Rates of protein turnover in vivo and in vitro in ventricular muscle of hearts from fed and starved rats

The insulin receptor family in the heart: new light on old insights

Insulin was discovered over 100 years ago. Whilst the first half century defined many of the physiological effects of insulin, the second emphasised the mechanisms by which it elicits these effects, implicating a vast array of G proteins and their regulators, lipid and protein kinases and counteracting phosphatases, and more. Potential growth-promoting and protective effects of insulin on the heart emerged from studies of carbohydrate metabolism in the 1960s, but the insulin receptors (and the related receptor for insulin-like growth factors 1 and 2) were not defined until the 1980s. A related third receptor, the insulin receptor-related receptor remained an orphan receptor for many years until it was identified as an alkali-sensor. The mechanisms by which these receptors and the plethora of downstream signalling molecules confer cardioprotection remain elusive. Here, we review important aspects of the effects of the three insulin receptor family members in the heart. Metabolic studies are set in the context of what is now known of insulin receptor family signalling and the role of protein kinase B (PKB or Akt), and the relationship between this and cardiomyocyte survival versus death is discussed. PKB/Akt phosphorylates numerous substrates with potential for cardioprotection in the contractile cardiomyocytes and cardiac non-myocytes. Our overall conclusion is that the effects of insulin on glucose metabolism that were initially identified remain highly pertinent in managing cardiomyocyte energetics and preservation of function. This alone provides a high level of cardioprotection in the face of pathophysiological stressors such as ischaemia and myocardial infarction.

The role of ubiquitin ligases in cardiac disease

Rigorous surveillance of protein quality control is essential for the maintenance of normal cardiac function, while the dysregulation of protein turnover is present in a diverse array of common cardiac diseases. Central to the protein quality control found in all cells is the ubiquitin proteasome system (UPS). The UPS plays a critical role in protein trafficking, cellular signaling, and most prominently, protein degradation. As ubiquitin ligases (E3s) control the specificity of the UPS, their description in the cardiomyocyte has highlighted how ubiquitin ligases are critical to the turnover and function of the sarcomere complex, responsible for the heart's required continuous contraction. In this review, we provide an overview of the UPS, highlighting a comprehensive overview of the cardiac ubiquitin ligases identified to date. We then focus on recent studies of new cardiac ubiquitin ligases outlining their novel roles in protein turnover, cellular signaling, and the regulation of mitochondrial dynamics and receptor turnover in the pathophysiology of cardiac hypertrophy, cardiac atrophy, myocardial infarction, and heart failure. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

AMP-activated protein kinase regulates E3 ligases in rodent heart

RATIONALE

The degradation of proteins by the ubiquitin proteasome system (UPS) is required for the maintenance of cellular homeostasis in the heart. An important regulator of metabolic homeostasis is AMP-activated protein kinase (AMPK). AMPK activation inhibits protein synthesis and activates autophagy, but whether AMPK plays a role in regulating protein breakdown through the UPS in the heart is not known.

OBJECTIVE

To determine whether AMPK enhances UPS-mediated protein degradation by directly regulating the ubiquitin ligases Atrogin-1 and muscle RING finger protein 1 (MuRF1) in the heart.

METHODS AND RESULTS

Nutrient deprivation and pharmacological or genetic activation of AMPK increased mRNA expression and protein levels of Atrogin-1 and MuRF1 and consequently enhanced protein degradation in neonatal cardiomyocytes. Inhibition of AMPK abrogated these effects. Using gene reporter and chromatin immunoprecipitation assays, we found that AMPK regulates MuRF1 expression by acting through the myocyte enhancer factor 2 (MEF2). We further validated these findings in vivo using MEF2-LacZ reporter mice. Furthermore, we demonstrated in adult cardiomyocytes that MuRF1 is necessary for AMPK-mediated proteolysis through the UPS in the heart. Consequently, MuRF1 knockout mice were protected from severe cardiac dysfunction during fasting.

CONCLUSIONS

AMPK regulates the transcription of Atrogin-1 and MuRF1 and enhances UPS-mediated protein degradation in heart. Specifically, AMPK regulates MuRF1 through the transcription factor MEF2. The absence of MuRF1 in the heart preserves cardiac function during fasting. The results strengthen the hypothesis that AMPK serves as a modulator of intracellular protein degradation in the heart.

Preserved protein synthesis in the heart in response to acute fasting and chronic food restriction despite reductions in liver and skeletal muscle

Whole body protein synthesis is reduced during the fed-to-fasted transition and in cases of chronic dietary restriction; however, less is known about tissue-specific alterations. We have assessed the extent to which protein synthesis in cardiac muscle responds to dietary perturbations compared with liver and skeletal muscle by applying a novel (2)H(2)O tracer method to quantify tissue-specific responses of protein synthesis in vivo. We hypothesized that protein synthesis in cardiac muscle would be unaffected by acute fasting or food restriction, whereas protein synthesis in the liver and gastrocnemius muscle would be reduced when there is a protein-energy deficit. We found that, although protein synthesis in liver and gastrocnemius muscle was significantly reduced by acute fasting, there were no changes in protein synthesis in the left ventricle of the heart for either the total protein pool or in isolated mitochondrial or cytosolic compartments. Likewise, a chronic reduction in calorie intake, induced by food restriction, did not affect protein synthesis in the heart, whereas protein synthesis in skeletal muscle and liver was decreased. The later observations are supported by changes in the phosphorylation state of two critical mediators of protein synthesis (4E-BP1 and eIF2alpha) in the respective tissues. We conclude that cardiac protein synthesis is maintained in cases of nutritional perturbations, in strong contrast to liver and gastrocnemius muscle, where protein synthesis is decreased by acute fasting or chronic food restriction.

