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 i nvestigated the hypothesis that this proximity results in a transient expos ure of mitochondrial Ca2+ uniporter (CaUP) to high concentrations of Ca2+ f ollowing Ca2+ release from the SR and thus an influx of Ca2+ into mitochond ria. Single ventricular myocytes of rat were skinned by exposing them to a physiological solution containing saponin (0.2 mg/ml). Cytosolic Ca2+ conce ntration ([Ca2+](c)) and mitochondrial Ca2+ concentration ([Ca2+](m)) were measured with fura-2 and rhod2, respectively. Application of caffeine (10 m M) induced a concomitant increase in [Ca2+](c) and [Ca2+](m). Ruthenium red , at concentrations that block CaUP but not SR release, diminished the caff eine 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 r educed substantially less than [Ca2+](c). Moreover, inhibition of SR Ca2+ p ump with two different concentrations of thapsigargin caused an increase in [Ca2+](m), which was related to the rate of [Ca2+](c) increase. Finally, e lectron microscopy showed that sites of junctions between SR and T tubules from which Ca2+ is released, or Ca2+ release units, CRUs, are preferentiall y located in close proximity to mitochondria. The distance between individu al SR Ca2+ release channels (feet or ryanodine receptors) is very short, ra nging between approximately 37 and 270 nm. These results are consistent wit h the idea that there is a preferential coupling of Ca2+ transport from SR to mitochondria in cardiac muscle cells, because of their structural proxim ity.