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The heart of tropical fishes is a particularly useful model system in which to investigate mechanisms of hypoxic tolerance. Here we focus on insights gained from two groups of fishes, cichlids and armoured catfishes. Cichlids respond to hypoxia by entering a sustained hypometabolism with decreased heart performance to match whole animal circulatory needs. Heart rate is decreased along with protein turnover to reduce adenosine triphosphate demand. This occurs despite the inherent capacity for high levels of cardiac power development. Although highly hypoxic tolerant at the whole animal level, the heart of cichlids does not have high constitutive activities of glycolytic enzymes compared to other species. Information is conflicting with respect to changes in glycolytic gene expression and enzyme activity following hypoxic exposure with some studies showing increases and others decreases. In contrast to cichlids, species of armoured catfish, that are routinely exposed to water of low oxygen content, do not display hypoxic bradycardia. Under hypoxia there are early changes in glucose trafficking suggestive of activation of glycolysis before lactate accumulation. Thereafter, heart glycogen is mobilized and lactate accumulates in both heart and blood, in some species to very high levels. Heart performance under hypoxia is enhanced by defense of intracellular pH. A functional sarcoplasmic reticulum and binding of hexokinase to the outer mitochondrial membrane may also play a role in cardioprotection. Maintenance of heart performance under hypoxia may relate to a tradeoff between air breathing via a modified stomach and circulatory demands for digestion. RESEARCH HIGHLIGHTS: Hypoxic cichlids enter hypometabolism in which heart performance matches circulatory needs; bradycardia along with lowered protein turnover reduces adenosine triphosphate demand. Armoured catfish maintain heart rate, carbohydrate metabolism is activated, lactate accumulates while heart pHi is defended. [ABSTRACT FROM AUTHOR]

When confined in pairs, juvenile rainbow trout (Oncorhynchus mykiss) form dominance hierarchies in which subordinate fish exhibit characteristic physiological changes including reduced growth rates and chronically elevated plasma cortisol concentrations. We hypothesized that alterations in protein metabolism contribute to the reduced growth rate of socially stressed trout, and predicted that subordinate trout would exhibit reduced rates of protein synthesis coupled with increases in protein degradation. Protein metabolism was assessed in dominant and subordinate fish after 4 days of social interaction, and in fish that were separated after 4 days of interaction for a 4 days recovery period, to determine whether effects on protein metabolism recovered when social stress was alleviated. Protein metabolism was assessed in liver and white muscle by measuring the fractional rate of protein synthesis and markers of protein degradation. In the white muscle of subordinate fish, protein synthesis was inhibited and activities of the ubiquitin-proteasome pathway (UPP) and the autophagy lysosomal system (ALS) were elevated. By contrast, the liver of subordinate fish exhibited increased rates of protein synthesis and activation of the ALS. When allowed to recover from chronic social stress for 4 days, differences in protein metabolism observed in white muscle of subordinate fish during the interaction period disappeared. In liver, protein synthesis returned to baseline levels during recovery from social stress, but markers of protein degradation did not. Collectively, these data support the hypothesis that inhibition of muscle protein synthesis coupled with increases in muscle protein breakdown contribute to the reduced growth rates of subordinate rainbow trout. [ABSTRACT FROM AUTHOR]

Young juvenile cuttlefish (Sepia officinalis) can grow at rates as high as 12% body weight per day. How the metabolic demands of such a massive growth rate impacts muscle performance that competes for ATP is unknown. Here, we integrate aspects of contractility, protein synthesis, and energy metabolism in mantle of specimens weighing 1.1 g to lend insight into the processes. Isolated mantle muscle preparations were electrically stimulated and isometric force development monitored. Preparations were forced to contract at 3 Hz for 30 s to simulate a jetting event. We then measured oxygen consumption, glucose uptake and protein synthesis in the hour following the stimulation. Protein synthesis was inhibited with cycloheximide and glycolysis was inhibited with iodoacetic acid in a subset of samples. Inhibition of protein synthesis impaired contractility and decreased oxygen consumption. An intact protein synthesis is required to maintain contractility possibly due to rapidly turning over proteins. At least, 41% of whole animal Ṁ O2 is used to support protein synthesis in mantle, while the cost of protein synthesis (50 μmol O2 mg protein–1) in mantle was in the range reported for other aquatic ectotherms. A single jetting challenge stimulated protein synthesis by approximately 25% (2.51–3.12% day–1) over a 1 h post contractile period, a similar response to that which occurs in mammalian skeletal muscle. Aerobic metabolism was not supported by extracellular glucose leading to the contention that at this life stage either glycogen or amino acids are catabolized. Regardless, an intact glycolysis is required to support contractile performance and protein synthesis in resting muscle. It is proposed that glycolysis is needed to maintain intracellular ionic gradients. Intracellular glucose at approximately 3 mmol L–1 was higher than the 1 mmol L–1 glucose in the bathing medium suggesting an active glucose transport mechanism. Octopine did not accumulate during a single physiologically relevant jetting challenge; however, octopine accumulation increased following a stress that is sufficient to lower Arg-P and increase free arginine. [ABSTRACT FROM AUTHOR]

