646 DIABETES, VOL. 47, APRIL 1998 Mouse Pancreatic -Cells Exhibit Preserved Glucose Competence After Disruption of the Glucagon-Like Peptide-1 Receptor Gen e Daisy Flamez, An Van Breusegem, Louise A. Scrocchi, Erik Quartier, Daniel Pipeleers, Daniel J. Drucker, and Frans Schuit Previous work suggested that glucagon-like peptide 1 (GLP-1) can acutely regulate insulin secretion in two ways, 1) by acting as an incretin, causing amplification of glucose-induced insulin release when glucose is given orally as opposed to intravenous glucose injection; and 2) by keeping the -cell population in a glucose- competent state. The observation that mice with homozygous disruption of the GLP-1 receptor gene are diabetic with a diminished incretin response to glucose underlines the first function in vivo. Isolated islets of Langerhans from GLP-1 receptor –/– mice were studied to assess the second function in vitro. Absence of pan- creatic GLP-1 receptor function was observed in GLP- 1 receptor –/– mice, as exemplified by loss of [ 1 2 5 I]GLP- 1 binding to pancreatic islets in situ and by the lack of GLP-1 potentiation of glucose-induced insulin secre- tion from perifused islets. Acute glucose competence of the -cells, assessed by perifusing islets with stepwise increases of the medium glucose concentration, was well preserved in GLP-1 receptor –/– islets in terms of insulin secretion. Furthermore, neither islet nor total pancreatic insulin content was significantly changed in the GLP-1 receptor –/– mice when compared with age- and sex-matched controls. In conclusion, mouse islets exhibit preserved insulin storage capacity and glucose- dependent insulin secretion despite the loss of func- tional GLP-1 receptors. The results demonstrate that the glucose responsiveness of islet -cells is well pre- served in the absence of GLP-1 receptor signaling. Diabetes 47:646–652, 1998 P ancreatic -cells require co-stimulation with nutrients and (neuro)hormones for the physiological control of insulin release ( 1–4). Part of the hormonal control of insulin release proceeds via receptor- mediat ed regulation of cAMP production in -cells (5,6). Cyclic AMP synergizes with signals derived from glucose metabolism at var- ious possible levels, including enhancement of voltage-depen- dent influx of calcium (7,8) and calcium-independent sensiti- zation of exocytosis (9). The physiological relevance of this syn- ergism between glucose and cAMP is underlined by the gluco-incretins glucagon-like peptide-1(7-36) amide (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), which are responsible for a greater insulin secretory response after oral gluco se than after intraveno us glucose (10–16). Sev- eral studies have underlined the importance of GLP-1 as an incretin. First, GLP-1 was found to be a potent stimulator of insulin secretion, both in ro dents (11) and in humans ( 12) ; the peptide bound to a specific class of high-affinity receptors (13) that were present in rat -cells (6). Second, occupation of the GLP-1 receptor (GLP-1R) with the antago nist exendin(9-39) amide (14) blocked at least part of the incretin effect in rats (15). Third, mice with homozygous disruption of the GLP-1 recep - tor gene ( GLP-1R –/– mice; 16) exhibited disturbed glucose tol- erance with high plasma gluco se and low plasma insulin levels, emphasizing the importance of GLP-1R in glucose homeosta- sis. However, the in vivo studies using exendin-(9-39)-amide or GLP-1R –/– mice are co mplex, in that loss of -cell stimulation is one of several mechanisms by which glucose homeostasis can be disturbed, considering that GLP-1 receptors have been implicated in the hypothalamic control of food intake (17), in the control of gastric emptying (18), and in peripheral glucose uptake (19). Furthermore, in addition to its effect as an incretin, it was proposed that GLP-1 can regulate -cell func- tion by maintaining at least part of the -cell population in a glu- cose-competent state, allowing the cells to respond to glucose in terms of hormone secretion (20). This concept merits further investigation, because individual pancreatic -cells have been reported to be heterogenous in their responsiveness to glucose, in terms of insulin biosynthesis (21), insulin release (22), and cytoplasmic calcium (23). The present in vitro study on islets of Langerhans isolated from diabetic GLP-1R–deficient mice was undertaken with the aim to assess the functional conse- quence of GLP-1R disruption in the endocrine pancreas. The results show that glucose-induced insulin secretion in pancre- atic islets obtained from GLP-1R–deficient mice is well pre- served at the age of 8–10 weeks, demonstrating that the GLP- 1R per se is no t required for maintaining gluco se competence in panc reatic - c ells. RESEARCH DESIGN AND METHODS Anim als. The study was conducted on 8- to 10-week-old male and female CD-1 mice fed ad libitum that were wild type (+/+) or homozygous (–/–) for the targeted From the Diabetes Research Center (D.F., A.V.B., E.Q., D.P., F.S.), Vrije Universiteit Brussel, Brussels, Belgium; and the Department of Medicine (L.A.S., D.J.D.), University of Toro nto, and Banting and Best Diabetes Cen- tre, The Toronto Ho spital, Toronto, Ontario, Canada. Address correspondence and reprint requests to Dr. Frans Schuit, Department of Biochemistry, Diabetes Research Center, Faculty of Medi- cine, Vrije Universiteit Brussel, Laarbeeklaan 103, B-1090 Brussels, Belgium. E-mail: fschuit@minf.vub.ac.be. Received fo r publication 30 October 1997 and accepted in revised form 12 December 1997. BSA, bovine serum albumin; GIP, glucose-dependent insulinotropic polypeptide; GLP-1, glucagon-like peptide-1(7–36) amide; GLP-1R, GLP-1 receptor; RT-PCR, reverse transcription–polymerase chain reaction; SSC, sodium chloride–sodium citrate buffer.