EFFECTS OF AGE AND GENDER ON SHORTENING VELOCITY AND MYOSIN ISOFORMS IN SINGLE-RAT MUSCLE-FIBERS

The maximum velocity of unloaded shortening (V-0) and the myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were determin ed in single fibres from soleus and extensor digitorum longus (EDL) mu scles 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 expressin g type IIX MyHC or a combination of IIX and IIB (IlXB) MyHC isoforms w ere predominant in old rats, while type lIB MyHC fibres predominated i n young individuals of both genders. The V-0 of soleus fibres expressi ng the type I MyHC isoform decreased (P < 0.001) by 40% with age in sp ite of an unchanged MyLC composition. In the EDL, the V-0 of fibres ex pressing IIX, IIXB and IIB MyHC isoforms did not change with age or di ffer between males and females. In conclusion, similar age-related cha nges in V-0 and MyHC composition were observed in single muscle cells from both male and female rats. The present results demonstrate that t he relationship between V-0 and MyHC isoform composition at the single fibre level is similar in male and female rats, and that similar qual itative changes take place during ageing in both genders.