using 3D ultrasound, and APD motility using manometry. Hyperglycaemia slowed gastric emptying (< 0.05), and this effect was abolished by l-NAME. l-NAME had no effect on gastric emptying during euglycaemia. Hyperglycaemia suppressed fasting antral motility [motility index: 3.9 ± 0.8 (hyperglycaemia) vs 6.5 ± 0.6 (euglycaemia); < 0.01]; l-NAME suppressed postprandial antral motility [motility index: 3.6 ± 0.2 (l-NAME) vs 5.1 ± 0.2 (placebo); < 0.001]. Postprandial basal pyloric pressure was higher during hyperglycaemia (< 0.001), and lower after administration of l-NAME (< 0.001). Slowing of gastric emptying induced by hyperglycaemia is mediated by NO, and may involve the modulation of tonic pyloric activity.

Although delayed gastric emptying occurs in 30–50% of outpatients with longstanding type 1 and type 2 diabetes,1 its pathogenesis is poorly understood. Acute changes in the blood glucose concentration have major, reversible, effects on gastrointestinal motor function in both healthy individuals and patients with diabetes.2–5 Hyperglycaemia is associated with slowing of gastric emptying,2,3,5 reduced proximal gastric tone,4,6 inhibition of antral pressure waves,7,8 and stimulation of tonic and phasic pyloric contractions.9 However, the mechanisms mediating these effects are unknown.

Nitric oxide (NO) is a major inhibitory neurotransmitter in the gastrointestinal tract, and appears to act as the final common pathway mediating enteric smooth muscle relaxation.10 In healthy subjects, an increase in NO availability has been reported to slow gastric emptying.11–13 There is, however, no information as to whether nitrergic mechanisms mediate the effects of hyperglycaemia on gastric emptying and antropyloroduodenal motility.

We aimed to determine the effects of the NO synthase inhibitor, Ng-nitro-l-arginine-methyl-ester (l-NAME), on the delay in gastric emptying and changes in antropyloroduodenal motility, associated with hyperglycaemia in healthy humans. As glucose-induced insulin secretion is influenced by NO availability in vitro and in animal models,14–16 we also evaluated the effects of l-NAME on the secretion of insulin and the incretin hormone glucose-dependent insulinotropic polypeptide (GIP).

Materials and methods

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References

Subjects

Seven healthy volunteers [four males; age 30.3 ± 3.8 years; body mass index (BMI) 23.6 ± 1.2 kg m−2 (mean ± SEM)] were recruited by advertisement. No subject was taking medication known to affect gastrointestinal function. The study protocol was approved by the Human Research Ethics Committee of the Royal Adelaide Hospital. Written, informed consent was obtained from each participant. All experiments were carried out in accordance with the Declaration of Helsinki.

Protocol

Each subject was studied on four occasions, each separated by at least 3 days, in a single-blinded, randomized cross-over design [euglycaemia (5 mmol L−1) vs hyperglycaemia (15 mmol L−1); l-NAME vs placebo]. Subjects attended the laboratory at 0830 h after an overnight fast (12 h for solids, 10 h for liquids). A manometric catheter (Dentsleeve International Ltd, Mui Scientific, Mississauga, Ontario, Canada) was inserted transnasally into the stomach and allowed to pass into the duodenum by peristalsis. The correct position of the catheter was determined by measurement of transmucosal potential difference (TMPD), monitored continuously throughout the study, using established criteria.17 The manometric catheter incorporated four antral sideholes (at 1.5 cm intervals), seven duodenal sideholes (3 cm intervals) and a 4.5-cm pyloric sleeve sensor. The most distal antral and most proximal duodenal sideholes were perfused with saline (0.9%) to measure TMPD.18 All other channels were perfused with degassed water.

