CULTURE DURATION AND CONDITIONS AFFECT THE OSCILLATIONS OF CYTOPLASMIC CALCIUM-CONCENTRATION INDUCED BY GLUCOSE IN MOUSE PANCREATIC-ISLETS

The pattern of the increase in cytoplasmic Ca-i(2+) that glucose produces in beta cells has been reported to be highly variable. Here, we evaluated the influence of the culture duration (1-4 days) and conditions (5-10 mmol/l glucose) on Ca-i(2+) in normal mouse islets stimulated by glucose. After 1 day of culture in 10 mmol/l glucose, a rise of the glucose concentration from 3 to 15 mmol/l induced a triphasic change of Ca-i(2+) in the islets. A small initial decrease was followed by a large peak increase and then by regular fast oscillations (similar to 2.5/min). When the culture was prolonged to 2, 3 and 4 days, the initial decrease became inconsistent and the peak occurred earlier, whereas the oscillations decreased in frequency, increased in duration and eventually disappeared; on day 4 the Ca-i(2+) rise was sustained. After culture in 5 mmol/l glucose, the pattern of Ca-i(2+) changes induced by 15 mmol/l glucose was different. The initial decrease was very pronounced, the first peak was delayed and clearly separated from the subsequent oscillations. These were of a mixed type (fast Ca2+ transients on top of slow ones) after 1 day, and of a slow type only after 4 days. These alterations in the Ca-i(2+) oscillations triggered by glucose could not be ascribed to desynchronization of the signal between different regions of the islets. In conclusion, culturing normal mouse islets in 5 or 10 mmol/l glucose for 1-4 days, markedly alters the characteristics of the changes in Ca-i(2+) produced by glucose. This pitfall must be borne in mind when studying stimulus-secretion coupling in beta cells from normal or diabetic animals, or from human islets.