Effect of reproduction on escape responses and muscle metabolic capacities in the scallop Chlamys islandica Müller 1776

In scallops, gametogenesis leads to mobilization of glycogen and proteins from the adductor muscle towards the gonad. This mobilization is likely to diminish the metabolic capacities of the adductor muscle and thereby the scallops' escape response. We examined the escape response in terms of number of valve claps until exhaustion, rate of clapping and the recovery during and after valve closure in adult scallops, Chlamys islandica, sampled at different stages in the reproductive cycle (immature, mature, before and after spawning). In parallel, we measured muscle glycogen, protein and phosphoarginine contents, the oxidative capacity of mitochondria isolated from the adductor muscle and levels of muscle enzymes which are active during exercise and recovery. The number of claps (24-26), rate of clapping ( approximately 13 clapsmin(-1)) and phosphoarginine and arginine kinase levels were similar during the different reproductive stages. All immature scallops responded to restimulation immediately after opening their valves, while only 62% of mature, 82% of prespawned and 38% of spawned scallops responded. Immature animals completely recovered their initial swimming capacity within 4 h of opening their valves, but mature, prespawned and spawned scallops needed 18, 12 and 18 h, respectively. Overall phasic adductor muscle from mature, prespawned and spawned animals showed decreased glycogen phosphorylase, phosphofructokinase, pyruvate kinase (except for prespawned), octopine dehydrogenase and citrate synthase levels, a deterioration of the oxidative capacity of mitochondria and a marked decrease in glycogen content compared to immature scallops. Therefore, during gonadal maturation and spawning, C. islandica did not change its clapping capacity, but slowed its recuperation from exhausting burst exercise, both during and after valve closure, likely due to the decreased metabolic capacity of the adductor muscle.     

Utilisation of glycogen,ATP and arginine phosphate in exercise and recovery in terrestrial red crabs,Gecarcoidea natalis

Intermittent locomotion by terrestrial crustaceans may under specific circumstances increase walking distance and may allow partial re-oxidization of anaerobic products, and replenishment of ATP and arginine phosphate. The Christmas Island red crab G. natalis undertakes a substantial breeding migration each year. The leg muscles of G. natalis subjected to bouts of 2.5 min walking and 2.5 min rest were severely anaerobic. Adenylate energy charge and the large arginine phosphate stores were greatly reduced. Walking for 4 min with pauses of only 1 min exacerbated the anaerobiosis and utilised 50% of the endogenous muscle glycogen. Post-exercise, the adenylate energy charge recovered before the arginine phosphate charge and a large and persistent hyperglycaemia accompanied the restoration of glycogen. Arginine phosphate functioned as a large, longer term, energy reservoir-almost as part of the adenylate pool. Gluconeogenesis is yet to be generally substantiated in decapod crustaceans but G. natalis appears to remove lactate slowly and to reincorporate exogenous glucose into muscle glycogen in the same time frame as lactate removal from the haemolymph. The 4:1 exercise/pause regimen facilitated access to energy stores and increased walking distance, and it allowed L-lactate and H(+) efflux from the muscle during pausing. These responses are similar to those of G. natalis in the field, except during the migration when walking was entirely aerobic. Determinations of adenylate, fuel and arginine phosphate reserves and usage during the migration are required together with more detailed behavioral analysis to resolve the dichotomy in metabolic response.     

The shift from glycogenolysis to glycogen resynthesis after escape swimming: studies on the abdominal muscle of the shrimp,Crangon crangon

