Sensitivity to oxidants of mitochondrial and sarcoplasmic reticular calcium uptake in saponin-treated cardiac myocytes

Calcium transport functions of mitochondria and sarcoplasmic reticulum (SR) were studied without prior extraction using isolated rat heart myocytes permeabilized with saponin. Calcium uptake by SR was rapid and its affinity was high in comparison to calcium uptake by mitochondria, which had a higher capacity. The sensitivity of uptake to two oxidants, H2O2 and HOCl (hypochlorous acid), depended on the cytosolic calcium concentration; when this was similar to the concentration in diastole (180 nM), HOCl inhibited calcium uptake by mitochondria and SR, whereas when the calcium concentration was 750 nM, mitochondrial calcium uptake showed relatively high resistance, although SR uptake was still markedly inhibited by HOCl. Calcium uptake of both mitochondria and SR was less sensitive to the action of H2O2 than to HOCl, and the H2O2 effect was less dependent on the cytosolic calcium concentration. Therefore, HOCl, when produced by activated leukocytes and supplied to the heart cells, may seriously impair the excitation-contraction coupling function of SR, whereas H2O2, possibly generated directly by mitochondria or generated from superoxide anions, may be tolerated relatively well by heart SR and mitochondria.