Fast muscle in squid (Loligo pealei): contractile properties of a specialized muscle fibre type.

The contractile properties of the transverse muscle of the tentacles and the transverse muscle of the arms of the squid Loligo pealei were investigated using small muscle fibre bundle preparations. In addition, transmission electron microscopy was used to measure the length of the thick myofilaments of the two muscle fibre types. The thick filament length of the cross-striated tentacle fibres was 0.81+/-0.08 microm (mean +/- S.D, N=51) while that of the obliquely striated arm muscle fibres was 7.41+/-0.44 microm (N=58). The difference in thick filament length of the two muscle types was predicted to result in a much higher shortening velocity of the tentacle muscle compared with the arm muscle. This was tested by investigating the force/velocity relationship for isotonic shortening of the two muscle types. Fitting Hill's equation to the results gave a maximum shortening velocity (V(max), the intercept on the velocity axis) of 15.4+/-1.0 L(0) s(-1) (mean +/- S.D., N=9) for the tentacle fibres and of 1.5+/-0.2 L(0) s(-1) (N=8) for the arm fibres, where L(0) is the length at which peak isometric force was recorded. The difference in thick filament length was also predicted to result in lower peak tension in the tentacle versus the arm muscle. For the tentacle, the mean peak tetanic tension during a brief isometric tetanus (0.2s) of 131+/-56 mN mm(-2) cross-sectional area (mean +/- S.D., N=12) was observed at a stimulus frequency of 80 Hz, whereas the mean peak tetanic tension of the arm fibres during a brief isometric tetanus (0.2s) was 468+/-91 mN mm(-2) (N=5) and was observed at a stimulus frequency of 160 Hz. The length/force relationships (expressed relative to L(0)) of the two muscle types were similar. The ratio of twitch force to peak tetanic force was 0.66 in the tentacle fibres, but only 0.03 in the arm fibres.