Cells of an established clonal line (RVF-SMC) derived from rat vena cava are described by light and electron microscope methods and biochemical analysis of the major proteins. The cells are flat, and they moderately elongate and form monolayers. They are characterized by prominent cables of microfilaments bundles decoratable with antibodies to actin and alpha-actinin. These bundles contain numerous densely stained bodies and are often flanked by typical rows of surface caveolae and vesicles. The cells are rich in intermediate-sized filaments of the vimentin type but do not show detectable amounts of desmin and cytokeratin filaments. Isoelectric focusing and protein chemical studies have revealed actin heterogeneity. In addition to the two cytoplasmic actins, beta and gamma, common to proliferating cells, two smooth muscle-type actins (an acidic alpha-like and a gamma-like) are found. The major (alpha-type) vascular smooth muscle actin accounts for 28% of the total cellular actin. No skeletal muscle or cardiac muscle actin has been detected. The synthesis of large amounts of actin and vimentin and the presence of at least three actins, including alpha-like actin, have also been demonstrated by in vitro translation of isolated poly(A)+ mRNAs. This is, to our knowledge, the first case of expression of smooth muscle-type actin in a permanently growing cell. We conclude that permanent cell growth and proliferation is compatible with the maintained expression of several characteristic cell features of the differentiated vascular smooth muscle cell including the formation of smooth muscle-type actin.

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