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

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 µm in unstretched fibers and 3.06 µm in stretched fibers) and smaller fiber diameter (256 µm) than F2 fibers (sarcomere lengths 3.48 µm in unstretched and 3.46 µm in stretched; 747 µm diameter). Moreover, F1 fibers showed a narrower range in sarcomere lengths than F2 fibers (2.81 to 3.28 µm vs. 2.47 to 4.05 µm 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 (pCa10=6.50±0.07) and at 50% maximum force (pCa50=6.43±0.07) than F2 fibers (pCa10=6.12±0.04 and pCa50=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 (nCa=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. J. Exp. Zool. 301A:588–598, 2004. © 2004 Wiley‐Liss, Inc.