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Rapid Oscillations in Plasma Glucagon-Like Peptide-1 (GLP-1) in Humans: Cholinergic Control of GLP-1 Secretion via Muscarinic Receptors¹

Zentrum Innere Medizin, Abteilung Klinische Endokrinologie, Medizinische Hochschule Hannover, Hannover, Germany; and Institute of Medical Physiology C, The Panum Institute, University of Copenhagen (J.J.H.), Copenhagen, Denmark

Address all correspondence and requests for reprints to: H. J. Balks, M.D., Department of Internal Medicine, Division of Clinical Endocrinology, Medizinische Hochschule Hannover, Konstanty-Gutschow-Strasse 8, D-30623 Hannover, Germany. E-mail: 106161.3402{at}compuserve.com

	Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
The mechanisms involved in the rapid glucagon-like peptide-1 (GLP-1) release following glucose ingestion are poorly defined. Besides a direct intestinal stimulation of L cells, humoral and neuronal mechanisms have been discussed. We investigated the temporal pattern of GLP-1 release in five healthy men (aged 27.8 ± 3.6 yr; body mass index, 23.4 ± 1.2 kg/m²) after an overnight fast for 60 min under basal conditions and for 60 min after an oral glucose load (OGL; 100 g) in both the presence and absence of atropine (80 ng/kg·min, iv). Blood was sampled every 2 min, and data were evaluated for the temporal pattern of GLP-1 secretion by several computer-assisted programs (deconvolution, Pulsar analysis, and Fourier transformation). With all methods a pulsatile pattern of plasma GLP-1 levels with a frequency of five to seven per h was detected; this remained unchanged in the different metabolic states and during atropine treatment. Glucose and GLP-1 plasma levels showed a parallel increase after OGL (OGL without atropine = control: 8.4 ± 2.9 and 7.9 ± 3.0 min, respectively). Atropine infusion delayed this increase significantly (16.8 ± 8.07 and 17.4 ± 6.61 min, respectively; P < 0.02). In contrast to plasma glucose concentrations (82.7 ± 0.3% of control; P < 0.05), atropine infusion reduced the integrated GLP-1 pulse amplitude to 56.0 ± 11.3% of the control levels (P < 0.05).

In conclusion, GLP-1 is secreted in a pulsatile manner with a frequency comparable to that of pancreatic hormones. Mean GLP-1 plasma concentrations increase after OGL due to augmented GLP-1 pulse amplitudes but not frequency. The differential effect of atropine on glucose and GLP-1 plasma levels suggest a direct cholinergic muscarinic control of L cells.

	Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
THE INCRETIN hormones gastrin inhibitory polypeptide (GIP) (1, 2, 3, 4) and glucagon-like peptide-1-(7, 8, 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) amide [GLP-1-(7, 8, 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) amide] (5, 6) are secreted in response to ingestion of mixed meals, potently stimulating glucose-induced insulin secretion from pancreatic ß-cells. In contrast to GIP, the insulinogenic property of GLP-1 is still conserved in type 2 diabetic patients (7). These metabolic effects have stimulated research on the potential of GLP-1 as a therapeutic agent for patients with type 2 diabetes mellitus (8, 10, 11, 12, 13, 14, 15), whereas the mechanisms involved in the release of GLP-1 have not been fully elucidated as yet. Previous studies demonstrated a rapid release of GLP-1 after glucose ingestion (8, 16, 17, 18). As GLP-1-positive L-cells are dispersed in increasing number in the distal jejunum, ileum, and throughout the large bowel (19, 20), luminal stimulation of GLP-1 release by nutrients has been questioned. The increment in plasma glucose, however, is unlikely to be the cause of this early GLP-1 stimulation as iv application of glucose mimicking the increment following an oral glucose load did not alter GLP-1 secretion (8, 11, 21). Several compounds, including GIP, the cholinergic agonist betanecol, bombesin, and calcitonin gene-related peptide, are stimulatory on GLP-1 secretion in vitro in rats (22, 23), opening the possibility for a regulatory enteroendocrine and neuroneuroendocrine loop between the proximal and the distal small intestine (22, 23). The cholinergic stimulation is dependent on muscarinic M3-subtype receptors, as indicated by a recent study using a murine intestinal cell line, STC1 (24). In contrast to the animal studies, infusion of synthetic GIP in humans has no effect on GLP-1 secretion (8).

To better define the mechanisms involved in GLP-1 release in vivo, we investigated the temporal pattern of GLP-1 release after glucose ingestion by high frequency blood sampling in healthy young volunteers.

	Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Subjects and experimental protocol

Five healthy men, aged 27.8 ± 3.6 yr, with a body mass index of 23.4 ± 1.2 kg/m², volunteered for the study. No subject had a family history of diabetes mellitus. They all were healthy at the time of investigation, had no history of gastrointestinal dysfunction, and had an unremarkable physical and biochemical examination. The investigations had been approved by the committee on medical ethics of the Medizinische Hochschule Hannover, and all subjects gave written consent.

The secretory pattern of GLP-1 was investigated after an overnight fast over 60 min under basal conditions and after an oral glucose load (OGL; 100 g glucose) for another 60 min by sampling blood every 2 min via an indwelling central venous catheter (25). Under identical conditions a second OGL was performed, but a bolus of 1 mg atropine was given iv 30 min before the OGL followed by a constant infusion of 80 ng atropine/kg BW·min.

Biochemical analysis

Plasma glucose levels were determined enzymatically (25). Plasma GLP-1 concentrations were measured after ethanol extraction of plasma samples (26) as previously described (27), using an antiserum (89890) specific for the C-terminal region of GLP-1-(7, 8, 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) amide with a cross-reactivity of less than 0.01% with the C-terminally truncated peptides GLP-1-(7, 8, 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) and GLP-1-(7, 8, 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) and 100% with GLP-1-(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) amide. Assay sensitivity was 1 pmol/L, intra- and interassay coefficients of variation were less than 5% and less than 11%, respectively.

Analysis of the temporal secretion pattern and statistical evaluation

The temporal pattern of GLP-1 secretion was analyzed by discrete deconvolution (DESADE) using a one-compartment model (28) with a plasma half-time of GLP-1 calculated from the original time series as previously described (25). The half-time of 4.8 ± 3.3 min (mean ± SD) found is comparable to data obtained in infusion and bolus injection studies using either a side-viewing or a C-terminal-specific GLP-1 antiserum (12, 29). For comparison, the Pulsar program (30) was used as an alternative pulse detection program, with G(1) 3.80, G(2) 2.26, G(3) 1.56, G(4) 1.13, and G(5) 0.83. Finally, spectral analysis was performed using the Matlab program (Math Works, Natick, MA).

To determine the OGL-induced increase in plasma glucose and hormone concentrations, the slopes of two succeeding regression lines (first regression line consisting of 12 points; second line of 8 points) were computed iteratively for succeeding points as previously described (25).

Statistical analyses were performed using the SPSS/PC 3.1 program. If not stated otherwise, data are given as the mean ± SD. ANOVA was used for analysis of changes in parameters within the time series, and P < 0.05 or lower was considered significant. Cross-correlation was calculated using the BMDP statistical package (31).

	Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Basal and glucose-stimulated GLP-1 plasma levels

In all subjects plasma GLP-1 concentrations showed a pulsatile pattern both under basal conditions and after glucose ingestion (Fig. 1) as analyzed by the DESADE or Pulsar program. Spectral analysis revealed a dominant frequency of 7.4 ± 0.58 min comparable to the rate of 9.17 ± 1.31 min detected by DESADE and 8.36 ± 0.57 min by the Pulsar program. OGL did not significantly alter the number of GLP-1 pulses, but GLP-1 pulse amplitudes were significantly increased by glucose ingestion (Table 1).

View larger version (40K): [in this window] [in a new window]   Figure 1. Individual patterns of glucose and GLP-1 during fasting and after an oral glucose challenge (100 g) in three healthy volunteers (A–C). Blood sampling was performed every 2 min starting at 0700 h ( = 0 min), glucose load was given at 0800 h (60 min; closed circles). Under identical experimental conditions, a bolus of 1 mg atropine at 0730 h (30 min) was administered iv, followed by a constant infusion of 80 ng atropine/kg BW·min (open circles). The upper panel shows plasma glucose levels, and the middle panels pulses obtained by DESADE and Pulsar analysis. In the bottom panel, plasma GLP-1 concentrations are shown.

View larger version (27K): [in this window] [in a new window]   Figure 2. Mean ± sd plasma levels of glucose and GLP-1 during an oral glucose load (100 g at 0800 h; 60 min) in five healthy volunteers. For methodology, see Fig. 1 (closed circles, control situation; open circles, atropine treatment).

