Electrokinetic and hydrodynamic properties of sarcoplasmic reticulum vesicles: a study by laser Doppler electrophoresis and quasi-elastic light scattering.

The electrical properties of sarcoplasmic reticulum vesicles were studied using electrophoretic mobility measurements by laser Doppler velocimetry. Combined with quasi-elastic light scattering analysis, this method enabled us to monitor changes in membrane surface charge density upon addition of NaCl, CaCl2, and MgCl2 solutions. At low ionic strength (4.7 mM), and in the absence of divalent salts, sarcoplasmic reticulum vesicles displayed a zeta potential value of -32.6 mV and a surface charge density of 5.4 10(-3) C/m2, corresponding to one elementary negative charge per 30 nm2 or about 2900 negative charges per vesicle. Upon addition of NaCl (up to 100 mM) vesicles became more negative (from 2900 to 6200 negative charges per vesicle), suggesting chloride binding or adsorption on the membrane. A noticeable increase of the vesicular size was observed upon addition of calcium or magnesium (25 to 30% increase of the hydrodynamic radius for 2.5 mM CaCl2 or MgCl2). This effect was prevented or suppressed by addition of relatively low concentrations of NaCl (50 mM). At low divalent cation concentrations one-third of the total net charge of the vesicles was neutralized suggesting a specific binding of cation on the membrane. At higher salt concentrations no modification of this parameter was observed. These results support the Gouy and Chapman theory about the electrical properties of sarcoplasmic reticulum vesicles.