Electrokinetic properties of synaptic vesicles and synaptosomal membranes

Using the technique of electrophoretic light scattering, we have measured the electrophoretic mobilities of synaptic vesicles and synaptosomal plasma membranes isolated from guinea-pig cerebral cortex. The electrophoretic mobility of synaptic vesicles is slightly greater than that of synaptosomal plasma membranes. Ca+2 and Mg+2 reduced the mobility of both species to the same extent at physiologically relevant concentrations (0-1 mM) and near-physiologic ionic strength. The extent of the reduction was not large (approximately 6% for synaptic vesicles in the presence of 100 mM KCl) at 1 mM divalent cation concentrations. At concentrations of approximately 2 mM and higher, Ca+2 reduced the mobility of synaptic vesicles more than did Mg/2. A similar but much smaller effect was observed in the case of synaptosomal plasma membranes. The addition of 1 mM Mg+2-ATP had no effect upon synaptic vesicle mobility either in the presence or absence of the ionophores nigericin or valinomycin. These data, together with earlier work (Siegel et al., 1978, Biophys. J. 22:341-346), demonstrate that substantial reduction of the average electrostatic surface charge density is not the most important role of divalent cations in promoting close approach of secretory granules and secretory cell membranes, and that it is certainly not the Ca+2-specific step in exocytosis.