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

Electrical activity and oscillations of cytosolic Ca concentrations ([Ca]) that trigger insulin release in response to glucose are key functions of pancreatic β cells. Although oscillatory Ca signals have been intensively studied in β cells, their lower frequency did not match that of electrical activity. In addition, the measured peak [Ca] 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 Ca 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 β cells, we resolved extraordinary fast trains of Ca spiking (frequency > 3 s) in response to glucose exposure. Using a low-affinity [Ca] indicator dye, we provide experimental evidence that Ca spikes reach low micromolar apparent concentrations in the vicinity of the plasma membrane Analysis of Ca spikes evoked by repeated depolarization for 10 ms closely matched the Ca dynamics observed upon glucose application. To our knowledge, this is the first study that experimentally demonstrates Ca spikes in β cells with velocities that resemble those of bursting or continuously appearing trains of action potentials (APs) in non-patched cells.