N-Ethylmaleimide-sensitive factor (NSF) has an important role in fusion processes within intracellular compartments and at the plasma membrane, but the exact role of this protein in the exocytotic machinery has not yet been determined. NSF was found to be present in the cytosol of rat pancreatic β-cells and rat insulinoma INS-1 cells. Capacitance measurements revealed that exocytosis of primed granules was not affected by the presence of a monoclonal antibody against NSF, mAb 2E5, suggesting that NSF is not involved in the fusion process. The antibody markedly decreased rapid refilling of new granules from a reserve pool during a first stimulation. However, slow refilling of primed granules occurred within a 2 min period between the first and second stimulations. We conclude that NSF is required in the exocytotic process in order to obtain a complete exocytotic response. Possible mechanisms by which NSF takes part in this process in insulin-secreting rat β-cells are discussed.

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