Kentish J C, ter Keurs H E, Ricciardi L, Bucx J J, Noble M I
Circ Res. 1986 Jun;58(6):755-68. doi: 10.1161/01.res.58.6.755.
To investigate the extent to which the properties of the cardiac myofibrils contribute to the length-force relation of cardiac muscle, we determined the sarcomere length-force relations for rat ventricular trabeculae both before and after the muscles were skinned with the detergent Triton X-100. Sarcomere length was measured continuously by laser diffraction. In the unskinned trabeculae stimulated at 0.2 Hz, the relation between active force and sarcomere length at an extracellular calcium concentration of 1.5 mM was curved away from the sarcomere length axis, with zero force at sarcomere length of 1.5-1.6 micron. At 0.3 mM calcium, the sarcomere length-force relation was curved toward the sarcomere length axis. Chemical skinning of the muscle with 1% Triton X-100 in a "relaxing solution" caused an increase in intensity and decrease in dispersion of the first order diffraction beam, indicating an increased uniformity of sarcomere length in the relaxed muscle. During calcium-regulated contractures in the skinned muscles, the central sarcomeres shortened by up to 20%. As the calcium concentration was increased over the range 1-50 microM, the relation between steady calcium-regulated force and sarcomere length shifted to higher force values and changed in shape in a manner similar to that observed for changes in extracellular calcium concentration before skinning. The sarcomere length-force relations for the intact muscles at an extracellular calcium concentration of 1.5 mM were similar to the curves at calcium concentration of 8.9 microM in the skinned preparations, whereas the curves at an extracellular calcium concentration of 0.3 mM in intact muscles fell between the relations at calcium concentrations of 2.7 and 4.3 microM in the skinned preparations. A factor contributing to the shape of the curves in the skinned muscle at submaximal calcium concentrations was that the calcium sensitivity of force production increased with increasing sarcomere length. The calcium concentration required for 50% activation decreased from 7.71 +/- 0.52 microM to 3.77 +/- 0.33 microM for an increase of sarcomere length from 1.75 to 2.15 micron. The slope of the force-calcium concentration relation increased from 2.82 to 4.54 with sarcomere length between 1.75 and 2.15 micron. This change in calcium sensitivity was seen over the entire range of sarcomere lengths corresponding to the ascending limb of the cardiac length-force relation. It is concluded that the properties of the cardiac contractile machinery (including the length-dependence of calcium sensitivity) can account for much of the shape of the ascending limb in intact cardiac muscle.
为了研究心肌肌原纤维的特性在多大程度上影响心肌的长度-张力关系，我们测定了用去污剂Triton X-100处理心肌前后大鼠心室肌小梁的肌节长度-张力关系。通过激光衍射连续测量肌节长度。在以0.2 Hz频率刺激的未处理肌小梁中，细胞外钙浓度为1.5 mM时，主动张力与肌节长度的关系曲线偏离肌节长度轴，肌节长度为1.5 - 1.6微米时张力为零。钙浓度为0.3 mM时，肌节长度-张力关系曲线朝向肌节长度轴弯曲。在“松弛溶液”中用1% Triton X-100对肌肉进行化学去膜处理，导致一级衍射光束的强度增加且离散度减小，表明松弛肌肉中肌节长度的均匀性增加。在去膜肌肉的钙调节收缩过程中，中央肌节缩短达20%。随着钙浓度在1 - 50 microM范围内增加，稳定的钙调节张力与肌节长度的关系向更高的张力值移动，并且形状改变，其方式类似于去膜前细胞外钙浓度变化时观察到的情况。细胞外钙浓度为1.5 mM时完整肌肉的肌节长度-张力关系与去膜制剂中钙浓度为8.9 microM时的曲线相似，而完整肌肉中细胞外钙浓度为0.3 mM时的曲线介于去膜制剂中钙浓度为2.7和4.3 microM时的关系曲线之间。在次最大钙浓度下去膜肌肉中曲线形状的一个影响因素是，力产生的钙敏感性随肌节长度增加而增加。肌节长度从1.75微米增加到2.15微米时，50%激活所需的钙浓度从7.71±0.52 microM降至3.77±0.33 microM。肌节长度在1.75和2.15微米之间时，力-钙浓度关系的斜率从2.82增加到4.54。在与心脏长度-张力关系上升支相对应的整个肌节长度范围内都观察到了这种钙敏感性的变化。结论是，心脏收缩机制的特性（包括钙敏感性的长度依赖性）可以解释完整心肌中上升支的大部分形状。
