Department of Public Health, Research Unit for Exercise Biology, Aarhus University, Aarhus, Denmark.
Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark.
Am J Physiol Endocrinol Metab. 2023 Dec 1;325(6):E700-E710. doi: 10.1152/ajpendo.00298.2023. Epub 2023 Oct 25.
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 × 45-s high-intensity cycling [∼100%-120% Watt (W)] combined with maximal repeated sprints (∼250%-300% W). M. vastus lateralis biopsies were obtained at baseline, after EX1, and EX3. During the complete exercise session, no net decline in either subsarcolemmal or intermyofibrillar lipid volume density occurred. However, a temporal relationship emerged for subsarcolemmal lipids with an ∼11% increase in droplet size after EX1 ( = 0.024), which reverted to baseline levels after EX3 accompanied by an ∼30% reduction in the numerical density of subsarcolemmal lipid droplets compared with both baseline ( = 0.019) and after EX1 ( = 0.018). Baseline distinctions were demonstrated with an approximately twofold higher intermyofibrillar lipid volume in type 1 versus type 2 fibers ( = 0.008), mediated solely by a higher number rather than the size of lipid droplets ( < 0.001). No fiber-type-specific differences were observed in subsarcolemmal lipid volume although type 2 fibers exhibited ∼17% larger droplets ( = 0.034) but a lower numerical density (main effect; = 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. Undertaking a severe repeated high-intensity intermittent exercise protocol led to no net decline in neither subsarcolemmal nor intermyofibrillar lipid content in the thigh muscle of young moderately to well-trained participants. However, a temporal remodeling of the subsarcolemmal pool of lipid droplets did occur indicative of potential transient lipid accumulation. Moreover, baseline fiber-type distinctions in subcellular lipid droplet deposition were present underscoring the diversity in lipid droplet storage among fiber types and subcellular regions.
肌内脂质以亚肌小节或肌原纤维内的滴状形式储存，在能量代谢中具有多种潜在作用。我们通过电镜观察了高强度间歇运动过程中肌内脂质的利用情况，这是一个以前尚未探索过的方面。17 名中等到高水平训练的男性接受了三个时期（EX1-EX3）的 10×45 秒高强度循环[约 100%-120%Watt（W）]，并结合最大重复冲刺[约 250%-300%W]。在基线、EX1 后和 EX3 后，分别从股外侧肌获得活检。在整个运动过程中，亚肌小节或肌原纤维内的脂质体积密度均未出现净下降。然而，亚肌小节脂质出现了时间上的关系，在 EX1 后，液滴大小增加了约 11%（=0.024），在 EX3 后，液滴的数量密度恢复到基线水平，与基线相比（=0.019）和 EX1 后（=0.018）相比，亚肌小节脂质滴的数量减少了约 30%。基线时，1 型纤维的肌间纤维脂质体积约为 2 型纤维的两倍（=0.008），这主要是由于数量而不是液滴大小（<0.001）的差异。尽管 2 型纤维的液滴大 17%（=0.034），但数量密度较低（主要效应；=0.010），包括基线时少 3%的液滴，但亚肌小节脂质体积没有观察到纤维类型特异性差异。总之，这些发现表明，在高强度间歇运动中，肌内脂质不是重要的底物；然而，重复的运动模式介导了亚肌小节脂质池的时间重塑。此外，基线时发现纤维类型和隔室特异性差异，突出了脂质滴沉积的异质性。在年轻的中等到高水平训练的参与者中，进行严重的重复高强度间歇运动方案后，大腿肌肉中的亚肌小节和肌原纤维内的脂质含量均无净下降。然而，亚肌小节脂质滴池确实发生了时间重塑，表明潜在的短暂脂质积累。此外，亚细胞脂质滴沉积的基线纤维类型差异存在，突出了不同纤维类型和亚细胞区域的脂质滴储存多样性。
