A mathematical model is described to accurately calculate vesicle size and neurotransmitter concentration distributions from electrochemical data. This model uses parameters from electrochemical exocytosis data obtained from PC12 cells in culture to calculate a size distribution that is then correlated to the size of vesicles obtained by electron microscopy. The relative standard deviation of the size distribution calculated from electrochemical data is 25% which matches the relative standard deviation of the vesicle size distribution measured by electron microscopy. The distribution calculated from electrochemical data is normalized to the vesicle size distribution of PC12 cell vesicles obtained from electron microscopy. Calculation of a vesicular catecholamine concentration histogram from the normalized size data and electrochemical parameters is then possible for individual exocytosis events. The average vesicular catecholamine concentration for PC12 cells as calculated by this method is 148±7 mM. More importantly, there is a distribution of concentration rather than a constant value. Additionally, the model permits evaluation of the concentration of transmitter in each individual vesicle and vesicle size for each vesicle from electrochemical data when the overall vesicle size distribution is known. Copyright (C) 1999 Elsevier Science B.V.
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