Physical Coupling Supports the Local Ca2+ Transfer between Sarcoplasmic Reticulum Subdomains and the Mitochondria in Heart Muscle

In many cell types, transfer of Ca2+ released via ryanodine receptors (RyR) to the mitochondrial matrix is locally supported by high [Ca2+] 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 [Ca2+] ([Ca2+](m)) responses to caffeine-induced Ca2+ release that were prevented when thapsigargin was added to predeplete the SR or by mitochondrial Ca2+ uptake inhibitors. Importantly, caffeine failed to increase [Ca2+] in the large volume of the incubation medium, suggesting that local Ca2+ transfer between the SR particles and mitochondria mediated the [Ca2+](m) signal. Despite the substantially reduced SR presence, pRHM still displayed a caffeine-induced [Ca2+] m rise comparable with the one recorded in cRHM. Thus, a relatively small fraction of the total SR is physically coupled and transfers Ca2+ locally to the mitochondria in cardiac muscle. The transferred Ca2+ stimulates dehydrogenase activity and affects mitochondrial membrane permeabilization, indicating the broad significance of the physical coupling in mitochondrial function.