Glycogen synthase kinase 3 (GSK3) in the heart: a point of integration in hypertrophic signalling and a therapeutic target? A critical analysis

Glycogen synthase kinase 3 (GSK3, of which there are two isoforms, GSK3alpha and GSK3beta) was originally characterized in the context of regulation of glycogen metabolism, though it is now known to regulate many other cellular processes. Phosphorylation of GSK3alpha(Ser21) and GSK3beta(Ser9) inhibits their activity. In the heart, emphasis has been placed particularly on GSK3beta, rather than GSK3alpha. Importantly, catalytically-active GSK3 generally restrains gene expression and, in the heart, catalytically-active GSK3 has been implicated in anti-hypertrophic signalling. Inhibition of GSK3 results in changes in the activities of transcription and translation factors in the heart and promotes hypertrophic responses, and it is generally assumed that signal transduction from hypertrophic stimuli to GSK3 passes primarily through protein kinase B/Akt (PKB/Akt). However, recent data suggest that the situation is far more complex. We review evidence pertaining to the role of GSK3 in the myocardium and discuss effects of genetic manipulation of GSK3 activity in vivo. We also discuss the signalling pathways potentially regulating GSK3 activity and propose that, depending on the stimulus, phosphorylation of GSK3 is independent of PKB/Akt. Potential GSK3 substrates studied in relation to myocardial hypertrophy include nuclear factors of activated T cells, beta-catenin, GATA4, myocardin, CREB, and eukaryotic initiation factor 2Bvarepsilon. These and other transcription factor substrates putatively important in the heart are considered. We discuss whether cardiac pathologies could be treated by therapeutic intervention at the GSK3 level but conclude that any intervention would be premature without greater understanding of the precise role of GSK3 in cardiac processes.

Signaling pathways mediating cardiac myocyte gene expression in physiological and stress responses

The contractile cells in the heart (the cardiac myocytes) are terminally differentiated. In response to pathophysiological stresses, cardiac myocytes undergo hypertrophic growth or apoptosis, responses associated with the development of cardiac pathologies. There has been much effort expended in gaining an understanding of the stimuli which promote these responses, and in identifying the intracellular signaling pathways which are activated and potentially involved. These signaling pathways presumably modulate gene and protein expression to elicit the end-stage response. For the regulation of gene expression, the signal may traverse the cytoplasm to modulate nuclear-localized transcription factors as occurs with the mitogen-activated protein kinase or protein kinase B/Akt cascades. Alternatively, the signal may promote translocation of transcription factors from the cytoplasm to the nucleus as is seen with the calcineurin/NFAT and JAK/STAT systems. We present an overview of the principal signaling pathways implicated in the regulation of gene expression in cardiac myocyte pathophysiology, and summarize the current understanding of these pathways, the transcription factors they regulate and the changes in gene expression associated with the development of cardiac pathologies. Finally, we discuss how intracellular signaling and gene expression may be integrated to elicit the overall change in cellular phenotype.

Liver dysfunction induced by bile duct ligation and galactosamine injection alters cardiac protein synthesis

Liver disease has been shown to affect the cardiovascular system and may influence cardiac protein metabolism. This hypothesis was tested by measuring rates of cardiac protein synthesis in 2 models of liver disease in rats. The study consisted of 5 groups--group 1: control, injected with saline and fed ad libitum; group 2: acute liver injury, by dosage with 400 mg/kg galactosamine; group 3: injected with saline and pair-fed to group 2; group 4: chronic liver disease, using bile duct ligation; and group 5: sham-operated and pair-fed to group 4. Rates of cardiac protein synthesis were measured using the flooding dose technique. After 1 week, galactosamine injection caused the following cardiac changes, i.e. group (2) versus (3): an increased RNA content, RNA/DNA ratio, and RNA/protein ratio. However, there was no change in DNA or protein content, or protein/DNA ratio. There was an increase in the fractional rate of protein synthesis, and the absolute synthesis rate. Cellular efficiency was increased, but RNA activity remained unchanged. Comparison of groups 4 and 5 showed that bile duct ligation caused no change in any parameters measured. Although comparison of the ad libitum-fed group 1 with the bile duct ligation group 4 showed reduced cardiac weight, protein, and RNA content, with decreased right ventricular absolute synthesis rates; this was also seen in the pair-fed group 5, suggesting that these effects were due solely to reduced oral intake. Thus, although galactosamine-induced acute liver injury caused marked changes in cardiac biochemistry, bile duct ligation per se did not. This study also illustrates the importance of including a pair-fed group.

Influence of starvation, surgery, and sepsis on cardiac protein synthesis in rats: effects of parenteral nutrition, glutamine, and growth hormone

The effect of sepsis on the rate of protein synthesis in the heart is poorly described. We have investigated changes in protein synthesis in the ventricles of the heart over time after cecal ligation and puncture (CLP) in rats in comparison with sham-operated and unoperated animals (ad libitum). All operated animals were starved from the time of surgery to the time of sacrifice. When operated animals were compared with ad libitum animals, ventricular weight and ventricular protein, and DNA and RNA contents were unchanged at 24 h, but were invariably reduced at 72 and 96 h. Fractional rate of protein synthesis (FSR), RNA activity, and cellular efficiency were reduced at 24 h and further reduced at 72 and 96 h. There were no differences, however, between septic and sham-operated animals. Eighteen hours after CLP, additional groups of rats were infused intravenously with 0.9% sodium chloride, parenteral nutrition (PN), or PN with glutamine, and were given a single dose of 400 microg recombinant human growth hormone (rhGH) or an equal volume of 0.9% sodium chloride. FSR was higher in animals given PN when compared with those given 0.9% sodium chloride only, and did not differ from FSR measured in unoperated animals. There was no additional benefit from the acute administration of either glutamine-enriched PN or rhGH. These results indicate that ventricular protein synthesis is markedly reduced by surgery and starvation, but that superimposed sepsis does not further influence these changes. PN can prevent the fall in cardiac protein synthesis associated with starvation, surgery, and sepsis, but neither glutamine nor rhGH produced any additional benefit.

