AJP - Heart and Circulatory Physiology, Vol 243, Issue 4 528-H535, Copyright © 1982 by American Physiological Society
Early changes in fiber profile and capillary density in long-term stimulated muscles
O. Hudlicka, L. Dodd, E. M. Renkin and S. D. Gray
Predominantly fast skeletal muscles of rabbits [tibialis anterior (TA),
extensor digitorum longus (EDL)] were stimulated at a frequency naturally
occurring in nerves to slow muscles (10 Hz continuously) for 8 h/day for
2--4 days. Such stimulation is known to convert all glycolytic fibers to
oxidative and to increase capillary density. Our aim was to study early
stages of conversion to investigate the factors responsible for the
changes. Staining of quick-frozen sections for myosin ATPase, succinic
dehydrogenase, and alkaline phosphatase was used to study the distribution
of different fiber types and to measure fiber cross-sectional areas,
capillaries per square millimeter, and capillary-to-fiber ratios in each
fiber category. TA but not EDL showed conversion of fast glycolytic to fast
oxidative fibers after 2 days, more after 4 days of stimulation. In both
muscles, the largest fast glycolytic fibers were diminished in number after
stimulation. There was significant increase in total capillaries per square
millimeter after 4 days and some increase after 2 days of stimulation. The
increase in capillaries per square millimeter exceeded the increase in the
number of fibers per square millimeter, and since there was no change in
mean fiber area, the increase is attributed to capillary growth. In EDL,
there was an increase in the number of capillaries supplying both fast
glycolytic and fast oxidative fibers, suggesting that capillary growth
precedes fiber type conversion. In TA, the number of capillaries supplying
fast oxidative fibers was increased but that to fast glycolytic fibers, was
not. This is consistent with capillary growth simultaneous with or
following fiber conversion. In both TA and EDL the number of capillaries
perfused after contraction was higher in stimulated muscles, suggesting
that increased capillary flow contributed to capillary growth.
