Gillis J M, Thomason D, Lefèvre J, Kretsinger R H
J Muscle Res Cell Motil. 1982 Dec;3(4):377-98. doi: 10.1007/BF00712090.
The distribution of Ca2+ and Mg2+ among the 'regulatory' cation binding sites of troponin (T-sites) and the strong, Ca2+-Mg2+ binding sites of troponin and parvalbumins (P-sites) in the sarcoplasm of a muscle was calculated. At rest, 60% of the T-sites were metal free, while 92% of the P-sites were loaded with Mg2+. In response to a Ca2+ pulse, troponin-calcium (T-Ca) complexes were rapidly formed, while the binding of Ca2+ to P-sites was limited by the slow rate of dissociation of the parvalbumin-magnesium (P-Mg) complexes. Muscle activation was not prevented by a high content of parvalbumins. Parvalbumin and the sarcoplasmic reticulum (SR) pump were complementary relaxing factors that removed Ca2+ from the cytosol and from the T-sites. Parvalbumins dominated the first part of relaxation, while the action of the SR was essential to ensure the return to a very low level of free Ca2+ ion and of T-Ca. After relaxation, a large fraction of the Ca2+ pulse was still bound to parvalbumins and returned slowly to the SR during the recovery. When the SR activity was reduced, the presence of parvalbumins preserved a fast rate of relaxation, at least for a few contractions. This may have a high adaptive value in cold-blooded animals.
计算了肌肉肌浆中肌钙蛋白“调节性”阳离子结合位点（T位点）以及肌钙蛋白和小清蛋白的强Ca²⁺-Mg²⁺结合位点（P位点）之间Ca²⁺和Mg²⁺的分布。静息时，60%的T位点无金属，而92%的P位点负载有Mg²⁺。响应Ca²⁺脉冲时，肌钙蛋白-钙（T-Ca）复合物迅速形成，而Ca²⁺与P位点的结合受小清蛋白-镁（P-Mg）复合物缓慢解离速率的限制。高含量的小清蛋白并不能阻止肌肉激活。小清蛋白和肌浆网（SR）泵是互补的舒张因子，可从细胞质和T位点去除Ca²⁺。小清蛋白在舒张的第一阶段起主导作用，而SR的作用对于确保游离Ca²⁺离子和T-Ca恢复到极低水平至关重要。舒张后，大部分Ca²⁺脉冲仍与小清蛋白结合，并在恢复过程中缓慢返回SR。当SR活性降低时，小清蛋白的存在至少在几次收缩期间保持了快速的舒张速率。这在冷血动物中可能具有很高的适应性价值。
