We measured exocytosis as increases in membrane capacitance in mouse α-cells, positively identified by glucagon immunostaining, in response to membrane depolarization. A, representative capacitance responses of mouse α-cells infected with green fluorescent protein (Ad-GFP; black traces) or SUMO1 (Ad-SUMO1; red traces). B, cumulative capacitance responses from mouse α-cells expressing Ad-GFP (open bars) and Ad-SUMO1 (black bars). Treatment with exendin 4 in the bath solution suppressed the exocytotic response, regardless of the presence of 0.1 mm or 1 mm cAMP in the patch pipette, but could not suppress the elevated responses observed in α-cells infected with Ad-SUMO1. *P < 0.05, **P < 0.01, ***P < 0.001 for comparisons with the respective control, or as indicated.