Lipid in skeletal muscle myotubes is associated to the donors' insulin sensitivity and physical activity phenotypes

Abstract

Objective: This study investigated the relationship between in vitro lipid content in myotubes and in vivo whole body phenotypes of the donors such as insulin sensitivity, intramyocellular lipids (IMCL), physical activity, and oxidative capacity.

Design and methods: Six physically active donors were compared to six sedentary lean and six T2DM. Lipid content was measured in tissues and myotubes by immunohistochemistry. Ceramides, triacylglycerols, and diacylglycerols (DAGs) were measured by LC-MS-MS and GC-FID. Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp (80 mU min⁻¹ m⁻²), maximal mitochondrial capacity (ATPmax) by ³¹P-MRS, physical fitness by VO₂max and physical activity level (PAL) by accelerometers.

Results: Myotubes cultured from physically active donors had higher lipid content (0.047 ± 0.003 vs. 0.032 ± 0.001 and 0.033 ± 0.001AU; P < 0.001) than myotubes from lean and T2DM donors. Lipid content in myotubes was not associated with IMCL in muscle tissue but importantly, correlated with in vivo measures of ATPmax (r = 0.74; P < 0.001), insulin sensitivity (r = 0.54; P < 0.05), type-I fibers (r = 0.50; P < 0.05), and PAL (r = 0.92; P < 0.0001). DAGs and ceramides in myotubes were inversely associated with insulin sensitivity (r = -0.55, r = -0.73; P < 0.05) and ATPmax (r = -0.74, r = -0.85; P < 0.01).

Conclusions: These results indicate that cultured human myotubes can be used in mechanistic studies to study the in vitro impact of interventions on phenotypes such as mitochondrial capacity, insulin sensitivity, and physical activity.

Trial registration: ClinicalTrials.gov NCT00401791 NCT00402012.

Figure 1

Lipid measured in myotubes from sedentary lean (n=5) and donors with type 2 diabetes (n=6) is significantly lower compared to active (n=6) donors (Figure 1A). Intramyocellular lipid measured in vastus lateralis muscle (Figure 1B) did is not correlate with lipids measured in myotubes (Figure 1C). *p<0.05.

Figure 2

Mitochondrial content in myotubes from active donors were significantly higher compared to sedentary lean donors (Figure 2A). Mitochondrial content was associated with lipid measured in myotubes (Figure 2B). *p<0.05.

Figure 3

Lipid measured in myotubes is correlated with donor’s phenotypes such as maximal ATP synthesis rate (ATPmax) (A), glucose disposal rate (B), percent of type-I fiber in vastus lateralis muscle (C) and physical activity level (PAL) (D).

Figure 4

Triacylglycerols measured in myotubes, were higher in active donors compared to sedentary lean donors (Figure 4A). Whereas, diacylglycerols (Figure 4B) and ceramides (Figure 4C), measured in myotubes, were significantly higher in donors with type 2 diabetes, compared to active. Ceramide species: C16, C18 and C24 were significantly higher in mytubes from donors with type 2 diabetes (Figure 4D). *p<0.05; **p<0.01

Figure 5

Diacylglycerols and ceramides content in myotubes were negatively correlated with donor’s ATP synthesis rate (Figure 5A-B) and glucose disposal rate (Figure 5C-D).