Calculation of transmitter concentration in individual PC12 cell vesicles with electrochemical data and a distribution of vesicle size obtained by electron microscopy

A mathematical model is described to accurately calculate vesicle size and neurotransmitter concentration distributions from electrochemical data. Thi s model uses parameters from electrochemical exocytosis data obtained from PC12 cells in culture to calculate a size distribution that is then correla ted to the size of vesicles obtained by electron microscopy. The relative s tandard deviation of the size distribution calculated from electrochemical data is 25% which matches the relative standard deviation of the vesicle si ze distribution measured by electron microscopy. The distribution calculate d from electrochemical data is normalized to the vesicle size distribution of PC12 cell vesicles obtained from electron microscopy. Calculation of a v esicular catecholamine concentration histogram from the normalized size dat a and electrochemical parameters is then possible for individual exocytosis events. The average vesicular catecholamine concentration for PC12 cells a s calculated by this method is 148 +/- 7 mM. More importantly, there is a d istribution of concentration rather than a constant value. Additionally, th e model permits evaluation of the concentration of transmitter in each indi vidual vesicle and vesicle size for each vesicle from electrochemical data when the overall vesicle size distribution is known. (C) 1999 Elsevier Scie nce B.V. All rights reserved.