The electrophysiological measurements were performed on α-cells within freshly isolated intact islets (from NMRI or C57Bl/6 mice), using an EPC-10 patch-clamp amplifier (HEKA Electronics, Lambrecht/Pfalz, Germany) and Pulse software. All electrophysiological experiments were performed at 34°C. α-Cells were identified as those active at low (3 mM) glucose and were differentiated from δ-cells (some of which fire action potentials, albeit at low frequency at this glucose concentration) by the distinct appearance of action potentials (Figure S2a). For the membrane potential recordings (Figure 2c), the perforated patch configuration was used as described previously (20 (link)) using solutions IC1 and EC1. Exocytosis was measured as increases in membrane capacitance in α-cells in intact islets as described previously using the standard whole-cell configuration and IC2 and EC2.