Differences in the oxygen consumption pattern suggest that nutrient-induced insulin secretion is differentially regulated in primary mouse islets and MIN6 pseudo-islets

Background and aims: In addition to plasma membrane depolarization metabolic amplification is required for the full extent of nutrient-induced insulin secretion. For metabolomic investigations insulin-secreting cell lines, such as MIN6, are utilized. The relevance of such data is, however, unclear.

Methods: All parameters were measured by perifusion of freshly isolated mouse islets or of MIN6 pseudo-islets. The oxygen consumption was measured by a fluorescence quenching technique, the free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) by imaging after loading with Fura and the insulin secretion by ELISA of the fractionated perifusate.

Results: MIN6 pseudo-islets perifused with 3 mM glucose showed a marked increase in [Ca²⁺]_i and insulin secretion when stimulated with 40 mM K⁺ for 10 min. After wash-out, the subsequent stimulation with 30 mM glucose was virtually ineffective on either parameter. However, when the sequence of exposure was reversed glucose was significantly effective, but much less so than K⁺. In normal islets, in contrast, both stimuli were nearly equi-effective. The oxygen consumption of islets responded to a 30 mM glucose with a marked, sustained increase whereas MIN6 pseudo-islets showed an increase only when a K⁺ depolarization had preceded the glucose challenge. K⁺ depolarization resulted in a biphasic response: a fast increase which was followed by a long-lasting depression. The latter phase was much stronger in MIN6 cells.

Conclusion: There are marked differences between the response patterns of MIN6 pseudo-islets and those of freshly isolated islets. This suggests that, whenever possible, primary islets should be preferred to study the mechanisms of metabolic amplification.