Ion movement accompanied by calcium uptake of sarcoplasmic reticulum vesicles studied through the osmotic volume...

The volume change of sarcoplasmic reticulum vesicles was induced by Ca2+ uptake. This volume change was measured by the light-scattering method. When vesicles were shrunk beforehand under the condition that anions are more permeable than cations, they swelled during Ca2+ uptake due to the concomitant incorporation of anions. On the contrary, they shrank with Ca2+ uptake due to the extrusion of cations under the condition that cations are more permeable than anions. From the analysis of the volume change it was concluded that all ions other than Ca2+ were transported passively in order to neutralize the membrane potential generated by the Ca2+ pump. These results support the idea that the Ca2+ pump is electrogenic. By using this technique, it became possible to measure the fast Ca2+ uptake rate. The dependence of the Ca2+ uptake rate on the Ca2+ concentration suggests that the site at which Ca2+ inhibits Ca2+ uptake is located inside the vesicle. From the osmotic response of the vesicles, the intravesicular concentration of free Ca2+ was estimated to be about 15 mM, when Ca2+ was fully taken up under the physiological condition.