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

1. Isolated soleus muscle fibres from aged rats contract more slowly than t hose from young rats. To deter mine whether this effect is due to a differe nce between the myosin molecules, we measured the rate at which actin filam ents are driven over a myosin coated surface in the presence of ATP by usin g a novel in vitro motility assay where myosin is extracted from single mus cle fibre segments. 2. Motility was dependent on the myosin density on the coverslip. In region s of high myosin density, actin motility was orientated parallel and anti-p arallel to the direction of flow during myosin adhesion to the coverslip. I n contrast, in regions of lower myosin density, actin motility was more ran dom. 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 o r 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 m onths old; v = 1.43 +/- 0.23 mu m s(-1); mean +/- S.D.) was faster (P < 0.0 01) than with myosin from aged rats (20-24 months: old; v = 1.27 +/- 0.23 m u m s(-1)). 4. No difference in myosin isoforms between young adult and aged fibres cou ld be detected using electrophoretic and immunocytochemical techniques. Fib res 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 (V-0) observed in soleus mus cle fibres expressing the beta/slow MyHC isoform.