Murine muscular dystrophy some histochemical and biochemical observations

Abstract

Histological and histochemical studies have been carried out on frozen transverse sections of whole muscles obtained from 3 to 4 month-old dystrophic mice of the Bar Harbor strain 129 and from control mice (littermate controls). Estimations of free fatty acid content in pectoral and abdominal muscle, heart, liver, brain and adipose tissue have been carried out in normal and dystrophic mice. In the triceps and gastrocnemius muscles of the normal mouse, there is an area deeply situated close to the bone surface which is mainly constituted of smaller (Type I) and the intermediate variety of muscle fibres; by contrast, the fibres of the more superficial region are mainly of the larger Type II variety. In the latter area the Type I and intermediate fibres are relatively few in number and are randomly distributed in a checker-board pattern. The entire bulk of the soleus muscle is made up of Type I and intermediate type fibres. In muscle from the 3 to 4 month-old dystrophic mice, most of the cellular abnormalities are seen in the area where there is a predominance of the Type I fibres (called by us the "Type I fibre area"). In the case of triceps and gastrocnemius muscles, the Type I fibre area is severely dystrophic, whereas the "Type II fibre area" appears to be relatively non-dystrophic and retains the normal checker-board pattern of fibre distribution. In soleus the entire muscle shows dystrophic changes at this age. Fibre "typing", using the techniques we have employed, has often been found to be a difficult matter in dystrophic muscle but there is some suggestive evidence which requires to be confirmed, that Type I fibres show a greater tendency to atrophy, and Type II fibres a greater tendency to enlargement in dystrophic muscle. A very high activity of glucose-6-phosphate dehydrogenase has been detected in dystrophic mouse muscle, and probably leads to an increased production of NADPH₂ in the system. From the staining pattern for fat and fatty acids, it is seen that cellular differentiation is lost and there is an overall increase in the staining intensity of the "dystrophic" region. Biochemical estimations showed that the free fatty acid content of the pectoral muscle (largely Type I) was increased by almost 300%. There were insignificant changes in the free fatty acid content of other tissues such as abdominal muscle (largely Type II), liver, brain and heart; for a reason which cannot be adduced, the free fatty acid content of adipose tissue obtained from dystrophic mice was reduced. Succinic dehydrogenase activity seemed to be slightly increased in the "dystrophic" area. The activity of this enzyme was localised in fibres undergoing atrophy as well as in fibres undergoing hypertrophy. β-Hydroxybutyrate dehydrogenase, which is responsible for the oxidation of free fatty acids (FFA), does not show a parallel increase in activity to the high concentration of FFA that has been found in dystrophic muscle. It seems probable that much of the FFA are being esterified into triglycerides. The concentration of glycogen in the dystrophic and non-dystrophic fibres did not seem to be significantly abnormal. The pentose shunt enzymes operating in dystrophic muscle are considered to be of great importance, as increased activity of these enzymes leads to an increased production of NADPH₂ in the system. The NADPH₂ formed via the pentose pathway in dystrophic muscle is probably responsible for the increased FFA synthesis. Some cellular damage and even destruction in the dystrophic muscle could conceivably be a secondary effect of FFA accumulation.