Contractile Properties of Obliquely Striated Muscle from the Mantle of Squid (Alloteuthis Subulata) and Cuttlefish (Sepia Officinalis) Available to Purchase

The mechanical properties of obliquely striated muscle fibres were investigated using thin slices of mantle from squid Alloteuthis subulata and cuttlefish Sepia officinalis. Brief tetani or twitch stimuli were used as this pattern is likely to occur during jetting of the intact animal. The length–active force relationship for twitches and tetani (0.1 s, 50 Hz) was similar to that of vertebrate cross-striated fibres. Passive force at the length giving maximum tetanic force was 0.13±0.05P₀ (mean ± S.E.M., N=6, where P₀ is maximum isometric tetanus force) and increased steeply at longer lengths. Peak force in a brief isometric tetanus (0.2 s, 100–150 Hz) was 262±16 mN mm⁻² cross-sectional area of wet tissue (N=6) for squid, and 226±19 mN mm⁻² (N=7) for cuttlefish. The force–velocity relationship for isotonic shortening during twitches of squid mantle slices was a ‘double hyperbolic’ relationship as described for cross-striated fibres by Edman. Fitting Edman’s equation to the results gave: P*=1.18±0.07, V_max=2.43±0.11L_tw s⁻¹ and 1/G=0.69±0.13 (N=8), where P* is the intercept on the force axis expressed relative to P_tw, peak isometric twitch force, V_max is the intercept on the velocity axis, L_tw is the length at which P_tw is produced and G is the constant expressing curvature. The large values of 1/G indicate that the force–velocity relationship is not very curved. Maximum power was produced during shortening at 0.45±0.03P_tw (N=8). Maximum power during twitch contraction was 18.3±1.7 mW g⁻¹ wet mass or, expressed in relative units, (V/V_max)(P/P_tw), where V is the velocity during shortening and P is the force during shortening, was 0.16±0.01 (N=8), which is higher than that of many cross-striated locomotor muscles.