Diarrhea reduces the rates of cardiac protein synthesis in myofibrillar protein fractions in rats in vivo

Although chronic diarrhea affects heart function and morphology, the pathogenic mechanisms are unknown. It was our hypothesis that diarrhea imposes metabolic stress to inhibit the synthesis of new contractile proteins. To test this hypothesis, we investigated the effects of lactose-induced diarrhea in rats. The groups were: 1) freely fed controls, 2) rats with lactose-induced diarrhea or 3) pair-fed rats. After 1 wk, hearts from the rats were subjected to subcellular fractionation techniques to isolate the major protein fractions, including myofibrillar proteins. The rates of protein synthesis were measured with concomitant assay of cardiac composition and plasma analytes. In comparison with the control group, diarrhea induced the following changes (P < 0.05): a decrease in heart weight, reduced RNA and mixed protein contents and a reduction in the fractional rate of mixed protein synthesis. There was a reduction in the content of all protein fractions. The fractional synthesis rate was reduced only for the myofibrillar fraction. Plasma insulin-like growth factor-I, but not corticosterone, was reduced. Plasma cholesterol and triglyceride concentrations were also reduced. In comparison with the pair-fed group, diarrhea induced the following changes (P < 0.05): a reduction in heart weight and fractional rate of mixed protein synthesis, reduced myofibrillar absolute synthesis rate and increased sarcoplasmic/myofibrillar fractional synthesis rate ratio. Plasma bicarbonate, triglyceride and urea concentrations were reduced, with an increase in albumin. Diarrhea impaired cardiac biochemistry, including a reduction in protein content and synthesis. A substantial proportion of these changes is due to anorexia, but the selective reduction in the synthesis of contractile proteins is a feature exclusive to the diarrhea group and may be due to reductions in plasma insulin-like growth factor-I.

Day length has a major effect on the response of protein synthesis rates to feeding in growing Japanese quail

We investigated the effect of day length on mixed protein fractional synthesis rates (K(S)) in 14- and 21-d-old Japanese quail (Coturnix c. japonica) habituated to either a long day length, 18 h light/6 h dark (LDL), or short day length, 6 h light/18 h dark (SDL), with free access to food during the light period. Rates of protein synthesis were measured by a flooding dose of L-[1-(13)C]leucine. In both groups, we measured K(S) of pectoral muscle, liver and heart after an overnight period of food deprivation and after 2-h food access at dawn. Rates of protein synthesis were also measured in LDL quail starved for 18 h and refed for 2 h. SDL chicks were smaller and had lower tissue weights at 2 wk of age than did LDL chicks (P<0.05). Starvation led to a lower rate of protein synthesis in those animals starved for 18 h. Food availability after starvation for 18 h induced a significant rise in tissue protein synthesis in both SDL and LDL quail (P<0.05). This increase was absent in LDL quail after a 6-h starvation period. There was an increase in K(S) to ad hoc changes in food supply. By determining the daily period in which feeding can occur, day length has a major effect on protein synthesis rates. This effect will determine the overall growth chicks are able to achieve that have been subjected to different day lengths.

Skeletal and heart muscle protein turnover during long-term exposure to high environmental temperatures in young rats

A study was undertaken to determine the long-term effects of a hot environment on protein turnover in skeletal and cardiac muscles of young homeothermic animals. Three groups of 36 male 28 day old rats were housed at 35°C (hot group), 25°C (control group), or 25°C but pair-fed to the intake of the hot group (pair-fed group). Rates of protein synthesis and degradation were measured in vivo on days 5, 10, 15, and 20. By day 20, soleus and gastrocnemius (skeletal muscle) protein masses were 7 and 14% lower in the hot group and 31 and 21% lower in the pair-fed group compared with the control group (P < 0.05). The fractional rate of protein synthesis (ksyn) was on average 11% lower (P < 0.05) in the hot group compared with control rats and was not different from pair-fed rats. The fractional rate of skeletal muscle protein degradation (kdeg) in hot rats was slightly lower than in control rats; kdegwas on average 18% higher (P < 0.05) in the pair-fed group compared with the hot group and this difference appeared to be most prominent on day 5. In heart, by day 20, protein mass was 30% lower in the hot group and 40% lower in the pair-fed group compared with control rats (P < 0.05). ksynwas on average 19% lower (P < 0.05) in the hot group compared with the control group, but not different from pair-fed rats. In the heart there were no differences in kdegamong treatments. Plasma triiodothyronine (T3) concentration was lower in the hot group, but not in the pair-fed group, compared with controls. In conclusion, chronic exposure to hot environments was associated with lower skeletal and cardiac muscle mass and protein turnover; lower protein mass in this tissue was due to decreased ksyn; this is consistent with lower plasma T3concentrations. In pair-fed rats, ksynwas also reduced, but interestingly kdegwas not, resulting in a greater loss of skeletal muscle mass. These results suggest that heat exposure invokes physiological adaptations to preserve skeletal muscle mass despite decreased food intake. In the heart, loss of protein was a result of decreased ksyn, which can be primarily ascribed to lower food intake.Key words: protein synthesis, protein metabolism, acclimation, heat stress.

Insulin-like growth factor I stimulates cardiac myosin heavy chain and actin synthesis in the awake rat

To determine the effect of insulin-like growth factor I (IGF-I) on cardiac contractile protein synthesis in vivo, we measured L-[ring-2, 6-3H]phenylalanine incorporation into myosin heavy chain and actin during intravenous infusions (4 h) of either saline or IGF-I (1 microgram. kg-1. min-1) in awake rats. After an overnight fast, IGF-I increased myosin synthesis by 29% compared with saline (11.5 +/- 0.8 vs. 8.9 +/- 0.6%/day, P < 0.01) and actin synthesis by 26% (7.2 +/- 0.3 vs. 5.7 +/- 0.3%/day, P < 0.01), with similar effects in left and right ventricles and a comparable effect on mixed cardiac protein. When amino acids were infused with IGF-I, a further increase in myosin synthesis was observed (P < 0.01). In fed rats, despite higher baseline synthesis rates than in fasted rats (P < 0. 01), IGF-I also increased the synthesis of myosin (12.3 +/- 0.5 vs. 9.9 +/- 0.5%/day, P < 0.01) and actin (8.8 +/- 0.3 vs. 7.5 +/- 0. 2%/day, P < 0.01) compared with saline. IGF-I infusion had no hypoglycemic effect and did not change heart rate or blood pressure. Thus relatively low-dose IGF-I has a direct action in vivo to acutely increase heart contractile protein synthesis in both fasted and fed awake rats.

