THE CONTRACTILE RESPONSE DURING STEADY LENGTHENING OF STIMULATED FROG-MUSCLE FIBERS
THE CONTRACTILE RESPONSE DURING STEADY LENGTHENING OF STIMULATED FROG-MUSCLE FIBERS
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DOI：
10.1113/jphysiol.1990.sp018324
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发表时间：
1990-12-01
影响因子：
5.5
通讯作者：
PIAZZESI, G
中科院分区：
文献类型：
--
作者：
LOMBARDI, V;PIAZZESI, G
1.通过扬声器电动机，在异构破伤风平台，以不同速度（每半肌节0.025-1.2 μ m/s;温度2 - 5.5 ℃）对分离的青蛙肌纤维施加稳定的延长。在肌节水平的延长是通过一个条纹跟随器在固定端或长度钳模式。力响应通过电容计传感器（谐振频率50 kHz）测量。排除了在延长过程中表现出总体不均匀性的制剂。2.在所有情况下，每半肌节延长约20 nm后达到稳定的张力。当张力为等长强直力（T0）的1.9-2倍时，在该稳定阶段期间的张力随着伸长速度上升至每半肌节0.25-0.4 μ m/s。速度的进一步增加只会使稳定张力增加很少。3.在过渡阶段期间，在达到稳定张力之前，如果延长速度小于0.25-0.3 μ m/s/半肌节，则张力单调上升;在更高的速度下，张力上升到稳定水平以上，当延长为10-14 nm/半肌节时达到峰值，然后下降到稳定水平。峰值处的张力继续升高，延长速度高于每半肌节0.3 μ m/s。4.在力响应的过渡阶段内的张力上升期间，由于肌腱顺应性，节段以比施加在整个纤维上的速度低15-20%的速度伸长。5. 在稳定阶段，非均匀性的延长速度开始以上的延长速度不同的纤维。在低于该值的速度下，段以与施加在纤维上的速度相同的速度伸长。6.张力反应大步拉伸（高达12纳米每半肌节），应用在高原的等长强直，表明contracitle机械的瞬时弹性是不负责的限制力达到高速延长。7.在稳态或力响应期间，通过在不同速度下的稳定延长上叠加小步长（每半肌节<1.5nm）来确定瞬时刚度。在延长过程中，刚度比等长强直平台期大10-20%，并且在所用速度范围内几乎保持恒定，与延长速度无关。8.结果表明，稳定的伸长强直纤维诱导一个跨桥周期，其特征在于快速脱离的跨桥扩展超过临界水平。以这种方式分离的跨桥的重新连接也非常迅速。9.收缩模型，包括跨桥连接，力的产生和分离的单独步骤，被认为是兼容的实验力和刚度-速度关系。在该模型中，分离的延长横桥发生在早期阶段的周期。这个分离过程的速率常数急剧增加超过临界量的跨桥应变;强制分离的跨桥的再附着比完成循环后分离的跨桥的附着快200倍。
1. Steady lengthenings at different velocities (0.025-1.2 .mu.m/s per half-sarcomere; temperature 2-5.5.degree.C) were imposed on isolated frog muscle fibers at the isomertic tetanus plateau by means of a loudspeaker motor. The lengthening at the sarcomere level was measured by means of a striation follower either in fixed-end or in length-clamp mode. The force response was measured by a capacitance gauge transducer (resonance frequency 50 kHz). Preparations showing gross non-homogeneity during lengthening were excluded. 2. A steady tension was in all cases reached after about 20 nm per half-sarcomere of lengthening. Tension during this steady phase rose with speed of elongation up to 0.25-0.4 .mu.m/s per half-sarcomere, when tension was 1.9-2 times isometric tetanic force (T0). Further increase in speed produced only very little increase in the steady tension. 3. During the transitory phase, before steady tension was reached, the tension rose monotonically if speed of lengthening was less than 0.25-0.3 .mu.m/s per half-sarcomere; at higher speed the tension rose above the steady level, reaching a peak when extension was 10-14 nm per half-sarcomere, and then fell to the steady level. Tension at the peak continued to rise with speed of lengthening above 0.3 .mu.m/s per half-sarcomere. 4. During the tension rise within the transitory phase of force reponse the segment elongated at a speed 15-20% lower than that imposed on the whole fibre, as a consequence of tendon compliance. 5. During the steady phase, non-homogeneity of lengthening speed began above a speed of lengthening which varied from fibre to fibre. At speeds below this value, segments elongated at the same speed as that imposed on the fibre. 6. Tension responses to large step stretches (up to 12 nm per half-sarcomere), applied at the plateau of isometric tetanus, showed that the instantenous elasticitiy of contracitle machinery is not responsible for the limit in force attained with high-speed lengthening. 7. Instantaneous stiffness was determined during the steady state or force response by superposing small steps (< 1.5 nm per half-sarcomere) on steady lengthening at different velocities. Stiffness was 10-20% larger during lengthening than at the plateau of isometric tetanus and remained practically constant, independent of lengthening velocity, in the range of velocities used. 8. The results indicate that steady lengthening of a tetanized fibre induces a crossbridge cycle characterized by fast-detachment of the cross-bridge extended beyond a critical level. Reattachment of cross-bridges detached in this way is also very rapid. 9. A model of contraction, including separate steps for cross-bridge attachment, force generation and detachment, was found to be compatible with the experimental force- and stiffness-velocity relations. In the model, detachment of extended crossbridges occurs at an early stage of the cycle. The rate constant of this detachment process increases sharply beyond a critical amount of cross-bridge strain; reattachment of forcibly detached cross-bridges is two hundred times faster than attachment of cross-bridges which detach after completion of the cycle.