SKELETAL MUSCLE PROTEASE ACTIVITIES ARE UNALTERED IN CIRRHOTIC RATS BUT ALTERED IN RESPONSE TO ETHANOL AND ACETALDEHYDE IN VITRO

This study was carried out in an attempt to differentiate betweenthe contribution of liver impairment and direct actions of alcoholin myopathy of alcoholic liver disease. Using an animal modelof cirrhosis we have previously shown that protein syntheticpotential in muscle was not significantly altered. We thereforeinvestigated the possibility that muscle degradation is increased.Cirrhosis was induced by carbon tetrachloride gavage in malerats receiving phenobarbitone in their drinking water. Controlswere given phenobarbitone alone. After 135 days the free, latentand total activities of the lysosomal enzymes cathepsin B andcathepsin D in gastrocnemius muscle were unaffected by the inductionof experimental cirrhosis when expressed relative to tissuewet weight, protein or DNA. The non-lysosomal enzyme neutralprotease was also measured in gastrocnemius muscle from controland cirrhotic rats. There was no difference between the twogroups in the free, latent or total activities. Addition ofethanol and acetaldehyde to the assay mixtures in some casessignificantly altered the relative activities of the proteasesin latent and free compartments of the cirrhotic tissues. Incontrol tissues a different pattern of response emerged. Itis concluded that in cirrhosis, at least in the carbon tetrachloride-inducedrat model, there is no change of the activity of cathepsin Band D and the neutral protease activity in gastrocnemius. Smallbut significant effects of ethanol and its metabolite acetaldehydeon latent and free muscle protease activity were demonstrated.     

In vivo rates of skeletal muscle protein synthesis in rats are decreased by acute ethanol treatment but are not ameliorated by supplemental alpha-tocopherol

Some studies have shown that reductions in tissue protein synthesis, under a variety of cytotoxic conditions, are ameliorated by alpha-tocopherol (ATC) supplementation. We have also shown evidence of increased oxidative stress and reduced protein synthesis rates in alcohol-exposed muscle. Serum levels of ATC fall and rates of muscle protein synthesis are reduced in patients with alcoholic myopathy. We therefore tested the hypothesis that treatment with ATC could ameliorate the ethanol-induced changes in muscle protein synthesis, a contributory event in the pathogenesis of alcoholic muscle disease. Studies were carried out on gastrocnemius (Type II fiber-predominant and usually considered representative of the musculature as a whole), soleus (Type I fiber-predominant) and plantaris (Type II fiber-predominant) muscles. For comparative purposes, we also investigated the liver. Young male Wistar rats (90 g body weight) were injected intraperitoneally (i.p.) daily with ATC (30 mg/kg body weight) in Intralipid fat emulsion (0.1 mL/100 g body, i.p.) for 5 d. Controls were similarly injected with the Intralipid vehicle alone. After ATC supplementation, rats were given ethanol (75 mmol/kg body weight, i.p., 2.5 h) or saline (0.15 mol/L NaCl, i. p.). Fractional rates of tissue protein synthesis (i.e., the percentage of the tissue protein pool renewed each day, k(s), %/d) and RNA activities [i.e., the amount of protein synthesis each day per unit RNA, k(RNA), mg protein/d/mg RNA)] were then measured. Supplementation increased ATC concentrations in plasma, gastrocnemius and liver. There was no effect of ATC supplementation alone on k(s) in any of the tissues. ATC supplementation in the absence of alcohol increased k(RNA) in the plantaris muscle. In nonsupplemented groups, acute ethanol treatment reduced skeletal muscle (soleus, plantaris and gastrocnemius) k(s). Hepatic k(s) was not altered by ethanol, although ATC concentrations in this tissue increased due to ethanol. However, none of the changes in muscle k(s) or k(RNA) due to ethanol were significantly affected by ATC supplementation. In conclusion, ATC supplementation does not appear beneficial in ameliorating acute alcohol toxicity in skeletal muscle as defined by reductions in protein synthesis.     

The acute effects of ethanol and acetaldehyde on rates of protein synthesis in type I and type II fibre-rich skeletal muscles of the rat.