Hyperinsulinemia inhibits myocardial protein degradation in patients with cardiovascular disease and insulin resistance

BACKGROUND

Insulin resistance, hyperinsulinemia, and myocardial hypertrophy frequently coexist in patients. Whether hyperinsulinemia directly affects myocardial protein metabolism in humans has not been examined, however. To test the hypothesis that hyperinsulinemia is anabolic for human heart protein, we examined the effects of insulin infusion on myocardial protein synthesis, degradation, and net balance in patients with ischemic heart disease.

METHODS AND RESULTS

Eleven men (aged 57 +/- 3 years) with coronary artery disease who had fasted for 12 to 16 hours received a constant infusion of insulin (50 mU.m-2.min-1) while plasma concentrations of glucose and amino acids were kept constant. Rates of myocardial protein synthesis, degradation, and net balance were estimated from steady state extraction and isotopic dilution of L-[ring-2,6-3H]phenylalanine across the heart basally and 90 minutes into infusion. Subjects had elevated fasting plasma insulin concentrations (173 +/- 21 pmol/L) and used little exogenous glucose during insulin infusion, suggesting resistance to the effects of insulin on whole-body carbohydrate metabolism. Basally, myocardial protein degradation, as estimated by phenylalanine release (133 +/- 28 nmol/min), exceeded protein synthesis, estimated by phenylalanine uptake (31 +/- 15 nmol/min), resulting in net negative phenylalanine balance (-102 +/- 17 nmol/min). Insulin infusion reduced myocardial protein degradation by 80% but did not affect protein synthesis, returning net phenylalanine balance to neutral.

CONCLUSIONS

Acute hyperinsulinemia markedly suppresses myocardial protein degradation in patients with cardiovascular disease who are resistant to its effects on whole-body glucose metabolism. This antiproteolytic action represents a potential mechanism by which hyperinsulinemia could contribute to the development of myocardial hypertrophy in patients with cardiovascular disease.

Protein synthesis in the heart in vivo, its measurement and patho-physiological alterations

Changes in cardiac protein composition occur in a variety of patho-physiological situations and are usually accompanied by modifications in protein synthesis. Although adjustments in protein synthesis during starvation may be adaptive, the alterations in protein synthesis seen in response to ethanol ingestion may be pathological and an important step in the genesis of alcoholic heart muscle disease. The alterations in heart muscle in hypertension are initially adaptive but in the long term they are deleterious, and involve both transcription and translation. While adequate methods exist for quantifying the amount of mRNA for contractile and non-contractile proteins, such studies of gene-expression provide no dynamic information on the rate at which tissue proteins are lost or accrued. This can only be determined by measuring the rate of protein turnover, i.e. either protein synthesis or protein breakdown. Techniques for directly determining the rates of protein breakdown are limited or involve surgical procedures. Methods for measuring the rate of protein synthesis are described, and are illustrated by their application to the investigation of starvation and ethanol toxicity. In particular, attention is focused on the fact that reliable rates of protein synthesis are obtained only if the specific radioactivity of the precursor at the site of protein synthesis (aminoacyl-tRNA) is assessed.

Ventricular muscle and lung protein synthesis in vivo in response to fasting, refeeding, and nutrient supply by oral and parenteral routes

BACKGROUND

Fasting and other catabolic states are characterized by reductions in the rate of protein synthesis. Most studies have investigated tissues such as skeletal muscle or liver, but impairments in the biochemistry of cardiovascular tissues also contribute to enhanced morbidity. The objectives of the present study were (1) to determine the response of protein synthesis in the heart and lung of young rats to overnight fasting; and (2) to determine whether protein synthesis could be ameliorated or modulated by refeeding or provision of enteral or parenteral nutrition.

METHODS

Fractional rates of protein synthesis (ie, the percentage of tissue protein renewed each day, ks) were measured in vivo in the ventricular muscle and lungs of young male Wistar rats (body weight, 100 to 130 g) with a "flooding" dose of L-[4(3)H]phenylalanine. Rats were fed ad libitum or fasted overnight. Fasted rats were subjected to various treatments.

RESULTS

When nutrient supply in fasted rats recommenced by refeeding for 1 hour, there were small but significant increases in the rates of ventricular protein synthesis, although the infusion of amino acids and glucose for 1 hour had no significant effect. Increases in ventricular ks were also obtained when amino acids were infused for 6 hours. Infusion of glucose alone for 6 hours did not cause a significant increase in ventricular ks. The effect of infusing a mixture of glucose plus amino acids for 6 hours was similar to the effects of amino acids alone. In all instances, ventricular ks in rats infused with amino acid for 6 hours did not attain rates observed in fed rats. ks was reduced in the lung after overnight fasting but was unresponsive to refeeding or to acute or chronic provision of amino acids and glucose by either IV or oral routes. Measurements also suggested that changes in neither insulin nor glucagon per se were responsible for the amino acid-induced increases in heart protein synthesis. However, acute treatment of rats with anti-insulin serum reduced rates of ventricular ks below values observed in fed rats. Anti-insulin serum also increased lung ks.

CONCLUSIONS

It was concluded that rates of heart protein synthesis could be increased by the chronic provision (ie, 6 hours) of nutrients by oral or IV routes. In contrast, the lung was insensitive to these treatments. The observations have important implications for clinical situations, which are characterized by diminished cardiopulmonary protein synthesis.

Ethanol toxicity and cardiac protein synthesis in vivo

Long- and short-term alcohol consumption induce a variety of cardiovascular changes, including alterations in hemodynamic variables and tissue biochemistry. In many instances some of the perturbations may be considered as compensatory adjustments, and indeed, there is some controversy that moderate long-term consumption may cause alterations in plasma lipid profiles, conferring cardiovascular protection by reducing the incidence of coronary artery disease. In the long term, however, ethanol misuse may induce a specific disease entity, namely alcoholic heart muscle disease, and short-term ethanol exposure may also perturb tissue contractility and hemodynamic indices. The mechanisms of these changes are unknown, but central to many of the metabolic and functional disturbances are alterations in tissue protein synthesis, perhaps precipitated or exacerbated by free radial formation or by the formation of protein-acetaldehyde adducts. Methods for measuring protein synthesis in vivo are reviewed, and their application to elucidating the mechanisms involved in cardiac abnormalities is described, including the effects of ethanol. Our results demonstrate that the effects of alcohol toxicity also occur at the subcellular level, and the synthesis of mitochondrial proteins are reduced in vivo, perhaps even contribution to defects in energy generation, the normal function of which is required to maintain contractility.

