Background and aims: After food intake, pancreatic islets secrete insulin with a biphasic pattern, which is impaired in type 2 diabetic patients. The mechanisms underlying this pattern have not been fully elucidated and the presence of distinct vesicle pools has been proposed as explanation. Electrical activity of islets consists of individual β cell activity (action potentials, APs) and the multicellular electrical response due to coupling between β cells (slow potentials, SPs). We addressed here the contribution of these two distinct activities to the 1 st and the 2 nd phase of β cell activity, and their modulation by physiological concentrations of GLP-1. Materials and methods: Electrical activity (SPs and APs) of entire mice (C57Bl6/J, age 10-14 weeks) or human islets have been recorded on polymer-coated microelectrode arrays (MEA). These new electrodes allow simultaneous detection of APs (of very low amplitude) and SPs at a high time resolution (10'000 points/s x60 electrodes) for a prolonged period mimicking physiological digestion (2 h). Specific filters differentially detect SPs and APs and 3 parameters were analyzed at the same time: SP frequencies, SP amplitudes and AP frequencies. To investigate synchrony of SPs between different regions of the same islet, we used high density MEAs with an inter-electrode distance of 30 instead of 200 µm followed by analysis via Matlab. Results: Islets were stimulated with glucose concentrations in the physiological range (5.5-8.2 mM). Electrical responses were biphasic for both SPs and APs. APs were mainly present during the 1 st phase while the transition between the 1 st and the 2 nd phase is driven by SPs. In 2 nd phase, the SP amplitude and synchronisation increased significantly (1 st phase: 18.1±2.3 µV; 2 nd phase: 47.4±5.5 µV, p<0.0001), reflecting further electrical coupling and synchronisation of β cells. The intra-islet synchronisation was also further correlate using high density MEAs. The incretin GLP-1, at a physiological postprandial concentration (50 pM), did not change the individual activity of cells (APs) but increased specifically coupling (SPs) and only in the 2nd phase (37.7±3.0 µV vs 47.0±4.2 µV with GLP-1, p<0.0001). Furthermore, when GLP-1 was applied in the presence of a subthreshold glucose concentration (5.5 mM), the hormone triggered only a 2 nd phase. The biphasic electric profile was confirmed in human islets. Their exposure to a glucotoxic medium (20 mM glucose, 65 h) considerably increased basal activity and abolished the biphasic response as well as the discrimination between glucose concentrations. These glucotoxic effects were partially reversible. Conclusion: Our data show that (i) electrical activity pattern shape the biphasic secretion and (ii) the transition period between the 1 st and the 2 nd phase results from increasing electrical synchronisation. Thus biphasic secretion is primarily dictated by changes in electrical activity rather than vesicle pools. The effects of GLP-1 on only coupling SP signals and only during the 2 nd phase explain its clinical effects.