Young rats were injected with either ethanol (75 mmol/kg), acetaldehyde (2.8 mmol/kg) or isovolumetric amounts of NaCl (0.15 mol/l, i.e. controls) with or without inhibitors of alcohol dehydrogenase (4-methylpyrazole) or aldehyde dehydrogenase (cyanamide). After 2.5 hr, fractional rates of protein synthesis (i.e. ks) in the soleus (Type I fibre-rich) and plantaris (Type II fibre-rich) muscles were measured. Ethanol alone reduced ks in both soleus and plantaris muscles, by approx. 25%. Pretreatment of ethanol-dosed rats with 4-methylpyrazole raised plasma ethanol levels and reduced ks in the soleus and plantaris by approx. 35%. Pretreatment of ethanol-dosed rats with cyanamide also increased plasma ethanol and further potentiated the effects of ethanol by reducing ks in the soleus and plantaris by approx. 65%. Acetaldehyde alone reduced ks by approx. 15%, and this effect was not significantly altered by 4-methylpyrazole pretreatment. In some instances, the plantaris was slightly more sensitive to ethanol and acetaldehyde than the soleus. Similar conclusions were derived when data were expressed relative to either RNA or DNA. The data thus suggest that the ethanol-induced inhibition of skeletal muscle protein synthesis may possibly be independently mediated by both ethanol and acetaldehyde.     

Indices of skeletal muscle growth in lean and obese Zucker rats

Skeletal muscle growth was studied in gastrocnemius muscle of lean and obese ad libitum or pair-fed Zucker rats. Skeletal muscle cellularity was determined by DNA, RNA and protein analysis. The length and weight of the tibia and femur were determined. All rats were killed at 15 weeks of age. Lean rats had heavier gastrocnemius, total DNA and total protein than either the ad libitum (ALO) or pair-fed (PFO) obese rats. Lean rats had longer and heavier tibias. Femur length was greater in lean animals while femur weight did not differ between lean and ALO groups. Both had heavier femur than the PFO rats. Comparison of the obese rats revealed that the ALO rats had greater gastrocnemius weight, total DNA and total protein than the PFO group. Total RNA was not different between the lean and ALO group. Comparison of tibias and femurs showed the ALO rats to have longer and heavier bones than the PFO rats. In summary, there are marked differences in the bones of the hindlimb and the cellularity of the gastrocnemius muscle between lean and obese Zucker rats. The differences were accentuated by pair feeding the obese rat.     

THE EFFECT OF CHRONIC ETHANOL FEEDING ON BODY AND PLASMA COMPOSITION AND RATES OF SKELETAL MUSCLE PROTEIN TURNOVER IN THE RAT

(1) Sexually immature and mature rats were fed a nutritionally-completeliquid diet or isovolumetric quantities of the same diet inwhich 36% of the calories as glucose were substituted by isocaloricethanol. (2) After 6 weeks ethanol feeding, significant reductionsin body weight (approx. 15%) occurred in both groups of rats.In immature rats there were significant reductions (7–21%)in bone, gastrocnemius, liver, and skin weights. The total skeletalmuscle mass was reduced by 20%. Lung and kidney weights werenot significantly altered. In mature rats smaller decreasesin organ weights were found, which were only significant forskeletal muscle and skin. The gastrocnemius protein contentwas significantly reduced in immature but not in mature rats.Plasma protein concentrations were unaltered in both groups.(3) Plasma aspartate aminotransaminase, glutamyl transferaseand creatine kinase activities in immature and mature rats werenot significantly altered by ethanol feeding, but there wereincreases in plasma alkaline phosphatase activities in immature,but not in mature, rats. Plasma glucose was slightly raisedby ethanol feeding in immature but not mature rats. Plasma triglyceridesand insulin were unaltered in both groups of rats. (4) Proteinsynthesis was measured with a flooding dose of L[4³H]-phenylalanine.Rates of protein breakdown were calculated from the differencebetween synthesis and growth. Fractional and absolute ratesof skeletal muscle protein synthesis were reduced by 13–30%by ethanol treatment, in immature and mature rats. Fractionalrates of protein breakdown were also reduced by ethanol, by13 and 19% in immature and mature rats, respectively.     

