| Abstract: | The distribution of in vivo injected 45Ca++ in the subcellular fractions of rat heart has been studied. Most of the radioactivity of the cell was found to be associated with the subcellular organelles; only a small fraction was recovered in the soluble phase. Mitochondria contained the greatest part of the total radioactivity associated with the subcellular organelles. After injection of 45Ca++ the specific activity of the mitochondrial calcium pool was several times higher than that of the calcium of the sarcoplasmic reticulum. Pentachlorophenol has been administered to rats to uncouple oxidative phosphorylation in heart mitochondria in vivo and its effect on the distribution of 45Ca++ in the heart studied. Under these conditions, it has been found that mitochondria contained much less 45Ca++ than the controls; this decrease was paralleled by an increase of the radioactivity associated with the microsomes and with the final supernatant. Experiments in which 45Ca++ was added to heart homogenates at 0° indicated that 45Ca++ also became bound to mitochondria and the other subcellular structures at 0°. However, PCP had no effect on the distribution of radioactivity among the subcellular fractions under these conditions. The results suggest that (1) energy-linked movements of Ca++ take place in mitochondria of the intact rat heart, (2) a part of the uptake of 45Ca++ by mitochondria does not depend on metabolism, and, (3) the movements of Ca++ in heart mitochondria in vivo are probably more active than those in the sarcoplasmic reticulum. |
