CHROMAFFIN CELL CORTICAL ACTIN NETWORK DYNAMICS CONTROL THE SIZE OF THE RELEASE-READY VESICLE POOL AND THE INITIAL RATE OF EXOCYTOSIS

Morphological, biochemical, and membrane capacitance measurements were used to study the role of cortical filamentous actin (F-actin) in exo ctyosis. Fluorescence and electron microscopy of resting chromaffin ce lls revealed a cortical actin network that excluded secretory vesicles from the subplasmalemmal area. Phorbol ester (PMA) treatment disrupte d cortical F-actin and increased both the number of vesicles within th e 0-50 nm subplasmalemmal zone and the initial rate of stimulated cate cholamine release. In PMA-pretreated cells, membrane capacitance studi es showed an increased number of vesicles fusing with the plasmalemma during the first two depolarizations of a train. PMA did not affect vo ltage-dependent Ca2+ influx. The total number of vesicles fused with t he plasma membrane correlated well with the number of vesicles occupyi ng the 0-50 nm cortical zone. Therefore, cortical F-actin disassembly allows translocation of vesicles to the plasmalemma in preparation for exocytosis.