Discrepancy in the regulation of respiratory capacity between chronically unloaded and overloaded hearts

Objective: We tested the effects of mechanical unloading and overloading on the regulation of respiratory capacity.

Methods: In male rats mechanical unloading was achieved by heterotopic heart transplantation and mechanical overloading by aortic banding. Subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria were isolated by differential centrifugation. Citrate synthase activity was used as mitochondrial marker enzyme. Maximal ADP-stimulated oxygen consumption (state 3) was measured with a Clark-type electrode. Expression of respiratory chain genes (NADH-DHsub2, COXIV), regulators of mitochondrial biogenesis (PGC-1α, PGC-1β, NRF1, NRF2, mtTFA, ERRα) and translocases of the mitochondrial membranes (TIM22, TOM40, TOM70) were investigated by real time RT-PCR.

Results: Unloaded hearts showed reduced maximal oxygen consumption of isolated mitochondria (natomsO/min/ml; SSM 203.5±78.6 vs. 803.7±147.2, p<0.001; IFM 467.6±157.9 vs. 1140.6±296.1, p<0.05), reduced expression of NADH-DHsub2 (–49%; p<0.05) and COXIV (–31%), and a 2-fold downregulation of PGC-1α (p<0.05), compared to controls. In contrast, there was no upregulation of NADH-DHsub2 and COXIV mRNA expression and unchanged maximal respiratory rates in overloaded hearts. PGC-1α mRNA expression was reduced (≈2-fold; p<0.01). In atrophy, expression of TOM70 was reduced (–42%; p<0.05), and expression of TIM22 was increased (≈2-fold; p<0.01). In hypertrophy, expression of the translocases was unchanged.

Conclusion: The regulatory changes observed with unloading matched energy demands. In contrast, chronic pressure overload did not result in a coordinate upregulation of respiratory capacity. This discrepancy suggests a possible dysregulation at the level of PGC-1α as mechanism for the onset of heart failure.