Muscle-specific differences in MyHC isoform abundance, metabolic profile and gene expression in cattle

Skeletal muscles in cattle exhibit variability in their structural and metabolic features, which are influenced by their anatomical location and function. This study aimed to characterise MyHC isoforms proportions, metabolic profile, adiposity and the expression of genes regulating muscle and lipid traits, and to analyse their interrelationships in two functionally distinct muscles -Longissimus thoracis (LT) and Masseter (MS)- in young Holstein and Pirenaica bulls raised for meat production. Eight young bulls from each group were sampled at the commercial slaughter age. The LT muscle exhibited a high proportion of fast-twitch glycolytic MyHC IIX (~ 47%) and fast-oxidative MyHC IIA (~ 36%) isoforms, whereas the MS contained only slow-oxidative MyHC I isoforms in both groups. Glycolytic enzyme activities were higher and oxidative enzyme activities lower in LT than in MS (P < 0.01). Adipocyte size distribution was unimodal in LT, whereas MS showed a more heterogeneous distribution, with indications of a second population of larger adipocytes. Gene expression of myosin heavy chain 1 (MYH1), myogenic differentiation 1 (MYOD1), and myostatin (MSTN) was significantly higher in LT than in MS muscle. Conversely, myosin heavy chain 7 (MYH7) along with lipoprotein lipase (LPL), fatty acid translocase (CD36) and carnitine palmitoyl transferase 1B (CPT1B), associated with lipid uptake, oxidation and triacylglycerol synthesis, were upregulated in MS in both groups (P < 0.05). Overall, the MS muscle, continuously active due to its role in mastication, showed a slow-twitch oxidative profile and molecular features associated with enhanced lipid metabolism. In contrast, the LT muscle, involved in locomotion, showed a glycolytic phenotype associated with markers of muscle growth and fast fibre differentiation, reflecting muscle-specific traits. These contrasting profiles reflect a physiological specialisation shaped by function and point to a coordinated regulation of structural, metabolic, and molecular traits.