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1 Departments of Exercise and Sport Sciences and 3 Physiology Center for Exercise Science, University of Florida, Gainesville, Florida 32611; and 2 Department of Biology, Williams College, Williamstown, Massachusetts 01267
MyoD
is one of four myogenic regulatory factors found exclusively in
skeletal muscle. In an effort to better understand the role that MyoD
plays in determining muscle contractile properties, we examined the
effects of MyoD deletion on both diaphragmatic contractile properties
and myosin heavy chain (MHC) phenotype. Regions of the costal diaphragm
from wild-type and MyoD knockout [MyoD (
/
)] adult male
BALB/c mice (n = 8/group) were removed, and in vitro
diaphragmatic contractile properties were measured. Diaphragmatic
contractile measurements revealed that MyoD (
/
) animals
exhibited a significant (P < 0.05) downward shift in
the force-frequency relationship, a decrement in maximal specific tension (Po;
33%), a decline in maximal shortening
velocity (Vmax;
37%), and concomitant decrease in peak
power output (
47%). Determination of MHC isoforms in the diaphragm
via gel electrophoresis revealed that MyoD elimination resulted in a
fast-to-slow shift (P < 0.05) in the MHC phenotype
toward MHC types IIA and IIX in MyoD (
/
) animals. These
data indicate that MyoD deletion results in a decrease in diaphragmatic
submaximal force generation and Po, along with decrements
in both Vmax and peak power output. Hence, MyoD plays an
important role in determining diaphragmatic contractile properties.
myogenic regulatory factors; myosin heavy chain; maximal specific tension; maximal shortening velocity; oxidative capacity
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