251-LB: Changes in Alpha-Cell Function in Defective Glucose Counterregulation

Increased glucagon secretion from the alpha cell is the first and most important defense against hypoglycemia. In type 1 diabetes this defensive mechanism is lost, increasing the mortality risk. Understanding what goes awry with alpha cells during disease progression is therefore of utmost importance. However, studying alpha cell physiology in type 1 diabetes has met major technological roadblocks, as methods conventionally used are not appropriate or are very difficult to apply to type 1 diabetic donors. Our project overcomes these limitations by using living pancreas slices, which allows functional assessments of damaged and infiltrated islets within their native environment. By destroying beta cells with streptozotocin (STZ) we established a mouse model in which counterregulatory responses to hypoglycemia are defective 2 weeks after treatment, as shown by insulin tolerance tests in vivo. To monitor alpha cell function, we prepared pancreas tissue slices from mice expressing a genetically encoded Ca²⁺ indicator in all alpha cells (GCaMP6). We measured Ca2+ responses in alpha cells in response to changes in glucose concentration, glutamate receptor agonists, GABA, serotonin, somatostatin, and KCl depolarization. When comparing alpha cell responses in slices obtained from STZ-treated mice to those in untreated mice, we found that fewer alpha cells responded. Responses to glutamate receptor agonists were particularly diminished, suggesting that autocrine alpha cell activation by glutamate is disrupted. We are currently testing human slices from control and type 1 diabetic donors to determine changes in alpha cell function.