Mouse cardiac mitochondria do not separate in subsarcolemmal and interfibrillar subpopulations

Cardiomyocytes consist of longitudinally oriented myofibril bundles with a misaligned composition caused by the uneven contours of the intercalated discs. The cytoplasmic space harbors the organelles, including mitochondria. This study investigated whether cardiomyocytes contain spatially and ultrastructurally discrete pools of mitochondria that can be separated for structurally and functionally appraisal in (patho)physiology. Transmission electron microscopy disclosed continuous transitions of mitochondria without attributable characteristics from beneath the sarcolemma directly into the barrier-free cytoplasmic space between myofibrils. The various shapes and sizes of mitochondria are formed by myofibril positioning and the space available independent of their localization within the cardiomyocytes. Furthermore, the established enzymatic isolation procedure including proteinase treatment resulted in loss of mitochondrial proteins, as evidenced by immunogold labeling of Connexin43 in situ, a postulated marker for distinguishing mitochondrial subpopulations. Moreover, mitochondrial ATP produced in those mitochondria was not different. These findings preclude a spatial and ultrastructural grading of cardiac mitochondria and their distinct separation and classification in subsarcolemmal and interfibrillar subpopulations.