Synthesis of ventricular mitochondrial proteins in vivo: effect of acute ethanol toxicity

Most studies on the pathological responses of the heart to ethanol have been conducted in isolated systems. The objectives of this study were to determine (1) the synthesis rate of ventricular mitochondrial proteins in vivo and (2) whether the synthesis rates of these proteins are perturbed by acute ethanol exposure in vivo. Fractional rates of protein synthesis [defined as the percentage of tissue protein renewed each day; i.e., ks (%/day)] were determined in male Wistar rats by in vivo injection of a flooding dose of L-[4-3H] phenylalanine. Subsarcolemmal mitochondria were released by polytron treatment, and the isolation of interfibrillar mitochondria involved treatment of the cardiac homogenate with the proteolytic enzyme Nagarse. In the control rats mean ks values of 22.4%/day were observed for mixed cardiac proteins. The synthesis rates of subsarcolemmal and interfibrillar mitochondrial proteins were lower, i.e., 16.9%/day and 10.9%/day, respectively. Acute ethanol administration (75 mmol/kg body weight ip, 2.5 hr) depressed the fractional rate of protein synthesis in all cardiac fractions, including those pertaining to the mitochondria, as follows: mixed fraction--21%, p < 0.01; subsarcolemmal mitochondria--23%, p < 0.01; interfibrillar mitochondria--26%, p < 0.05; and nuclear fraction--20%, p < 0.05. In conclusion, the reduced synthesis rate of the mitochondrial proteins in response to acute ethanol exposure may in some way be partly connected with the depression in myocardial contractility and associated functional damage of mitochondrial metabolism.

Physiological hyperinsulinemia inhibits myocardial protein degradation in vivo in the canine heart

Myocardial protein turnover in vivo was examined in anesthetized dogs following a 16- or 36-hour fast and again during a hyperinsulinemic (2 mU/kg per minute) euglycemic clamp with or without amino acid replacement or during saline infusion. We measured myocardial phenylalanine balance and rates of protein synthesis and degradation, using the extraction of intravenously infused L-[ring-2,6-3H]phenylalanine and the dilution of its specific activity across the heart at isotopic steady state. After both a 16-hour (n = 19) and 36-hour fast (n = 10), there was net myocardial release of phenylalanine indicated by the negative balances for phenylalanine of -52 +/- 9 (p less than 0.001) and -38 +/- 9 (p less than 0.005) nmol/min, respectively. Overall, the basal rate of myocardial protein degradation was lower in the 36-hour-fasted animals (81 +/- 13 versus 121 +/- 12 nmol/min, p less than 0.05). Myocardial phenylalanine balance and rates of protein synthesis and degradation did not change during insulin and glucose infusion in the 36-hour-fasted animals (n = 10). In these animals, there was a 30-40% decline in plasma amino acid concentrations, including branched chain (p less than 0.001) and essential amino acids (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial protein turnover in patients with coronary artery disease. Effect of branched chain amino acid infusion

The regulation of protein metabolism in the human heart has not previously been studied. In 10 postabsorptive patients with coronary artery disease, heart protein synthesis and degradation were estimated simultaneously from the extraction of intravenously infused L-[ring-2,6-3H]phenylalanine (PHE) and the dilution of its specific activity across the heart at isotopic steady state. We subsequently examined the effect of branched chain amino acid (BCAA) infusion on heart protein turnover and on the myocardial balance of amino acids and branched chain ketoacids (BCKA) in these patients. In the postabsorptive state, there was a net release of phenylalanine (arterial-cardiac venous [PHE] = -1.71 +/- 0.32 nmol/ml, P less than 0.001; balance = -116 +/- 21 nmol PHE/min, P less than 0.001), reflecting protein degradation (142 +/- 40 nmol PHE/min) in excess of synthesis (24 +/- 42 nmol PHE/min) and net myocardial protein catabolism. During BCAA infusion, protein synthesis increased to equal the degradation rate (106 +/- 24 and 106 +/- 28 nmol PHE/min, respectively) and the phenylalanine balance shifted (P = 0.01) from negative to neutral (arterial-cardiac venous [PHE] = 0.07 +/- 0.36 nmol/ml; balance = 2 +/- 25 nmol PHE/min). BCAA infusion stimulated the myocardial uptake of both BCAA (P less than 0.005) and their ketoacid conjugates (P less than 0.001) in proportion to their circulating concentrations. Net uptake of the BCAA greatly exceeded that of other essential amino acids suggesting a role for BCAA and BCKA as metabolic fuels. Plasma insulin levels, cardiac double product, coronary blood flow, and myocardial oxygen consumption were unchanged. These results demonstrate that the myocardium of postabsorptive humans is in negative protein balance and indicate a primary anabolic effect of BCAA on the human heart.

Protein synthesis in liver and extra-hepatic tissues after partial hepatectomy

Effects of partial hepatectomy on protein synthesis were defined in liver and extra-hepatic tissues of the mature rat. Studies were performed at 24 h and 48 h after surgery in the absence of the dietary input. Protein accretion in the regenerating liver preceded mitosis, but was accompanied by increases in RNA content and fractional rates of protein synthesis (ks). A positive relationship existed between protein-synthetic capacity and ks over the period of study. Increases in ks also bore a positive relationship with increases in translational efficiency. Extra-hepatic tissues showing decreased rates of protein synthesis after liver resection included kidney, striated muscles and brain. Effects were observed mainly at 24 h after surgery and resulted from decreased translational efficiency. Partial hepatectomy increased ks in diaphragm and tibia at both 24 h and 48 h after surgery. In diaphragm, there was net protein accretion, and, as in liver, increases in ks were due to increases in both protein-synthetic capacity and efficiency.

