Fujita H, Ishiwata S
Department of Physics, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan.
Biophys J. 1999 Sep;77(3):1540-6. doi: 10.1016/S0006-3495(99)77001-3.
We investigated the effect of pH on isometric tension in actin filament-reconstituted and thin filament-reconstituted bovine cardiac muscle fibers in the pH range of 6.0-7.4. Thin filament was reconstituted from purified G-actin with either bovine cardiac tropomyosin (Tm) or rabbit skeletal Tm in conjunction with cardiac or skeletal troponin (Tn). Results showed that isometric tension decreased linearly with a decrease in pH. The slope of the pH-tension relation, DeltaF/DeltapH (Deltarelative tension/Deltaunit pH), was 0.28 and 0.44 in control cardiac fibers and skeletal fibers, respectively. In actin filament-reconstituted fibers without regulatory proteins, DeltaF/DeltapH was 0.62, namely larger than that in cardiac or skeletal fibers. When reconstituted with cardiac Tm-Tn complex (nTm), DeltaF/DeltapH recovered to 0.32, close to the value obtained in control cardiac fibers. When reconstituted with skeletal nTm, DeltaF/DeltapH recovered to 0.48, close to the value for control skeletal fibers. To determine whether Tm or Tn is responsible for the inhibitory effects of nTm on the tension decrease caused by reduced pH, thin filament was reconstituted with cardiac Tm and skeletal Tn, or with skeletal Tm and cardiac Tn. When cardiac Tm was used, pH dependence of isometric tension coincided with that of control cardiac fibers. When skeletal Tm was used, the pH dependence coincided with that of control skeletal fibers. Furthermore, closely similar results were obtained in fibers reconstituted with actin and either cardiac or skeletal Tm without Tn. These results demonstrate that Tm but not Tn modulates the pH dependence of active tension.
我们研究了pH值在6.0 - 7.4范围内对肌动蛋白丝重构和细肌丝重构的牛心肌纤维等长张力的影响。细肌丝由纯化的G - 肌动蛋白与牛心肌原肌球蛋白（Tm）或兔骨骼肌Tm联合心肌或骨骼肌肌钙蛋白（Tn）重构而成。结果表明，等长张力随pH值降低呈线性下降。pH - 张力关系的斜率，即ΔF/ΔpH（Δ相对张力/Δ单位pH），在对照心肌纤维和骨骼肌纤维中分别为0.28和0.44。在没有调节蛋白的肌动蛋白丝重构纤维中，ΔF/ΔpH为0.62，即大于心肌或骨骼肌纤维中的值。当用心肌Tm - Tn复合物（nTm）重构时，ΔF/ΔpH恢复到0.32，接近对照心肌纤维中获得的值。当用骨骼肌nTm重构时，ΔF/ΔpH恢复到0.48，接近对照骨骼肌纤维的值。为了确定是Tm还是Tn导致了nTm对pH降低引起的张力降低的抑制作用，细肌丝用心肌Tm和骨骼肌Tn或骨骼肌Tm和心肌Tn进行重构。当使用心肌Tm时，等长张力的pH依赖性与对照心肌纤维的一致。当使用骨骼肌Tm时，pH依赖性与对照骨骼肌纤维的一致。此外，在用肌动蛋白和心肌或骨骼肌Tm（无Tn）重构的纤维中也获得了非常相似的结果。这些结果表明，调节活性张力pH依赖性的是Tm而非Tn。