Summary The mobilization of glycogen and phosphoarginine during work and their resynthesis during periods of recovery were investigated in abdominal muscles of the shrimpCrangon crangon. All parameters, metabolite levels as well as glycogen phosphorylase (EC 2.4.1.1) and synthase (EC 2.4.1.11) activities were determined in each individual shrimp investigated. At the onset of work both glycogen and phosphoarginine were degraded with the rate of phosphoarginine utilization being more than 80-fold faster than glycogen. After exhaustive work phosphoarginine stores were replenished within 30 min and seemed to exceed the resting level thereafter. In contrast, glycogen was not resynthesized immediately after work, but was further degraded during recovery leading to the accumulation of lactate. Only when the phosphagen level had reached the resting level did glycogenolysis shift to its resynthesis. The shift is characterized by: (1) a change in the mass action ratio of phosphoglucomutase from values below the equilibrium constant to values above the constant, (2) a dramatic decrease in the ratio fructose 1,6-bisphosphate/fructose 6-phosphate indicating phosphofructokinase inhibition, (3) an increase in the glucose concentration, and (4) an increase in the proportion of glycogen synthase I. The inactivation of glycogen phosphorylase by dephosphorylation during recovery was 2.4-fold. 36±8% (n=5) of total activity remained in the phosphorylated form. It is proposed that this part of the enzyme was inactivated by the drop in inorganic phosphate level due to the restoration of phosphoarginine.     

Locomotory,ventilatory and metabolic responses of the subterranean Stenasellus virei (Crustacea,Isopoda) to severe hypoxia and subsequent recovery

The locomotory and ventilatory activities and the intermediary and energy metabolism modifications of the hypogean aquatic isopod crustacean Stenasellus virei were investigated in severe hypoxia (Po₂ < 0.03 kPa) and subsequent recovery. The aims of this study were i) to determine why the subterranean species displayed a greater tolerance of hypoxia than numerous other epigean crustaceans, ii) to confirm previous results obtained with four hypogean and epigean crustaceans, iii) to compare the responses to severe hypoxia in hypogean amphipods and isopods, and iv) to better understand the ecological problems of the hypogean organisms survival in subterranean habitats. S. virei responded to experimental long-term, severe hypoxia with classical anaerobic metabolism mainly characterized by a decrease in adenosine triphosphate (ATP) and phosphagen, utilization of glycogen and glutamate, and accumulation of lactate and alanine. Lactate was also largely excreted by this organism, which is unusual for crustaceans in general. Compared to most other epigean crustaceans, the isopod S. virei showed high amounts of stored glycogen and arginine phosphate. These differences in glycogen and phosphagen stores, and the ability to reduce energetic expenditures linked to locomotion and ventilation, extended the survival of S. virei under experimental anaerobiosis. During recovery, the isopod S. virei showed a higher capacity for glyconeogenesis from lactate and a faster and total replenishment of ATP and arginine phosphate levels than epigean crustaceans. Data concerning responses to hypoxia and subsequent recovery in S. virei are similar to those previously obtained with two other hypogean amphipods, except that this isopod did not synthesize succinate in anaerobiosis.     

Tonic contractions allow metabolic recuperation of the adductor muscle during escape responses of giant scallop Placopecten magellanicus

To escape from starfish predators, giant scallops, Placopecten magellanicus, swim using series of strong phasic contractions interrupted by tonic contractions. To investigate whether these tonic contractions allow metabolic recuperation of the adductor muscle, we sampled scallops at rest (Control), after an initial series of phasic contractions (Phasic) and after 1 min of tonic contraction following their initial phasic contractions (Phasic + Tonic) and compared muscle levels of phosphoarginine, adenylate nucleotides (ATP, ADP and AMP) and adenylate energy charge (AEC). Scallops in the two active groups did not differ in the numbers of phasic contractions or the mean phasic force production. Phosphoarginine concentrations in the adductor muscle decreased with phasic activity and remained low after 1 min of tonic contraction. ATP and ADP and total adenylate levels did not differ between the three groups, but AMP levels were higher in the scallops sampled after phasic contractions than in control scallops. The AEC was reduced by phasic contractions but returned to control levels after 1 min of tonic contraction. A significant negative correlation between AEC and the number of claps in the Phasic group disappeared in the Phasic + Tonic group. Thus, tonic contractions following phasic contractions allow partial metabolic recovery of the adductor muscle by returning AEC to control levels. However, phosphoarginine levels did not recover during tonic contractions, and a negative correlation between the number of claps and phosphoarginine levels remained in the Phasic + Tonic group. By interspersing tonic contractions between series of phasic contractions, scallops improved muscle energetic status, which should help maintain phasic force production during the remainder of the escape response.     