Alcohol affects the skeletal muscle proteins,titin and nebulin in male and female rats

Alcoholic myopathy is characterized by decreased protein synthesis and contents resulting in atrophy of muscle fibers. We investigated the effect of alcohol on the cytoskeletal muscle proteins, nebulin and titin. Because women are more susceptible than men to the toxic effects of alcohol, male and female rats were included. Four groups were investigated: alcoholic males, pair-fed males, alcoholic females, pair-fed females. Alcohol consumption per unit body weight was 12.9 g/kg.d, with no difference between males and females. After 10 wk, male and female rats fed alcohol had lower gastrocnemius and plantaris protein and RNA contents (P < 0.001), with no effect on soleus, indicating myopathy of type II fibers. The gastrocnemius was fractionated to measure myofibrillary protein contents. Low percentage SDS-gel electrophoresis was performed to determine myosin heavy chain (MHC), nebulin and titin contents. Alcohol reduced gastrocnemius myofibrillary protein and MHC contents, and the plantaris RNA/protein ratio (P < 0.01). The titin/MHC and nebulin/MHC ratios were unaffected, suggesting a concomitant reduction in titin and nebulin. The decreases in titin and nebulin contents may affect muscle function. An interaction between gender and alcohol was noted for the plantaris RNA/protein ratio (P < 0.025), suggesting a reduced capacity for muscle protein synthesis in females.     

Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats

ObjectiveThis study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation.MethodsOn the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-¹³C phenylalanine).ResultsAs expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis.ConclusionThe rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.     

Comparison of the chemical and biochemical composition of thirteen muscles of the rat after dietary protein restriction

1. The objective of this study was to determine whether the chemical and biochemical changes induced by muscle wasting caused by dietary protein restriction are different in various skeletal muscles. 2. Rats were fasted for 3 d and then fed on a 10 g protein/kg diet for 21 d. Thirteen muscles from the trunk, forelimb, and hind-limb regions were analysed for muscle weight, and the content of water, fat, cellular and extracellular protein, DNA and RNA. Results were compared to values for an 'initial' control group killed at the start of the experiment. 3. Weight loss was greatest in trunk muscles and least in the distal forelimb muscles. Water content decreased in most muscles, but increased in three forelimb muscles. A significant loss of lipid was found in the gastrocnemius, while the biceps brachii gained lipid. Changes in lipid content of the muscles did not form a distinctive pattern. 4. All muscles except the distal forelimb muscles lost a significant amount of cellular protein, while all muscles except the diaphragm gained extracellular protein. 5. DNA content was unchanged in all muscles. The value for cellular protein:DNA was significantly reduced in the rectus abdominis and the diaphragm. A significant loss of RNA was found in all muscles; the percentage change was greatest in trunk muscles and least in the distal forelimb muscles. The values for RNA:protein and RNA:DNA were significantly lower in all muscles except two distal forelimb muscles. 6. With the exception of the water and lipid content of the muscles, the directions of the changes in the experimental animals were the same for all muscles. The results suggested, however, that the magnitude of changes in certain chemical and biochemical indices of composition may depend to some extent on the anatomical location of the muscle: trunk muscles tended to show the greatest percentage change, while the distal forelimbs changed the least.     

Growth hormone improves body mass recovery with refeeding after chronic undernutrition-induced muscle atrophy in aging male rats

