Localization of calcium antimonate in the atrial and ventricular muscle fibers of the cat heart

The technique of using potassium antimonate to precipitate sodium ions selectively in neutral or in weakly alkaline solutions has been used in preparing cardiac tissues for electronmicroscopic studies. We have observed, however, that potassium antimonate also precipitates calcium ions in these solutions. Since this reaction is not specific for sodium ions, the electron-dense precipitates in antimonate-treated tissues probably contain both sodium and calcium antimonate. But the calcium antimonate can be chelated by EDTA (ethylenediamine tetraacetic acid) or EGTA [(ethylene-bis-oxyethylenenitrilo) tetraacetic acid], separating it from the sodium antimonate. This permitted study of the localization of calcium as an antimonate precipitate in the atrial and ventricular muscle fibers of the cat heart. Calcium antimonate was found mainly in the nuclei and in the I bands of both the atrial and ventricular muscle fibers. Mitochondria contain a modest amount of calcium antimonate precipitate. Calcium is also clearly localized to the longitudinal tubules and the lateral sacs of the sarcoplasmic reticulum of ventricular muscle fibers; less clearly so in atrial muscle fibers. In addition, a significant amount of calcium is found in L lines on both sides of the M bands in ventricular muscle fibers.