Effect of inactivity and passive stretch on protein turnover in phasic and postural rat muscles

Abstract

A state of hypokinesia and hypodynamia has been induced in the hindlimb muscles of the rat (100 g) using a suspension model. The ensuing muscle atrophy was assessed by reference to muscles in fully mobile control animals, which were either fed ad libitum or fed the same lower food intake of the suspended animals. Over a total of 7 days of suspension the slow-twitch postural soleus muscle underwent a much greater atrophy than the fast-twitch phasic extensor digitorum longus. Changes with respect to the position of the suspended foot, and hence muscle length, necessitate caution in comparing the extent of the atrophy between different muscle types. After 3 days of inactivity the atrophy of the soleus muscle was explained by a 21% decrease in the fractional rate of synthesis (measured in vivo) and a 100% increase in the rate of protein breakdown. The reduction in the synthetic rate was associated with a net loss (23%) of RNA and hence muscle ribosomes. In contrast when this inactive soleus muscle was permanently stretched the RNA content (44%) and protein synthetic rate increased (59%) markedly above control values. Although protein breakdown remained elevated in this stretched muscle, the extent of the atrophy in response to hypokinesia and hypodynamia was greatly reduced.

Stimulation of myofibrillar protein synthesis in hindlimb suspended rats by resistance exercise and growth hormone

“…Current literature suggests that stimulation of protein synthesis following exercise may be related to the mode and intensity of loading (21,22,23), as well as the fiber type of the muscle in which protein synthesis is measured (23). To our knowledge, our measurements of soleus and gastrocnemius myofibrillar protein synthesis (Ks) 15-360' post-exercise, represents the first attempt to measure in vivo contractile protein synthesis immediately following resistance exercise, and are consistent with results indicating that muscle stretch increases protein synthesis (21,24). Our results indicate that resistance exercise stimulated soleus Ks in hindiimb suspended (HLS) rats 60' post-exercise (Figure 1), and Ks decayed towards the level seen in non-exercised HLS rats over the next 5 hours.…”

Effect of denervation on mitochondrially mediated apoptosis in skeletal muscle

“…Muscle disuse, in the presence or absence of the nerve, has been shown to activate numerous well-known proteolytic pathways (i.e., ubiquitin-proteasome, lysosomal, and calpain) to increase the rate of protein degradation. A concurrent reduction in the rate of protein synthesis leads to a marked reduction in fiber size (16,24,31,46). In addition to these pathways, apoptosis has recently emerged as a significant contributor to the atrophic response associated with chronic muscle disuse, sarcopenia, and pathological muscular diseases (1,13,34,40).…”

Two myogenic regulatory factor transcripts exhibit muscle‐specific responses to disuse and passive stretch in adult rats

“…In the model we have employed disuse causes muscle fibre atrophy which is prevented by passive stretch and over longer time periods causes fibre hypertrophy [13]. Whether MRFs also play a role in the determination of this muscle fibre size is at present unclear.…”