Inulin exclusion volume of sarcoplasmic reticulum vesicles under various solvent conditions

The inulin exclusion volume of sarcoplasmic reticulum vesicles was studied using the ultracentrifugation method, after equilibrium penetration of solutes had been established under various solvent conditions. The intravesicular volume in equilibrium decreases with increasing salt concentration, while it does not change with neutral molecules. The concentrations of salt ions which caused the intravesicular volume to decrease to a half of the maximal volume were as follows; about 200 mM for KCl and NaCl, about 30 mM for CaCl2 and MgCl2. The mechanism of the volume change can be explained in terms of the Donnan pressure model on the basis of the existence of fixed charges inside the vesicles.