Oxygen conserving mitochondrial adaptations in the skeletal muscles of breath hold divers

Background
The performance of elite breath hold divers (BHD) includes static breath hold for more than 11 minutes, swimming as far as 300 m, or going below 250 m in depth, all on a single breath of air. Diving mammals are adapted to sustain oxidative metabolism in hypoxic conditions through several metabolic adaptations, including improved capacity for oxygen transport and mitochondrial oxidative phosphorylation in skeletal muscle. It was hypothesized that similar adaptations characterized human BHD. Hence, the purpose of this study was to examine the capacity for oxidative metabolism in skeletal muscle of BHD compared to matched controls.
Methods
Biopsies were obtained from the lateral vastus of the femoral muscle from 8 Danish BHD and 8 non-diving controls (Judo athletes) matched for morphometry and whole body VO2max. High resolution respirometry was used to determine mitochondrial respiratory capacity and leak respiration with simultaneous measurement of mitochondrial H2O2 emission. Maximal citrate synthase (CS) and 3-hydroxyacyl CoA dehydrogenase (HAD) activity were measured in muscle tissue homogenates. Western Blotting was used to determine protein contents of respiratory complex I-V subunits and myoglobin in muscle tissue lysates.
Results
Muscle biopsies of BHD revealed lower mitochondrial leak respiration and electron transfer system (ETS) capacity and higher H2O2 emission during leak respiration than controls, with no differences in enzyme activities (CS and HAD) or protein content of mitochondrial complex subunits myoglobin, myosin heavy chain isoforms, markers of glucose metabolism and antioxidant enzymes.
Conclusion
We demonstrated for the first time in humans, that the skeletal muscles of BHD are characterized by lower mitochondrial oxygen consumption both during low leak and high (ETS) respiration than matched controls. This supports previous observations of diving mammals demonstrating a lower aerobic mitochondrial capacity of the skeletal muscles as an oxygen conserving adaptation during prolonged dives.

研究背景
顶级屏息潜水员（Breath Hold Divers, BHD）的表现包括单次屏息静态憋气时长超11分钟、游泳距离可达300米，或单次屏息下潜深度突破250米。潜水哺乳动物已通过多种代谢适应性机制，进化出在低氧环境下维持氧化代谢的能力，包括增强氧气运输能力与骨骼肌线粒体氧化磷酸化能力。有假说认为人类屏息潜水者也具备此类适应性特征。因此本研究旨在对比屏息潜水者与匹配对照组的骨骼肌氧化代谢能力。

研究方法
研究人员从8名丹麦屏息潜水者与8名在形态计量学指标、整体最大摄氧量（VO2max）上匹配的非潜水对照者（柔道运动员）的股外侧肌获取活检组织。采用高分辨率呼吸测定法，在检测线粒体呼吸容量与质子漏呼吸的同时，同步测定线粒体过氧化氢（H2O2）释放量。通过肌肉组织匀浆测定柠檬酸合酶（CS）与3-羟酰基辅酶A脱氢酶（HAD）的最大活性。采用免疫印迹法（Western Blotting）检测肌肉组织裂解液中呼吸链复合体I-V亚基与肌红蛋白的蛋白含量。

研究结果
与对照组相比，屏息潜水者的肌肉活检样本显示出更低的线粒体质子漏呼吸与电子传递系统（ETS）容量，且质子漏呼吸过程中的过氧化氢释放量更高；两组的酶活性（CS与HAD）、线粒体复合体亚基、肌红蛋白、肌球蛋白重链亚型、糖代谢标志物及抗氧化酶的蛋白含量均无显著差异。

研究结论
本研究首次在人类群体中证实，与匹配对照组相比，屏息潜水者的骨骼肌在低质子漏呼吸与高电子传递系统呼吸状态下的线粒体氧消耗量均更低。该结果支持此前关于潜水哺乳动物的研究结论：骨骼肌较低的有氧线粒体能力是长时间潜水过程中的氧储备适应性机制。