Fig. 5.
Correlating junctional SR(jSR)-mitochondria junctions and SR-mitochondrial tethers with OMM-IMM contact points using transmission electron microscopy (TEM). Cardiac muscle samples from rat left ventricle were fixed and processed for TEM. High-magnification (76,000× at 7 inches print size) images were taken from longitudinal as well as transversal sections of regions containing well recognizable dyads and mitochondria. A: representative high-magnification (left) TEM image of a jSR-OMM junction showing the cross section of a T-tubule (T) and a jSR sack (SR) in association with a mitochondrion (M). Note the contact points (cp) between the OMM and IMM and the discrete electron densities interconnecting the SR and OMM corresponding to tethers (white arrows: the bottom one attaching to a cp, whereas the top one attaches to an OMM segment free of IMM). The high-magnification image at left is zoomed out by a factor of 10× at right to show the organellar context. B: bar graph analyzing the likelihood of an OMM-IMM contact point being located at a jSR-OMM interface or at size-matched SR-free OMM segments selected as shown in the scheme. For each segment the presence and absence of a contact point scores 1 and 0, respectively. For each mitochondrion there is one OMM-jSR junction, and 1–3 SR-free OMM segments are averaged (in some cases it was not possible to have 3 SR-free OMM segments equal in size to the jSR-OMM junction and having well recognizable membranes). Nineteen mitochondria from 3 animals were analyzed. The difference between the 2 groups is significant (P < 0.005, using Wilcoxon signed rank test). C: bar graph summarizing the number of SR-OMM tethers observed in each jSR-mitochondria junction that ended at an OMM-IMM contact point or at an IMM-free OMM region. Data are means ± SE (n = 19; P > 0.05 using Wilcoxon signed rank test).