Pattern of rise in subplasma membrane Ca2+ concentration determines type of fusing insulin granules in pancreatic β cells

We simultaneously analyzed insulin granule fusion with insulin fused to green fluorescent protein and the subplasma membrane Ca2+ concentration ([Ca2+]PM) with the Ca2+ indicator Fura Red in rat β cells by dual-color total internal reflection fluorescence microscopy. We found that rapid and marked elevation in [Ca2+]PM caused insulin granule fusion mostly from previously docked granules during the high KCl-evoked release and high glucose-evoked first phase release. In contrast, the slow and sustained elevation in [Ca2+]PM induced fusion from newcomers translocated from the internal pool during the low KCl-evoked release and glucose-evoked second phase release. These data suggest that the pattern of the [Ca2+]PM rise directly determines the types of fusing granules.