Once the catheter was positioned correctly, 100 mL of either 25% dextrose (Baxter Healthcare Pty. Ltd, Old Toongabbie, NSW, Australia) or 0.9% saline (Baxter Healthcare Pty. Ltd.) was infused intravenously over 2 min, followed by an infusion of the same solution commencing at 150 mL h−1, adjusted every 5 min according to the glucometer readings (Medisense Optium; Medisense Inc., Waltham, MA, USA) on the ‘hyperglycaemic’ days, or remaining at 150 mL h−1 on the ‘euglycaemic’ days, for the remainder of the study. In addition, insulin (Actrapid Penfill; Novo Nordisk Pharmaceuticals Pty. Ltd, Baulkham Hills, NSW, Australia) in 4% gelatin solution (Gelofusine; B. Braun Australia Pty. Ltd, Bella Vista, NSW, Australia), made up to 0.2 IU mL−1, was infused intravenously according to a sliding scale.19

Once the blood glucose concentration had been stabilized for 30 min, and during phase II activity of the migrating motor complex (MMC), an intravenous infusion of either l-NAME (180 μg kg−1 h−1) (Clinalfa, Merck Pty Ltd, Kilsyth, VIC, Australia) or placebo (0.9% saline) was commenced (T = −30 min) and continued for 150 min (i.e. until T = 120 min).20 At T = −2 min, subjects consumed a drink containing 50 g glucose monohydrate (170 kcal), made up to 300 mL with water, over 2 min.

Gastric emptying was quantified by three-dimensional (3D) ultrasound:21 images of the stomach were acquired at T = −30, −15, −2 (immediately before the glucose drink), 0 (immediately after the drink), 15, 30, 45, 60, 90, 120 and 150 min, while antropyloroduodenal motility was recorded between T = −30 and 150 min.

Venous blood was sampled every 15 min from T = −30 to 60 min, and then at 30-min intervals until T = 150 min, for measurement of blood glucose, plasma insulin and plasma GIP concentrations. Blood pressure and heart rate were recorded every 15 min, using an automated device (DINAMAP ProCare 100; GE Medical Systems, Milwaukee, WI, USA), and mean values calculated for the entire study duration.

Measurements

Gastric emptying and intragastric distribution  Images of the stomach were acquired using a LogiqTM9 Ultrasound System (GE Healthcare Technologies, Milwaukee, WI, USA) with Truscan Architecture.22 The 3D positioning and orientation measurement (POM) was performed with a magnetic sensor.22 Scans were performed in the sitting position, during breath-holding in inspiration. Images were later transferred to a Microsoft Windows workstation, and processed using the EchoPAC3D software® (GE Vingmed Sound, Horten, Norway).22

Changes in gastric volume over time were used to generate total gastric emptying curves, expressed as percentage retention relative to the initial postprandial volume at T = 0 min. The stomach was separated into proximal and distal portions by a sagittal plane joining the incisura to the greater curvature.23 Changes in proximal and distal gastric volumes over time were calculated as percentages of the initial postprandial total gastric volume.

Manometric analysis  Manometric signals were recorded using commercially available software [Flexisoft, Version 3; Assoc. Prof. G.S. Hebbard, Royal Melbourne Hospital, Melbourne, Australia, written in LabVIEW 3.1.1 (National Instruments Australia, North Ryde, NSW, Australia)].24 Manometric data were analysed using established software (Professor A.J. Smout, Utrecht, The Netherlands)24 to calculate the: (a) number of antral pressure waves ≥10 mmHg; (b) basal pyloric pressure; (c) number of duodenal pressure waves ≥10 mmHg; (d) number of propagated antral and duodenal wave sequences; and (e) antral and duodenal wave amplitudes,24 during 10-min periods between T = −30 and T = 150 min. The number of isolated pyloric pressure waves (IPPWs) per 10 min was determined visually, according to established criteria.25 Propagated wave sequences were further divided into those that propagated over short (<4.5 cm) and long (≥4.5 cm) distances, and these were analysed separately.5 Antral and duodenal motility indices were calculated using the formula:5

  • image

Blood glucose, plasma insulin, and GIP concentrations  Blood glucose concentrations were determined immediately using a portable glucometer (MediSense Optium, MediSense Inc., Waltham, MA, USA). Blood samples for hormone measurements were collected in ice-chilled tubes containing ethylenediaminetetraacetic acid (EDTA) and 400 kIU aprotinin (Trasylol; Bayer Australia, Pymble, Australia) per litre of blood. Plasma was separated by centrifugation (1488 g, 15 min, 4 °C) and stored at −70 °C for subsequent analysis. Plasma GIP was measured by radioimmunoassay,26 and insulin by enzyme-linked immunosorbent assay (ELISA; Diagnostics Systems Laboratories Inc., Webster, TX, USA).27

