Rapid oscillations in plasma glucagon-like peptide-1 (GLP-1) in humans : Cholinergic control of GLP-1 secretion via muscarinic receptors

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.