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Sustained glucose-stimulated insulin secretion in mouse islets is not culture-dependent To the Editor: The process of sustained glucose-evoked insulin secretion has been extensively studied, although the molecular mechanisms implicated are still poorly understood. Biphasic insulin release, and more specifically its associated second phase, is a subject of long-standing and intensive research [1]. In this context, it is intriguing that the dynamics of insulin secretion in a mouse model differ, depending on which of two widely used experimental preparations are employed. Using in situ pancreatic perfusion, glucose-stimulated insulin release is essentially monophasic, with a very weak second phase following the 4-to 5-min first phase [2, 3]. In contrast, once isolated, pancreatic islets exhibit sustained secretory responses to glucose in a perifusion protocol. How the mouse beta cell can switch from the transient (in situ) to the sustained (in vitro) secretory mode is unknown. Investigators commonly keep the islets in culture between the isolation procedure and perifusion experiments. This suggests the possibility that culture conditions might induce changes in gene expression and/or enzyme activity, thereby modifying the machinery of metabolism-secretion coupling. For instance, glucokinase activity is induced in mouse islets cultured at high glucose [4]. Such culture-dependent changes are supposed to modify glucose metabolism, thereby promoting signal generation implicated in the sustained phase of insulin release. Here, we questioned the effects of culture periods on the secretory profiles of isolated mouse islets in response to glucose stimulation. The kinetics of glucose-stimulated insulin release were measured in mouse islets at different time periods following isolation. Pancreatic islets were isolated by collagenase digestion from male BALB/c mice weighing 25 to 30 g [5]. For Day 0, freshly isolated islets were used 1 to 2 h after isolation. For cultured islets, isolated mouse islets were cultured free-floating in RPMI-1640 medium (11.1 mmol/l glucose) with 5% fetal calf serum for 3 and 7 days before the secretion assay. Islet perifusions were carried out as described [5] using 15 to 30 hand-picked islets per chamber of 250-µl volume maintained by thermostat at 37 °C (Brandel, Gaithersburg, Md., USA). The flux was set at 0.5 ml/min and after a 20-min washing period at basal glucose, fractions were collected every minute with glucose changes as detailed below. Insulin levels were determined by radioimmunoassay using rat insulin as standard [5]. Insulin secretion was expressed as the fraction of insulin collected every minute, normalised per 10 islets (ng in-sulin/10 islets). Mouse …