RNA Content and Protein Synthesis in Skeletal Muscles of Young Rats: Effects of Protein Deficiency

Abstract

HOWARTH, R. E. 1972. RNA content and protein synthesis in skeletal muscles of young rats: effects of protein deficiency. Can. J. Pkysiol. Phar~nacol. 50,[59][60][61][62][63][64] A protein-free diet was fed to weanling rats for 1, 2, 3, 4, or 7 days before sacrifice in order to determine the time course of RNA depletion from their gastroca~ernius muscles. A 34(% Boss of total RNA occurred on the first 2 days and thereafter depletion occurred at approximately 8% per day.The time course of RNA repletion was determined by refeeding a stock diet to protein-starved rats. A significant increase in total RNA content occurred on the 2nd day and repletion of total RNA content was complete on the 4th day of refeding the stock diet. Rates of muscle protein synthesis in vivo were estimated by 14C incorporation from L-leucine into muscle proteins, and correlations between RNA concentrations and rate of protein synthesis were evaluated by linear regression anlysis. There was no significant relationship between soluble RNA concentration and protein synthesis. The relationship between protein synthesis and microsomal RNA concentration was significant. The resulks skipport the conclusion that availability of ribosomal WNA is a major factor in the long-term regulation of muscle protein biosyntkesis.HOWARTH, R. E. 1972. RNA content and protein synthesis in skeletal muscles of youiag rats: efkcts of protein deficiency. Can. J. Pkysiol. Pharmacol. 50. 59-64.Avant d'etse sacrifit5, des jeunes rats ont ete nourris pendant 1, 2, 3,4, OLI 7 Q O I~~S d'un sCgirne sans psoteines afin de determiner 1e laps dc temps rcquis avant que les m. gwsfrocrtemii soient Cpuises de AWN. Une perte de 34% du total ARN a etC observe aprks les deux premiers jours et par la suite environ 8% d7@puisement est survenue par jour. La periode de temps requise pour restorer B e ARN a Ct C dkterrninee en nourissant des rats privCs de protkines d'un rCyime rkgulier. Un gain significatif en contenu total d'ARN a Ct C not6 le deuxiitme jour et la restoration totale du sontenu AWN etait complete le quatrikmc jour du rkgirne. Ee rnontant de protCines musculaires synthCtisCs in vivo ont Ct C estime en incorporant 14C de L-leucine dans les proteines muscuPaires et %a relation entre Ies concentrations ARN et Ie montant de protkl'nes synthCtisCs ont etC 6valuCs par une analyse de rkgression linkaire. I1 n9existait pas de relation significative entre la concentration AWN soluble et la synthkse protkine. La relation entre la synthkse protkine et la concentration ARN microsomique Ctctit significative. Les resultats s~rpporte la conclusion que le rnontant de ARN ribosomique est un facteklr nnajeur dans la rkg~rlation dc langue durCe de biosynthese de muscle protkine.

Assay of factors affecting rate of protein biosynthesis in bovine and porcine skeletal muscle

“…Although the results of many of these studies are based on assay of the amount of unsedimentable RNA and, therefore, might be subject to the same experimental difficulties as the studies described in this thesis, the results of Tsai et al, (1973) on porcine muscle (Table 16) The tRNA content of skeletal muscle cells increases with increasing age up to maturity (Tsai et al, 1973; see also Table 16). Although amount of protein biosynthesis in muscle is related to amount of total and microsomal RNA in the same tissue, it seems unrelated to total amount of tRNA (Howarth, 1972). As indicated in the Review of Literature in this thesis.…”

“…As indicated in the Review of Literature in this dissertation, ribosomal content is positively correlated with rate of protein synthesis in muscle tissue. This relationship accounts for the positive correlation between ribosomal content of muscle and muscle growth rate (Howarth, 1972). Table 12 also show that only a very small proportion of the initially very small ribosome content of muscle cells can be isolated in the purified form.…”

“…DNA content in muscle is not decreased by a protein-free diet (Howarth and Baldwin, 1971;Wannemacher and Allison, 196 8). Both total and microsomal RNA but not tRNA are related to rate of muscle protein synthesis (Howarth, 1972). Therefore, it has been concluded that rRNA concentra-' tion, which is the major proportion of microsomal RNA, is a major determinant of the rate of muscle protein synthesis (Allsion et al, 1963;Wannemacher and Allison, 1968).…”

“…The effects of dietary amino acid deficiency on muscle ribosome content are readily reversible because feeding a single protein-containing meal to proteindeficient rats restores protein synthesis by isolated muscle ribosomes (Von der Decken and Omstedt,19 70). Continued protein refeeding causes a linear increase in muscle RNA content (Howarth, 1972) . When supplied with a nutritionally adequate diet, maximum growth rate is limited by the accompany ing high rate of protein degradation.…”

“…Comparison of protein synthesis in muscle from normal and "double-muscled" animals began with a comparison of the muscle RNA concentrations in these two groups of animals because higher muscle RNA content has been reported to be related to a higher rate of muscle protein synthesis (Howarth, 1972;Millward et al, 1975;Omstedt et al, 1973;Srivastava and Chaudhary, 1969;Tsai et al, 1973;Wannemacher, 1972).…”