Yamada Kazuhiro
Department of Physiology, University of Oita Faculty of Medicine, Oita 879 -5593, Japan.
Adv Exp Med Biol. 2005;565:193-202; discussion 203, 379-95. doi: 10.1007/0-387-24990-7_15.
For muscle heat measurements the methods available are sensitive and rapid, and the heat is related to the chemical changes in a manner that provides a firm outline for understanding the mechanism of contraction. For example linear dependence of the shortening heat on the sarcomere length has shown that the rate of turnover of cross-bridges increases during shortening. However, heat is bound to lack specificity. In order to cope with this problem, various methods such as rigorous chemical analyses, phosphorus NMR and microcalorimetry have been introduced. As a result of ultra-rapid freezing and chemical analysis by D. R. Wilkie (Gilbert, Kretzchmar, Wilkie and Woledge, 1971), the energy balance discrepancy between (heat + work) and the amount of phosphocreatine (PCr) split emerged, i.e. the unexplained enthalpy. Calcium ions move from the sarcoplasmic reticulum to the calcium-receptive proteins in the sarcoplasm during contraction. In an attempt to find the cause of the unexplained enthalpy, microcalorimetry of calcium binding to calcium-receptive proteins has been performed. The results have shown that calcium ions dislocated between sites within the sarcoplasm on activation may produce about 1/3 of the unexplained heat. In addition calcium pump should operate by consuming PCr to relocate the calcium after the contraction. Time-resolved phosphorus NMR has also shown that a certain amount of PCr splitting continues during early minute of recovery period after the contraction without Pi released. This delayed splitting of PCr is most likely caused by the kinetic properties of the contractile proteins and can explain another 1/3 of the unexplained enthalpy. The mechanism of how muscle is regulated is another important question. Studies of calcium binding to calcium-receptive proteins in the sarcoplasm by using titration microcalorimetry has shown that troponin C has a characteristic single calcium-binding site that is most likely to be involved in the regulation of contraction.
对于肌肉热量测量，现有的方法灵敏且快速，并且热量与化学变化相关，这种关联方式为理解收缩机制提供了坚实的框架。例如，缩短热与肌节长度的线性关系表明，在缩短过程中横桥的周转速率增加。然而，热量必然缺乏特异性。为了解决这个问题，人们引入了各种方法，如严格的化学分析、磷核磁共振和微量量热法。通过D. R. 威尔基（吉尔伯特、克雷茨马尔、威尔基和沃利奇，1971年）进行的超快速冷冻和化学分析，出现了（热+功）与磷酸肌酸（PCr）分解量之间的能量平衡差异，即无法解释的焓。在收缩过程中，钙离子从肌浆网移动到肌浆中的钙受体蛋白。为了找出无法解释的焓的原因，人们对钙离子与钙受体蛋白的结合进行了微量量热法研究。结果表明，激活时肌浆内不同位点之间移位的钙离子可能产生约1/3无法解释的热量。此外，钙泵在收缩后应通过消耗PCr来重新定位钙。时间分辨磷核磁共振也表明，在收缩后的恢复早期，一定量的PCr分解仍在继续，且没有释放无机磷（Pi）。PCr的这种延迟分解很可能是由收缩蛋白的动力学特性引起的，并且可以解释另外1/3无法解释的焓。肌肉如何被调节的机制是另一个重要问题。通过滴定微量量热法研究肌浆中钙离子与钙受体蛋白的结合表明，肌钙蛋白C具有一个特征性的单一钙结合位点，该位点最有可能参与收缩调节。
