Glutamine Triggers and Potentiates Glucagon-Like Peptide-1 Secretion by Raising Cytosolic Ca2+ and cAMP

Abstract

Abstract L-glutamine stimulates glucagon-like peptide 1 (GLP-1) secretion in human subjects and cell lines. As recent advances have enabled the study of primary GLP-1–releasing L cells, this study aimed to characterize glutamine-sensing pathways in native murine L cells. L cells were identified using transgenic mice with cell-specific expression of fluorescent markers. Cells were studied in primary colonic cultures from adult mice, or purified by flow cytometry for expression analysis. Intracellular Ca2+ was monitored in cultures loaded with Fura2, and cAMP was studied using Förster resonance energy transfer sensors expressed in GLUTag cells. Asparagine, phenylalanine, and glutamine (10 mm) triggered GLP-1 release from primary cultures, but glutamine was the most efficacious, increasing secretion 1.9-fold with an EC50 of 0.19 mm. Several amino acids triggered Ca2+ changes in L cells, comparable in magnitude to that induced by glutamine. Glutamine-induced Ca2+ responses were abolished in low Na+ solution and attenuated in Ca2+ free solution, suggesting a role for Na+ dependent uptake and Ca2+ influx. The greater effectiveness of glutamine as a secretagogue was paralleled by its ability to increase cAMP in GLUTag cells. Glutamine elevated intracellular cAMP to 36% of that produced by a maximal stimulus, whereas asparagine only increased intracellular cAMP by 24% and phenylalanine was without effect. Glutamine elevates both cytosolic Ca2+ and cAMP in L cells, which may account for the effectiveness of glutamine as a GLP-1 secretagogue. Therapeutic agents like glutamine that target synergistic pathways in L cells might play a future role in the treatment of type 2 diabetes.