Expression of αSM actin in terrestrial ectothermic vertebrates.

α-Smooth muscle (αSM) actin of endothermic vertebrates is selectively recognized by the monoclonal antibody anti-αSM-1. Immunoreactivity to this antibody has been shown to be localized in the NH-terminal sequence Ac-EEED (Chaponnier et al. 1994). Among terrestrial ectothermic vertebrates, two amphibian ( Triturus vulgaris, Rana esculenta) and three reptilian species ( Pseudemys scripta elegans, Natrix natrix, Podarcis sicula) were screened to investigate if their vascular and visceral smooth muscles were stained by anti-αSM-1. In all the specimens tested, Western-blot analysis of tissue extracts immunodecorated with anti-αSM-1 revealed a single polypeptide chain having the same electrophoretic mobility as bovine αSM actin. The binding to amphibian and reptilian tissue extracts was inhibited by the synthetic peptide Ac-EEED, but not Ac-DEED, as occurs in mammals. αSM actin expression was found in vascular and visceral smooth muscle cells of the species tested. The media of small and large blood vessels was labelled by anti-αSM-1. In the stomach and intestine the outer longitudinal and inner circular layers of the muscularis and of the muscularis mucosae were stained. In addition, myofibroblasts of the subepithelial layer were labelled. A more restricted expression of this isoactin was detected in turtle ( P. scripta elegans) visceral smooth muscle cells, which may be related to the involvement of the digestive system in respiratory activity. These data suggest that in vertebrate evolution αSM actin arose earlier than previously proposed. [ABSTRACT FROM AUTHOR]