Distribution of glucagon receptors on hormone-specific endocrine cells of rat pancreatic islets

Glucagon is insulinotropic, but it remains uncertain whether the insulinotropic action is mediated directly by glucagon receptors expressed on beta-cells or by cross-binding to the insulinotropic glucagonlike peptide-1 (GLP-1) receptor known to be expressed on beta-cells. Binding of [I-125]glucagon to GLP-1 receptors and not to glucagon receptors has been reported in tumor-derived beta-cells (15). The objectives of the current study were to use receptor-binding techniques and a glucagon receptor-specific antiserum to determine whether glucagon receptors are present on beta-cells. Specific binding (7.2 +/- 0.8%) of [I-125]GLP-1 to beta TC-3 cells was displaced equivalently with GLP-1 and exendin-(9-39) (K-d = 0.9 and 0.4 nM, respectively), whereas approximately 700-fold higher concentrations of glucagon were required for equal displacement (K-d = 400 nM). Binding of[I-125]glucagon to beta TC-3 cells (similar to 1%) was displaced equivalently with 1 mu M glucagon, GLP-1, or exendin-(9-39). These observations support earlier findings that beta TC-3 cells do not express functional glucagon receptors. However, specific binding of [I-125]glucagon was observed on INS-1 cells (2.3 +/- 0.2%); this was displaced with glucagon (K-d = 1 nM), but not 1 mu M GLP-1 or exendin-(9-39). To examine the distribution of glucagon receptors on native beta-cells, dispersed cultured rat islets were immunostained for glucagon, somatostatin, or insulin in combination with a polyclonal rabbit antiserum raised to an extracellular portion of the glucagon receptor (KD-14). The glucagon receptor antiserum colocalized staining with approximately 97% of immunoreactive insulin cells, 9% of immuoreactive glucagon cells, and 11% of immunoreactive somatostatin cells. Perfusion of the rat pancreas with concentrations of glucagon as low as 10(-12) M resulted in significant insulin release. These results suggest that whereas the tumor-derived beta-cell line beta TC-3 does not express functional glucagon receptors, INS-1 cells and isolated rat pancreatic beta-cells have specific glucagon receptors, as do a subpopulation of alpha- and delta-cells. A model is proposed for the role of glucagon in islet hormone secretion during feeding and fasting.