Muscle Fibers in Rostral and Caudal Myotomes of the Atlantic Cod (Gadus morhuaL.) Have Different Mechanical Properties

Muscle fibers were isolated from myotomes of the Atlantic cod atpoints 0.35 and 0.90 of the distance from the snout to the caudalpeduncle (Ls). The time from delivery of the first stimulus to halfpeak force (HPT) and from the last stimulus to half relaxation (HFT) for isometric tetani were significantly shorter for muscle fibers from rostral than from caudal myotomes (24. 7 ± 1.8 ms for HPT and 54.0 ± 2.9 ms for HFT for fibers at 0.35 Ls, and 42.1 ± 2.4 ms for HPT and 88.3 ± 8.0 ms for HFT for fibers at 0.90 Ls at 5° C; mean ± standard error of the mean [SEM], n = 15). Maximum (unloaded) contraction velocities were found to be 2.4 times higher for rostral than for caudal muscle fibers (6.9 ± 1.3 and 2.9 ± 0.8 muscle lengths · s⁻¹, respectively, at 5°C; mean ± SEM, n = 8). A quantitative ultrastructural analysis showed that differences in relaxation rates were not correlated with differences in the volume and surface densities of T-tubules and sarcoplasmic reticulum. The sarcoplasmic reticulum and T-tubules constituted around 6% and 0.23% of myofibrillar volume, respectively, in both rostral and caudal fibers. Sinusoidal length changes approximating those found during steady swimming were imposed on isolated muscle fibers. Activation phase (defined in degrees from the point that the muscle passed through the resting length) was varied from 15° to 360° at oscillation frequencies of 4 Hz and 9 Hz. Caudal fibers, by virtue of their longer HPT and HFT, generate tension over a greater portion of the oscillation cycle and so produce net negative work at earlier values of activation phase than rostral fibers. Caudal muscle fibers are activated while lengthening during subcarangiform swimming, which, combined with the present results, suggests that they may play a role in propagating rostrally produced forces to the tail blade. No significant differences were found between the contractile properties of muscle fibersfrom summer- and winter-caught cod.