Identification of Expression and Function of the Glucagon‐like Peptide‐1 Receptor in Gastrointestinal Smooth Muscle
Authors: May Alexander T , Wang Hongxia , Mahavadi Sunila , Grider John R. , Murthy Karnam S. 
Published: November 2023

In response to ingestion of nutrients enteroendocrine L cells secrete a potent incretin hormone, glucagon‐like peptide‐1 (GLP‐1) to enhance glucose‐dependent release of insulin and regulate glucose metabolism. Therapies related to GLP‐1 (e.g., exendin‐4) are currently approved for type 2 diabetes. The GLP‐1 receptor (GLP‐1R) is expressed in cells of the gastrointestinal tract (enteric neurons, enterocytes) and elsewhere (pancreatic b cells, heart, and vascular smooth muscle). In pancreatic b cells, GLP‐1R are coupled to stimulatory G protein, Gs, generation of cAMP and activation of cAMP‐dependent protein kinase. Although a decrease in the motility of stomach and colon are commonly associated with diabetes, the expression and function of GLP‐1R in gastrointestinal smooth muscle are not known.

Aim

To test the hypothesis that GLP‐1 regulates smooth muscle function by acting on GLP‐1R expressed on smooth muscle.

Methods

Intestine and colon were removed from mice, and smooth muscle cells isolated by collagenase digestion, and cultured in DMEM‐10. Expression of GLP‐1R mRNA was measured by RT‐PCR using specific primers and the expression of GLP‐1R protein was measured by western blot using GLP‐1R specific antibody (sc‐66911 and 1:200 dilution). The effect of GLP‐1 on basal activity and on acetylcholine (ACh)‐induced contraction was measured in muscle strips and intact colon in organ bath experiments.

Results

Amplification of GLP‐1R mRNA was obtained in mRNA isolated from mucosa derived from colon and intestine and also in mRNA obtained from tissue preparations devoid of mucosa suggesting expression of GLP‐1R mRNA in mucosal as well as non‐mucosal cells (enteric neurons or muscle cells) of the colon and intestine. The specificity of GLP‐1R primers was corroborated using mRNA from a pancreatic b cell line (MIN‐6) shown to express GLP‐1R in our previous studies. A similar pattern of GLP‐1R protein expression was obtained with western blot. To identify the GLP‐1R expression specifically in muscle cells devoid of other cells such as enteric neurons, expression was examined in cultured smooth muscle. Expression of GLP‐1R mRNA was confirmed in these pure smooth muscle cell cultures of colon and intestine. The role of GLP‐1R in the regulation of smooth muscle function was analyzed in organ bath studies. The addition of GLP‐1 (1 nM‐1 mM) caused dose‐dependent relaxation of basal tone in colonic muscle strips and relaxation of acetylcholine (ACh, 1 mM)‐induced phasic activity and tonic contractions in both muscle strips and intact colon. The effect of GLP‐1 on ACh‐induced contraction suggests a role of smooth muscle Gs‐coupled GLP‐1R in mediating relaxation.

Conclusion

Colonic smooth muscle cells express GLP‐1R and GLP‐1 inhibits both basal and acetylcholine‐induced contraction. Studies are underway to identify the mechanism of action of GLP‐1 to inhibit colonic muscle function.

Support or Funding Information

Supported by grants DK15564, DK28300 and DK34153.