Priming of insulin granules for exocytosis by granular Cl- uptake and acidification

ATP-dependent priming of the secretory granules precedes Ca2+-regulated neu roendocrine secretion, but the exact nature of this reaction is not fully e stablished in all secretory cell types. We have further investigated this r eaction in the insulin-secreting pancreatic B-cell and demonstrate that gra nular acidification driven by a V-type H+-ATPase in the granular membrane i s a decisive step in priming, This requires simultaneous Cl- uptake through granular ClC-3 Cl- channels. Accordingly, granule acidification and primin g are inhibited by agents that prevent transgranular Cl- fluxes, such as 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and an antibody again st the ClC-3 channels, but accelerated by increases in the intracellular AT P:ADP ratio or addition of hypoglycemic sulfonylureas, We suggest that this might represent an important mechanism for metabolic regulation of Ca2+-de pendent exocytosis that is also likely to be operational in other secretory cell types.