The influence of aging on the myosin type of the rabbit soleus and longissimus dorsi muscles.

The influence of aging on the myosin type, and on the fibre composition of both the slowly contracting ("red") M. soleus and the fast ("white") M. longissimus dorsi was examined in the rabbit. For myosin characterization isolated myofibrils were electrophoresed on SDS-polyacrylamide gels, and the fibre pattern within the respective muscles was analyzed with an immunocytochemical method. Antisera against either fast or slow rabbit myosin were collected from guinea pigs after longterm immunization. After incubation of the paraformaldehyde-fixed muscle thin sections the fibres containing either fast or slow myosin could be distinguished from each other by indirect immunofluorescence. The soleus muscles of 1 day old rabbits were composed of 25% slow and 75% fast fibres. In young-adult (5--8 mo.) rabbits the fibres were mostly slow (over 90%), while in old age (4--7 y.) again up to 50% of the soleus fibres contained fast myosin. In contrast, in the longissimus dorsi muscle constantly around 95% of the fibres contained fast myosin. In accordance with the immunocytochemical finding of an increase of fast fibres in the aging soleus muscle, the presence of fast myosin could also be demonstrated electrophoretically. With this method, soleus myofibrils from young-adult animals were observed to contain virtually slow myosin only. No slow, but only fast myosin was identified in SDS-gels of longissimus dorsi myofibrils at all ages. These results are discussed in relation to the well known metabolic alterations occurring in the mammalian skeletal muscle during aging.