Martyn D A, Gordon A M
Center for Bioengineering, University of Washington, Seattle 98195.
J Muscle Res Cell Motil. 1988 Oct;9(5):428-45. doi: 10.1007/BF01774069.
The calcium sensitivity of force was measured in glycerinated rabbit psoas fibres at sarcomere lengths (SL) from 2.3 to 3.4 micron. Increased SL caused calcium sensitivity to increase and the slope of force-calcium relations to decrease. We have hypothesized that length-dependent changes in myofilament lattice spacing and the presence of fixed charge on the myofilaments are important in determining calcium sensitivity. Lattice spacing changes were monitored by measuring fibre diameter (D). D was decreased by increasing SL, decreasing bathing solution pH and by osmotic compression with 3% PVP. 3% PVP caused D to decrease by about 15% at all SLs and pH values tested. Force-calcium relations were measured at different SLs and pH values, with and without 3% PVP in the bathing solutions. At all pH values D at SL 2.3 micron with 3% PVP was comparable to the value at 3.4 micron, without PVP. At pH 7.5 and 7.0 calcium sensitivity was about the same at both SL, although the slope of the force-calcium relation was less at longer SL. The similarity of the calcium sensitivity at the same D, but much different SL, indicates that lattice spacing is important in determining calcium sensitivity, while SL and the degree of myofilament overlap are important in determining the slope of force-calcium relations. In order to test for the role of myofilament charge in determining calcium sensitivity, pH and ionic strength were varied. Decreasing pH caused decreased maximum force and calcium sensitivity. In addition, the influence of SL on calcium sensitivity decreased as pH was lowered, with minimal SL dependence at pH 5.5; even though lattice spacing still decreased with increasing SL. When D was decreased with PVP, calcium sensitivity increased at all SLs in pH 7.5 and 7.0 while the same lattice spacing changes at pH 6.0 and 5.5 resulted in greatly reduced shifts in calcium sensitivity. These results indicate that the effect of lattice spacing on calcium sensitivity depends on myofilament charge. At pH 6.0, even though osmotic compression of the lattice has no effect, increasing SL causes about half the shift in calcium sensitivity seen at pH 7.0. Lowering ionic strength from 200 to 110 mM caused an increase in both the magnitude and length dependence of calcium sensitivity at pH 7.0, while at pH 5.5 both decreased.(ABSTRACT TRUNCATED AT 400 WORDS)
在肌节长度（SL）为2.3至3.4微米的甘油处理兔腰大肌纤维中测量了力的钙敏感性。肌节长度增加导致钙敏感性增加，力-钙关系的斜率降低。我们推测肌丝晶格间距的长度依赖性变化以及肌丝上固定电荷的存在对确定钙敏感性很重要。通过测量纤维直径（D）来监测晶格间距变化。通过增加肌节长度、降低浴液pH值以及用3%聚乙烯吡咯烷酮（PVP）进行渗透压压缩，纤维直径减小。在所有测试的肌节长度和pH值下，3% PVP使纤维直径降低约15%。在不同的肌节长度和pH值下，在浴液中有或没有3% PVP的情况下测量力-钙关系。在所有pH值下，含3% PVP的2.3微米肌节长度处的纤维直径与不含PVP的3.4微米处的值相当。在pH 7.5和7.0时，两个肌节长度处的钙敏感性大致相同，尽管较长肌节长度处力-钙关系的斜率较小。相同纤维直径但肌节长度差异很大时钙敏感性相似，这表明晶格间距对确定钙敏感性很重要，而肌节长度和肌丝重叠程度对确定力-钙关系的斜率很重要。为了测试肌丝电荷在确定钙敏感性中的作用，改变了pH值和离子强度。降低pH值导致最大力和钙敏感性降低。此外，随着pH值降低，肌节长度对钙敏感性的影响减小，在pH 5.5时肌节长度依赖性最小；尽管晶格间距仍随肌节长度增加而减小。当用PVP使纤维直径减小时，在pH 7.5和7.0的所有肌节长度处钙敏感性均增加，而在pH 6.0和5.5时相同的晶格间距变化导致钙敏感性变化大大减小。这些结果表明晶格间距对钙敏感性的影响取决于肌丝电荷。在pH 6.0时，即使晶格的渗透压压缩没有影响，增加肌节长度也会导致约为pH 7.0时所见钙敏感性变化的一半。将离子强度从200 mM降低到110 mM会导致pH 7.0时钙敏感性的幅度和长度依赖性均增加，而在pH 5.5时两者均降低。（摘要截取自400字）