The effects of growth hormone (GH) administration and refeeding after chronic undernutrition (UN) were investigated in Fischer 344 male rats aging into senescence (24.5 mo of age) during UN initiated at 12.5 mo of age that produced muscle atrophy and a 50% decrease in body mass. Muscle mass, protein, myosin heavy-chain (MHC) composition and circulating testosterone levels were measured and compared to controls with free access to food. Within 9 wk, refeeding + GH restored body mass to control levels, whereas it was still decreased with refeeding alone. By 24.5 mo of age, refeeding alone restored body mass, while addition of GH resulted in overshoot. UN uniformly decreased mass of the gastrocnemius, extensor digitorum longus, soleus and diaphragm muscles to 50-60% of controls. Refeeding and refeeding + GH restored these losses with some overshoot of gastrocnemius muscle suggesting hypertrophy. UN more than doubled slow Type I MHC composition and approximately halved fast Type IIB and IIX MHC in the deep gastrocnemius muscle while it increased Type IIA MHC in the diaphragm. Refeeding and refeeding + GH reversed these shifts. MHC shifts in the extensor digitorum longus and soleus muscles were not statistically significant, whereas UN increased fast Type IIA MHC followed by decrease with refeeding + GH. UN decreased testosterone levels to nearly zero followed by restoration with refeeding and refeeding + GH. We conclude that the phenotype of mixed-MHC muscles such as the gastrocnemius and diaphragm are most affected by chronic UN, which is reversible with refeeding and refeeding + GH. These alterations were associated with changes in circulating testosterone, which may be a key regulatory element in these processes.     

Muscle fiber composition and length-tension relationships in rats chronically exposed to alcohol

Adult Fischer 344 rats were fed an alcohol diet (7-9 g/kg/day) for 12 weeks and were compared to pair-fed controls with regard to the contraction and fiber characteristics of the gastrocnemius and plantaris muscles of the leg. Muscles were isolated in situ with blood and nerve supplies intact. The muscles were stretched by 1 mm increments and were stimulated at each muscle length with a voltage (1 msec pulse) that had been observed to produce maximal twitch force at the initial muscle length. Maximal twitch tension was found to be only 10% less in alcohol than pair-fed rats and the increase in force resulting from stretching was approximately 15% less in alcohol than pair-fed rats. No significant changes in sciatic nerve conduction velocities were produced by alcohol exposure. Moreover, no significant differences in muscle weight or the number and size of Type I, Type IIa or Type IIb muscle fibers were observed. Although the 12 weeks of alcohol exposure affected muscle physiology and histology in the direction of increased impairment, the differences were not large enough to be statistically significant.     

Exercise training and dietary chromium effects on glycogen, glycogen synthase, phosphorylase and total protein in rats

The effects of exercise training and dietary chromium intake on rat liver and muscle glycogen metabolism, tissue and body weight and feed consumption were examined. After 16 wk of training, liver, gastrocnemius and biceps femoris glycogen concentrations were higher in the trained compared to sedentary groups, independent of dietary chromium. There was a chromium x training interaction on glycogen synthase activities in the liver and gastrocnemius muscle. Liver glycogen phosphorylase activities (expressed per g liver) were lower in the chromium-supplemented rats as compared to the non-supplemented rats after 5 wk of dietary treatment, but were similar after 8 wk and higher after 18 wk. Gastrocnemius phosphorylase activity (expressed per mg protein) was lower in the trained rats as compared to the sedentary rats after 16 wk, independent of dietary chromium. Biceps femoris phosphorylase activities were not altered due to training or dietary chromium. Total protein concentration increased in the liver but decreased in the gastrocnemius due to dietary chromium. In summary, liver glycogen synthase and phosphorylase activities were dependent upon dietary chromium. Dietary chromium altered gastrocnemius synthase, but not phosphorylase activities. Changes in enzyme activities may be related to the chromium-dependent effects on liver protein and the chromium and training-dependent effects on gastrocnemius total protein.     

Differential response of protein metabolism in splanchnic organs and muscle to pectin feeding

The aim of the present study was to determine whether the addition of soluble fibre in the diet affected protein metabolism in the intestinal tissues, some visceral organs and in skeletal muscle. A diet supplemented with pectin (80 g/kg) was fed to young growing rats and the effect on organ mass and protein metabolism in liver, spleen, small and large intestines and gastrocnemius muscle was monitored and compared with the control group. Protein synthesis rates were determined by measuring [13C]valine incorporation in tissue protein. In the pectin-fed rats compared with the controls, DM intake and body weight gain were reduced (9 and 20 %, respectively) as well as gastrocnemius muscle, liver and spleen weights (6, 14 and 11 %, respectively), but the intestinal tissues were increased (64 %). In the intestinal tissues all protein metabolism parameters (protein and RNA content, protein synthesis rate and translational efficiency) were increased in the pectin group. In liver the translational efficiency was also increased, whereas its protein and RNA contents were reduced in the pectin group. In gastrocnemius muscle, protein content, fractional and absolute protein synthesis rates and translational efficiency were lower in the pectin group. The stimulation of protein turnover in intestines and liver by soluble fibre such as pectins could be one of the factors that explain the decrease in muscle turnover and whole-body growth rate.     