Statistical analysis

Data were evaluated by repeated-measures analysis of variance (anova) with ‘glycaemic state’ (euglycaemia or hyperglycaemia), ‘treatment’ (l-NAME or placebo), and ‘time’ as within-subject factors. Post hoc comparison of mean values was performed for individual time points in the event of significant interactions between either ‘glycaemic state’ or ‘treatment’ and ‘time’. The presence of a significant interaction between ‘glycaemic state’ and ‘treatment’ indicates a differential effect of l-NAME depending on ‘glycaemic state’. In the event of no interaction, a significant effect of either ‘glycaemic state’ or ‘treatment’ indicated an overall difference between hyerglycaemia and euglycaemia, or l-NAME and placebo, averaged over both conditions of the other factor. In such case, a single mean value for each condition was obtained. A statistical software package (SPSS 15.0; SPSS Inc, Chicago, IL, USA) was used for all analyses. Statistical significance was accepted at < 0.05, and data are presented as mean values ± standard error of the mean (SEM). The primary endpoint was the rate of emptying from the total stomach. Subject numbers were based on power calculations performed in similar studies.5,22

Results

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References

All subjects tolerated the study well. The volume of glucose or saline solutions infused intravenously was greater on the hyperglycaemic, compared with euglycaemic, days (< 0.05), but did not differ between the two hyperglycaemic, or between the two euglycaemic, days [945.3 ± 116.9 mL (hyperglycaemia/l-NAME), 864.0 ± 150.2 mL (hyperglycaemia/placebo), 479.8 ± 10.2 mL (euglycaemia/l-NAME), 485.0 ± 12.0 mL (euglycaemia/placebo)].

Blood pressure and heart rate

Systolic blood pressure was not affected by either l-NAME or hyperglycaemia. However, l-NAME was associated with a modest increase in the mean diastolic blood pressure, not specific to any ‘glycaemic state’ [65.6 ± 1.8 mmHg (l-NAME) vs 62.8 ± 1.7 mmHg (placebo); < 0.05], and hyperglycaemia was associated with a slight decrease in the mean diastolic blood pressure, not specific to any ‘treatment’ [63.5 ± 1.7 mmHg (hyperglycaemia) vs 64.9 ± 1.6 mmHg (euglycaemia); < 0.05]. The mean heart rate was lower with l-NAME [63.0 ± 6.0 bpm (l-NAME) vs 69.0 ± 5.7 bpm (placebo); < 0.01], but was unaffected by glycaemic state.

Gastric emptying and intragastric distribution

Total gastric emptying  Gastric emptying on the hyperglycaemia/placebo day was slower compared with the remaining 3 days (< 0.05). l-NAME abolished the effect of hyperglycaemia on gastric emptying, so that on the hyperglycaemia/l-NAME day, gastric emptying was not different from that of the euglycaemic days. There was an interaction between ‘glycaemic state’ and ‘treatment’ (< 0.05), i.e. l-NAME affected gastric emptying during hyperglycaemia (< 0.05) and not during euglycaemia (= N.S.) (Fig. 1).

image

Figure 1.  (A) Total gastric emptying, (B) proximal retention, and (C) distal retention, expressed as a percentage of the total gastric volume immediately after the glucose drink (T = 0 min). l-NAME was associated with an acceleration of total gastric emptying and a reduction in proximal gastric retention only during hyperglycaemia, but had no effect during euglycaemia. *Time points that are significantly different between the hyperglycaemia/placebo day and each of the remaining 3 days. Data are mean ± SEM (= 7).

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Proximal gastric retention  During hyperglycaemia, proximal retention was less with l-NAME than placebo (< 0.05), while l-NAME had no effect on proximal retention during euglycaemia (Fig. 1). The overall emptying patterns from the proximal stomach resembled those from the total stomach.

Distal gastric retention  There was no effect of either ‘glycaemic state’ or ‘treatment’ on retention in the distal stomach (Fig. 1).

Antral pressure waves

Fasting  The number [10.1 ± 3.7 (hyperglycaemia) vs 21.6 ± 3.8 (euglycaemia); = 0.01], mean amplitude [35.7 ± 10.4 mmHg (hyperglycaemia) vs 67.2 ± 9.8 mmHg (euglycaemia); < 0.05] and mean motility index (MI) [3.9 ± 0.8 (hyperglycaemia) vs 6.5 ± 0.6 (euglycaemia); < 0.01] of antral waves were less during hyperglycaemia than euglycaemia (Fig. 2). Propagated wave sequences were suppressed during hyperglycaemia (7.6 ± 3.3 compared with 17.3 ± 4.0; < 0.05), particularly over short distances (<4.5 cm, < 0.01; ≥4.5 cm, = 0.09) (Fig. 2).5l-NAME had no effect on fasting antral motility.

image

Figure 2.  Effects of hyperglycaemia on fasting antral motility. Number of waves is the mean of the number per 10 min, ±SEM, between T = −30 min and T = 0 min. Propagated sequences, amplitude and motility index are the mean values ±SEM, between T = −30 min and T = 0 min (= 7).

