Albumin in rabbit skeletal muscle. Origin, distribution and regulation by contractile activity

A 68-kDa protein is present in increasing amounts in the soluble fractions of fast, slow and cardiac muscles of the rabbit. Induced contractile activity by chronic 10-Hz stimulation caused in rabbit fast-twitch muscle a rapid increase of this protein, reaching fivefold to sixfold higher levels than normal after one week of stimulation. The 68-kDa protein was identified as albumin by its mobility in one-dimensional and two-dimensional electrophoreses and by its reactivity with a polyclonal antibody directed against rabbit serum albumin. Immunohistochemistry revealed a localisation pattern corresponding to a distribution mainly in the interstitium. The assumption that albumin is not an intrinsic protein of the muscle fiber was strongly supported by the failure to show its synthesis by in vivo labeling. Injection of [35S]methionine into normal and chronically stimulated muscles did not lead to a detectable incorporation of the precursor into muscle albumin. In addition, albumin could not be detected as a product formed by in vitro translation of RNA preparations from normal and chronically stimulated muscles.