Atropine treatment

Intravenously administered atropine did not affect basal plasma glucose or GLP-1 levels. The frequency of pulses during the fasting period was almost identical between the 30-min segment without and with iv atropine within and between comparable time segments of the two test situations, indicating no influence of atropine on the basal dynamics of GLP-1 secretion. However, atropine significantly attenuated both the increase in plasma glucose and GLP-1 concentrations in all subjects (Figs. 1 and 2) and reduced the area under the GLP-1 curve to 56.0 ± 11.3% of controls. In contrast, the area under the curve for glucose was only reduced by approximately 17.3 ± 0.3% (Table 2). Again, the frequency of GLP-1 pulses after glucose stimulation remained unaltered, but the lower plasma GLP-1 levels resulted from a selective effect on the amplitude of GLP-1 pulses (Table 1).

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
The secretion of pancreatic hormones is pulsatile, with a frequency of one pulse approximately every 10–15 min (25, 32). The temporal pattern of GLP-1 secretion has not been elucidated as yet. In the present study we used three different and independent analytical methods to analyze the secretion dynamics of GLP-1. All three independent techniques used for pulse analysis revealed a pulsatile pattern of hormone release. One of these methods, the deconvolution technique DESADE, is based on the plasma half-life of the hormone, which we estimated from the original time series. The plasma half-life of GLP-1 may vary with the specificity of the antibody used, but as all three techniques independently revealed a comparable frequency of GLP-1 pulses, such technical problems are unlikely (29, 33).

The mechanisms underlying GLP-1 pulse generation and the coordinated release from dispersed L cells in the distal parts of the intestinal tract (19, 20) are unclear. Stimulation of GLP-1 secretion by circulating glucose concentrations, luminal contact of L cells to ingested glucose, or neuronal modulations are possible explanations. Direct stimulation of basal GLP-1 secretion by circulating plasma glucose levels is an unlikely explanation, as the threshold to induce GLP-1 release in cell culture experiments (34) is higher than the circulating glucose concentrations. Moreover, iv glucose infusion in man failed to increase plasma GLP-1 concentrations even when mimicking the glucose increment achieved by OGL (18, 21).

L Cells are located in the distal jejunum, with increasing density throughout the ileum and colon (19, 20). Rapid filling of the distal small intestine is a prerequisite for GLP-1 stimulation by intestinal luminal contact with glucose. Therefore, an extremely rapid gastric emptying and intestinal glucose transport have to be postulated to explain the parallel increase in plasma glucose and GLP-1 only 8 min after glucose ingestion. Using a -scintillation technique, we investigated in a preliminary study the gastric emptying of a liquid glucose load (100 g) containing a liquid tracer (40 millibecquerels ^99mTc-DTPA) into the small intestine. The proximal duodenum was reached only 6–8 min after oral ingestion of the glucose load (Balks H. J. and Mihlau E., unpublished results), closely resembling the time interval observed for the initial increase in either plasma glucose or GLP-1 in the present study. This fits with other findings questioning such rapid intestinal transport (18, 20).

The autonomic nervous system is involved in the modulation of glucose homeostasis, coordinating gastric emptying, intestinal transport of ingested nutrients, and coupling of glucoregulatory hormones from the gut (e.g. GIP) and pancreas (22, 23, 35, 36, 37, 38). In rats GIP is directly stimulating GLP-1 secretion in vitro (22, 23, 39), an effect that has not been confirmed in humans in vivo (21). However, cholinergic agonists potently stimulated GLP-1 secretion in vitro in the rat (23, 24, 39) through acetycholine M3 subtype receptors as reported recently (24). Thus, the cholinergic system may be a good candidate for the physiological regulation of GLP-1. This is supported by our finding of an attenuated increase in GLP-1 after glucose administration in the presence of atropine, suggesting a direct inhibition beyond the effects on intestinal motility, as GLP-1 pulse amplitude was disproportionally lower for a given glucose plasma level. Neuronal integration of GLP-1 release from L cells would help in understanding the pulsatile release mechanism in a dispersed cellular system such as L cells. This, furthermore, may explain the pulsatile nature of GLP-1 release and fits observations on the release pattern of duodenal K cells (40, 41) and pancreatic ß-cells (36, 37, 38), where the endogenous dynamic of single cells appears to be integrated by intrinsic and extrinsic humoral and neuronal factors (42, 43, 44).

In summary, we have shown that GLP-1 secretion is pulsatile and that the mechanisms involved in controlling and coordinating L cells and in the formation of a pulsatile secretion pattern of GLP-1 depend on the parasympathetic nervous system in vivo in man.

	Acknowledgments

 
We thank Mrs. D. Becker, Mrs. S. Baars, and Mrs. L. Albaek for their excellent assistance with the analytical procedures.

	Footnotes

 
¹ This work was supported in part by grants from the Danish Medical Research Council.

Received August 8, 1996.

Revised November 15, 1996.

Accepted November 26, 1996.

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