Interaction between sarcomere and mitochondrial length in normoxic and hypoxic rat ventricular papillary muscles

We hypothesized that the mitochondrial length may be altered according to changes in the sarcomere length, and that this relationship may be affected by exposure to hypoxia. Rat ventricular papillary muscles were isolated and immersed in normoxic or hypoxic solutions for 10 min. Sarcomeres of various lengths were obtained by fixing the papillary muscles in a slack or stretched state, or after exposure to a contracture solution containing saponin and CaCl2. The mitochondrial length measured using electron microscopy significantly correlated to the length of the adjacent sarcomere in both the normoxic (n=767) and hypoxic (n=1145) groups (P<.0001). The slope of the regression line, however, was significantly less steep, and its intercept was significantly larger in the hypoxic group than in the normoxic group (analysis of covariance). When we analyzed the mitochondrial lengths among the three sarcomere-length subgroups (<1.5, 1.5–2.0, and >2.0 μm), the mitochondrial length was significantly shorter in the hypoxic condition than in the normoxic condition at sarcomere lengths greater than 2.0 μm. Staining for desmin, the major muscle-type intermediate filament, the longitudinal system of which connects the mitochondria with the Z bands of sarcomeres, showed a clear cross-striation pattern in both papillary muscles with and without the exposure to hypoxia, suggesting that desmin was preserved after the exposure to hypoxia. These data indicate that the mitochondrial length changes according to changes in the sarcomere length, suggesting the possible role of mitochondria as an internal load against myocyte contraction. It is also suggested that mitochondria exposed to hypoxia may be more resistive to both compression and stretch in a longitudinal direction than those in the normoxic condition.