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Synchronization and entrainment of cytoplasmic Ca²⁺ oscillations in cell clusters prepared from single or multiple mouse pancreatic islets

Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, B-1200 Brussels, Belgium

Submitted 12 February 2004 ; accepted in final form 20 April 2004

In contrast to pancreatic islets, isolated -cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i). Here, clusters of 5–30 cells were prepared from a single mouse islet or from pools of islets, loaded with fura-2, and studied with a camera-based system. [Ca²⁺]_i oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index . During perifusion with 12 mM glucose, the clusters exhibited regular [Ca²⁺]_i oscillations that were quasi-perfectly synchronized ( period of 1.4% and index close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized, even when prepared from one single islet. Pairs of clusters neighboring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12 mM glucose by applying external signals. A single pulse of 20 mM glucose, 10 mM amino acids, or 10 µM tolbutamide transiently altered [Ca²⁺]_i oscillations but did not reset the clusters to oscillate synchronously. On a background of 12 mM glucose, repetitive applications (1 min/5 min) of 10 µM tolbutamide, but not of 20 mM glucose, synchronized separate clusters. Our results identify a level of -cell heterogeneity intermediate between single -cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.

-cells; intracellular calcium; insulin secretion

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