Pham Toan, Tran Kenneth, Mellor Kimberley M, Hickey Anthony, Power Amelia, Ward Marie-Louise, Taberner Andrew, Han June-Chiew, Loiselle Denis
Department of Physiology, University of Auckland, Auckland, New Zealand.
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand.
J Physiol. 2017 Jul 15;595(14):4725-4733. doi: 10.1113/JP274174. Epub 2017 Jun 1.
The heat of activation of cardiac muscle reflects the metabolic cost of restoring ionic homeostasis following a contraction. The accuracy of its measurement depends critically on the abolition of crossbridge cycling. We abolished crossbridge activity in isolated rat ventricular trabeculae by use of blebbistatin, an agent that selectively inhibits myosin II ATPase. We found cardiac activation heat to be muscle length independent and to account for 15-20% of total heat production at body temperature. We conclude that it can be accurately estimated at minimal muscle length.
Activation heat arises from two sources during the contraction of striated muscle. It reflects the metabolic expenditure associated with Ca pumping by the sarcoplasmic reticular Ca -ATPase and Ca translocation by the Na /Ca exchanger coupled to the Na ,K -ATPase. In cardiac preparations, investigators are constrained in estimating its magnitude by reducing muscle length to the point where macroscopic twitch force vanishes. But this experimental protocol has been criticised since, at zero force, the observed heat may be contaminated by residual crossbridge cycling activity. To eliminate this concern, the putative thermal contribution from crossbridge cycling activity must be abolished, at least at minimal muscle length. We achieved this using blebbistatin, a selective inhibitor of myosin II ATPase. Using a microcalorimeter, we measured the force production and heat output, as functions of muscle length, of isolated rat trabeculae from both ventricles contracting isometrically at 5 Hz and at 37°C. In the presence of blebbistatin (15 μmol l ), active force was zero but heat output remained constant, at all muscle lengths. Activation heat measured in the presence of blebbistatin was not different from that estimated from the intercept of the heat-stress relation in its absence. We thus reached two conclusions. First, activation heat is independent of muscle length. Second, residual crossbridge heat is negligible at zero active force; hence, the intercept of the cardiac heat-force relation provides an estimate of activation heat uncontaminated by crossbridge cycling. Both results resolve long-standing disputes in the literature.
心肌的激活热反映了收缩后恢复离子稳态的代谢成本。其测量的准确性关键取决于横桥循环的消除。我们通过使用blebbistatin（一种选择性抑制肌球蛋白II ATP酶的药物）消除了离体大鼠心室肌小梁中的横桥活性。我们发现心脏激活热与肌肉长度无关，并且在体温下占总产热的15 - 20%。我们得出结论，在最小肌肉长度时可以准确估计激活热。
在横纹肌收缩过程中，激活热有两个来源。它反映了与肌浆网Ca -ATP酶泵出Ca以及与Na,K -ATP酶偶联的Na /Ca交换体转运Ca相关的代谢消耗。在心脏标本中，研究人员通过将肌肉长度缩短至宏观抽搐力消失的程度来估计其大小，但这种实验方案受到了批评，因为在零力时，观察到的热量可能被残余的横桥循环活性所污染。为了消除这一担忧，至少在最小肌肉长度时，必须消除横桥循环活性产生的假定热贡献。我们使用blebbistatin（肌球蛋白II ATP酶的选择性抑制剂）实现了这一点。使用微量热计，我们测量了来自两个心室的离体大鼠肌小梁在5Hz等长收缩和37°C下，作为肌肉长度函数的力产生和热输出。在存在blebbistatin（15μmol/l）的情况下，所有肌肉长度下的主动力均为零，但热输出保持恒定。在存在blebbistatin的情况下测量的激活热与在不存在blebbistatin时根据热-应力关系的截距估计的激活热没有差异。因此，我们得出了两个结论。第一，激活热与肌肉长度无关。第二，在零主动力时，残余横桥热可忽略不计；因此，心脏热-力关系的截距提供了一个未被横桥循环污染的激活热估计值。这两个结果解决了文献中长期存在的争议。
