Histoenzymatic characterization of sub-types of Type I fibres in fast muscles of rats

Abstract

The histochemical activities of succinic dehydrogenase (SDH), myofibrillar Adenosine triphosphatase (ATPase) and alpha glycerophosphate dehydrogenase were studied in serial sections of rat vastus lateralis (red) (RVL), gastrocnemius and diaphragm muscles. Three main fibre-types were distinguished. The “Type I” fibres of RVL and gastrocnemius muscles fell into two distinct groups: one category-”Type IA” showed very low ATPase activity. The second category of “Type IB” fibres displayed moderate ATPase reaction. The “Type IA” fibres were divisible into two sub-groups when tested for SDH reaction. “Type IA₁” fibres possessed a homogenous distribution of diformazan·granules throughout the fibre: “Type IA₂” fibres displayed characteristic “moth-eaten” pattern of diformazan localization. The diaphragm muscle did not show either “Type IB” or “Type IA₂” varieties. The great majority of Type I fibres were sub-type IA₁ in the three fast muscles studied. It is also demonstrated here that an inherent heterogeneity exists between Type I fibres of diaphragm and leg muscles in regard to α-GPD localization. This histochemical data emphasizes the fact that subdivision of Type I striated muscle fibres of mammalian animals into two sub-types is only approximate and that a further subcategorization is possible.