Effects of catecholamines on protein synthesis in cardiac myocytes and perfused hearts isolated from adult rats. Stimulation of translation is mediated through the alpha 1-adrenoceptor

Protein-synthesis rates in freshly isolated cardiac myocytes from adult rats were acutely stimulated by 20-30% by 1 microM-adrenaline, by 1 microM-noradrenaline or by 1 microM-phenylephrine, but were not stimulated by 1 microM-isoprenaline. Stimulation by 1 microM-adrenaline was completely prevented by 100 nM-prazosin. Yohimbine was much less effective in preventing stimulation, and 20 microM-DL-propranolol was completely ineffective. The stimulation of protein synthesis by adrenaline was still observed after inhibition of transcription by actinomycin D. None of these manipulations affected myocyte ATP contents. In anterogradely perfused hearts, protein-synthesis rates were stimulated by 1-2 microM-adrenaline in the presence of 10 microM-DL-propranolol (to decrease the beta-adrenergic effects of adrenaline). ATP contents were not altered, but phosphocreatine contents were increased. These observations lead us to conclude that cardiac protein synthesis can be stimulated acutely at the level of translation by alpha 1-adrenergic stimulation. We discuss possible roles for protein kinase C and intracellular alkalinization in the mediation of this effect.

The acute and chronic effects of ethanol on cardiac muscle protein synthesis in the rat in vivo

An investigation was made into the acute and chronic effects of ethanol on rates of protein synthesis in the hearts of young rats (80-100 g body weight). Acute ethanol administration (75 mmol/kg body weight, IP) significantly reduced the fractional rate of protein synthesis by 20% after 2.5 hr, compared with saline-treated controls. Chronic ethanol feeding (36% of total calories) for 6 weeks significantly reduced cardiac wet weight by 11%, when compared to rats fed isovolumetric amounts of the same diet in which ethanol was substituted by isocaloric glucose. Neither the concentration nor the content of mixed cardiac proteins relative to body weight were overtly altered by chronic ethanol feeding, although, the total content of mixed cardiac proteins were significantly decreased. RNA concentrations and RNA relative to body weight increased slightly, but total cardiac DNA decreased. Indices for the capacity or potential of the heart to synthesis protein (indicated by the RNA/protein and RNA/DNA ratios) and the "DNA-unit" (protein/DNA ratio) were increased in response to chronic ethanol treatment. The fractional and absolute rates of mixed protein synthesis in the heart were (relatively) unaltered by chronic ethanol treatment, as was RNA efficiency and synthesis relative to DNA. It was concluded that the heart displays contrasting responses to acute and chronic ethanol exposure.

Stimulation of protein synthesis by raised extracellular pH in cardiac myocytes and perfused hearts

Protein synthesis was stimulated in freshly-isolated rats cardiac myocytes by increasing the extracellular pH of Hepes-buffered Tyrode's solutions over the range pH 7.4-8.4. The maximal stimulation was about 45%. Protein synthesis in anterogradely-perfused rat hearts was stimulated by 11% by increasing the pH of the bicarbonate-containing perfusion medium from pH 7.4 to 7.8. This manoeuvre increased intracellular pH by 0.12 units. A concomitant increase in phosphocreatine concentration was observed. These findings are consistent with the hypothesis that intracellular pH may exert profound effects on tissue protein synthesis rates.

Protein synthesis in rat cardiac myocytes is stimulated at the level of translation by phorbol esters

12-O-Tetradecanoylphorbol 13-acetate acutely stimulated the rate of protein synthesis maximally by about 43% in terminally differentiated myocytes freshly isolated from adult rat hearts. Stimulation was rapidly expressed (within 20 min). The relative effects of four phorbol esters on protein synthesis was consistent with a specific effect on protein kinase C. Inhibition of transcription with actinomycin D had no effect on the absolute stimulation of the protein synthesis rate by 12-O-tetradecanoylphorbol 13-acetate. We conclude that protein kinase C may be involved in the regulation of the translational process.

Effects of increasing extracellular pH on protein synthesis and protein degradation in the perfused working rat heart

Increasing the extracellular pH over the range pH 7.4-8.9 stimulated protein synthesis by about 60% in the rat heart preparation anterogradely perfused in vitro. Protein degradation was inhibited by this pH increase. The magnitudes of the effects at pH 8.9 on protein synthesis and degradation were similar to those of high concentrations of insulin. Cardiac outputs were increased, as were cardiac phosphocreatine contents, indicating that the alterations in extracellular pH did not adversely affect the physiological viability of the preparation. ATP contents were unaltered. The creatine kinase equilibrium was used to assess the magnitude of the change in intracellular pH induced by these treatments. The increase in intracellular pH was about 0.2 for a 1-unit increase in extracellular pH. Thus small changes in intracellular pH have dramatic effects on cardiac protein turnover.

The effects of fasting or hypoxia on rates of protein synthesis in vivo in subcellular fractions of rat heart and gastrocnemius muscle

We measured rates of protein synthesis in vivo in subcellular fractions (soluble, myofibrillar and stromal fractions) of the heart and the gastrocnemius from rats after fasting or under hypoxic conditions (i.e. atmospheres containing 5% or 10% O2). Such interventions are known to inhibit protein synthesis under some circumstances. The recovery of tissue protein after fractionation was 80-100%. The proportions of protein present in the soluble and stromal fractions were different in the two muscles. The rates of protein synthesis in the myofibrillar and stromal fractions were less than those for total mixed tissue protein, whereas the rate for soluble protein was greater. Both fasting and moderate hypoxia (10% O2 for 24 h) inhibited protein synthesis in the gastrocnemius. In this tissue, the synthesis of the myofibrillar fraction was apparently the most sensitive to inhibition, and this resulted in some significant increases in the soluble-fraction/myofibrillar-fraction protein-synthesis rate ratios. In the heart, fasting inhibited protein synthesis, but moderate hypoxia (10% O2 for 24 h) did not. The rate of protein synthesis in the cardiac myofibrillar fraction was again more sensitive to fasting than were the rates in the other fractions, but it was not as sensitive as that in the gastrocnemius. Under severely hypoxic conditions (5% O2 for 1 or 2 h), protein synthesis was decreased in all fractions in both tissues. These results suggest that the rates of protein synthesis in these relatively crude subcellular fractions vary.