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Postprandial  After the drink, antral motility was suppressed on all study days, when compared with fasting. There were fewer short propagated antral sequences during hyperglycaemia (11.3 ± 1.8) than euglycaemia (34.3 ± 7.3) (< 0.05), but no other effect of hyperglycaemia was observed on antral waves. l-NAME, compared with placebo, was associated with the following effects: fewer antral waves (10.8 ± 2.9 vs 18.5 ± 2.9; = 0.05), a reduction in amplitude (23.5 ± 2.0 mmHg vs 32.8 ± 2.6 mmHg; < 0.01) and motility index (3.6 ± 0.2 mmHg vs 5.1 ± 0.2 mmHg; < 0.001), and fewer propagated antral sequences (32.6 ± 11.5 vs 67.2 ± 11.6; < 0.05), particularly over long distances (14.1 ± 5.9 vs 40.1 ± 9.2; < 0.05) (Fig. 3). These were overall effects not specific to any ‘glycaemic state’.

image

Figure 3.  Effects of l-NAME on postprandial antral motility. Number of waves is the mean of the number per 10 min, ±SEM, between T = 0 min and T = 150 min. Propagated sequences, amplitude and motility index are the mean values ± SEM, between T = 0 min and T = 150 min (= 7).

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Duodenal pressure waves

Fasting  There was no effect of ‘glycaemic state’ on duodenal motility. l-NAME was associated with a small reduction in the duodenal motility index (8.2 ± 0.2), compared with placebo (8.8 ± 0.2) (< 0.01).

Postprandial  The mean amplitude of duodenal waves was slightly greater during hyperglycaemia (29.5 ± 1.0 mmHg) than euglycaemia (26.4 ± 0.5 mmHg) (< 0.01), although the number of waves and propagated sequences, and the motility index, did not differ. l-NAME was associated with a reduction in long propagated sequences (≥4.5 cm) only during hyperglycaemia [31.7 ± 9.6 (l-NAME) vs 127.4 ± 25.9 (placebo); < 0.01], but had no other effect on postprandial duodenal motility.

Isolated pyloric pressure waves and basal pyloric pressures

Fasting  During fasting, the number of isolated pyloric pressure waves and the basal pyloric pressure did not differ with ‘glycaemic state’ or ‘treatment’ (Fig. 4).

image

Figure 4.  Basal pyloric pressure. Values are the mean over either the entire fasting (T = −30 min to T = 0 min), or the entire postprandial (T = 0 min to T = 150 min), period. Data are mean ± SEM (= 7).

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Postprandial  The number of isolated pyloric pressure waves was not different between the four study days. Basal pyloric pressure was higher during hyperglycaemia (4.5 ± 0.4 mmHg) than euglycaemia (1.9 ± 0.6 mmHg) (< 0.001), and was less during l-NAME administration (1.6 ± 0.6 mmHg) than placebo (4.8 ± 0.4 mmHg) (< 0.001), not specific to any ‘glycaemic state’ (Fig. 4).

Plasma insulin and GIP concentrations

Insulin  During fasting, plasma insulin levels were higher during hyperglycaemia (75.6 ± 15.7 IU), compared with euglycaemia (16.1 ± 1.8 IU) (= 0.01), but were not affected by l-NAME. Postprandially, the incremental area under the insulin curve was greater during hyperglycaemia (11 793.9 ± 1273.8 IU min), compared with euglycaemia (5234.1 ± 1201.1 IU min) (< 0.01). During hyperglycaemia, l-NAME was associated with a smaller area under the insulin curve, compared with placebo (9287.1 ± 1628.1 IU min vs 14 300.7 ± 1381.6 IU min; < 0.05); however, there was no difference during euglycaemia [5610.8 ±1593.5 IU min (l-NAME) vs 4857.5 ± 861.4 IU min (placebo); = N.S.] (Fig. 5).

image

Figure 5.  Plasma concentrations of (A) insulin, and (B) glucose-dependent insulinotropic polypeptide (GIP). Data are mean ± SEM (= 7).

