The involvement of sarcotubular membranes in genetic muscular dystrophy

Microsomal preparations from breast muscle of normal and dystrophic chickens are characterized with regard to ultrastructural features, protein composition, Ca2+ transport and ATPase activity. Dystrophic muscle yields a greater microsomal dry weight, with a reduced protein to lipid ratio. This is related to the presence of a considerable number of low density microsomes, in addition to seemingly normal microsomes. The low density microsomes display a reduced number of protein particles on freeze fracture faces. Electrophoretic analysis reveals nearly identical patterns in normal and dystrophic microsomes. Furthermore, normal and dystrophic microsomes sustain equal rates of Ca2+ transport and ATPase, demonstrating an identical protein specific activity. However, the dystrophic microsomes have a lower capacity to retain transported Ca2+. The high yield of low density microsomes with reduced capacity for Ca2+ uptake is attributed to the presence of membranes proliferated in the junctional and tubular sarcomere regions of the dystrophic muscle. It is suggested that proliferation of such membranes accounts for the altered excitation-contraction coupling and cable properties of genetically dystrophic muscle.