Fiber type composition is a significant and critical characteristic of skeletal muscle. The purpose of this study was to investigate the differences in metabolic characteristics in athletes, lean and T2DM determined in human primary myotubes, and to compare metabolic characteristics such as fiber type composition, lipid content, mitochondrial capacity, physical fitness and activity measured in previous studies in vivo versus fiber type-I and lipid content measured in primary myotubes cultured from the same donors. Moreover, we determined the effect of progressive passaging of myotubes on fiber type-I content, lipid content and mitochondrial mass.
Six athletes, 6 sedentary lean and 6 subjects with T2DM were studied. Muscle samples were obtained from Vastus Lateralis (VL) from which myoblasts were sorted, cultured and differentiated into myotubes. Type-I fiber and lipid content were quantified by immunohistochemistry technique using MHC-I antibody and bodipy respectively. Mitochondrial mass was measured by Mitotracker Green.
Myotubes from athletes had a significant higher amount of fiber type-I (9.9±1.8 vs. 1.6±1.0 vs. 2.0±0.8 AU; p<0.001), lipid content (0.047±0.003 vs. 0.032±0.001 vs. 0.033±0.001 AU; p<0.001) and mitochondrial mass (105.2±3.8 vs. 80.0±3.0 vs. 90.0±3.9 AU; p<0.01) compared to myotubes cultured from lean and T2DM subjects; fiber type-I measured in myotubes correlated with fiber type-I measured directly in tissue and also with mitochondrial capacity measured as ATPmax. Lipid content measured in myotubes was not correlated with intramyocellular lipid (IMCL) measured in tissue. However, lipid measured in myotubes strongly correlated with insulin sensitivity measured by hyperinsulinemic euglycemic clamp (r2=0.29; p<0.05), physical fitness, measured by VO2max (r2=0.54; p<0.05) and physical activity level (PAL) (r2=0.85; p<0.01). No significant passage effect on fiber type-I content, lipid content and mitochondrial mass was observed in myotubes through passage 4 to passage 7. However, a decreasing trend was observed in fiber type-I (P=0.14) and mitochondrial mass (P=0.05) with multiple passaging.
In conclusion, primary myotubes retain fiber type characteristics of skeletal muscle in vivo. Lipid measured in cultured myotubes, although not associated with IMCL content measured in tissue, it was strongly associated with insulin sensitivity and physical fitness and activity of the donor.