Mathieu-Costello O, Szewczak J M, Logemann R B, Agey P J
Department of Medicine, University of California, San Diego, La Jolla 92093-0623.
Am J Physiol. 1992 Jun;262(6 Pt 2):R955-65. doi: 10.1152/ajpregu.1992.262.6.R955.
We investigated the relationship between capillary-to-fiber geometry and muscle aerobic capacity by comparing the bat flight muscle (pectoralis muscle), i.e., an ultimate case of extreme O2 demand in a mammalian skeletal muscle, with bat hindlimb and rat soleus muscles. At a given sarcomere length (2.1 microns), fiber cross-sectional area was considerably smaller in bat muscles (pectoralis 318 +/- 10 microns 2, hindlimb 447 +/- 35 microns 2) than in rat soleus muscle (2,027 +/- 125 microns 2). Capillary number per fiber cross-sectional area was much greater in bat pectoralis (6,394 +/- 380/mm2) than in bat hindlimb and rat soleus muscle (2,865 +/- 238 and 1,301 +/- 129/mm2, respectively; all values normalized to 2.1-microns sarcomere length). At the same sarcomere length (2.1 microns), the degree of tortuosity and branching of capillaries were significantly greater in bat pectoralis than in bat hindlimb and rat soleus muscle. In bat flight muscle, capillary length per fiber volume was very high (9,025 +/- 342/mm2). It was 2.2- and 5.4-fold larger than in bat hindlimb and rat soleus, respectively. Mitochondria occupied 35.3 +/- 1.2, 16.5 +/- 1.3, and 6.1 +/- 0.9% of the muscle fiber volume in bat pectoralis, hindlimb, and rat soleus muscles, respectively. There was a strong correlation between capillary length (as well as capillary surface) per fiber volume and mitochondrial volume density in all muscles. Considering capillary supply and mitochondrial volume on an individual fiber basis, we found that 1) the number of capillaries around a fiber was linearly related to mitochondrial volume per micron length of fiber in the muscles but that 2) capillary surface per fiber surface, at given mitochondrial volume per micron length of fiber, was about twice as large in bat pectoralis as in rat soleus muscle, whereas in bat hindlimb it was intermediate between that in bat pectoralis and that in rat soleus muscle. This was due to the differences in fiber size (rat soleus greater than bat muscles) and capillary-to-fiber ratio (bat pectoralis greater than hindlimb) between the muscles. It is notable that in the bat, the substantially greater O2 transfer capacity of the flight muscle compared with hindlimb was achieved by increasing the size of the capillary-to-fiber interface, i.e., capillary-to-fiber surface, via an increase in capillary number rather than by substantially reducing fiber size.
我们通过比较蝙蝠的飞行肌肉（胸肌），即哺乳动物骨骼肌中对氧气需求极高的极端情况，与蝙蝠的后肢肌肉和大鼠的比目鱼肌，研究了毛细血管与肌纤维几何结构和肌肉有氧能力之间的关系。在给定的肌节长度（2.1微米）下，蝙蝠肌肉（胸肌318±10平方微米，后肢肌肉447±35平方微米）的肌纤维横截面积明显小于大鼠比目鱼肌（2027±125平方微米）。每单位肌纤维横截面积的毛细血管数量在蝙蝠胸肌（6394±380/mm²）中比在蝙蝠后肢肌肉和大鼠比目鱼肌中（分别为2865±238/mm²和1301±129/mm²；所有数值均已归一化至2.1微米的肌节长度）多得多。在相同的肌节长度（2.1微米）下，蝙蝠胸肌中毛细血管的曲折度和分支程度明显大于蝙蝠后肢肌肉和大鼠比目鱼肌。在蝙蝠的飞行肌肉中，每单位肌纤维体积的毛细血管长度非常高（9025±342/mm²）。分别比蝙蝠后肢肌肉和大鼠比目鱼肌大2.2倍和5.4倍。线粒体分别占蝙蝠胸肌、后肢肌肉和大鼠比目鱼肌肌纤维体积的35.3±1.2%、16.5±1.3%和6.1±0.9%。所有肌肉中，每单位肌纤维体积的毛细血管长度（以及毛细血管表面积）与线粒体体积密度之间存在很强的相关性。从单个肌纤维的角度考虑毛细血管供应和线粒体体积，我们发现：1）肌肉中围绕一根肌纤维的毛细血管数量与每微米肌纤维长度的线粒体体积呈线性相关，但2）在每微米肌纤维长度的线粒体体积给定的情况下，每单位肌纤维表面积的毛细血管表面积在蝙蝠胸肌中约为大鼠比目鱼肌的两倍，而在蝙蝠后肢肌肉中则介于蝙蝠胸肌和大鼠比目鱼肌之间。这是由于不同肌肉之间肌纤维大小（大鼠比目鱼肌大于蝙蝠肌肉）和毛细血管与肌纤维比例（蝙蝠胸肌大于后肢肌肉）的差异。值得注意的是，在蝙蝠中，飞行肌肉与后肢相比，氧气转运能力显著增强是通过增加毛细血管与肌纤维的界面大小，即毛细血管与肌纤维的表面积，通过增加毛细血管数量而不是大幅减小肌纤维大小来实现的。