Changes in the composition of mammary tissue, liver and muscle of rat dams during lactation and after weaning.

This investigation was conducted to evaluate the changes in the total content of protein and RNA in liver and muscles of rat dams before and after acute separation from their litters. Groups of 8-12 rats fed ad libitum were killed on the 12th (group L12) and 20th d (group L20) of lactation and on the 1st (group W1) and 7th d (group W7) after weaning. Nonpregnant, nonlactating rats paired for age served as controls. Dry matter, protein, DNA and RNA levels of the mammary gland, liver and muscles of the right hindlimb were determined. Wet weight and total organ or tissue protein, DNA and RNA were higher in mammary glands of L12 and L20 than in age-matched controls. These values were lower in groups W1 and W7 than in the lactating groups. No changes were noted in the total liver protein or DNA content, but total liver RNA was greater in groups L12 and L20 than in controls or group W1. Total muscle dry matter, DNA and RNA were significantly lower in groups L12 and L20 than in groups W1 and W7. Muscle protein content increased progressively from the 12th to 20th d of lactation to a peak in group W1, and it decreased to values found in age-matched controls in group W7. Although the muscle protein mass of the hindlimb during peak lactation (group L12) was only 63% of that in nonlactating control rats, within 1 d of weaning it was significantly higher than in nonlactating rats. Similar changes in RNA suggest that these changes in protein content are related to an adaptative mechanism designed to handle the surplus of plasma amino acids not used by the mammary gland after weaning.     

Changes in protein, RNA and DNA and rates of protein synthesis in muscle-containing tissues of the mature rat in response to ethanol feeding: a comparative study of heart, small intestine and gastrocnemius muscle.

1. The relative sensitivity of heart, small intestine and skeletal muscle to chronic ethanol feeding was investigated in mature Wistar rats fed ethanol as 36% of total energy intake; controls were fed the same diet in which ethanol was substituted by isoenergetic glucose. 2. Chronic ethanol feeding had no apparent effect on the protein, RNA and DNA contents of heart homogenates (atria and ventricles). The ratios of RNA/protein (synthetic capacity), RNA/DNA (synthetic material per nucleus) and protein/DNA (DNA-unit or apparent cell size) were also unaltered in the hearts of alcohol-fed rats. Fractional rates of cardiac protein synthesis (ks), and synthesis relative to RNA (kRNA) and DNA (kDNA) and absolute rates of protein synthesis (Vs) were unaffected by ethanol feeding. The total content of cardiac soluble proteins was unaltered by chronic ethanol feeding, but there were small and statistically significant decreases in the contents of the myofibrillar and stromal protein fractions. There were no differences in ks in any of the cardiac subcellular protein fractions. 3. In the small intestine, ethanol feeding had no statistically significant effect on either protein or RNA contents, but there was an apparent increase in RNA when expressed relative to either protein or DNA, though the DNA-unit was unaltered. There were also substantial decreases in ks, kRNA, kDNA and Vs of approximately 15-35%. 4. In the gastrocnemius, RNA contents were significantly reduced by ethanol feeding but protein and DNA contents were unaffected. Indices of the synthetic capacity and synthetic material per nucleus were also reduced, but the DNA-unit was unaltered. These observations were accompanied by approx. 15-30% reductions in ks, kRNA, kDNA and Vs in response to ethanol feeding. 5. It is concluded that various aspects of protein metabolism in the heart, small intestine and skeletal muscle are adversely affected by chronic ethanol toxicity. The characteristics and magnitude of the responses in each tissue differ. Effects in the heart may be subtle, though haemodynamic indices may ensue. The ethanol-induced alterations in the small intestine and skeletal muscle may be responsible for gastrointestinal disturbances in motility and skeletal muscle weakness, respectively.     

