Spatial Homogeneity in Metabolic Activity Controls Electrical Activity in Pancreatic Islets

Abstract

Pancreatic islets are micro-organs which secrete hormones including insulin and glucagon to maintain glucose homeostasis. Regulated secretion of insulin is managed by β-cells when glucose is taken into the cell and enters the metabolic pathway to generate NADH and ATP. The increased ATP/ADP ratio triggers the closure of ATP sensitive potassium channels which activates voltage gated calcium channels, depolarizing the membrane and increasing cytosolic calcium levels [Ca2+] to induce insulin release. Oscillatory depolarizations of β-cells are synchronized by connexin 36 gap junctions that allow the passage of cations between cells. Travelling waves of depolarization and [Ca2+] influx emerge from one spatial origin and terminate at another, strengthening the collective effect of insulin secretion. Individual β-cells show large amounts of electrical heterogeneity; in both the level of glucose required to induce electrical oscillations and in oscillatory dynamics. To test if cells of similar heterogeneity are grouped together forming regions of spatial homogeneities a mouse line expressing the optogenetic tool Chanelrhodopsin-2 (ChR2) was created to induce membrane depolarizations in subregions of the islet. Two-photon NADH imaging was used with ChR2 activation to determine if subregions showed preferential excitability. β-cell electrical activity measured through ChR2 activation was found to be spatially variable, and this intra-islet variation corresponded with NADH levels in that subregion. A computational electrophysiological model of the islet was used to determine that variations in ChR2 stimulated spatial electrical activity and NADH levels could come from subregions of similar cells. Furthermore, ChR2 stimulated activity was less in the depolarizing wave origin – consistent with the computational model. These data suggest islets are grouped in neighborhood regions of like heterogeneity which provide functional control over electrical activity and dynamics.