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GIP   Plasma GIP concentrations increased substantially after the drink on all study days (< 0.001), but were not affected by either the ‘glycaemic state’ or ‘treatment’ (Fig. 5).

Discussion

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References

Our study has confirmed that acute hyperglycaemia delays gastric emptying of a nutrient liquid, and has established for the first time that this effect is mediated by NO. The dose of l-NAME used was based on previous studies20,28 and, as previously observed,20,28 changes in cardiovascular function were consistent with NO synthase inhibition. We cannot, however, exclude the possibility that some of the effects of l-NAME that we observed were not related to nitrergic mechanisms.29 Three-dimensional ultrasound was chosen to measure gastric emptying because of its superior accuracy over 2D ultrasound,21 and non-invasive nature, and because no ionizing radiation is required, unlike scintigraphy.22 Three-dimensional ultrasound has also been validated against scintigraphy for the measurement of gastric emptying of a liquid meal in healthy subjects.22 Although this method could potentially overestimate retention, as it is unable to differentiate between the liquid meal and gastric secretions, we do not believe that l-NAME had any major effect on gastric secretions, as the gastric volume was not significantly different between the two euglycaemic days. We acknowledge that we have studied only healthy subjects, and that our observations may not necessarily apply to patients with diabetes, in whom there may be changes to gastrointestinal signalling pathways secondary to either chronic hyperglycaemia or diabetes per se.

In this study, the marked slowing of gastric emptying observed with hyperglycaemia is consistent with previous reports.2,3,5,30 The observation that proximal, but not distal, gastric retention closely resembled total gastric emptying probably reflects the fact that the majority of the drink was retained, at least initially, in the proximal stomach. It should be recognized that our findings may not apply to the emptying of solid meals, although after ingestion of a mixed solid/liquid meal, solids also tend to remain in the proximal stomach until the majority of the liquid has emptied.31 The increased proximal gastric retention associated with hyperglycaemia in our study may indicate the suppression of fundic tone as a potential mechanism, although tone was not measured directly, which would have required the use of a barostat.32 Our observations regarding the effect of hyperglycaemia on fasting antral motility are also largely in keeping with the existing literature.7–9 Postprandially, however, we found that only short propagated antral wave sequences were suppressed during hyperglycaemia. This observation may reflect the fact that antral motility is normally suppressed after nutrient liquid ingestion,5,17,33 making any further difference between varying glycaemic states difficult to demonstrate. Although we did not find any effect of hyperglycaemia on the number of isolated pyloric pressure waves, in contrast to a previous report,9 we did observe an increase in basal pyloric pressure associated with hyperglycaemia postprandially, which has hitherto not been reported. Information on the effect of hyperglycaemia on proximal small intestinal motility is limited and inconsistent,19,34–36 but our study has not identified any major effect of hyperglycaemia on duodenal motility.

The role of NO in the gastropyloroduodenal region has been controversial. In humans, several studies involving either NO donors, such as intravenous nitroglycerin13 or sublingual glyceryl trinitrate,37 or inhibitors of NO production such as Ng-monomethyl-l-arginine (l-NMMA),11 indicate that NO mediates relaxation of the gastric fundus,37 slows gastric emptying,11–13,38–42 and decreases antral11,12 and pyloric13 contractions. Conversely, another study reported that l-NAME had no effect on antropyloroduodenal motility,28 while in the healthy elderly, l-NAME did not influence gastric emptying of a glucose drink.20 In animals, pyloric relaxation is impaired in neuronal nitric oxide synthase (nNOS)-deficient mice,43 and the relaxation of the rat gastric antrum appears to be dependent on nNOS expression,44 indicating an inhibitory effect of NO. However, other animal studies suggest that l-NAME is associated with slowing of gastric emptying.42,45 Furthermore, there may be sex-dependent differences in the contribution of nitrergic mechanisms to gastric motor function, with female rats having greater levels of nitrergic activity during health, and a greater propensity for disordered gastric motor function during diabetes, than male rats.44

