β-oxidation, esterification, and secretion of radiolabeled fatty acids in cultivated Atlantic salmon skeletal muscle cells

The white muscle of Atlantic salmon metabolizes FA with different chain lengths and different saturations at different rates, but few details are available on the processes involved or the products formed. We have investigated how multinucleated muscle cells (myotubes) in culture metabolize [1-C-14] 8:0, [1-C-14] 18:1n-9, and [1-C-14] 20:5n-3. The myotubes were formed by the differentiation of isolated myosatellite cells from the white skeletal muscle of salmon fry. Almost all (98%) of the [1-C-14] 8:0 substrate was oxidized to acid-soluble products (ASP) and (CO2)-C-14 after 48 h of incubation, whereas only approximately 50% of the [1-C-14] 18:1n-9 and [1-C-14] 20:5n-3 substrates were oxidized. However, only one cycle of beta-oxidation was measured by the method used. For all three substrates, the main ASP were acetate and a combined fraction of oxaloacetate and malate. Nearly twice as much radioactivity from the [1-C-14] 20:5n-3 substrate was found in the cellular lipids as radioactivity from [1-C-14] 18:1n-9, indicating that [1-C-14] 20:5n-3 was taken up into muscle cells more rapidly than [1-C-14] 18:1n-9. Approximately 10% of the added [1-C-14] 20:5n-3 substrate and 5% of the added [1-C-14] 18:1n-9 substrate was secreted from the muscle cells into the culture media as esterified lipids. Immunocytochemical staining showed that the cells synthesized apolipoprotein A-1. Differentiated muscle cells also expressed peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARbeta, two transcription factors that are involved in regulating beta-oxidation.