Microgravity induced transformations of myosin isoforms and
contractile properties of skeletal muscle.
Caiozzo, Vincent J., Fadia Haddad, Michael J. Baker, Robert E.
Herrick, Noelani Prietto, and Kenneth M. Baldwin.
Departments of Physiology and Biophysics and Orthopaedics, College
of Medicine, University of California, Irvine, CA 92717
APStracts 2:0459A, 1995.
This study examined the effects of microgravity (14 days) on: i) the
contractile properties of the soleus (SOL), an antigravity skeletal
muscle; and ii) the myosin heavy chain (MHC) protein and mRNA isoform
content of the SOL, vastus intermedius (VI), plantaris (PLAN), and
tibialis anterior (TA) muscles. The force-velocity relationships of
the flight SOL muscles had a significant reduction in Po (-37%) and a
corresponding increase in Vmax (+20%). Additionally, the force
-frequency relationship of the flight (SOL) muscles was shifted to the
right of the ground based control group. Microgravity had the
greatest effect upon muscle fiber composition in the SOL muscle, with
a reduction in slow muscle fibers and a corresponding increase in
muscle fibers categorized as hybrid fibers. The estimated absolute
MHC isoform content was altered to the greatest extent in the SOL and
VI muscles, with significant decreases and elevations in the slow
Type I and fast Type IIX MHC protein isoforms, respectively.
Consistent with the protein data, both the flight SOL and VI muscles
exhibited significant elevations in the fast Type IIX MHC mRNA
isoform. In contrast, however, the flight PLAN and TA groups had
significant increases in the fast Type IIB MHC mRNA isoform content
without corresponding changes at the protein level. The results of
this study suggest that spaceflight of even short duration produces
important changes in the contractile properties of antigravity
skeletal muscle. These changes are mediated by alterations in MHC
phenotype and reductions in muscle mass. In some instances, the
alterations in MHC mRNA isoform content seemed to be uncoupled from
those occurring at the protein level. This apparent uncoupling
between mRNA and protein expression demonstrates that the effects of
microgravity must be better understood at the transcriptional,
translational, and posttranslational levels.
Received 23 December 1994; accepted in final form 11 October
1995.
APS Manuscript Number A1310-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95