The effects of severe zinc deficiency on protein turnover in muscle and thymus

Measurements have been made of protein turnover, RNA and DNA in thymus and skeletal muscle from rats fed on a zinc-deficient diet (ZD) for 10 and 17 d, in pair-fed controls (CI) and in muscle from rats fed on the ZD diet for 24 d and then fed on restricted amounts of the deficient diet with (RIZS) or without (RIZD) Zn supplementation, for 8 d. In thymus the ZD diet induced a loss of DNA and protein which was not observed with the CI rats. Accumulation of RNA was less affected but protein synthesis was reduced. In muscle the accumulation of DNA and protein was slowed by the ZD diet, particularly in glycolytic muscles compared with oxidative muscles, and Zn supplementation increased DNA and protein. Protein synthesis and RNA concentrations were reduced in the ZD rats compared with the CI rats, but Zn supplementation at constant restricted food intake did not increase protein synthesis. Muscle protein synthesis per unit RNA varied markedly in the ZD rats after 10 d when the characteristic cycling of the food intakes and body-weight was most pronounced, the highest values being observed in the anabolic phase of the cycle although these were less than values for well-fed controls. The variability was inversely correlated with the plasma Zn levels. The extent of the variability was much less after 17 d and was not apparent in the food-restricted ZD animals. Protein degradation in muscle, assessed as the difference between overall and net protein synthesis, was faster in the ZD rats compared with the CI rats and fluctuated considerably, partly accounting for the cyclic changes in muscle after 10 d, and was entirely responsible after 17 d. The concentration of muscle-free 3-methylhistidine and its urinary excretion rate indicated inconsistent results which could not be satisfactorily interpreted. Plasma insulin was reduced in the ZD rats compared with the CI rats and was insensitive to food intake in contrast to urinary corticosterone excretion which was inversely correlated with the cyclic changes in body-weight and food intake. Furthermore, adrenalectomized rats exhibited increased mortality and reduced cycling of body-weight and food intake. Thus Zn deficiency impairs growth by a combination of reduced food intake, a reduced anabolic response to food due to a reduced capacity for protein synthesis and reduced activation of protein synthesis, possibly reflecting impaired insulin secretion, and an increased catabolic response to the reduced intake in which corticosterone may play a role.     

Effects of leucine supplementation on the body composition and protein status of rats submitted to food restriction

OBJECTIVE: Acute administration of leucine has been shown to stimulate certain protein synthesis related anabolic processes. However, the effect of chronic leucine administration in a catabolic situation caused by food restriction (FR) has not been established. We therefore evaluated the effect of chronic leucine supplementation on the body composition and some indicators of protein nutritional status of rats submitted to FR. METHODS: Adult male Wistar rats were submitted to 50% FR for 6 weeks. The control group received the AIN-93M diet and the leucine group received the same diet supplemented with 5.91 g L-leucine/kg ration. We then determined carcass chemical composition, serum leptin, albumin and total protein concentrations, and protein, DNA and RNA concentrations in gastrocnemius muscle and liver. RESULTS: No difference in final body weight was observed between groups. However, the leucine group presented a lower amount of body fat (P < 0.05). Leptin concentration showed a directly proportional correlation with the amount of body fat (r = 0.88, P < 0.05), but no significant difference in serum leptin concentration was observed between groups (P = 0.08). Regarding protein nutritional status, liver protein concentration was higher in the leucine group (P < 0.05). In the gastrocnemius muscle, a higher RNA concentration (P < 0.05) and a tendency towards higher DNA concentration (P = 0.06) were observed in the leucine group. CONCLUSION: The results indicate that low-dose leucine supplementation increases body fat loss and improves liver protein status and the capacity of muscle protein synthesis in rats submitted to FR.     

