Ca2+ has a Permissive Effect on Glycolytic Oscillations in Pancreatic Beta Cells

The secretion of insulin from pancreatic islet beta cells is pulsatile (∼5 min period) and reflects upstream oscillations in metabolism and Ca2+. Of the oscillating pathways in the beta cell, glycolytic oscillations are the least well studied. We recently developed a FRET biosensor for pyruvate kinase M2 activity (PKAR, Pyruvate Kinase Activity Reporter), which is activated by fructose 1,6-bisphosphate. We used PKAR FRET to measure oscillations in glycolysis stimulated by 10 mM glucose. After abolishing Ca2+ oscillations with diazoxide (Dz), the oscillations in PKAR were terminated. However, in some cases the oscillations could be restored by raising extracellular KCl, which elevated the intracellular Ca2+ levels but did not restore Ca2+ oscillations. These results indicate that glycolytic oscillations can persist in the absence of Ca2+ oscillations, and suggest the presence of a Ca2+ threshold that is permissive of glycolytic oscillations. To address this question, we varied the extracellular Ca2+ from 1.25 to 5 mM, which increased the amplitude of intracellular Ca2+ oscillations in a dose-dependent manner. Parallel measurements using PKAR indicated that the amplitude of glycolytic oscillations in pyruvate kinase M2 activity are augmented by intracellular Ca2+, however PKAR oscillations were terminated if extracellular Ca2+ dropped too low. Taken together, these results indicate that glycolytic oscillations in beta cells are dependent upon on a threshold level of intracellular Ca2+ level, and represent a distinct oscillating compartment of the cell. Supported by R01DK46409 (L.S.).