Light microscopy of the pectoralis muscle of rats on a low protein diet did not show such morphological alterations as atrophy, degeneration, or sarcoplasmic edema, but electron microscopy occasionally demonstrated ultrastructural changes only in the sarcomeres of myofibrils, In the affected sarcomeres, the Z-line was disrupted and often showed a jagged structure. The Z-substance with electron opacity was frequently present flowing along the long axis of myofibrils, here referred to as the streaming of Z-lines, In addition, regular striations formed by the reciprocal arrangement of thick and thin filaments disappeared from the affected sarcomeres, though these filaments were still discernible. Two or more consecutive sarcomeres in a single myofibril were occasionally involved in these changes. A further two or more neighboring sarcomeres at the same level of myofibrils were affected transversely by these structural alterations. On the other hand, the ultrastructure of the intestinal smooth muscle was not affected by protein deficiency. The study suggests that the ultrastructural damage induced by a low protein diet is attributed to the activation of endogenous protease by the excess leaking of Ca2+- into the cytosol as a result of lipid peroxidation of cell membrane by raised free radicals, owing to the depletion of glutathione production by protein deficiency. It also suggests that the smooth muscle cells differ in their susceptibility to protein deficiency from the skeletal muscle cells.
