Contractile properties of a white- and a red-fibre type of the m. hyohyoideus of the carp (Cyprinus carpio L.)

Isometric contraction parameters were measured for white and red fibre bundles isolated from the m. hyohyoideus of the carp. The two fibre types, which have multiterminal innervation, were stimulated via the nerve as well as epimuscularly. Both red and white fibres reacted to a single stimulus with a twitch. Stimulation via the nerve revealed: 1. Twitches and tetani of white fibres have shorter contraction and relaxation times than those of red fibres. 2. Both types reach similar maximal tetanic tensions (about 12 N/cm²) but red fibres require a higher stimulus frequency to reach this tension. 3. The ratio of twitch tension to maximum tetanic tension is 0.42 for white and 0.27 for red fibres. 4. The maximum slope of tension rise in white fibres is independent of the stimulus frequency; in red fibres it increases at high stimulus frequencies. 5. White fibres are more susceptible to fatigue than red fibres. After about 45 s of repeated tetanization (22 tetani) white fibres had lost half their tension. Red fibres had lost half their tension after about 10 min (300 tetani). 6. Sag, the decline of tension during a tetanus, is greater in white than in red fibres. It has a different frequency dependence in both types. 7. Epimuscular stimulation resulted in a slow, incomplete contraction and a very slow decline of tension, especially in red fibres. 8. In agreement with existing biochemical, electromyographical and ultrastructural data, white fibres are adapted for quick short duration activity and red fibres for slow, sustained activity.