Histochemical and immunohistochemical properties of skeletal muscle fibres fromRana andXenopus

Summary Modern histochemical and immunohistochemical techniques have been used to type skeletal muscle fibres from threeRana species andXenopus laevis.Differing myosin properties and metabolic capacities (representing various contractile properties) define a minimum of four fibre types inRana and five inXenopus. TheRana andXenopus types are sufficiently similar so that a single nomencclature can be applied to them. This nomenclature uses an initial letter indicating the probable contractile performance (F=fast-twitch, S=slow-twitch and T=tonic), and a number indicating rank order of presumed shortening velocity.The largest, fastest fibres-F1-have low oxidative and, at best, moderate glycolytic capacities. Commonly adjacent to them are smaller, F2 fibres with variable but at least moderate metabolic capacities. F3 fibres are rarer and have on average the highest oxidative capacity, and at least moderate glycolytic capacity. They usually occur in the reddest parts of the muscle and, inRana, only in the vicinity of tonic fibres.Metabolically weak, classical amphibian tonic fibres (T5) occur in bothXenopus andRana, but onlyXenopus also has an S4 fibre type. This has moderate metabolic capacity and myosin properties suggesting it is probably capable of slow shortening as well as tonic hold. Immunohistochemically, S4 fibres are most similar to avian slow-twitch fibres.