Conley K E
Department of Radiology, University of Washington Medical Center, Seattle 98195.
Adv Vet Sci Comp Med. 1994;38A:1-39.
How is the muscle fiber designed to accomplish the diversity of tasks performed by striated muscle? Basically, a common contractile mechanism and a similar organization of metabolism in striated muscles are used to generate a wide spectrum of speeds and durations of contraction. The speed of contraction ranges from manyfold within an animal to over a hundred-fold between animals, owing to variation in the intrinsic velocities of the myosin isoforms. Recruiting fibers that contain the myosin isoform that contracts at the appropriate velocity varies the speed of locomotion at minimal cost. The magnitude and duration of the energy supply required to meet this contractile demand depends on the size of the cellular energy buffer and the capacities of the metabolic pathways. The faster the contractile speed, the larger the PCr pool and the greater the glycolytic capacity to meet a high rate of ATP use. Slower-contracting fibers have a smaller buffer for the short term, but an increased oxidative capacity for continuous energy supply to maintain energy balance over the long term. In general, fibers trade contractile speed for duration of performance, but a number of exceptions exist where rapid contractions are maintained for extended periods. In the face of this heterogeneity of properties, common features are found that assure an energy balance. The PCr/ATP buffer system offers a simple mechanism of feedback control of energy supply despite the wide range of high-energy phosphate concentrations and oxidative capacities found in skeletal muscle. An oxygen balance system also appears to be present in the terminal structures of the respiratory system, the capillaries, and mitochondria. Despite the diversity of these structures, a rather constant ratio of oxygen delivery capacity to mitochondrial oxidative capacity is found in vertebrate striated muscles. Finally, a critical feature of muscle energy balance that remains unresolved is (are) the mechanism(s) controlling mitochondrial respiration in heart. Feedback control appears to account for linking ATP supply to demand in skeletal muscle, but the mechanisms governing respiratory control in heart are still under vigorous investigation. Thus, the links between contractile demand and oxidative phosphorylation are still unresolved in this tissue, which may indicate that a key element is missing in our understanding of the cellular energetics of exercise.
肌纤维是如何设计以完成横纹肌所执行的各种任务的？基本上，横纹肌利用共同的收缩机制和相似的代谢组织来产生广泛的收缩速度和持续时间。由于肌球蛋白同工型的内在速度不同，收缩速度在动物体内的差异可达数倍，在不同动物之间可达一百倍以上。招募含有以适当速度收缩的肌球蛋白同工型的纤维，能以最小的成本改变运动速度。满足这种收缩需求所需的能量供应的大小和持续时间取决于细胞能量缓冲池的大小和代谢途径的能力。收缩速度越快，磷酸肌酸（PCr）储备越大，糖酵解能力越强，以满足高ATP使用速率。收缩较慢的纤维短期内缓冲较小，但氧化能力增强，以长期持续供应能量以维持能量平衡。一般来说，纤维在收缩速度和表现持续时间之间进行权衡，但也有一些例外情况，即快速收缩能持续较长时间。面对这种特性的异质性，可以发现一些共同特征来确保能量平衡。尽管骨骼肌中存在广泛的高能磷酸盐浓度和氧化能力，但PCr/ATP缓冲系统提供了一种简单的能量供应反馈控制机制。呼吸系统的终末结构、毛细血管和线粒体中似乎也存在氧平衡系统。尽管这些结构各不相同，但在脊椎动物的横纹肌中，氧输送能力与线粒体氧化能力的比例相当恒定。最后，肌肉能量平衡中一个尚未解决的关键特征是控制心脏线粒体呼吸的机制。反馈控制似乎解释了骨骼肌中ATP供应与需求的联系，但心脏呼吸控制的机制仍在深入研究中。因此，在这个组织中，收缩需求与氧化磷酸化之间的联系仍未解决，这可能表明我们对运动细胞能量学的理解中缺少一个关键要素。
