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Correlation between Vesicle Quantal Size and Fusion Pore Release in Chromaffin Cell Exocytosis

Christian Amatore ^*, Stéphane Arbault ^*, Imelda Bonifas ^*, Yann Bouret ^*, Marie Erard ^*, Andy G. Ewing  and Leslie A. Sombers 

^* Ecole Normale Supérieure, Département de Chimie, UMR CNRS-ENS-UPMC 8640 "PASTEUR", Paris, France; and The Pennsylvania State University, Departments of Chemistry and Neural & Behavioral Sciences, University Park, Pennsylvania

Correspondence: Address reprint requests to C. Amatore, Fax: 33-1-4432-3863; E-mail: Christian.Amatore{at}ens.fr.

A significant number of exocytosis events recorded with amperometry demonstrate a prespike feature termed a "foot" and this foot has been correlated with messengers released via a transitory fusion pore before full exocytosis. We have compared amperometric spikes with a foot with spikes without a foot at chromaffin cells and found that the probability of detecting a distinct foot event is correlated to the amount of catecholamine released. The mean charge of the spikes with a foot was found to be twice that of the spikes without a foot, and the frequency of spikes displaying a foot was zero for small spikes increasing to 50% for large spikes. It is hypothesized that in chromaffin cells, where the dense core is believed to nearly fill the vesicle, the expanding core is a controlling factor in opening the fusion pore, that prefusion of two smaller vesicles leads to excess membrane, and that this slows pore expansion leading to an increased observation of events with a foot. Clearly, the physicochemical properties of vesicles are key factors in the control of the dynamics of release through the fusion pore and the high and variable frequency of this release makes it highly significant.

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