Possible interactions between polymerized (F-) actin and insulin-storage granules from rat islets of Langerhans were examined in vitro by comparing the sedimentation of the granules in the presence of various actin concentrations. Actin in the concentration range 0.1–0.5 mg/ml produced a retardation in granule-sedimentation rates consistent with binding of the granules to the actin filaments. The interaction was increased by addition of ATP (2mM), but was decreased by CaCl2 (0.1 mM). Binding of granules to actin was unaffected by cyclic AMP or by preincubation of the granules with phospholipase C. Specificity of the interaction was confirmed by the use of depolymerized (G-) actin and of myosin to provide a solution of comparable viscosity; neither of these caused any alteration of granule sedimentation. Possible implications of this interaction of insulin-storage granules with actin for the mechanism of insulin secretion are briefly discussed.

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