Glucose-induced early changes in cytoplasmic calcium of pancreatic beta-cells studied with time-sharing dual-wavelength fluorometry.

The cytoplasmic calcium concentration (Ca2+i) was measured in suspensions of fura-2-loaded mouse pancreatic beta-cells by recording the 340:380 nm fluorescence excitation ratio. Exposure to 20 mM glucose resulted in an initial reduction and later increase of Ca2+i irrespective of preincubation in medium containing 0.5 or 1.28 mM Ca2+ and 0 or 3 mM glucose. When elevating the Ca2+ concentration to 5 or 10 mM only 5 min before raising glucose to 20 mM, the sugar-induced reduction of Ca2+i became more pronounced like the subsequent increase. However, when the Ca2+ concentration was increased from 1.28 to 10 mM 2 min after stimulation with glucose, there was a sudden pronounced Ca2+i transient, which was followed by a decrease and a slower secondary rise. After preincubation in 20 mM glucose the glucose-induced initial reduction of Ca2+i was only seen in a Ca2+-deficient medium. Reintroduction of the sugar in the presence of extracellular Ca2+ resulted in an immediate rise of Ca2+i, the rapidity of which depended on the transmembrane Ca2+ gradient. The results emphasize the role of a saturable beta-cell pool of Ca2+ in glucose-induced reduction of Ca2+i and indicate that the first phase of insulin release depends on an influx of extracellular Ca2+.