Glucose effects on cytoplasmic Ca2+ of individual pancreatic beta-cells recorded by two procedures for dual-wavelength fluorometry.

Two different dual-wavelength microfluorometric systems were used to measure the cytoplasmatic Ca2+ concentration (Cai2+) of individual pancreatic beta-cells loaded with the indicator fura-2. Fluorescence was excited continuously at 340 nm with frequent manual checks at 360 nm. Alternatively, the 340/380 nm fluorescence excitation ratio was recorded based on automated samples at the two wavelengths every 10 msec. Irrespective of the experimental technique a rise of the glucose concentration from 3 to 20 mM resulted in an initial lowering of Cai2+ followed by a rise with a peak and a subsequent sustained intermediary level. The increase of Cai2+ depended on the presence of extracellular Ca2+. In some cells the pattern differed and the increase of the glucose concentration induced large oscillations of Cai2+. Glucose-induced increase of Cai2+ was associated with small rapid fluctuations, which were detectable only with the fast time-sharing approach. The characteristic Cai2+ response to glucose was maintained by some beta-cells after culture for more than 3 weeks. The results indicate that the glucose sensitivity of individual beta-cells varies, and that many electrically coupled cells within an islet collectively determine the characteristic pattern of rhythmic depolarization.