Glucose Effects on Cytoplasmic Ca²⁺ of Individual Pancreatic β-Cells Recorded by Two Procedures for Dual-Wavelength Fluorometry

Summary

Two different dual-wavelength microfluorometric systems were used to measure the cytoplasmatic Ca²⁺ concentration (Ca_i ²⁺) of individual pancreatic β-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 Ca_i ²⁺ followed by a rise with a peak and a subsequent sustained intermediary level. The increase of Ca_i ²⁺ depended on the presence of extracellular Ca²⁺. In some cells the pattern differed and the increase of the glucose concentration induced large oscillations of Ca_i ²⁺. Glucose-induced increase of Ca_i ²⁺ was associated with small rapid fluctuations, which were detectable only with the fast time-sharing approach. The characteristic Ca_i ²⁺ response to glucose was maintained by some β-cells after culture for more than 3 weeks. The results indicate that the glucose sensitivity of individual β-cells varies, and that many electrically coupled cells within an islet collectively determine the characteristic pattern of rhythmic depolarization.

Key Words

Pancreatic islets - Calcium content - Glucose