AJP - Cell Physiology, Vol 263, Issue 1 C86-C94, Copyright © 1992 by American Physiological Society
Response of slow and fast muscle to hypothyroidism: maximal shortening velocity and myosin isoforms
V. J. Caiozzo, R. E. Herrick and K. M. Baldwin
Department of Physiology and Biophysics, University of California, Irvine 92717.
This study examined both the shortening velocity and myosin isoform
distribution of slow- (soleus) and fast-twitch (plantaris) skeletal muscles
under hypothyroid conditions. Adult female Sprague-Dawley rats were
randomly assigned to one of two groups: control (n = 7) or hypothyroid (n =
7). In both muscles, the relative contents of native slow myosin (SM) and
type I myosin heavy chain (MHC) increased in response to the hypothyroid
treatment. The effects were such that the hypothyroid soleus muscle
expressed only the native SM and type I MHC isoforms while repressing
native intermediate myosin and type IIA MHC. In the plantaris, the relative
content of native SM and type I MHC isoforms increased from 5 to 13% and
from 4 to 10% of the total myosin pool, respectively. Maximal shortening
velocity of the soleus and plantaris as measured by the slack test
decreased by 32 and 19%, respectively, in response to hypothyroidism. In
contrast, maximal shortening velocity as estimated by force-velocity data
decreased only in the soleus (-19%). No significant change was observed for
the plantaris.