Metabolic responses in two species of crayfish (Parastacus defossus and Parastacus brasiliensis) to post-hypoxia recovery

The period of post-hypoxia recovery is essential for the rapid replenishment of energy reserves and for the removal of metabolic end products formed during hypoxia. Periods of post-hypoxia recovery were analyzed in two crayfish species, where Parastacus defossus is a fossorial species, and Parastacus brasiliensis lives in lotic environments with higher oxygen levels. After 4 h of hypoxia (2 mg O(2)/L), groups of animals were placed in tanks with oxygenated water and were then removed at intervals of 1, 3, 6, and 9 h. Hemolymph and tissues (hepatopancreas, muscle, and anterior and posterior gills) were extracted for the determination of glucose, lactate, free glucose, glycogen, total proteins, total lipids, arginine phosphate, and arginine. As expected, lactate levels were restored more rapidly in P. defossus than in P. brasiliensis. P. defossus restored its glycogen reserves of the hepatopancreas and muscle tissue. Free glucose was quickly restored in all tissues of both species. In relation to arginine phosphate reserves, P. defossus showed a greater ability to restore this metabolite in the hepatopancreas. Both species recovered their arginine phosphate reserves, but they also used this metabolite in longer periods of recovery. Mainly in P. brasiliensis the reserves of total lipids seem to be an important source of energy during the recovery period. The animals developed various metabolic strategies to post-hypoxia recovery, mainly P. defossus which restored its reserves more completely and more rapidly than did P. brasiliensis.     

Sublethal effects of pentachlorophenol in the abalone (Haliotis rufescens) as measured by in vivo 31P NMR spectroscopy

The sublethal biochemical effects of pentachlorophenol (PCP) were investigated in live, intact red abalones (Haliotis rufescens), using a flow-through exposure system, by in vivo 31P NMR spectroscopy. Based on rangefinding tests (6-hr LC50 = 1.6 mg/L; 6-hr no-observable-effect-level (NOEL) = 0.8 mg/L), three abalones were separately exposed to a sublethal concentration (1.2 mg/L) for 5 hr, followed by a 13 hr recovery period. Effects in foot muscle included both a decrease in phosphoarginine and an increase in inorganic monophosphate concentrations ([PA] and [Pi], respectively); both foot muscle concentrations of adenosine triphosphate [ATP] and intracellular pH (pHi) also declined. Parallel in vitro experiments revealed that concentrations of glycerol 3-phosphate, lactate, citrate, succinate, malate, and alanine (Ala) all increased, while those of glyceraldehyde 3-phosphate and glutamine (Gln) remained stable. Also, these effects were not evident until 2 hr into exposure, possibly the time required for PCP to attain an effective concentration in foot muscle. During recovery, while Pi declined to pre-exposure levels, [PA] completely recovered in only one individual. Also, realkalinization of pHi was similar to recovery of [Pi], and ATP returned to near-initial levels, as did glycerol 3-phosphate, lactate, succinate, malate, and Ala; glyceraldehyde 3-phosphate, citrate, and Gln levels declined. Recovery responses corresponded to the time for PCP clearance from foot muscle. The effects of PCP were similar to those of hypoxia, fatigue, hypersalinity, and arginine kinase inhibitors, and so sublethal PCP concentrations may also inhibit electron transport and arginine kinase as well as uncouple mitochondrial oxidative phosphorylation in intact molluscs. Thus, the effects of pollutants on key biochemical processes may now be measured in intact aquatic organisms as they occur, improving our ability to accurately assess the environmental effects of pollutants in the laboratory.     

