Glucose-induced [Ca2+]i oscillations in β cells are composed of trains of spikes within a subplasmalemmal microdomain

Electrical activity and oscillations of cytosolic Ca2+ concentrations ([Ca2+]i) that trigger insulin release in response to glucose are key functions of pancreatic beta cells. Although oscillatory Ca2+ signals have been intensively studied in beta cells, their lower frequency did not match that of electrical activity. In addition, the measured peak [Ca2+]i did not reach levels that are typically required by synaptotagmins to elicit the release of insulin containing vesicles in live-cell experiments. We therefore sought to resolve the Ca2+ dynamics in the subplasmalemmal microdomain that is critical for triggering fast exocytosis. Applying total internal reflection fluorescence (TIRF) microscopy in insulin-producing INS-1E and primary mouse beta cells, we resolved extraordinary fast trains of Ca2+ spiking (frequency > 3 s(-1)) in response to glucose exposure. Using a low-affinity [Ca2+]i indicator dye, we provide experimental evidence that Ca2+ spikes reach low micromolar apparent concentrations in the vicinity of the plasma membrane. Analysis of Ca2+ spikes evoked by repeated depolarization for 10 ms closely matched the Ca2+ dynamics observed upon glucose application. To our knowledge, this is the first study that experimentally demonstrates Ca2+ spikes in beta cells with velocities that resemble those of bursting or continuously appearing trains of action potentials (APs) in non-patched cells.