Hypothyroid-mediated changes in adult rat diaphragm muscle
contractile properties and myosin heavy chain isoform expression.
Gosselin, Luc E., Wen-Zhi Zhan, and Gary C. Sieck.
Department of Physical Therapy and Exercise Science, State
University of New York at Buffalo, Buffalo, NY 14214 and Departments
of Anesthesiology and Physiology, and Biophysics, Mayo Clinic and
Foundation, Rochester, MN 55905
APStracts 3:0062A, 1996.
The purpose of the present study was to examine the effect of acute
hypothyroidism on myosin heavy chain (MHC) isoform composition and
contractile properties in the adult rat diaphragm muscle.
Hypothyroidism was induced by addition of propylthiouracil (0.05%) in
the drinking water for a period of three weeks. MHC isoform
composition of control and hypothyroid diaphragm muscle was assessed
by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In
vitro isometric contractile properties of midcostal diaphragm muscle
segements were measured at 26oC whereas maximal unloaded shortening
velocity (Vo) was measured at 15oC using the "slack test"
method. Serum T3 and T4 values were significantly lower in the
hypothyroid group compared to control. A small but significant
increase in the percentage of slow MHC isoform in the diaphragm was
observed with acute hypothyroidism whereas the percentage of the fast
MHC isoforms (2A, 2X, and 2B) did not significantly differ between
groups. Peak twitch force did not differ between groups. However,
twitch contraction and half relaxation times were significantly
prolonged in the hypothyroid group compared with control. Maximal
specific force was reduced in the hypothyroid group compared to
control, averaging 15.7 and 19.8 N/cm2, respectively (P < 0.05).
Vo averaged 4.3 and 8.2 muscle lengths per second in the hypothyroid
and control groups, respectively (P<0.05). We conclude that
acute hypothyroidism results in alterations in adult diaphragm muscle
contractile properties that cannot be attributed soley to changes in
MHC isoform composition.
Received 20 September 1995; accepted in final form 18 January
1996.
APS Manuscript Number A1018-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 February 96