Edman K A P, Caputo C
Department of Experimental Medical Science, Biomedical Centre, University of Lund, Lund, Sweden.
Acta Physiol (Oxf). 2017 Oct;221(2):142-148. doi: 10.1111/apha.12876. Epub 2017 Apr 11.
The study was undertaken to explore whether shortening of striated muscle during activity is associated with release of bound Ca into the myofibrillar space as has previously been proposed in order to explain the depressant effect of active shortening.
The experiments were carried out on single muscle fibres isolated from the anterior tibialis muscle of Rana temporaria. The fibres were loaded with the calcium sensitive indicator Fluo-3. The fibres, stimulated to produce a partially fused isometric tetanus, were subjected to a shortening ramp or, alternatively, to a stretch ramp during activity while force, fibre length, sarcomere length and the Fluo-3 signal were recorded.
A shortening ramp performed during a partially fused tetanus caused an increase in the myofibrillar free calcium concentration and produced, simultaneously, a decrease in active force. The isometric force recovered gradually after the shortening ramp, while the intracellular Ca concentration stayed above the control level during the remainder of the stimulation period. A stretch ramp applied during a partially fused tetanus caused a considerably smaller change in the myofibrillar Ca concentration.
The results provide evidence that the myosin cross-bridges interact with the calcium binding sites on the thin filaments during active shortening, causing sustained release of calcium and reduced contractile strength.
本研究旨在探讨横纹肌在活动期间的缩短是否与结合钙释放到肌原纤维空间有关，正如之前所提出的那样，以此来解释主动缩短的抑制作用。
实验在从林蛙胫前肌分离出的单根肌纤维上进行。这些纤维用钙敏感指示剂Fluo - 3进行加载。纤维被刺激产生部分融合的等长强直收缩，在活动期间对其施加缩短斜坡或拉伸斜坡，同时记录力、纤维长度、肌节长度和Fluo - 3信号。
在部分融合的强直收缩期间进行的缩短斜坡导致肌原纤维游离钙浓度增加，同时主动力下降。缩短斜坡后等长力逐渐恢复，而在刺激期的剩余时间内细胞内钙浓度保持在对照水平之上。在部分融合的强直收缩期间施加的拉伸斜坡导致肌原纤维钙浓度的变化小得多。
结果提供了证据，表明肌球蛋白横桥在主动缩短期间与细肌丝上的钙结合位点相互作用，导致钙的持续释放和收缩强度降低。
