Morris Carl A, Tobacman Larry S, Homsher Earl
Department of Physiology, School of Medicine, University of California, Los Angeles, CA 90095, USA.
J Physiol. 2003 Jul 1;550(Pt 1):205-15. doi: 10.1113/jphysiol.2003.040899. Epub 2003 May 2.
The unloaded shortening velocity of skinned rabbit psoas muscle fibres is sensitive to [Ca2+]. To determine whether Ca2+ affects the unloaded shortening velocity via regulation of crossbridge kinetics or crossbridge number, the shortening velocity was measured following changes in either [Ca2+] or the number of active thin filament regulatory units. The native troponin C (TnC) was extracted and replaced with either cardiac TnC (cTnC) or a mixture of cTnC and an inactive mutant cardiac TnC (CBMII TnC). The unloaded shortening velocity of the cTnC-replaced fibres was determined at various values of [Ca2+] and compared with different cTnC:CBMII TnC ratios at a saturating [Ca2+]. If Ca2+ regulates the unloaded shortening velocity via kinetic modulation, differences in the velocity-tension relationship between the cTnC fibres and the cTnC:CBMII TnC fibres would be apparent. Alternatively, Ca2+ control of the number of active crossbridges would yield similar velocity-tension relationships when comparing the cTnC and cTnC:CBMII TnC fibres. The results show a decline in the unloaded shortening velocity that is determined by the relative tension, defined as the level of thin filament activation, rather than the [Ca2+]. Furthermore, at lower levels of relative tension, the reduction in unloaded shortening is not the result of changes in any cooperative effects of myosin on Ca2+ binding to the thin filament. Rather, it may be related to a decrease in crossbridge-induced activation of the thin filament at the level of the individual regulatory unit. In summary, the results suggest that Ca2+ regulates the unloaded shortening velocity in skinned fibres by reducing the number of crossbridges able to productively bind to the thin filament without affecting any inherent property of the myosin.
去皮肤的兔腰大肌纤维的无负荷缩短速度对[Ca2+]敏感。为了确定Ca2+是通过调节横桥动力学还是横桥数量来影响无负荷缩短速度，在[Ca2+]或活性细肌丝调节单位数量发生变化后测量缩短速度。提取天然肌钙蛋白C（TnC），并用心脏TnC（cTnC）或cTnC与无活性突变体心脏TnC（CBMII TnC）的混合物进行替换。在不同的[Ca2+]值下测定cTnC替换纤维的无负荷缩短速度，并与饱和[Ca2+]下不同的cTnC:CBMII TnC比率进行比较。如果Ca2+通过动力学调节来调节无负荷缩短速度，那么cTnC纤维和cTnC:CBMII TnC纤维之间的速度-张力关系差异将很明显。或者，当比较cTnC和cTnC:CBMII TnC纤维时，Ca2+对活性横桥数量的控制将产生相似的速度-张力关系。结果表明，无负荷缩短速度的下降取决于相对张力，相对张力定义为细肌丝激活水平，而不是[Ca2+]。此外，在较低的相对张力水平下，无负荷缩短的减少不是肌球蛋白对Ca2+结合到细肌丝的任何协同效应变化的结果。相反，它可能与单个调节单位水平上横桥诱导的细肌丝激活减少有关。总之，结果表明Ca2+通过减少能够有效结合到细肌丝上的横桥数量来调节去皮肤纤维的无负荷缩短速度，而不影响肌球蛋白的任何固有特性。
