The NOS/NO system in cardiac and skeletal muscle remodelling of the lungfish Protopterus annectens: Switch from aquatic to aestivating conditions

During aestivation, characterized by prolonged torpor, metabolic depression and biochemical and morpho-functional readjustments, the lungfish heart continues to pump, while the skeletal muscle stops to function but can immediately contract during arousal. The mechanisms of rearrangement occurring in myotomal and myocardial muscles during aestivation and arousal are unknown. Nitric oxide (NO), for its universal role in cardio-circulatory and muscle homeostasis, could be involved in coordinating these stress-induced adaptations. Western blotting and immunofluorescence on cardiac and skeletal muscles of Protopterus annectens (freshwater, six months of aestivation and six days after arousal) showed that expression, localization and activity of the endothelial-like nitric oxide synthase (eNOS) isoform and its partners Akt and Hsp-90 are tissue-specifically modulated. During aestivation, phospho-eNOS/eNOS and phospho-Akt/Akt ratios increased in the heart but decreased in the skeletal muscle. Hsp-90 increased in both muscle types during aestivation. TUNEL assay revealed that increased apoptosis occurred in the skeletal muscle of aestivating lungfish, but the myocardial apoptotic rate of the aestivating lungfish remained unchanged as compared with the freshwater control. Consistent with the preserved cardiac activity during aestivation, the expression of apoptosis repressor (ARC) remained unchanged in the heart of aestivating and aroused fish as compared with the freshwater. Contrarily, ARC expression was reduced in the skeletal muscle of aestivating lungfish. Our data indicate that changes in the eNOS/NO system and cell turnover are implicated in the morpho-functional readjustments occurring in lungfish cardiac and skeletal muscle during the switch from freshwater to aestivation, and between the maintenance and arousal phases of aestivation