Insulinotropic Glucagon-Like Peptide I Receptor Expression in Glucagon-Producing α-Cells of the Rat Endocrine Pancreas

Glucagon-like peptide I (GLP-I), an intestine-derived incretin hormone, is a potent stimulator of insulin and somatostatin secretion. In some studies, GLP-I is an inhibitor of glucagon secretion. It remains uncertain, however, whether the effect of GLP-I on the inhibition of glucagon secretion is direct, owing to interactions with GLP-I receptors on ケ-cells, or indirect, via paracrine suppression by insulin or somatostatin. The localization of the GLP-I receptor on insulin and somatostatin-producing cells in the islets is well established. Whether the GLP-I receptor also resides on the glucagon-producing α-cells remains controversial and is reported to be absent on rat α-cells. To investigate the distribution of the GLP-I receptor on islet cells, we examined the expression of GLP-I receptor mRNA in phenotypically distinct islet cell lines and islets, and the presence of immunoreactive GLP-I receptor in dispersed rat islet cells using a specific antiserum. GLP-I receptor mRNA was readily detected by reverse transcription–polymerase chain reaction (RT-PCR) in both rat islets and in established islet cell lines representing distinct α-, β, and δ-cell phenotypes. In addition, GLP I receptor expression was detected in single rat α-cells by single-cell RT-PCR. In dispersed rat islet cells analyzed by double immunofluorescent staining, 90% of the insulin, 76% of the somatostatin, and 20% of the glucagon positive cells colocalized with the GLP-I receptor immunoreactivity. Thus, a substantial population of glucagon immunoreactive α-cells express the GLP-I receptor. These findings imply that GLP-I may have a direct receptor-mediated action in the regulation of the physiological functions on a substantial subpopulation of α-cells. We suggest that a possible role for GLP-I receptors on α-cells may be to provide positive autocrine feedback control on glucagon secretion during fasting and/or to dampen the potent paracrine suppression of glucagon secretion by insulin during feeding.