FRET-based voltage probes for confocal imaging: membrane potential oscillations throughout pancreatic islets

Insulin secretion is dependent on coordinated pancreatic islet physiology. In the present study, we found a way to overcome the limitations of cellular electrophysiology to optically determine cell membrane potential (V-m) throughout an islet by using a fast voltage optical dye pair. Using laser scanning confocal microscopy (LSCM), we observed fluorescence (Forster) resonance energy transfer ( FRET) with the fluorescent donor N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidylethanolamine and the acceptor bis-(1,3-diethylthiobarbiturate) trimethine oxonol in the plasma membrane of essentially every cell within an islet. The FRET signal was approximately linear from V-m - 70 to +50 mV with a 2.5-fold change in amplitude. We evaluated the responses of islet cells to glucose and tetraethylammonium. Essentially, every responding cell in a mouse islet displayed similar time-dependent changes in V-m. When V-m was measured simultaneously with intracellular Ca2+, all active cells showed tight coupling of V-m to islet cell Ca2+ changes. Our findings indicate that FRET-based, voltage-sensitive dyes used in conjunction with LSCM imaging could be extremely useful in studies of excitation-secretion coupling in intact islets of Langerhans.