In vitro motility speed of slow myosin extracted from single soleus fibres from young and old rats.

1. Isolated soleus muscle fibres from aged rats contract more slowly than those from young rats. To determine whether this effect is due to a difference between the myosin molecules, we measured the rate at which actin filaments are driven over a myosin coated surface in the presence of ATP by using a novel in vitro motility assay where myosin is extracted from single muscle fibre segments. 2. Motility was dependent on the myosin density on the coverslip. In regions of high myosin density, actin motility was orientated parallel and anti-parallel to the direction of flow during myosin adhesion to the coverslip. In contrast, in regions of lower myosin density, actin motility was more random. The speed was about 20 % higher in the high density regions (P < 0.001). Further, the speed of filaments in the high density region, moving away or towards the fibre was less variable (P < 0.05) than that of more randomly moving filaments in the low density region. 3. The speed with myosin from slow soleus fibres of young adult rats (3-6 months old; v = 1.43 +/- 0.23 microm s-1; mean +/- s.d.) was faster (P < 0.001) than with myosin from aged rats (20-24 months old; v = 1.27 +/- 0.23 microm s-1). 4. No difference in myosin isoforms between young adult and aged fibres could be detected using electrophoretic and immunocytochemical techniques. Fibres of both ages expressed the beta/slow myosin heavy chain (MyHC) isoform and slow isoforms of essential and regulatory myosin light chains (MyLCs). 5. It is concluded that an age-related alteration in myosin contributes to the slowing of the maximum shortening velocity (V0) observed in soleus muscle fibres expressing the beta/slow MyHC isoform.