Effects of age and gender on shortening velocity and myosin isoforms in single rat muscle fibres

The maximum velocity of unloaded shortening (V-0) and the myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were determined in single fibres from soleus and extensor digitorum longus (EDL) muscles of male and female rats 3-6 and 22-24 months old. In the soleus muscle, the beta/slow (type I MyHC) isoform predominated in both young and old animals, irrespective of gender. In the EDL, fibres expressing type IIX MyHC or a combination of IIX and IIB (IlXB) MyHC isoforms were predominant in old rats, while type lIB MyHC fibres predominated in young individuals of both genders. The V-0 of soleus fibres expressing the type I MyHC isoform decreased (P < 0.001) by 40% with age in spite of an unchanged MyLC composition. In the EDL, the V-0 of fibres expressing IIX, IIXB and IIB MyHC isoforms did not change with age or differ between males and females. In conclusion, similar age-related changes in V-0 and MyHC composition were observed in single muscle cells from both male and female rats. The present results demonstrate that the relationship between V-0 and MyHC isoform composition at the single fibre level is similar in male and female rats, and that similar qualitative changes take place during ageing in both genders.