The release of calcium from heart mitochondria by sodium

The release of Ca²⁺ from heart mitochondria could be induced by addition of 20 to 50 mm Na⁺ to the incubation medium. Of the other cations tested, K⁺, Rb⁺, Cs⁺, and Mg²⁺ were without effect, whereas some release was induced by Li⁺. In the presence of ruthenium red, which prevented the re-binding of the released Ca²⁺, 2 to 5 mm Na⁺ were sufficient to produce a measurable release of Ca²⁺. The amount of Ca²⁺ freed from the mitochondria was proportional to the amount of Na⁺ added, and to the concentration of Ca²⁺ present in the mitochondria. The release appeared to be a biphasic process, and in the first 15 s up to 3 nmol Ca²⁺ per mg of mitochondrial protein could be dissociated from the mitochondria.The phenomenon was more evident at pH 6.5 than under slightly alkaline conditions. The significance of this release in the process of heart contraction is discussed.