Effect of sarcomere length on step size in relaxed rabbit psoas muscle

Recent experiments have shown that shortening and stretching of sarcomeres in single activated and unactivated myofibrils occur in stepwise fashion (Yang et al. (1998) Biophys J 74: 1473-1483; Blyakhman et al. (2001) Biophys J 81: 1093-1100; Yakovenko et al. (2002) Am J Physiol Cell Physiol 283: 735-742). Here, we carried out measurements on single myofibrils from rabbit psoas muscle to investigate steps in unactivated specimens in more detail. Activated and unactivated myofibrils were released and stretched in ramp-like fashion. The time course of length change in the single sarcomere was consistently stepwise. We found that in the unactivated myofibrils, step size depended on initial sarcomere length, diminishing progressively with increase of initial sarcomere length, whereas in the case of activated sarcomeres, step size was consistently 2.7 nm.