Dietary protein does not affect overloaded skeletal muscle in rats

We compared the effects of three levels of dietary protein, i.e., 7% (low protein; LP); 17.5% (adequate protein; CON); or 30% (high protein; HP) on growth of functionally overloaded muscle in Sprague-Dawley male rats. Growth of plantaris and soleus muscles was induced by the surgical removal of gastrocnemius muscles in one hindlimb; muscles in the other leg were used as sham-operated, intra-animal controls. After 4 wk, rats fed the 7% LP diet gained less weight (-29%) and had lighter livers (-20%) and kidneys (-16%) than rats fed the CON diet (P < 0.05). Measurements of rats fed the 30% HP diet were not different from those of CON rats except that their kidneys were larger (+6%) (P < 0.05). The level of dietary protein did not affect the experimentally induced muscular growth in either plantaris or soleus muscles. Gains in overloaded plantaris muscles over sham-operated muscles were not different among rats fed LP, CON and HP diets for muscle mass (+42 to +45%), total protein (+42 to +46%) and myofibrillar protein (+40 to +44%). Soleus muscles also did not differ among diet groups for gains in mass (+20 to +33%), total protein (+20 to +33%) and myofibrillar protein (+21 to +33%). No dietary protein effects were found on myosin heavy chain isoform (I, IIa, IIx, IIb) expression in either plantaris or soleus muscles. We conclude that gains in plantaris and soleus muscle mass, total protein and myofibrillar protein induced by functional overload are not affected by low (7%) or high (30%) protein feeding in young male rats for 4 wk.     

Adaptations of adult lean and obese zucker rats during food restriction

The adaptations that occur during prolonged food restriction of adult lean and obese Zucker rats were examined. Lean Zucker rats were restricted until they lost 31% (RL-31) or 42% (RL-42) of their starting weight. Obese Zucker rats were restricted until they lost 53% (RO-53) or 78% (RO-78) of their initial weight. Weights of five skeletal muscles, three fat pads and two organs were determined. During restriction, the obese rats catabolized less muscle protein per day than the lean animals. The percentage of muscle lost by the obese was lower than the percentage body weight loss. After prolonged restriction, both lean and obese rats were essentially depleted of fat stores. Obese rats relied to a greater extent on fat stores for energy and to a lesser extent on catabolism of lean body mass than lean rats. The ratios of protein/DNA and RNA/DNA were significantly reduced in muscles from all restricted animals and the DNA content was lower in the groups that lost the most weight (RL-42 and RO-78). In conclusion, obese Zucker rats have an enhanced ability to adapt to restriction with a slower rate of skeletal muscle catabolism. However, “survival” during prolonged restriction is dependent on the initial amount of adipose tissue and not loss of a critical amount of lean body mass.     

The effects of dietary energy restriction on overloaded skeletal muscle in rats

We evaluated the effects of three levels of energy intake, 73 % (CON73), 81 % (CON81) and 100 % (CON100) of the ad libitum intake of the control diet, on skeletal muscle growth induced by functional overload in male rats. Unlike most previous studies which have employed chronic or acute food restriction where all nutrients are reduced in the diet, the present study tested the effects of energy deprivation as a single factor without inducing other nutritional deficiencies. Muscular growth of plantaris and soleus muscles was induced by removal of synergist gastrocnemius muscles in one hindlimb; muscles in the other leg were used as sham-operated intra-animal controls. After 30 d, rats on the energy-restricted CON73 and CON81 diets gained less weight and had smaller livers, kidneys, hearts and fat pads (epididymal, retroperitoneal and omental) than CON100 rats They also had smaller sham-operated plantaris muscles (CON73 --13 %, CON81 --9 %) containing less total protein (CON73 --14 %; CON81 --10 %) than CON100 rats However, the same measurements in overloaded plantaris muscles were similar among groups. Soleus muscle mass and protein contents were not significantly affected by energy restriction in our study. Percentage distributions of myosin heavy-chain isoforms (types I, IIa, IIx and IIb) were similar among rats in CON100, CON81 and CON73 groups for both plantaris and soleus muscles. We conclude that the growth reduction of plantaris muscle induced by energy restriction at 73 % and 81 % for 30 d was prevented by functional overload in male rats.