In Vitro and In Vivo Effects of Leucine on Skeletal Muscle Protein Synthesis and Degradation
Chang Hong, Soon-Ok
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Permalink
https://hdl.handle.net/2142/71742
Description
Title
In Vitro and In Vivo Effects of Leucine on Skeletal Muscle Protein Synthesis and Degradation
Author(s)
Chang Hong, Soon-Ok
Issue Date
1983
Department of Study
Human Resources and Family Studies
Discipline
Human Resources and Family Studies
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Health Sciences, Nutrition
Abstract
The regulatory role of leucine in skeletal muscle protein synthesis and degradation was examined using in vitro and in vivo experimental methods. Two skeletal muscles from young Sprague-Dawley rats were used in all experiments. These muscles were the soleus, a slow-twitch, oxidative muscle and the extensor digitorum longus (EDL), a fast-twitch, glycolytic muscle.
In vitro experiments examined the effects of leucine on protein synthesis and degradation in muscles isolated from fed, food deprived, protein deprived, and diabetic rats. Leucine (0.5 mM) stimulated protein synthesis by 22-69% in these incubated muscles and the stimulatory effects were greater in the soleus than in the EDL. Leucine did not alter the rate of protein degradation in any of the muscles examined. These results indicate that leucine has anabolic effects on protein synthesis in isolated skeletal muscles, but leucine did not have anticatabolic effects on these muscles.
In vivo experiments examined the effects of leucine on skeletal muscle protein synthesis in fed and 1-day starved rats. The rate of protein synthesis was measured using a single large dose injection of ('14)C-tyrosine. Leucine was administered to rats by a single i.p. injection of 80 or 160 (mu)moles of leucine which elevated the concentration of free leucine in soleus muscles by 4- and 6.8-fold, respectively. Leucine did not stimulate protein synthesis in either the soleus or the EDL of fed rats at either of these levels. However, a single i.p. injection of leucine (160 (mu)moles) stimulated in vivo protein synthesis of EDL and plantaris muscles by 24-29% in 1-day starved rats. The soleus muscles in 1-day starved rats did not respond to the leucine injection.
After 1-day of starvation, the synthesis rate of the EDL was suppressed to one-half of the normal level while the soleus maintained a relatively normal rate of protein synthesis. Thus, in vivo stimulatory effects of leucine appear to be related to the degree of the catabolic condition in muscles. Stimulatory effects of leucine demonstrated in vitro regardless of nutritional or hormonal states of the animals may also be related to the inherent catabolic state of isolated muscles. The results of the present studies suggest that an anabolic effect of leucine on skeletal muscle protein synthesis is not a general phenomenon. However, leucine may have a nitrogen sparing effect in skeletal muscles under certain catabolic conditions and the specific effects of leucine in skeletal muscle protein turnover appear to be stimulation of protein synthesis rather than inhibition of proteolysis.
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