No Net-Utilization of Intramuscular Lipid Droplets during Repeated High-Intensity Intermittent Exercise.

Abstract

Intramuscular lipids are stored as subsarcolemmal or intramyofibrillar droplets with potential diverse roles in energy metabolism. We examined intramuscular lipid utilization through transmission electron microscopy during repeated high-intensity intermittent exercise, an aspect that is hitherto unexplored. Seventeen moderately-to-well-trained males underwent three periods (EX1-EX3) of 10 x 45-s high-intensity cycling (~100-120% watt max) combined with maximal repeated sprints (~250-300% watt max). M. vastus lateralis biopsies were obtained at baseline, after EX1, and EX3. During the complete exercise session, no net decline in neither subsarcolemmal nor intermyofibrillar lipid volume density occurred. However, a temporal relationship emerged for subsarcolemmal lipids with an ~11% increase in droplet size after EX1 (P=0.024), which reverted to baseline levels after EX3 accompanied by a ~30% reduction in the numerical density of subsarcolemmal lipid droplets compared to both baseline (P=0.019) and after EX1 (P=0.018). Baseline distinctions were demonstrated with a ~2-fold higher intermyofibrillar lipid volume in type 1 vs. type 2 fibers (P=0.008), mediated solely by a higher number rather than size of lipid droplets (P<0.001). No fiber type-specific differences were observed in subsarcolemmal lipid volume although type 2 fibers exhibited ~17% larger droplets (P=0.034) but a lower numerical density (main effect; P=0.010) including 3% less droplets at baseline. Collectively, these findings suggest that intramuscular lipids do not serve as an important substrate during high-intensity intermittent exercise, however, the repeated exercise pattern mediated a temporal remodeling of the subsarcolemmal lipid pool. Furthermore, fiber type- and compartment-specific differences were found at baseline underscoring the heterogeneity in lipid droplet deposition.