Martyn D A, Chase P B
Center for Bioengineering, University of Washington, Seattle 98195.
Biophys J. 1995 Jan;68(1):235-42. doi: 10.1016/S0006-3495(95)80179-7.
The early, rapid phase of tension recovery (phase 2) after a step change in sarcomere length is thought to reflect the force-generating transition of myosin bound to actin. We have measured the relation between the rate of tension redevelopment during phase 2 (r), estimated from the half-time of tension recovery during phase 2 (r = t0.5(-1)), and steady-state force at varying [Ca2+] in single fibers from rabbit psoas. Sarcomere length was monitored continuously by laser diffraction of fiber segments (length approximately 1.6 mm), and sarcomere homogeneity was maintained using periodic length release/restretch cycles at 13-15 degrees C. At lower [Ca2+] and forces, r was elevated relative to that at pCa 4.0 for both releases and stretches (between +/- 8 nm). For releases of -3.4 +/- 0.7 nm.hs-1 at pCa 6.6 (where force was 10-20% of maximum force at pCa 4.0), r was 3.3 +/- 1.0 ms-1 (mean +/- SD; N = 5), whereas the corresponding value of r at pCa 4.0 was 1.0 +/- 0.2 ms-1 for releases of -3.5 +/- 0.5 nm.hs-1 (mean +/- SD; N = 5). For stretches of 1.9 +/- 0.7 nm.hs-1, r was 1.0 +/- 0.3 ms-1 (mean +/- SD; N = 9) at pCa 6.6, whereas r was 0.4 +/- 0.1 ms-1 at pCa 4.0 for stretches of 1.9 +/- 0.5 (mean +/- SD; N = 14). Faster phase 2 transients at submaximal Ca(2+)-activation were not caused by changes in myofilament lattice spacing because 4% Dextran T-500, which minimizes lattice spacing changes, was present in all solutions. The inverse relationship between phase 2 kinetics and force obtained during steady-state activation of skinned fibers appears to be qualitatively similar to observations on intact frog skeletal fibers during the development of tetanic force. The data are consistent with models that incorporate a direct effect of [Ca2+] on phase 2 kinetics of individual cross-bridges or, alternatively, in which phase 2 kinetics depend on cooperative interactions between cross-bridges.
肌节长度发生阶跃变化后，张力恢复的早期快速阶段（阶段2）被认为反映了与肌动蛋白结合的肌球蛋白产生力的转变过程。我们测量了阶段2期间张力重新发展的速率（r）与稳态力之间的关系，其中r是根据阶段2期间张力恢复的半衰期估算得出（r = t0.5(-1)），实验对象是来自兔腰大肌的单根纤维，测量在不同[Ca2+]浓度下进行。通过对纤维段（长度约1.6毫米）进行激光衍射来连续监测肌节长度，并在13 - 15摄氏度下使用周期性的长度释放/再拉伸循环来维持肌节的均匀性。在较低的[Ca2+]浓度和力的条件下，对于释放和拉伸（在+/- 8纳米之间），r相对于pCa 4.0时均有所升高。在pCa 6.6时，对于-3.4 +/- 0.7纳米·小时-1的释放（此时力为pCa 4.0时最大力的10 - 20%），r为3.3 +/- 1.0毫秒-1（平均值+/-标准差；N = 5），而在pCa 4.0时，对于-3.5 +/- 0.5纳米·小时-1的释放（平均值+/-标准差；N = 5），相应的r值为1.0 +/- 0.2毫秒-1。对于1.9 +/- 0.7纳米·小时-1的拉伸，在pCa 6.6时r为1.0 +/- 0.3毫秒-1（平均值+/-标准差；N = 9），而在pCa 4.0时，对于1.9 +/- 0.5（平均值+/-标准差；N = 14）的拉伸，r为0.4 +/- 0.1毫秒-1。在亚最大Ca(2+)激活状态下更快的阶段2瞬变并非由肌丝晶格间距的变化引起，因为所有溶液中都含有4%的葡聚糖T - 500，它能将晶格间距的变化降至最低。在对去表皮纤维进行稳态激活期间获得的阶段2动力学与力之间的反比关系，在定性上似乎与在完整青蛙骨骼肌纤维产生强直力过程中的观察结果相似。这些数据与以下模型一致：该模型要么包含[Ca2+]对单个横桥阶段2动力学的直接影响，要么阶段2动力学取决于横桥之间的协同相互作用。
