Isomyosins and isoactins in mammalian myocardium

Myosin and actin are the major components of the thick and the thin filaments of the sarcomere, respectively. The existence of several isomyosins with different heavy chains and different enzymatic activities was demonstrated more than 20 years ago when adult fast-twitch and slow-twitch muscles were compared to cardiac ventricular tissue and it is clear now that the sarcomeric myosin heavy chains (MHC) are coded by a highly conserved multigene family (l). The classical experiments of Barany (2) who showed that maximal velocity of a given muscle fiber is correlated to the ATPase activity of its constituent myosin, and of Buller, Eccles and Eccles (3) in which the nerve was shown to modulate the properties of a muscle, opened an exciting new era in muscle research and neurobiology.