Persistence of an embryonic intermediate filament-associated protein in the smooth muscle cells of elastic arteries and in Purkinje fibres.

During differentiation of most myogenic tissues, vimentin is transiently expressed as the intermediate filament (IF) protein subunit and is progressively replaced by desmin. However, smooth muscle cells of mature vascular tissue contain variable amounts of vimentin whose cellular content decreases as function of the distance from the heart. IFAPa-400 is a developmentally regulated IF crosslinker protein whose expression appears to parallel that of vimentin during chick myogenesis. Immunohistological and immunoblot techniques were employed to study the expression of this protein in cardiac and vascular smooth muscle cells of the adult chicken. As observed for vimentin, the expression of IFAPa-400 persists in the mature smooth muscle cells of the large arteries as they leave the heart. However, both of these proteins are down-regulated according to a proximo-distal gradient with respect to the distance from the heart. Conversely, desmin is much more abundant in the distal segments of the aorta. Thus, the co-ordinate expression of vimentin with IFAPa-400 may be a characteristic feature of elastic vascular tissue in which it could meet mechanical requirements close to those of the embryonic cells expressing them. This hypothesis is supported by the observation that the single cell type which continues to express the vimentin-IFAPa-400 combination in the mature heart is the Purkinje fibres, which are also subjected to high mechanical tensions but in which myofibrils are generally sparse compared to working myocytes.