Transport of Ca2+ from sarcoplasmic reticulum to mitochondria in rat ventricular myocytes

Studies with electron microscopy have shown that sarcoplasmic reticulum (SR) and mitochondria locate close to each other in cardiac muscle cells. We investigated the hypothesis that this proximity results in a transient exposure of mitochondrial Ca2+ uniporter (CaUP) to high concentrations of Ca2+ following Ca2+ release from the SR and thus an influx of Ca2+ into mitochondria. Single ventricular myocytes of rat were skinned by exposing them to a physiological solution containing saponin (0.2 mg/ml). Cytosolic Ca2+ concentration ([Ca2+](c)) and mitochondrial Ca2+ concentration ([Ca2+](m)) were measured with fura-2 and rhod2, respectively. Application of caffeine (10 mM) induced a concomitant increase in [Ca2+](c) and [Ca2+](m). Ruthenium red, at concentrations that block CaUP but not SR release, diminished the caffeine induced increase in [Ca2+](m) but not [Ca2+](c). In the presence of 1 mM BAPTA, a Ca2+ chelator, the caffeine-induced increase in [Ca2+](m) was reduced substantially less than [Ca2+](c). Moreover, inhibition of SR Ca2+ pump with two different concentrations of thapsigargin caused an increase in [Ca2+](m), which was related to the rate of [Ca2+](c) increase. Finally, electron microscopy showed that sites of junctions between SR and T tubules from which Ca2+ is released, or Ca2+ release units, CRUs, are preferentially located in close proximity to mitochondria. The distance between individual SR Ca2+ release channels (feet or ryanodine receptors) is very short, ranging between approximately 37 and 270 nm. These results are consistent with the idea that there is a preferential coupling of Ca2+ transport from SR to mitochondria in cardiac muscle cells, because of their structural proximity.