A small fraction of the sarcoplasmic reticulum is physically coupled to mitochondria in cardiac muscle, allowing local Ca2+ transfer that stimulates dehydrogenase activity.
Physical coupling between a small fraction of the sarcoplasmic reticulum and mitochondria in cardiac muscle supports local Ca2+ transfer, which is crucial for mitochondrial function.
In many cell types, transfer of Ca(2+) released via ryanodine receptors (RyR) to the mitochondrial matrix is locally supported by high Ca(2+) microdomains at close contacts between the sarcoplasmic reticulum (SR) and mitochondria. Here we studied whether the close contacts were secured via direct physical coupling in cardiac muscle using isolated rat heart mitochondria (RHMs). "Immuno-organelle chemistry" revealed RyR2 and calsequestrin-positive SR particles associated with mitochondria in both crude and Percoll-purified "heavy" mitochondrial fractions (cRHM and pRHM), to a smaller extent in the latter one. Mitochondria-associated vesicles were also visualized by electron microscopy in the RHMs. Western blot analysis detected greatly reduced presence of SR markers (calsequestrin, SERCA2a, and phospholamban) in pRHM, suggesting that the mitochondria-associated particles represented a small subfraction of the SR. Fluorescence calcium imaging in rhod2-loaded cRHM revealed mitochondrial matrix Ca(2+) (Ca(2+)(m)) responses to caffeine-induced Ca(2+) release that were prevented when thapsigargin was added to predeplete the SR or by mitochondrial Ca(2+) uptake inhibitors. Importantly, caffeine failed to increase Ca(2+) in the large volume of the incubation medium, suggesting that local Ca(2+) transfer between the SR particles and mitochondria mediated the Ca(2+)(m) signal. Despite the substantially reduced SR presence, pRHM still displayed a caffeine-induced Ca(2+)(m) rise comparable with the one recorded in cRHM. Thus, a relatively small fraction of the total SR is physically coupled and transfers Ca(2+) locally to the mitochondria in cardiac muscle. The transferred Ca(2+) stimulates dehydrogenase activity and affects mitochondrial membrane permeabilization, indicating the broad significance of the physical coupling in mitochondrial function.
Garcı́a-Pérez et al. (Sat,) reported a other. Caffeine-induced Ca2+ release was evaluated on Mitochondrial matrix [Ca2+] responses. A small fraction of the sarcoplasmic reticulum is physically coupled to mitochondria in cardiac muscle, allowing local Ca2+ transfer that stimulates dehydrogenase activity.