Regular articleCompared effects of hindlimb unloading versus terrestrial deafferentation on muscular properties of the rat soleus
Introduction
Hindlimb unloading (HU) is a well-known model to mimic the effects of real weightlessness (Morey et al., 1979). In hindlimb muscles, an exposure to HU induces marked changes in extensor muscles such as soleus or vastus intermedius, whereas the HU impact is less important in fast muscles such as the tibialis anterior and the extensor digitorum longus (for review see Thomason and Booth 1990, Edgerton and Roy 1996, Fitts et al 2000, Ohira 2000. In the rat soleus, after HU, decreases in muscle mass and cross-sectional area of muscle fiber have been currently described. A shift of the muscle type from a slow to a fast one is reported; the contractile properties and the contractile protein isoforms evolve toward those characterizing a fast muscle Stevens et al 1999, Fitts et al 2000, Ohira 2000, Bastide et al 2002, Stevens et al 2002. Some muscular changes observed after HU could be attributed to modifications in EMG activity. Indeed, it has been demonstrated by Alford et al. (1987) that this experimental condition induced, in the soleus muscle, a total EMG disappearance in the 3 s following HU. However, these authors described a progressive return of EMG activity closer to normal values after only 7 days of HU (81% of control EMG activity). This result is in partial disagreement with those of Blewett and Elder (1993), who have demonstrated more recently that the EMG soleus activity remained quantitatively and qualitatively decreased even after 28 days of HU. Moreover, the EMG pattern was shifted from a tonic to a phasic-type characteristic of a fast muscle type (Riley et al., 1990). Consequently, taking into account these latter results, it was tempting to speculate that some muscle changes observed after HU are directly related to both the nature and the level of EMG activity, as suggested by Leterme and Falempin (1994). The muscle position during HU could also be a trigger to changes observed in simulated microgravity. In this situation the soleus muscle is often in a shortened position (Riley et al., 1990), and it can be supposed that muscle spindles which are stretch sensors, are probably not activated. During real or simulated microgravity this deficiency in the muscular afferent message may be considered as a functional deafferentation as previously suggested by several authors Money and Cheung 1991, De-Doncker et al 2000. As the contractile properties of the soleus muscle and its composition in myosin isoforms have never been studied after deafferentation, the aims of this study were thus (1) to determine whether a chronical deafferentation induces significant modifications in the morphological and contractile properties of the soleus muscle and in its myosin heavy-chain (MHC) content and (2) to compare the effects of a terrestrial deafferentation with changes induced by a HU period in the soleus muscle. We were then able to predict whether afferent activity had a real impact on muscular modifications observed in HU.
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Animal groups
Male Wistar rats (Iffa Credo, l’Arbresle, France) weighing 280–300 g were divided randomly into three groups: control (CON, n = 6), hindlimb unloading (HU, n = 6), and terrestrial deafferentation (DEAF, n = 6). The rats were housed individually in conventional plastic cages and had free access to food and water. The rats were kept at a 25°C room temperature and under a 12:12 h light:dark cycle. All procedures described below were approved by both the Agricultural and Forest Ministry and
Muscle weight and cross-sectional area of muscle fibers
The data of each experimental group are reported in Table 1. For all three groups, the animals’ body weight (BW) remained similar after 14 days of experimentation with values between 284 and 300 g. However, compared with the CON group, the DEAF and HU groups showed a significant decrease in both MWW and MWW to BW ratio. These two parameters remained significantly different when DEAF was compared to HU group. MWW and MMW/BW in the DEAF group presented intermediate values between those of CON and
Discussion
The aim of this study was to determine whether the abolition of the afferent message induced muscular changes similar to those observed after a HU period.
Our results showed, after HU, a muscle atrophy assessed by a decrease in the CSA of all fiber types. This atrophy, correlated with a decrease in normalized muscle force, has usually been attributed to a decrease in muscle protein synthesis and to an increase in protein degradation Thomason et al 1987, Bock 1998. Moreover, the muscle type
Acknowledgements
This study was supported by grants from the “Centre National d’Etudes Spatiales” (3194) and the “Conseil Régional du Nord-Pas-de-Calais.” The authors thank Dr. G.S. Butler-Browne for the growing of hybridomas producing SC-71 and BF-F3 antibodies initially developed by Dr. Schiaffino et al. (1989). The hybridomas are commercialized by DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen Gmbh, Germany).
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