Stress fiber sarcomeres of fibroblasts are contractile

Living cultured fibroblasts were microinjected with rhodamine-labeled smooth muscle alpha-actinin and visualized by video-intensified fluorescence microscopy. The alpha-actinin incorporated into the stress fibers and exhibited a regularly striped arrangement. The fluorescently labeled stress fibers remained intact despite glycerol or digitonin extraction of the cells; furthermore, these cell models contracted upon addition of MgATP. During this process, sections of alpha-actinin-labeled stress fibers contracted up to 25%; shortening proceeded in the nonfluorescent part of the stress fiber sarcomeres. In glycerol-extracted cell models, adenylylimidodiphosphate, ADP and pyrophosphate inhibited but vanadate, N-ethylmaleimide-modified heavy meromyosin, cytochalasin B, colchicine, phalloidin and DNAase I did not. Cytochalasin B was inhibitory when added to the intact cells before glycerol extraction. These morphological and biochemical findings demonstrate that stress fiber sarcomeres of fibroblasts are contractile elements and support the concept that an actomyosin system may be involved.