Fink R H, Stephenson D G, Williams D A
J Physiol. 1986 Jan;370:317-37. doi: 10.1113/jphysiol.1986.sp015937.
A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+-activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast- and slow-twitch fibres and toad twitch fibres. The contractile apparatus of rat slow-twitch fibres was affected differently from that of rat fast-twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as gamma/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow-twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mM and from 56 to 316 mM respectively. Maximum Ca2+-activated force, however, declined only by a factor of 2.2 in slow-twitch fibres compared with 4.2 in the other fibre types, when I was increased from 154 to 354 mM. In slow-twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50% of the fast-twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow-twitch fibres. Rigor force levels were found to decline markedly with increasing iota and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross-bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross-bridges that can be formed) is influenced by the ionic conditions. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+-sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than gamma/2) influences markedly the maximal Ca2+-activated force, the maximum steepness of the pCa-force relations and the oscillatory processes of myofibrillar origin.
开展了一项研究，以调查离子强度和单价阳离子对机械去膜肌纤维的等长收缩、Ca2+激活的力及强直反应的影响。使用了三种类型的骨骼肌纤维：大鼠快肌纤维和慢肌纤维以及蟾蜍的快肌纤维。当改变离子强度（以离子当量表示为I或形式上表示为γ/2）和[K+]时，大鼠慢肌纤维的收缩装置受到的影响与大鼠快肌纤维和两栖类快肌纤维不同。因此，当I和[K+]分别从94 mM增加到354 mM以及从56 mM增加到316 mM时，慢肌纤维对Ca2+的表观敏感性降低幅度（降低20倍）远大于其他纤维类型（降低12倍）。然而，当I从154 mM增加到354 mM时，慢肌纤维中最大Ca2+激活的力仅下降2.2倍，而其他纤维类型下降4.2倍。在慢肌纤维中，发现肌原纤维起源的力振荡在低I值时幅度、持续时间和频率大幅增加，而在高离子强度时几乎消失。在低I值时，还发现约50%的快肌纤维在次最大激活时会出现肌原纤维力振荡。这些振荡的特征与慢肌纤维不同。在所有纤维类型中，发现强直力水平随I和[K+]的增加而显著下降。出乎意料的是，一旦在特定离子环境中建立强直力，力的水平是稳定的，而与离子强度和单价阳离子浓度的进一步变化无关。这些结果表明，强直横桥可以在不同的稳定位置形成，并且在某些位置附着的概率（而非可形成的横桥总数）受离子条件影响。本研究提供的进一步实验证据表明，[K+]的增加主要导致收缩装置Ca2+敏感性的降低，并且离子强度（表示为I而非γ/2）显著影响最大Ca2+激活的力、pCa-力关系的最大斜率以及肌原纤维起源的振荡过程。
