Functional Heterogeneity Among Pancreatic Alpha Cells

Historically, endocrine cells in the pancreatic islets have been assumed to function as relatively homogenous populations largely because we lacked the ability to measure individual cell activity with sufficient throughput to reliably detect heterogeneity within each population. The glucagon-secreting alpha cells play a vital role in regulating glycemia, but the mechanisms that control alpha cell activity and whether the alpha cells behave as a single unit or heterogeneously remain incompletely understood. To overcome the limitations in throughput that have to date prevented the study of alpha cells at the population level, we used genetically-encoded fluorescent indicators selectively expressed in alpha cells. Imaging intact mouse islets with these indicators in 3D responding to treatments in real time yields hundreds of individual alpha cell recordings per experiment. Calcium imaging showed reproducible heterogeneous responses to a panel of known physiological potentiators of glucagon secretion such as arginine vasopressin, epinephrine, and amino acids. Separate dose response experiments revealed that the proportion of alpha cells responding to each signal plateaus at different proportions of alpha cells. The calcium data correlate both with direct glucagon secretion levels as well as cAMP measurement. Our findings highlight previously unappreciated levels of functional heterogeneity among alpha cells and demonstrate that alpha cells are not a single uniform unit. Our observations suggest that dose-dependent increases in glucagon secretion in response to different physiological cues may be the result of mobilizing progressively larger proportions of the total alpha cell mass. We hypothesize that this functional heterogeneity is a built-in mechanism through which different physiological cues elicit graded glucagon responses from the alpha cells.