Insulin granule mobility, cytosolic calcium concentration, and stimulated insulin secretion in MIN6 cells and beta cells: differences and similarities

Background and aims: The conventional view that insulin granules are sequentially recruited to release insulin has been superseded by newer, more complex models, resulting from TIRF imaging. Here, we compared insulin secretion, cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and granule mobility in MIN6 cells and in primary mouse beta cells.

Methods: Insulin secretion was measured in perifused MIN6 pseudo-islets or cultured mouse islets. [Ca²⁺]_i was measured by the Fura technique, granule mobility (insulin-EGFP-label) was imaged by TIRF microscopy. In these experiments the temperature was 32 °C.

Results: With both cell types, there was a moderate secretion response to 30 mM glucose and a strong secretion when 40 mM KCl was subsequently applied. The reversed sequence of exposure slightly decreased the effect of KCl and strongly decreased the response to 30 mM glucose. With MIN6 pseudo-islets the secretion was higher at 32 °C than at 37 °C, while the opposite was true for mouse islets. KCl produced a marked increase in [Ca²⁺]_i both in MIN6 cells and beta cells, while glucose was nearly inefficient in MIN6 cells but not in beta cells. In MIN6 cells the total granule number, the number of arriving granules at the plasma membrane and the short-term resident granules were markedly increased by both glucose and KCl. In beta cells in contrast these parameters were increased by glucose, but only after a preceding KCl depolarization.

Conclusion: There are marked differences between the pre-fusion behaviour of insulin granules in MIN6 cells and that in beta cells, in spite of similar patterns of [Ca²⁺]_i.