EXPRESSION OF ALPHA-SM ACTIN IN TERRESTRIAL ECTOTHERMIC VERTEBRATES

alpha-Smooth muscle (alpha SM) actin of endothermic vertebrates is sel ectively recognized by the monoclonal antibody anti-alpha SM-1. Immuno reactivity to this antibody has been shown to be localized in the NH2- terminal sequence Ac-EEED (Chaponnier et al. 1994). Among terrestrial ectothermic vertebrates, two amphibian (Triturus vulgaris, Rana escule nta) and three reptilian species (Pseudemys scripta elegans, Natrix na trix, Podarcis sicula) were screened to investigate if their vascular and visceral smooth muscles were stained by anti-alpha SM-1. In all th e specimens tested, Western-blot analysis of tissue extracts immunodec orated with anti-alpha SM-1 revealed a single polypeptide chain having the same electrophoretic mobility as bovine alpha SM actin. The bindi ng to amphibian and reptilian tissue extracts was inhibited by the syn thetic peptide Ac-EEED, but not Ac-DEED, as occurs in mammals. alpha S M actin expres sion was found in vascular and visceral smooth muscle c ells of the species tested. The media of small and large blood vessels was labelled by anti-alpha SM-1. In the stem ach and intestine the ou ter longitudinal and inner circular layers of the muscularis and of th e muscularis mucosae were stained. In addition, myofibroblasts of the subepithelial layer were labelled. A more restricted expression of thi s isoactin was detected in turtle (P. scripta elegans) visceral smooth muscle cells, which may be related to the involvement of the digestiv e system in respiratory activity. These data suggest that in vertebrat e evolution alpha SM actin arose earlier than previously proposed.