THAPSIGARGIN INHIBITS THE GLUCOSE-INDUCED DECREASE OF INTRACELLULAR CA2+ IN MOUSE ISLETS OF LANGERHANS

Stimulation of pancreatic islets of Langerhans with glucose results in changes in intracellular Ca2+ concentration ([Ca2+](i)). With the use of mouse islets loaded with fura 2, the earliest glucose-induced alte ration of [Ca2+](i) was a pronounced decline in [Ca2+](i). This effect (phase 0) was evident 1 min after increasing extracellular glucose fr om 2 to 12 mM and was sustained for 3-5 min. Phase 0 was also observed when glucose was increased from 5 to 12 mM, indicating that it was no t an experimental artifact resulting from substrate depletion. The [Ca 2+](i)-lowering effect of glucose was mimicked by D-glyceraldehyde but not by 2-deoxyglucose, pyruvate, glyburide, or 30 mM extracellular KC l. Manno-heptulose inhibited phase 0, whereas diazoxide, sodium azide, calmidazolium, or increasing extracellular [Ca2+] to 10 mM were all w ithout effect. After the elevation of islet [Ca2+](i) with 5 mu M glyb uride, 12 mM glucose caused a considerable transient decrease in [Ca2](i). Under similar conditions, 5 mM caffeine attenuated phase 0, wher eas 1 mu M thapsigargin, a specific inhibitor of the sarcoplasmic and endoplasmic reticulum family of Ca2+-adenosinetriphosphatases (SERCA), almost completely inhibited any glucose-induced reduction of [Ca2+](i ). These observations suggest that glucose causes an elevation of beta -cell SERCA activity triggered by factors generated during the cytosol ic stages of glycolysis.