marcz90

According to the last sentence of the abstract of the paper that you linked to (Evidence that glucagon stimulates insulin secretion through its own receptor in rats), "glucagon stimulates insulin release mainly through glucagon receptors but not GLP-1 receptors on islet beta cells." So glucagon has both a primary and secondary insulin secreting effect -- the primary being through the glucagon receptors on pancreatic islet cells, and the secondary being due to increased serum glucose. Having clarified the ways that glucagon stimulates insulin secretion, the initial question still remains: "Why is glucagon a stimulator of insulin secretion?" In my own case, I really only ask this question with regard to the primary insulin secreting effect of glucagon, which I think is probably also true for OP (what we have been calling glucagon's "secondary insulin stimulating effect" is really the primary way that insulin secretion is stimulated—no confusion there. High blood-glucose = insulin release). So the question, "why is glucagon a stimulator of insulin secretion," could be directed more bluntly at the point of confusion by asking, "why does glucagon directly stimulate the secretion of insulin?" While doing research into this question I found the following relevant excerpt from p. 1410 of Williams Textbook of Endocrinology 13th edition: The hormones produced by pancreatic alpha and beta cells also modulate insulin release. Whereas glucagon has a stimulatory effect on the beta cell,541 somatostatin suppresses insulin release.542 It is currently unclear whether these hormones reach the beta cell by traveling through the islet cell interstitium (thus exerting a paracrine effect) or through islet cell capillaries. Indeed, the importance of these two hormones in regulating basal and postprandial insulin levels under normal physiologic circumstances is in doubt. Paradoxically, the low insulin levels observed during prolonged periods of starvation have been attributed to the elevated glucagon concentrations seen in this setting.500,543-546 541. Samols E, Marri G, Marks V. Promotion of insulin secretion by glucagon. Lancet. 1965;2:15-16. 542. Alberti K, Christensen NJ, Christensen SE, et al. Inhibition of insulin secretion by somatostatin. Lancet. 1973;2:1299-1301. 543. Aguilar-Parada E, Eisentraut A, Unger R. Effects of starvation on plasma pancreatic glucagon in normal man. Diabetes. 1969;18:717-723. 544. Marliss E, Aoki TT, Unger RH, et al. Glucagon levels and metabolic effects in fasting man. J Clin Invest. 1970;49:2256-2270. 545. Malaisse W, Malaisse L, Wright P. Effect of fasting upon insulin secretion in the rat. Am J Physiol. 1967;213:843-848. 546. Zawalich W, Dye ES, Pagliara AS, et al. Starvation diabetes in the rat: onset, recovery and specificity of reduced responsiveness of pancreatic β-cells. Endocrinology. 1979;104:1344-1351.