The effect of chronic ethanol ingestion on protein metabolism in type-I- and type-II-fibre-rich skeletal muscles of the rat

1. The effects of chronic ethanol feeding on muscles containing a predominance of either Type I (aerobic, slow-twitch) or Type II (anaerobic, fast-twitch) fibres were studied. Male Wistar rats, weighing approx. 90 g or 280 g, were pair-fed on a nutritionally complete liquid diet containing 36% of total energy as ethanol, or isovolumetric amounts of the same diet in which ethanol was replaced by isoenergetic glucose. After 6 weeks feeding, fractional rates of protein synthesis were measured with a flooding dose of L-[4-(3)H]-phenylalanine and muscles were analysed for protein, RNA and DNA. 2. Ethanol feeding decreased muscle weight, protein, RNA and DNA contents in both small and large rats. Type-II-fibre-rich muscles showed greater changes than did Type-I-fibre-rich muscles. Changes in protein paralleled decreases in DNA. 3. The capacity for protein synthesis (RNA/protein), fractional rates of protein synthesis and absolute rates of protein synthesis were decreased by ethanol feeding in both small and large rats. The amounts of protein synthesized relative to RNA and DNA were also decreased. Changes were less marked in Type-I than in Type-II-fibre-rich muscles. Loss of protein, RNA and DNA was greater in small rats, but protein synthesis was more markedly affected in large rats. 4. It was concluded that chronic ethanol feeding adversely affects protein metabolism in skeletal muscle. Fibre composition and animal size are also important factors in determining the pattern of response.

The effects of surgical stress and short-term fasting on protein synthesis in vivo in diverse tissues of the mature rat

1. We measured fractional rates of protein synthesis, capacities for protein synthesis (i.e. RNA/protein ratio) and efficiencies of protein synthesis (i.e. protein-synthesis rate relative to RNA content) in fasted (24 or 48 h) or fasted/surgically stressed female adult rats. 2. Of the 15 tissues studied, fasting caused decreases in protein content in the liver, gastrointestinal tract, heart, spleen and tibia. There was no detectable decrease in the protein content of the skeletal muscles studied. 3. Fractional rates of synthesis were not uniformly decreased by fasting. Rates in striated muscles, uterus, liver, spleen and tibia were consistently decreased, but decreases in other tissues (lung, gastrointestinal tract, kidney or brain) were inconsistent or not detectable, suggesting that, in many tissues in the mature rat, protein synthesis was not especially sensitive to fasting. 4. In fasting, the decreases in fractional synthesis rate resulted from changes in efficiency (liver and tibia) or from changes in efficiency and capacity (heart, diaphragm, plantaris and gastrocnemius). In the soleus, the main change was a decrease in capacity. 5. Surgical stress increased fractional rates of protein synthesis in diaphragm (where there were increases in both efficiency and capacity) by about 50%, in liver by about 20%, in spleen by about 40%, and possibly also in the heart. In liver and spleen, capacities were increased. In other tissues (including the skeletal muscles), the fractional rates of protein synthesis were unaffected by surgical stress.

Protein synthesis and degradation during starvation-induced cardiac atrophy in rabbits

To determine the relative importance of protein degradation in the development of starvation-induced cardiac atrophy, in vivo fractional synthetic rates of total cardiac protein, myosin heavy chain, actin, light chain 1, and light chain 2 were measured in fed and fasted rabbits by continuous infusion of [3H] leucine. In addition, the rate of left ventricular protein accumulation and loss were assessed in weight-matched control and fasted rabbits. Rates of total cardiac protein degradation were then estimated as the difference between rates of synthesis and growth. Fasting produced left ventricular atrophy by decreasing the rate of left ventricular protein synthesis (34.8 +/- 1.4, 27.3 +/- 3.0, and 19.3 +/- 1.2 mg/day of left ventricular protein synthesized for 0-, 3-, and 7-day fasted rabbits, respectively). Inhibition of contractile protein synthesis was evident by significant reductions in the fractional synthetic rates of all myofibrillar protein subunits. Although fractional rates of protein degradation increased significantly within 7 days of fasting, actual amounts of left ventricular protein degraded per day were unaffected. Thus, prolonged fasting profoundly inhibits the synthesis of new cardiac protein, including the major protein constituents of the myofibril. Both this inhibition in new protein synthesis as well as a smaller but significant reduction in the average half-lives of cardiac proteins are responsible for atrophy of the heart in response to fasting.

Effects of pressure overload and insulin on protein turnover in the perfused rat heart. Prostaglandins are not involved although their synthesis is stimulated by insulin

A modified anterogradely perfused rat heart preparation is described in which all the cardiac output passes through the coronary circulation. Such a preparation develops hypertensive aortic pressures. Hypertensive aortic pressures or insulin stimulate the rate of cardiac protein synthesis and inhibit the rate of protein degradation. Aortic pressure and insulin may be important in the regulation of cardiac nitrogen balance in vivo. By abolishing cardiac prostaglandin synthesis with 4-biphenylacetate, we were able to investigate the possible involvement of prostaglandins in the modulation of protein turnover by pressure overload or insulin. There was no evidence of any involvement. However, insulin stimulated and cycloheximide inhibited cardiac prostaglandin synthesis. These findings are consonant with an enzyme involved in prostaglandin synthesis being short-lived and prostaglandin synthesis being rapidly influenced by activators and inhibitors of protein synthesis and degradation.

Contrasting response of protein degradation to starvation and insulin as measured by release of N tau-methylhistidine or phenylalanine from the perfused rat heart

An isotope-dilution method is described for the measurement of N tau-methylhistidine release from the perfused rat heart. We argue that release of N tau-methylhistidine is indicative of cardiac actin degradation. N tau-Methylhistidine release is compared with phenylalanine release in the presence of cycloheximide (phenylalanine release being a measure of degradation of mixed proteins). In hearts perfused with glucose plus acetate, the rate of actin degradation was increased by starvation and was not inhibited by insulin. In contrast, the rate of mixed-protein degradation was decreased by starvation and was inhibited by insulin. The fractional rate of degradation of mixed proteins in hearts from fed or starved rats was greater than that for actin. It is suggested that there are at least two pools of intracellular protein, the degradation rates of which differ in terms of their response to insulin and starvation.