In this study, the administration of l-NAME abolished the slowing of gastric emptying induced by hyperglycaemia, but had no effect on gastric emptying during euglycaemia, suggesting that the actions of NO may be glucose-dependent, which may partly account for the discrepancies in the literature regarding its actions, and indicates that in future studies the glycaemic state should be specified. The lack of effect of l-NAME on gastric emptying during euglycaemia, however, does not completely discount a potential role of NO in the normal feedback mechanism that regulates gastric emptying after a meal, particularly as the ‘physiological’ postprandial hyperglycaemia that normally occurs after a meal was prevented by the euglycaemic insulin/glucose clamp. We found no effect of l-NAME on antral motility during the fasting period, although the duration of observation was relatively short (i.e. 30 min). However, l-NAME suppressed all measures of postprandial antral motility. This contrasts with previous reports that increased NO availability was associated with a decrease, rather than an increase, in the postprandial antral motility index,11,12 although methodological differences may account for the discrepancy. For example, other studies either used a different NO synthase inhibitor (l-NMMA),11 or an NO donor,12 and one study used a semi-liquid, rather than a liquid, meal.11 The fact that l-NAME was associated with a suppression of postprandial antral motility, but not with a slowing of gastric emptying, suggests that antral motility probably does not play a major role in the emptying of a liquid meal. The number of isolated pyloric pressure waves was unaffected by l-NAME during both the fasting and postprandial periods. However, basal pyloric pressure was reduced by l-NAME postprandially, with a similar trend during fasting, indicating that l-NAME may have a suppressive effect on tonic, but not phasic, pyloric contractions. This could represent the dominant effect through which l-NAME abolishes the delay in gastric emptying induced by hyperglycaemia. Duodenal motility appeared little affected by l-NAME, suggesting limited involvement of nitrergic mechanisms in this region. Volunteers in our study were predominantly male, and in future studies relating to NO, it would be interesting to evaluate male and female responses separately.

The higher plasma insulin levels on the hyperglycaemic, compared with euglycaemic, days were anticipated. The lower incremental area under the insulin curve on the hyperglycaemia/l-NAME, compared with the hyperglycaemia/placebo, day, is modest, but is consistent with our previous observation that l-NAME is associated with less stimulation of insulin after oral glucose in healthy elderly subjects, compared with placebo,20 and implies that NO plays a role in mediating insulin secretion. The mechanism of this effect is unknown, but appears unlikely to involve the incretin hormone GIP, the levels of which did not differ between the study days, nor did our previous study indicate that glucagon-like peptide-1 (GLP-1) was involved.20 Given the latter observation, we did not measure GLP-1 in the current study. The involvement of NO in insulin secretion warrants further exploration, particularly in patients with diabetes, as both hyperglycaemia46 and diabetes47per se have been associated with impaired nitrergic activity. There are well-established links between NO and insulin release in animal and in vitro models, with one study suggesting that NO may contribute to glucose-stimulated insulin release,16 but others supporting an inhibitory effect of NO on beta-cell function.14,15 The lack of difference in plasma GIP between the four study days is somewhat surprising, given the markedly slower rate of gastric emptying on the hyperglycaemia/placebo day, compared with the other three days. We cannot provide a logical explanation for this. Hyperglycaemia and hyperinsulinaemia are unlikely to be involved in the secretion of GIP, given that levels on the hyperglycaemia/l-NAME day did not differ from the two euglycaemic days. Nitric oxide is also unlikely to be involved in GIP release, given the lack of difference in GIP levels between the euglycaemia/l-NAME and euglycaemia/placebo days.

In summary, we showed that the slowing of gastric emptying of a nutrient liquid, induced by acute hyperglycaemia, was abolished by the NO synthase inhibitor, l-NAME. Basal pyloric pressure was elevated during hyperglycaemia, and reduced by l-NAME. Insulin secretion appeared to be influenced by NOS inhibition. Further studies relating to the role of NO on gastric emptying and insulin release are now warranted in patients with diabetes.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References

Associate Professor Karen Jones’ salary is funded by a project grant provided by the National Health and Medical Research Council of Australia (NH&MRC). Doctor Gentilcore was supported by an Australian Clinical Research Fellowship (456441) from the NH&MRC of Australia. Prof. Gilja was supported by The Norwegian Medical Association (CM Aarsvold Fund) and Meltzer Foundation in Bergen. This work was supported by funding from the National Health & Medical Research Council of Australia, and by a Sylvia & Charles Viertel Clinical Investigator Grant awarded to A/Prof C. Rayner.

References

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments
  7. Conflicts of interest
  8. References
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