Goodman M N
Biochem J. 1987 Jan 1;241(1):121-7. doi: 10.1042/bj2410121.
The influence of Ca2+ on myofibrillar proteolysis was evaluated in the isolated extensor digitorum longus muscle incubated in vitro with agents previously shown to increase the intracellular concentration of Ca2+. Myofibrillar proteolysis was evaluated by measuring the release of N tau-methylhistidine, and total proteolysis was evaluated by measuring tyrosine release by incubated muscles after the inhibition of protein synthesis with cycloheximide. Incubated muscles released measurable quantities of N tau-methylhistidine, and muscle contents of the amino acids remained stable over 2 h of incubation. The release of N tau-methylhistidine by incubated muscles was similar to its release by perfused rat muscle in response to brief starvation, indicating the integrity of the incubated muscles. Ca2+ ionophore A23187, dibucaine, procaine, caffeine and elevated K+ concentration increased lactate release by incubated muscles and decreased tissue contents of ATP and phosphocreatine to varying degrees, indicating the metabolic effectiveness of the agents tested. Only A23187 and dibucaine increased total cell Ca2+, and they increased tyrosine release. Caffeine and elevated [K+] increased neither cell Ca2+ nor tyrosine release; however, only A23187 and dibucaine increased tyrosine release significantly. On the other hand, these agents were without effect on myofibrillar proteolysis as assessed by N tau-methylhistidine release by incubated muscles and changes in tissue contents of the amino acid. In fact, some of the agents tested tended to decrease myofibrillar proteolysis slightly. These results indicate that acute elevation of intracellular Ca2+ is associated with increased breakdown of non-myofibrillar but not myofibrillar proteins. Because of this, the role of elevated Ca2+ in muscle atrophy in certain pathological states is questioned. The data also indicate that the breakdown of myofibrillar and non-myofibrillar proteins in muscle is regulated independently and by different pathways, a conclusion reached in previous studies with perfused rat muscle.
在体外培养的离体趾长伸肌中，用先前已证明能增加细胞内钙离子浓度的试剂进行孵育，评估钙离子对肌原纤维蛋白水解的影响。通过测量N-τ-甲基组氨酸的释放来评估肌原纤维蛋白水解，在用环己酰亚胺抑制蛋白质合成后，通过测量孵育肌肉中酪氨酸的释放来评估总蛋白水解。孵育的肌肉释放出可测量量的N-τ-甲基组氨酸，并且在2小时的孵育过程中氨基酸的肌肉含量保持稳定。孵育肌肉释放的N-τ-甲基组氨酸与其在短暂饥饿时灌注大鼠肌肉的释放相似，表明孵育肌肉的完整性。钙离子载体A23187、丁卡因、普鲁卡因、咖啡因和升高的钾离子浓度不同程度地增加了孵育肌肉中乳酸的释放，并降低了ATP和磷酸肌酸的组织含量，表明所测试试剂的代谢有效性。只有A23187和丁卡因增加了细胞总钙离子含量，并且它们增加了酪氨酸的释放。咖啡因和升高的[K⁺]既没有增加细胞钙离子含量也没有增加酪氨酸释放；然而，只有A23187和丁卡因显著增加了酪氨酸释放。另一方面，根据孵育肌肉释放的N-τ-甲基组氨酸以及氨基酸组织含量的变化评估，这些试剂对肌原纤维蛋白水解没有影响。事实上，一些测试试剂倾向于略微降低肌原纤维蛋白水解。这些结果表明，细胞内钙离子的急性升高与非肌原纤维蛋白而非肌原纤维蛋白的分解增加有关。因此，在某些病理状态下钙离子升高在肌肉萎缩中的作用受到质疑。数据还表明，肌肉中肌原纤维蛋白和非肌原纤维蛋白的分解是独立调节的，并且通过不同的途径调节，这一结论是先前对灌注大鼠肌肉的研究所得出的。