Stimulation of protein synthesis, glucose uptake and lactate output by insulin and adenosine deaminase in the rat heart

In the anterogradely perfused rat heart, physiological concentrations of insulin stimulated the rates and efficiencies of protein synthesis in both ventricles and atria. Half-maximal stimulation of ventricular protein synthesis was obtained at about 35 microU/ml. Glucose uptake and lactate release were also stimulated over this range of insulin concentrations. Adenosine deaminase increased protein synthesis rates in ventricles and atria in the presence of submaximally stimulating insulin concentrations (40 microU/ml) but had no effect in the absence of insulin or in the presence of maximally stimulating concentrations. The insulin sensitivities of glucose uptake and lactate release were also increased by adenosine deaminase. Adenosine may be a modulator of insulin sensitivity in the heart.

The effects of lactate, acetate, glucose, insulin, starvation and alloxan-diabetes on protein synthesis in perfused rat hearts

Compared with glucose, lactate + acetate stimulated ventricular protein synthesis in anterogradely perfused hearts from fed or 72 h-starved rats. Stimulation was greater on a percentage basis in starved rats. Atrial protein synthesis was not detectably stimulated by lactate + acetate. Insulin stimulated protein synthesis in atria and ventricles. The stimulation of protein synthesis by lactate + acetate and insulin was not additive, the percentage stimulation by insulin being less in the ventricles of lactate + acetate-perfused hearts than in glucose-perfused hearts. Perfusion of hearts from 72 h-starved or alloxan-diabetic rats with glucose + lactate + acetate + insulin did not increase protein-synthesis rates or efficiencies (protein synthesis expressed relative to total RNA) to values for fed rats, implying there is a decrease in translational activity in these hearts. In the perfused heart, inhibition of protein synthesis by starvation and its reversal by re-feeding followed a relatively prolonged time course. Synthesis was still decreasing after 3 days of starvation and did not return to normal until after 2 days of re-feeding.

Effects of glucocorticoid treatment on cardiac protein synthesis and degradation

We treated rats with dexamethasone (DEX, 1 mg . kg-1 . day-1) and examined the effects of this glucocorticoid on heart protein metabolism using atrial explant and Langendorff perfusion preparations. Fasted rats treated with DEX for 2 days had significantly lower body weights (92% of control, P less than 0.001) and larger hearts (106% of control, P less than 0.005) than fasted control animals. Protein and RNA concentrations remained constant. In atrial explants, DEX treatment produced a 19% increase in protein synthesis (P less than 0.001) and a 13% increase in protein degradation (P less than 0.002). In Langendorff-perfused hearts, DEX treatment caused a 36% increase in protein synthesis (P less than 0.02), while protein degradation was 8% above control (P greater than 0.05). Thus, in contrast to their catabolic effects on skeletal muscle, glucocorticoids are anabolic on the heart. The increased accumulation of total cardiac protein during early glucocorticoid administration is mediated entirely via increased rates of synthesis.

A comparison of rates of protein turnover in rat diaphragm in vivo and in vitro

Protein synthesis and degradation rates in diaphragms from fed or starved rats were compared in vivo and in vitro. For fed rats, synthesis rates in vivo were approximately twice those in vitro, but for starved rats rates were similar. Degradation rates were less in vivo than in vitro in diaphragms from either fed or starved rats.

The effects of 6 hours of hypoxia on protein synthesis in rat tissues in vivo and in vitro

Rates of protein synthesis were measured in vivo in several tissues (heart, skeletal muscles, liver, tibia, skin, brain, kidney, lung) of fed rats exposed to O2/N2 (1:9) for 6 h starting at 08:00-11:00 h. Protein synthesis rates were depressed by 15-35% compared with normoxic controls in all of the tissues studied. The decreases were greatest in the brain and the skin. Although hypoxia inhibited gastric emptying, its effects on protein synthesis could probably not be attributed to its induction of a starved state, because protein-synthesis rates in brain and skin were not decreased by a 15-18 h period of starvation initiated at 23:00 h. Furthermore, we showed that protein synthesis was inhibited by hypoxia in the rat heart perfused in vitro, suggesting a direct effect. The role of hypoxia in perturbing tissue nitrogen balance in various physiological and pathological states is discussed.

Regional variation and differential sensitivity of rat heart protein synthesis in vivo and in vitro

In vivo, fractional rates of protein synthesis in atrial muscle of hearts taken from fed rats were 70% greater than in ventricular muscle. After 3 days starvation, atrial protein synthesis is inhibited, but the inhibition is less than in ventricles. A crude subcellular fractionation of the aqueous homogenates by centrifugation at 32000g showed that the supernatant and precipitate proteins were synthesized at the same rate in the ventricles. The fractional rates of protein synthesis and RNA/protein ratios in the right ventricle were 10% greater than in the left ventricle. Protein synthesis in both of these regions was inhibited equally by starvation. In vitro, rates of protein synthesis in atria and ventricles of anterogradely perfused rat hearts were stimulated by saturating insulin concentrations and were inhibited by starvation, but the effects in atria were smaller than in ventricles. Rates of protein synthesis in atria in vitro were 80-95% of rates in vivo. The heart therefore shows considerable regional variation in rates of protein synthesis in vivo and in vitro, and the sensitivity of protein synthesis in the various regions to interventions such as insulin and starvation differs.

Control of tissue carnitine contents: effects of partial hepatectomy and liver regeneration on carnitine concentrations in liver and extrahepatic tissues of the rat

The liver is the sole site of carnitine biosynthesis in the rat. However, the first 24 h after the surgical removal of two-thirds of the liver mass are not associated with depletion of carnitine either in the liver remnant or in a number of extrahepatic tissues with relatively short turnover times of carnitine (less than 24 h; heart, spleen, kidney). Dietary carnitine was not supplied. The results suggest that the capacity of the remnant liver for carnitine biosynthesis is sufficient to maintain tissue carnitine contents. Liver regeneration influenced the relative proportions of hepatic free and acylated carnitines in a manner compatible with changes in fat disposition in the proliferating tissue.