Gap junctions and other mechanisms of cell-cell communication regulate basal insulin secretion in the pancreatic islet

Cell-cell communication in the islet of Langerhans is important for the regulation of insulin secretion. Gap-junctions coordinate oscillations in intracellular free-calcium ([Ca ²⁺] _i) and insulin secretion in the islet following elevated glucose. Gap-junctions can also ensure that oscillatory [Ca ²⁺] _i ceases when glucose is at a basal levels. We determine the roles of gap-junctions and other cell-cell communication pathways in the suppression of insulin secretion under basal conditions. Metabolic, electrical and insulin secretion levels were measured from islets lacking gap-junction coupling following deletion of connexion36 (Cx36 ^-/-), and these results were compared to those obtained using fully isolated β-cells. K _ATP loss-of-function islets provide a further experimental model to specifically study gap-junction mediated suppression of electrical activity. In isolated β-cells or Cx36 ^-/- islets, elevations in [Ca ²⁺] _i persisted in a subset of cells even at basal glucose. Isolated β-cells showed elevated insulin secretion at basal glucose; however, insulin secretion from Cx36 ^-/- islets was minimally altered. [Ca ²⁺] _i was further elevated under basal conditions, but insulin release still suppressed in K _ATP loss-of-function islets. Forced elevation of cAMP led to PKA-mediated increases in insulin secretion from islets lacking gap-junctions, but not from islets expressing Cx36 gap junctions. We conclude there is a redundancy in how cell-cell communication in the islet suppresses insulin release. Gap junctions suppress cellular heterogeneity and spontaneous [Ca ²⁺] _i signals, while other juxtacrine mechanisms, regulated by PKA and glucose, suppress more distal steps in exocytosis. Each mechanism is sufficiently robust to compensate for a loss of the other and still suppress basal insulin secretion.