Koenders Annette, Lamey Tina M, Medler Scott, West Jan M, Mykles Donald L
School of Natural Sciences, Edith Cowan University, Joondalup, WA 6027, Australia.
J Exp Zool A Comp Exp Biol. 2004 Jul 1;301(7):588-98. doi: 10.1002/jez.a.86.
One type of fast fiber and two types of slow (slow-twitch, S1 and slow-tonic, S2) fibers are found in decapod crustacean skeletal muscles that differ in contractile properties and myofibrillar protein isoform compositions. In this study the structural characteristics, protein isoform compositions, and Ca2+-activation properties of fast fibers in the claw closer (F1) and abdominal deep flexor (F2) muscles of Cherax destructor were analyzed. For comparison, myofibrillar protein isoform compositions of slow (long-sarcomere) fibers from claw and abdomen were also determined; our results indicate that the slow fibers in the claw closer were the slow-twitch (S1) type and those in the abdominal superficial flexor were primarily slow-tonic (S2) type. F1 fibers had shorter resting sarcomere lengths (2.93 microm in unstretched fibers and 3.06 microm in stretched fibers) and smaller fiber diameter (256 microm) than F2 fibers (sarcomere lengths 3.48 microm in unstretched and 3.46 microm in stretched; 747 microm diameter). Moreover, F1 fibers showed a narrower range in sarcomere lengths than F2 fibers (2.81 to 3.28 microm vs. 2.47 to 4.05 micro m in unstretched fibers). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting showed that the fast fibers from claw and abdomen differed in troponin-I composition; F1 fibers expressed two isoforms of troponin-I (TnI1 and TnI2) in approximately equal amounts, whereas F2 fibers expressed primarily TnI3 and lower levels of TnI1. F1 fibers were more sensitive to Ca2+, as shown by higher pCa values at threshold activation (pCa(10)=6.50+/-0.07) and at 50% maximum force (pCa(50)=6.43+/-0.07) than F2 fibers (pCa(10)=6.12+/-0.04 and pCa(50)=5.88+/-0.03, respectively). F1 fibers also had a greater degree of co-operativity in Ca2+ activation, as shown by a higher maximum slope of the force-pCa curve (n(Ca)=12.98+/-2.27 vs. 4.34+/-0.64). These data indicate that there is a greater fast fiber-type diversity in crustacean muscles than was previously supposed. Moreover, the differences in activation properties suggest that the TnI isoform composition influences the Ca2+ sensitivity of the contractile mechanism.
在十足目甲壳类动物的骨骼肌中发现了一种快肌纤维和两种慢肌纤维（慢缩肌，S1和慢张力肌，S2），它们在收缩特性和肌原纤维蛋白同工型组成上存在差异。在本研究中，分析了澳洲淡水龙虾螯闭合肌（F1）和腹部深屈肌（F2）中快肌纤维的结构特征、蛋白同工型组成和Ca2+激活特性。为作比较，还测定了螯部和腹部慢（长肌节）肌纤维的肌原纤维蛋白同工型组成；我们的结果表明，螯闭合肌中的慢肌纤维为慢缩（S1）型，而腹部浅屈肌中的慢肌纤维主要是慢张力（S2）型。F1纤维的静息肌节长度较短（未拉伸纤维中为2.93微米，拉伸纤维中为3.06微米），纤维直径较小（256微米），而F2纤维的静息肌节长度较长（未拉伸时为3.48微米，拉伸时为3.46微米；直径为747微米）。此外，F1纤维的肌节长度范围比F2纤维窄（未拉伸纤维中为2.81至3.28微米，而F2纤维为2.47至4.05微米）。十二烷基硫酸钠-聚丙烯酰胺凝胶电泳和免疫印迹显示，螯部和腹部的快肌纤维在肌钙蛋白I组成上存在差异；F1纤维表达两种等量的肌钙蛋白I同工型（TnI1和TnI2），而F2纤维主要表达TnI3，TnI1水平较低。F1纤维对Ca2+更敏感，在阈激活时（pCa(10)=6.50±0.07）和最大力的50%时（pCa(50)=6.43±0.07）的pCa值高于F2纤维（分别为pCa(10)=6.12±0.04和pCa(50)=5.88±0.03）。F1纤维在Ca2+激活方面也具有更高的协同性，力-pCa曲线的最大斜率更高（n(Ca)=12.98±2.27，而F2纤维为4.34±0.64）。这些数据表明，甲壳类动物肌肉中的快肌纤维类型多样性比以前认为的更大。此外，激活特性的差异表明，肌钙蛋白I同工型组成影响收缩机制对Ca2+的敏感性。