The physiological causes of intraspecific differences in fitness components such as growth rate are currently a source of debate. It has been suggested that differences in energy metabolism may drive variation in growth, but it remains unclear whether covariation between growth rates and energy metabolism is: (i) a result of certain individuals acquiring and consequently allocating more resources to growth, and/or is (ii) determined by variation in the efficiency with which those resources are transformed into growth. Studies of individually housed animals under standardized nutritional conditions can help shed light on this debate. Here we quantify individual variation in metabolic efficiency in terms of the amount of adenosine triphosphate (ATP) generated per molecule of oxygen consumed by liver and muscle mitochondria and examine its effects, both on the rate of protein synthesis within these tissues and on the rate of whole-body growth of individually fed juvenile brown trout (Salmo trutta) receiving either a high or low food ration. As expected, fish on the high ration on average gained more in body mass and protein content than those maintained on the low ration. Yet, growth performance varied more than 10-fold among individuals on the same ration, resulting in some fish on low rations growing faster than others on the high ration. This variation in growth for a given ration was related to individual differences in mitochondrial properties: a high whole-body growth performance was associated with high mitochondrial efficiency of ATP production in the liver. Our results show for the first time, to our knowledge, that among-individual variation in the efficiency with which substrates are converted into ATP can help explain marked variation in growth performance, independent of food intake. This study highlights the existence of inter-individual differences in mitochondrial efficiency and its potential importance in explaining intraspecific variation in whole-animal performance. [ABSTRACT FROM AUTHOR]

Protein metabolism, including the interrelated processes of synthesis and degradation, mediates the growth of an animal. In ectothermic animals, protein metabolism is responsive to changes in both biotic and abiotic conditions. This study aimed to characterise responses of protein metabolism to food deprivation that occur in the coldwater salmonid, Arctic charr, Salvelinus alpinus. We compared two groups of Arctic charr: one fed continuously and the other deprived of food for 36 days. We measured the fractional rate of protein synthesis (KS) in individuals from the fed and fasted groups using a flooding dose technique modified for the use of deuterium-labelled phenylalanine. The enzyme activities of the three major protein degradation pathways (ubiquitin proteasome, lysosomal cathepsins and the calpain systems) were measured in the same fish. This study is the first to measure both KS and the enzymatic activity of protein degradation in the same fish, allowing us to examine the apparent contribution of different protein degradation pathways to protein turnover in various tissues (red and white muscle, liver, heart and gills). KS was lower in the white muscle and in liver of the fasted fish compared to the fed fish. There were no observable effects of food deprivation on the protease activities in any of the tissues with the exception of liver, where the ubiquitin proteasome pathway seemed to be activated during fasting conditions. Lysosomal proteolysis appears to be the primary degradation pathway for muscle protein, while the ubiquitin proteasome pathway seems to predominate in the liver. We speculate that Arctic charr regulate protein metabolism during food deprivation to conserve proteins. [ABSTRACT FROM AUTHOR]

The effects of temperature on protein metabolism have been studied mostly with respect to protein synthesis. Temperature generally has a parabolic effect on protein synthesis with a maximum rate being observed at optimal growth temperature. The effect of temperature on protein degradation is poorly understood. The 20S proteasome is mainly responsible for the degradation of short-lived and oxidatively modified proteins and has been recently identified as a potentially good proxy for protein degradation in fish. The aim of this experiment was to examine the relationships between the rate of;protein synthesis, activity of the 20S proteasome, oxidative stress markers and antioxidant capacity in white muscle of juvenile spotted wolffish (Anarhichas minol) acclimated at three temperatures (4, 8 and 12°C). The rate of protein synthesis was lower at 4°C than at 8°C while it was intermediate at 12°C. Despite the decrease of protein synthesis at low temperature, the activity of 20S proteasome activity was maintained high in fish acclimated at lower temperature (4°C), reaching levels 130% of that of fish acclimated at 8°C when measured at a common temperature. The oxidative stress markers TBARS and protein-carbonyl content did not change among temperature groups, but reduced glutathione concentration was higher in cold-acclimated fish, suggesting a higher antioxidant capacity in this group. Our data suggest that lower growth rate in cold temperature results from both high 20S proteasome activity and a reduced rate of protein synthesis. [ABSTRACT FROM AUTHOR]

This article studies whether digestive and metabolic enzyme activity levels predict growth rate and survival of newly hatched Atlantic wolffish. The existence of a strong link between performances and enzymatic activity early in the life of wolffish could provide a valuable indicator of juvenile quality at first feeding, enabling the prediction of growth rate and survival capacities and a better understanding of the physiological processes that determine the performance of juvenile Atlantic wolffish. The findings indicate that the use of enzymatic indicators to assess juvenile quality at hatching is worth further investigation.