Ion movement accompanied by calcium uptake of sarcoplasmic reticulum vesicles studied through the osmotic volume change by the light scattering method

Abstract

Summary 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 15mm, when Ca²⁺ was fully taken up under the physiological condition.