The function of mitochondrial F(O)F(1) ATP-synthase from the whiteleg shrimp Litopenaeus vannamei muscle during hypoxia

The effect of hypoxia and re-oxygenation on the mitochondrial complex F(O)F(1)-ATP synthase was investigated in the whiteleg shrimp Litopenaeus vannamei. A 660 kDa protein complex isolated from mitochondria of the shrimp muscle was identified as the ATP synthase complex. After 10h at hypoxia (1.5-2.0 mg oxygen/L), the concentration of L-lactate in plasma increased significantly, but the ATP amount and the concentration of ATPβ protein remained unaffected. Nevertheless, an increase of 70% in the ATPase activity was detected, suggesting that the enzyme may be regulated at a post-translational level. Thus, during hypoxia shrimp are able to maintain ATP amounts probably by using some other energy sources as phosphoarginine when an acute lack of energy occurs. During re-oxygenation, the ATPase activity decreased significantly and the ATP production continued via the electron transport chain and oxidative phosphorylation. The results obtained showed that shrimp faces hypoxia partially by hydrolyzing the ATP through the reaction catalyzed by the mitochondrial ATPase which increases its activity.     

Hexokinase 2, glycogen synthase and phosphorylase play a key role in muscle glycogen supercompensation

Background

Glycogen-depleting exercise can lead to supercompensation of muscle glycogen stores, but the biochemical mechanisms of this phenomenon are still not completely understood.

Methods

Using chronic low-frequency stimulation (CLFS) as an exercise model, the tibialis anterior muscle of rabbits was stimulated for either 1 or 24 hours, inducing a reduction in glycogen of 90% and 50% respectively. Glycogen recovery was subsequently monitored during 24 hours of rest.

Results

In muscles stimulated for 1 hour, glycogen recovered basal levels during the rest period. However, in those stimulated for 24 hours, glycogen was supercompensated and its levels remained 50% higher than basal levels after 6 hours of rest, although the newly synthesized glycogen had fewer branches. This increase in glycogen correlated with an increase in hexokinase-2 expression and activity, a reduction in the glycogen phosphorylase activity ratio and an increase in the glycogen synthase activity ratio, due to dephosphorylation of site 3a, even in the presence of elevated glycogen stores. During supercompensation there was also an increase in 5′-AMP-activated protein kinase phosphorylation, correlating with a stable reduction in ATP and total purine nucleotide levels.

Conclusions

Glycogen supercompensation requires a coordinated chain of events at two levels in the context of decreased cell energy balance: First, an increase in the glucose phosphorylation capacity of the muscle and secondly, control of the enzymes directly involved in the synthesis and degradation of the glycogen molecule. However, supercompensated glycogen has fewer branches.     

Metabolic modulation induced by chronic hypoxia in rats using a comparative proteomic analysis of skeletal muscle tissue

Hypoxia-induced changes of rat skeletal muscle were investigated by two-dimensional difference in-gel electrophoresis (2D-DIGE) and mass spectrometry. The results indicated that proteins involved in the TCA cycle, ATP production, and electron transport are down-regulated, whereas glycolytic enzymes and deaminases involved in ATP and AMP production were up-regulated. Up-regulation of the hypoxia markers hypoxia inducible factor 1 (HIF-1alpha) and pyruvate dehydrogenase kinase 1 (PDK1) was also observed, suggesting that in vivo adaptation to hypoxia requires an active metabolic switch. The kinase protein, mammalian target of rapamycin (mTOR), which has been implicated in the regulation of protein synthesis in hypoxia, appears unchanged, suggesting that its activity, in this system, is not controlled by oxygen partial pressure.     

Glycogen repletion following burst activity: a carbohydrate-sparing mechanism in animals adapted to arid environments?

