Mechanical unloading by heterotopic heart transplantation reduces maximal respiratory capacity of isolated mitochondria in rat

Objectives: Workload determines energy production and thereby oxygen consumption of the heart. We tested whether the respiratory capacity of mitochondria from unloaded hearts is decreased and that of overloaded hearts is increased.

Material and Methods: Mechanical unloading was achieved by heterotopic rat heart transplantation for 7 days and overlaoding by aortic banding for 14 days. Subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria were isolated by differential centrifugation. Maximal ADP-stimulated oxygen consumption was measured with a Clark electrode. Citrate synthase activity (CS; total, free, latent) was used as mitochondrial marker enzyme. CS ratio (CSR: CSlatent/CSfree) served as index for structural integrity of the mitochondria. Expression of respiratory chain genes (RCG: NADH dehydrogenase and cytochrome c oxidase) was assessed by northern blot analysis.

Results: Transplantation resulted in atrophy and banding in hypertrophy (mg heart/g body weight: 2.1±0.4, 2.6±0.1, 4.0±0.9, p<0.05). Citrate synthase activity was decreased in atrophied hearts (U/gwet: 58.6±10.6 vs. 118±14.1, p<0,01) but was not different in hypertrophied hearts. CSR were not different among groups. Maximal oxygen consumption was significantly reduced in mitochondria from atrophied hearts compared to controls (natomsO/min/UCSlatent; SSM 29.0±6.1 vs. 47.3±6,1, p<0.05; IFM 35.0±7.4 vs. 57.5±6.6, p<0.05) but unchanged in mitochondria from hypertrophied hearts. The reduction of respiratory activity in atrophied hearts was accompanied by reduced expression of RCG.

Conclusions: Mechanical unloading reduces maximal respiratory capacity of isolated mitochondria. We suggest that mitochondria of overloaded, hypertrophied hearts in vivo may exploit a greater fraction of their maximal respiratory capacity than mitochondria of normal or atrophied hearts.