Maximum velocity of unloaded shortening as a function of myosin
isoform composition in single fibers from young and old rats.
Li, Xiaopeng, and Lars Larsson.
Departments of Clinical Neurophysiology and Neurology, Karolinska
Hospital, Stockholm, Sweden
APStracts 2:0268C, 1995.
Maximum velocity of unloaded shortening (V0) and myosin heavy (MyHC)
and light chain (MyLC) isoform compositions were determined in 185
single fibers from the extensor digitorum longus (EDL) and soleus
muscles in rats aged 3-6 and 20-24 months. In the soleus, fibers
expressing the type I MyHC isoform dominated in both young and old
animals. In the EDL, most fibers in the young animals expressed type
IIB MyHC, or a combination of type IIX and IIB (IIXB), whereas in the
old animals type IIXB MyHC fibers predominated. V0 was significantly
(p<0.01) lower (0.59 +/- 0.28 ML/s, n=55) in soleus fibers from
old as compared with young animals (1.12 +/- 0.46 ML/s, n=48) in
spite of the fact that all fibres expressed the type I MyHC and slow
MyLC isoforms. In the EDL, V0 values in single fibers did not differ
between young (2.18 +/- 0.58 ML/s, n=43) and old animals (2.10 +/-
0.53 ML/s, n=39). The mechanism underlying age-related slowing in
soleus fibers is not known, but is suggested that there could be more
than one [beta]/slow MyHC isoform and that there is an age-related
transition within these isoforms.
Received 4 January 1995; accepted in final form 10 July 1995.
APS Manuscript Number C6-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.