Abstract
ABSTRACTFish may be extremely hypoxia resistant. We investigated how muscle fibre size and oxidative capacity in zebrafish (Danio rerio) adapt during severe chronic hypoxia. Zebrafish were kept for either 3 or 6 weeks under chronic constant hypoxia (CCH) (10% air/90%N2 saturated water). We analyzed cross-sectional area (CSA), succinate dehydrogenase (SDH) activity, capillarization, myonuclear density, myoglobin (Mb) concentration and Mb mRNA expression of high and low oxidative muscle fibres. After 3 weeks of CCH, CSA, SDH activity, Mb concentration, capillary and myonuclear density of both muscle fibre types were similar as under normoxia. In contrast, staining intensity for Mb mRNA of hypoxic high oxidative muscle fibres was 94% higher than that of normoxic controls (P<0.001). Between 3 and 6 weeks of CCH, CSA of high and low oxidative muscle fibres increased by 25 and 30%, respectively. This was similar to normoxic controls. Capillary and myonuclear density were not changed by CCH. However, in high oxidative muscle fibres of fish maintained under CCH, SDH activity, Mb concentration as well as Mb mRNA content were higher by 86%, 138% and 90%, respectively, than in muscle fibres of fish kept under normoxia (P<0.001). In low oxidative muscle fibres, SDH activity, Mb and Mb mRNA content were not significantly changed. Under normoxia, the calculated interstitial oxygen tension required to prevent anoxic cores in muscle fibres (PO2crit) of high oxidative muscle fibres was between 1.0 and 1.7 mmHg. These values were similar at 3 and 6 weeks CCH. We conclude that high oxidative skeletal muscle fibres of zebrafish continue to grow and increase oxidative capacity during CCH. Oxygen supply to mitochondria in these fibres may be facilitated by an increased Mb concentration, which is regulated by an increase in Mb mRNA content per myonucleus.
Highlights
Diseases such as chronic obstructive pulmonary disease, chronic heart failure and pulmonary hypertension are associated with hypoxia and considerable reductions in body and skeletal muscle mass and mitochondrial density (Gosker et al, 2000; Green et al, 2008; Schols, 2000; Steffensen and Farrell, 1998; Whittom et al, 1998)
Calibrated histochemistry reveals differential responses for high and low oxidative muscle fibres during chronic constant hypoxia (CCH) Sections of the zebrafish tail were incubated for succinate dehydrogenase (SDH) activity, Mb peroxidase activity and Mb mRNA content
In conclusion, we found that high oxidative muscle fibres of zebrafish enhance metabolic power during chronic constant hypoxia by increasing the oxidative capacity of muscle fibres while muscle fibre size remained unaltered
Summary
Diseases such as chronic obstructive pulmonary disease, chronic heart failure and pulmonary hypertension are associated with hypoxia and considerable reductions in body and skeletal muscle mass and mitochondrial density (Gosker et al, 2000; Green et al, 2008; Schols, 2000; Steffensen and Farrell, 1998; Whittom et al, 1998). Certain fish species can adapt very well to chronic constant hypoxia (CCH) (Johnston and Bernard, 1982; Roesner et al, 2006; Treberg et al, 2007; van der Meer et al, 2005). The CCH zebrafish model may be useful to identify mechanisms underlying skeletal muscle adaptation in response to chronic hypoxia and identify signalling targets for treatment of cachexia
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