The intra-islet theory states that glucagon secretion is suppressed when insulin secretion is stimulated, but glucagon's role in intra-islet paracrine regulation is controversial. This study investigated intra-islet functions of glucagon in mice. We examined glucagon-induced insulin secretion using isolated perfused pancreata from wild-type, GLP-1 receptor (GLP-1R) knockout, diphtheria toxin-induced proglucagon knockdown, β cell-specific glucagon receptor (Gcgr) knockout, and global Gcgr knockout (Gcgr−/−) mice. We found that glucagon stimulates insulin secretion through both Gcgr and GLP-1R. Moreover, loss of either Gcgr or GLP-1R does not change insulin responses, whereas combined blockage of both receptors significantly reduces insulin secretion. Active GLP-1 is identified in pancreatic perfusate from Gcgr−/− but not wild-type mice, suggesting that β cell GLP-1R activation results predominantly from glucagon action. Our results suggest that combined activity of glucagon and GLP-1 receptors is essential for β cell secretory responses, emphasizing a role for paracrine intra-islet glucagon actions to maintain appropriate insulin secretion. Glucose-stimulated insulin secretion can be regulated by glucagon and GLP-1 receptors on paracrine β-cells. Svendsen et al. find that complete blockade of glucagon signaling in islets severely limits insulin secretion but establish that paracrine glucagon signaling involves both the glucagon and GLP-1 receptors.
Svendsen, B., Larsen, O., Gabe, M. B. N., Christiansen, C. B., Rosenkilde, M. M., Drucker, D. J., & Holst, J. J. (2018). Insulin Secretion Depends on Intra-islet Glucagon Signaling. Cell Reports, 25(5), 1127-1134.e2. https://doi.org/10.1016/j.celrep.2018.10.018