Two fast-type fibers in claw closer and abdominal deep muscles of the Australian freshwater crustacean,Cherax destructor, differ in Ca2+ sensitivity and troponin-I isoforms

✍ Scribed by Koenders, Annette ;Lamey, Tina M. ;Medler, Scott ;West, Jan M. ;Mykles, Donald L.

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 226 KB
• Volume: 301A
• Category: Article
• ISSN: 0022-104X
• DOI: 10.1002/jez.a.86

✦ Synopsis

One type of fast fiber and two types of slow (slow-twitch, S 1 and slow-tonic, S 2 ) 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 Ca 2þ -activation properties of fast fibers in the claw closer (F 1 ) and abdominal deep flexor (F 2 ) 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 (S 1 ) type and those in the abdominal superficial flexor were primarily slow-tonic (S 2 ) type. F 1 fibers had shorter resting sarcomere lengths (2.93 mm in unstretched fibers and 3.06 mm in stretched fibers) and smaller fiber diameter (256 mm) than F 2 fibers (sarcomere lengths 3.48 mm in unstretched and 3.46 mm in stretched; 747 mm diameter). Moreover, F 1 fibers showed a narrower range in sarcomere lengths than F 2 fibers (2.81 to 3.28 mm vs. 2.47 to 4.05 mm 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; F 1 fibers expressed two isoforms of troponin-I (TnI 1 and TnI 2 ) in approximately equal amounts, whereas F 2 fibers expressed primarily TnI 3 and lower levels of TnI 1 . F 1 fibers were more sensitive to Ca 2þ , as shown by higher pCa values at threshold activation (pCa 10 ¼6.5070.07) and at 50% maximum force (pCa 50 ¼6.4370.07) than F 2 fibers (pCa 10 ¼6.1270.04 and pCa 50 ¼5.8870.03, respectively). F 1 fibers also had a greater degree of cooperativity in Ca 2þ activation, as shown by a higher maximum slope of the force-pCa curve (n Ca ¼12.9872.27 vs. 4.3470.64). 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 Ca 2þ sensitivity of the contractile mechanism.