Duty S, Allen D G
Department of Physiology, University of Sydney, Australia.
Pflugers Arch. 1994 May;427(1-2):102-9. doi: 10.1007/BF00585948.
Fluorescence microscopy has been used to examine the distribution of intracellular calcium concentration ([Ca2+]i) in isolated single fibres from the mouse flexor brevis muscle during fatiguing stimulation. Under control conditions there was a virtually uniform distribution of [Ca2+]i in fura-2 loaded fibres either at rest or during short (0.35 s, 100 Hz) tetani. Fatigue produced by repeated short tetani was accompanied by an early rise, followed by a marked fall, in tetanic [Ca2+]i. Throughout the period of fatiguing stimulation the distribution of [Ca2+]i remained uniform with no detectable gradients observed. In contrast, when fatigue was produced by continuous 100 Hz stimulation, a small gradient of [Ca2+]i developed across the fibre with the [Ca2+]i in the centre of the fibre lower than that at the edge of the fibre. This gradient was apparent after 1.7 s, persisted for at least 11 s and was superimposed on a rise followed by a fall in spatially averaged [Ca2+]i. Reduction of the extracellular Na+ to 50% caused reduced force production and a reduced [Ca2+]i in the centre of the fibre. To assess the contribution of reduced response of the myofibrillar proteins to [Ca2+]i during continuous tetani, the relation between [Ca2+]i and force throughout the long tetanus was compared with that obtained in short, unfatigued tetani. These results show that in long tetani, reduced tetanic [Ca2+]i and reduced responsiveness of the myofibrillar proteins to [Ca2+]i each make important contributions to the decline of force, whereas the gradients of [Ca2+]i make only a small contribution.
荧光显微镜已被用于检测小鼠短屈肌分离出的单根纤维在疲劳刺激过程中细胞内钙浓度（[Ca2+]i）的分布情况。在对照条件下，无论是在静息状态还是在短暂（0.35秒，100赫兹）强直收缩期间，fura-2负载的纤维中[Ca2+]i的分布几乎是均匀的。由重复短强直收缩产生的疲劳伴随着强直收缩期[Ca2+]i的早期升高，随后显著下降。在整个疲劳刺激期间，[Ca2+]i的分布保持均匀，未观察到可检测到的梯度。相比之下，当通过持续100赫兹刺激产生疲劳时，纤维中会形成一个小的[Ca2+]i梯度，纤维中心的[Ca2+]i低于纤维边缘的[Ca2+]i。这个梯度在1.7秒后变得明显，持续至少11秒，并叠加在空间平均[Ca2+]i先升高后下降的过程中。将细胞外Na+浓度降低至50%会导致力产生减少以及纤维中心的[Ca2+]i降低。为了评估在持续强直收缩期间肌原纤维蛋白对[Ca2+]i反应性降低的作用，将长时间强直收缩期间[Ca2+]i与力之间的关系与在短暂、未疲劳的强直收缩中获得的关系进行了比较。这些结果表明，在长时间强直收缩中，强直收缩期[Ca2+]i降低以及肌原纤维蛋白对[Ca2+]i的反应性降低均对力的下降起重要作用，而[Ca2+]i梯度的作用较小。
