Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells

1. The cytoplasmic Ca2+ concentration ([Ca2+](i)) was measured in single ce lls and cell clusters of different sizes prepared from mouse pancreatic isl ets. 2. During stimulation with 15 mM glucose, 20% of isolated cells were inert, whereas 80% showed [Ca2+](i) oscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min(- 1) and a less prominent one of 0.27 min(-1). 3. In contrast, practically all clusters (2-50 cells) responded to glucose, and no inert cells were identified within the clusters. ils compared to si ngle cells, mean [Ca2+](i) was more elevated, [Ca2+](i) oscillations were m ore regular and their major frequency was slightly higher (but reached a pl ateau at similar to 0.25 min(-1)). In some cells and clusters, faster oscil lations occurred on top of the slow ones, between them or randomly. 4. Image analysis revealed that the regular [Ca2+](i) oscillations were wel l synchronized between all cells of the clusters. Even when the Ca2+ respon se was irregular, slow and fast [Ca2+](i), oscillations induced by glucose were also synchronous in all cells. 5. In contrast, [Ca2+](i) oscillations resulting from mobilization of intra cellular Ca2+ by acetylcholine were restricted to certain cells only and we re not synchronized. 6. Heptanol and 18 alpha-glycyrrhetinic acid, two agents widely used to blo ck gap junctions, altered glucose-induced Ca2+ oscillations, but control ex periments showed that they also exerted effects other than a selective unco upling of the cells. 7. The results support theoretical models predicting an increased regularit y of glucose-dependent oscillatory events in clusters as compared to isolat ed islet cells, but contradict the proposal that the frequency of the oscil lations increases with the number of coupled cells. Islet cell clusters fun ction better as electrical than biochemical syncytia. This may explain the co-ordination of [Ca2+](i) oscillations driven by depolarization-dependent Ca2+ influx during glucose stimulation.