Expression of Myosin Heavy Chain Isoforms Along Intrafusal Fibers of Rat Soleus Muscle Spindles After 14 Days of Hindlimb Unloading

Abstract

Morphological, contractile, histochemical, and electrophoretical characteristics of slow postural muscles are altered after hindlimb unloading (HU). However, very few data on intrafusal fibers (IFs) are available. Our aim was to determine the effects of 14 days of hindlimb unloading on the morphological and immunohistochemical characteristics of IF in rat soleus muscle. Thirty-three control and 32 unloaded spindles were analyzed. The number and distribution of muscle spindles did not appear to be affected after unloading. There was no significant difference in number, cross-sectional area, and histochemical properties of IF between the two groups. However, after unloading, a significant decrease in slow type 1 MHC isoform and a slight increase in slow-tonic MHC expression were observed in the B and C regions of the bag1 fibers. The alpha-cardiac MHC expression was significantly decreased along the entire length of the bag2 fibers and in the B and C regions of the bag1 fibers. In 12 muscle spindles, the chain fibers expressed the slow type 1 and alpha-cardiac MHC isoforms over a short distance of the A region, although these isoforms are not normally expressed. The most striking finding of the study was the relative resistance of muscle spindles to perturbation induced by HU.

Fiber Content and Myosin Heavy Chain Composition of Muscle Spindles in Aged Human Biceps Brachii

“…Instead, they were mostly unstained or occasionally stained weakly to moderately, indicating a significant reduction in the My-HC ␣ -c content in the aged nuclear bag 1 and bag 2 fibers. A similar reduction has also been observed in rat nuclear bag 1 fibers after neonatal and adult deefferentation (Pedrosa et al 1990;Walro et al 1997;Wang et al 1997) and in both nuclear bag 1 and bag 2 fibers after a 14-day period of hypogravity (De-Doncker et al 2002). In contrast to the nuclear chain fibers in young adult biceps, some nuclear chain fibers in old muscle spindles showed novel expression of MyHC␣-c isoform.…”

Physiologically adaptive changes of the L5 afferent neurogram and of the rat soleus EMG activity during 14 days of hindlimb unloading and recovery

“…This is probably due to the presence of the capsule surrounding intrafusal fibers (Yellin and Eldred, 1970;Maier et al, 1972). In a study on the morphological and histochemical properties of intrafusal fibers after HU (De-Doncker et al, 2002), we have demonstrated that there was no significant difference in number, cross-sectional area and histochemical properties of intrafusal fibers in comparison with soleus muscle spindles of control rats. Moreover, in the rat soleus muscle, the number (HU: 13.5±1.3; CONT: 14.3±1.5) and length (HU: 2.75±0.2·mm; CONT: 2.82±0.3·mm) of muscle spindles were not changed after a HU period.…”

Aging effects on joint proprioception: the role of physical activity in proprioception preservation

“…It is interesting to note that similar myosin heavy chain protein adaptations were found in rats in response to hind-limb unloading [18] and denervation [78,79]. The loss of nuclear chain fibers may impair the static sensitivity of the spindle and ultimately the ability to correctly interpret muscle length [33].…”