In chromaffin cells, adrenaline is known to be released through docking and then fusion of a secretory vesicle to the cytoplasmic membrane of the cell. Here we propose a method for the calculation of the dynamics of the vesicle membrane during the fusion from amperometric currents observed during individual exocytotic secretion events. The method is based on recognition of the fact that the overall current spike shape results from the convolution of the membrane dynamics with the rate of diffusion and exchange of the catecholamine cation inside the matrix core of the vesicle. This convolution can be treated analytically thanks to a reasonable approximation on the relative time scales of the opening function and diffusion; this leads to a convolution integral with which one deconvolutes the experimental amperometric data. An alternative numerical treatment through Brownian motion simulations dispenses with the need for this simplifying approximation. Combination of both approaches yields the membrane dynamics with a precision and a time resolution never achieved before. The peculiar dynamics of the vesicle membrane hint that exocytotic events are regulated by the swelling of the matrix polyelectrolyte core of the vesicle (although this important component is transparent in the analysis proposed here); this points to the important role of matrix swelling in exocytotic behavior. In particular, the effect may be elaborated to offer a new energetic interpretation of the transition between pore release and fusion release: secretory vesicles which involve pores and matrices similar to those of the adrenal cells investigated here can be separated into two classes according to their radius and catecholamine content. Small vesicles (`ca. 25 nm radius, and containing ca. 20 000 molecules) should always release their contents through pore docking; larger vesicles should always fuse, unless another mechanism closes the pore before ca. 20 000 molecules of catecholamine have been released.
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