? There is a wealth of information about the physiology of rodent islets, but the biology of human islets of Langerhans remains poorly understood. Because there is growing evidence that human islets are strikingly different in terms of architecture and cellular composition, human islet biology needs to be reevaluated. Our long-term goal is to contribute to the understanding of human islet biology. This proposal addresses one aspect of human islet function: the regulation of glucagon release. Despite its importance to prevent hypoglycemia and therefore for survival, the mechanisms for the regulation of glucagon secretion by pancreatic islets are still controversial. Glucagon secretion is stimulated by decreasing plasma glucose levels. However, when isolated, glucagon-secreting alpha cells are stimulated by high glucose levels. To reconcile these contradictory observations, it has been suggested that paracrine or autocrine signaling is the primary regulatory mechanism governing alpha cell activity and glucagon release. ? Glutamate is a major neurotransmitter in the brain that has been recently proposed as a paracrine signaling molecule providing negative feedback on glucagon secretion in rodent islets. Studies on rodent islets have shown that alpha cells release glutamate and that beta cells and alpha cells express ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors, respectively. In striking contrast to these results, our preliminary studies on human and monkey islets show that glutamate induces Ca2+ responses exclusively in alpha cells. Glutamate strongly stimulates glucagon release but not insulin release. These responses to glutamate are mediated by iGluRs of the AMPA type. Glucagon release in response to decreasing glucose levels is strongly reduced when the AMPA receptors are blocked. Based on these results, we hypothesize that glutamate signaling provides an autocrine loop that is necessary for alpha cell responses to hypoglycemia. This positive feedback allows for propagation of glucagon responses throughout the islet.
Three Specific Aims will test this innovative idea: ? 1) Determine that glutamate is released by alpha cells in primate islets ? 2) Determine that primate alpha cells express functional glutamate receptors ? 3) Determine that glutamate autocrine loops in primate islets are necessary for glucagon responses ? If validated, this research will direct clinicians to consider therapies that activate AMPA receptors to prevent recurrent hypoglycemia in type 1 diabetic patients treated with insulin. ? ?
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