ADAPTATION OF MUSCLE FIBRE SIZE: DIFFEREN

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ADAPTATION OF MUSCLE FIBRE SIZE DIFFEREN- TIAL
EFFECTS OF MUSCLE FIBRE LENGTH AND INSULIN-LIKE
GROWTH FACTOR 1 Jaspers Richard1, Testerink
Janwillem1, Bagowski Christophe2, van der Laarse
Willem3 (Research Institute Move, Vrije
Universiteit Amsterdam1, Institute of Biology,
University of Leiden2, Department of Physiology,
VU University medical center3, Netherlands)
Mechanical loading of muscle in vivo by
immoblization at extended length causes
hypertrophy and an increase in the number of
sarcomeres in series1,2. Mechanical loading may
affect protein turnover directly or indirectly by
stimulat-ing the expression of insulin-like
growth factor 1 (IGF-1)3 which is an autocrine
hypertrophic growth factor. The aim of this study
was to investigate whether mechanical loading by
high strain and IGF-1 induce hypertrophy and
increase in the number of sarcomeres in series
and whether IGF-1 and high strain interact. For
this purpose, we developed a culture system which
allows long-term culture of mature, isolated
muscle fibres while muscle fibre length,
contractile activity and medium composition are
manipulated indepen-dently4,5. Single Xenopus
leavis muscle fibres were dissected from m.
iliofibularis and attached to a force transducer
in a cul-ture chamber (20oC). Fibres were
cultured either at in-termediate length at a mean
sarcomere length of 2.3µm (referred to as l2.3µm)
or at extended lengths (12 over l2.3µm, referred
to as high length) for 10 to 24 days. At both
lengths, fibres were cultured in a serum-free
medium with or without human recombinant IGF-1
(1µg/ml). For muscle fibres cultured at l2.3µm
in medium with-out IGF-1, tetanic force and fibre
cross-sectional area (CSA) remained unchanged
during culture. Fibres cul-tured at high lengths
in medium without IGF-1 caused a reduced tetanic
force by 1.40.2 (meanSEM) per day (Plt0.001),
whereas fibre CSA was constant. In contrast,
tetanic force of fibres cultured at l2.3µm in
medium with IGF-1, increased substantially by
1.00.1 per day up to 14.82.8 after 16.60.6
days of culture (Plt0.001). This was accompanied
by an increase in fi-bre CSA of 33.43.8
(Plt0.001). CSA of fibres cultured at high
lengths, with IGF-1 increased to a similar extend
(28.83.7 after 16.61.4 days of culture).
However, the IGF-1 induced increase in tetanic
force at high lengths (0.60.2 per day) was
significantly lower than that at l2.3µm. For all
conditions, the number of sarcomeres in series as
well as the number of myonuclei remained
un-changed during culture. We conclude that
IGF-1 induces hypertrophy in Xenopus muscle
fibres without an increase in the number of
myonu-clei. High muscle strain does not induce
hypertrophy or in-crease the number of myonuclei
but reduces tetanic force. The number of
sarcomeres in series in muscle fibres is not
increased by either IGF-1, high muscle fibre
strain or their combination. 1. P. E. Williams,
G. Goldspink, J. Anat. 127, 459-468 (1978).
2. S. Yang et al., J. Anat. 190, 613-622 (1997).
3. P. A. Huijing, R. T. Jaspers, Scand. J. Med.
Sci. Sports 15, 349-380 (2005). 4. M. B. Lee-de
Groot, W. J. Van der Laarse, J. Muscle Res. Cell
Motil. 17, 439-448 (1996). 5. R. T. Jaspers et
al., Cell Tissue Res 326, 795-808 (2006).
Keywords Hypertrophy, Anabolic Steroids,
Biomove ses-sion
12thAnnual Congress of the ECSS, 1114 July 2007,
Jyväskylä, Finland I