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Effects of hindlimb unweighting and aging on rat semimembranosus muscle and myosin

Department of Physical Medicine and Rehabilitation, University of Minnesota, Minneapolis, Minnesota

Submitted 5 May 2005 ; accepted in final form 10 May 2006

We tested the hypothesis that lower specific force (force/cross-sectional area) generated by type II fibers from hindlimb-unweighted rats resulted from structural changes in myosin (i.e., a change in the ratio of myosin cross bridges in the weak- and strong-binding state during contraction). In addition, we determined whether those changes were age dependent. Permeabilized semimembranosus muscle fibers from young adult and aged rats, some of which were hindlimb unweighted for 3 wk, were studied for Ca²⁺-activated force generation and maximal unloaded shortening velocity. Fibers were also spin labeled specifically at myosin Cys707 to assess the structural distribution of myosin during maximal isometric contraction using electron paramagnetic resonance spectroscopy. Myosin heavy chain isoform (MHC) expression and the ratio of MHC to actin were evaluated in each fiber. Fibers from the unweighted rats generated 34% less specific force than fibers from weight-bearing rats (P < 0.001), independent of age. Electron paramagnetic resonance analyses showed that the fraction of myosin heads in the strong-binding structural state during contraction was 11% lower in fibers from the unweighted rats (P = 0.019), independent of age. More fibers from unweighted rats coexpressed MHC IIB-IIX compared with fibers from weight-bearing rats (P = 0.049). Unweighting induced a slowing of maximal unloaded shortening velocity and an increase in the ratio of MHC to actin in fibers from young rats only. These data indicate that altered myosin structural distribution during contraction and a preferential loss of actin contribute to unweighting-induced muscle weakness. Furthermore, the age of the rat has an influence on some parameters of changes in muscle contractility that are induced by unweighting.

specific force; hindlimb suspension; myosin heavy chain; electron paramagnetic resonance spectroscopy

This article has been cited by other articles:

				S. Zhong and L. V. Thompson The roles of myosin ATPase activity and myosin light chain relative content in the slowing of type IIB fibers with hindlimb unweighting in rats Am J Physiol Cell Physiol, August 1, 2007; 293(2): C723 - C728. [Abstract] [Full Text] [PDF]