The Western chestnut mouse (Pseudomys nanus ferculinus) is one of several native rodent species adapted to the arid environments of Australia. Since these environments are often associated with a paucity in dietary carbohydrate, the problem arises as to the mechanism whereby these rodents replete their stores of muscle glycogen when recovering from high intensity physical activity. This is an important issue since the maintenance of adequate stores of muscle glycogen is crucial to support the energy demands associated with 'flight or fight' responses. Whilst it is known that food ingestion post-exercise is required for the total repletion of muscle glycogen in rats and humans, our findings indicate that the Western chestnut mouse has the impressive capacity to replete completely its stores of muscle glycogen, even in the absence of food intake. Indeed during recovery from burst activity which results in the massive breakdown of the stores of muscle glycogen, the levels of glycogen return back to pre-exercise levels within only 50 minutes despite the absence of food intake. This capacity is important in the broader context of nutritional adaptation to arid/seasonally-arid regions since it allows muscles to replete their fuel stores even when food is not available. How common is this strategy among desert-adapted mammal species is a question yet to be answered.     

Metabolic recovery in goldfish: A comparison of recovery from severe hypoxia exposure and exhaustive exercise

Severe hypoxia exposure and exhaustive exercise in goldfish both elicit a strong activation of substrate-level phosphorylation with the majority of the metabolic perturbations occurring in the white muscle. Approximately half of the muscle glycogen breakdown observed during severe hypoxia exposure was accounted for by ethanol production and loss to the environment, which limited the extent of muscle glycogen recovery when animals were returned to normoxic conditions. Ethanol production in goldfish is not solely a response to anoxia/hypoxia exposure however, as a transient increase in ethanol production was observed during the early stages of recovery from exhaustive exercise. These data suggest that ethanol production is a ubiquitous "anaerobic" end product, which accumulates whenever metabolic demands exceed mitochondrial oxidative potential. Exhaustive exercise and hypoxia exposure both caused a 7 to 8 micromol g(-1) wet mass increase in muscle [lactate] and the rates of recovery following these perturbations were similar. The rates of muscle PCr and pHi recovery after hypoxia exposure and exhaustive exercise were similar with levels returning to controls values within 0.5 h. Surprisingly, liver [glycogen] was not depleted during exposure to severe hypoxia, however, during recovery from both hypoxia and exercise dramatically different responses in liver [glycogen] were noted. During the early stages of recovery, liver [glycogen] transiently increased to high levels after exhaustive exercise, while during recovery from hypoxia there was a transient decrease in liver glycogen over the same time frame. Overall, this points to the liver playing a dramatically different role in facilitating recovery from exercise compared with hypoxia exposure.     

Functional expression of phosphagen kinase systems confers resistance to transient stresses in Saccharomyces cerevisiae by buffering the ATP pool

Phosphagen kinase systems provide different advantages to tissues with high and fluctuating energy demands, in particular an efficient energy buffering system. In this study we show for the first time functional expression of two phosphagen kinase systems in Saccharomyces cerevisiae, which does not normally contain such systems. First, to establish the creatine kinase system, in addition to overexpressing creatine kinase isoenzymes, we had to install the biosynthesis pathway of creatine by co-overexpression of L-arginine:glycine amidinotransferase and guanidinoacetate methyltransferase. Although we could achieve considerable creatine kinase activity, together with more than 3 mM intracellular creatine, this was not sufficient to confer an obvious advantage to the yeast under the specific stress conditions examined here. Second, using arginine kinase, we successfully installed an intracellular phosphagen pool of about 5 mM phosphoarginine. Such arginine kinase-expressing yeast showed improved resistance under two stress challenges that drain cellular energy, which were transient pH reduction and starvation. Although transient starvation led to 50% reduced intracellular ATP concentrations in wild-type yeast, arginine kinase overexpression stabilized the ATP pool at the pre-stress level. Thus, our results demonstrate that temporal energy buffering is an intrinsic property of phosphagen kinases that can be transferred to phylogenetically very distant organisms.     

Effects of exercise in normoxia and acute hypoxia on respiratory muscle metabolites

We determined changes in rat plantaris, diaphragm, and intercostal muscle metabolites following exercise of various intensities and durations, in normoxia and hypoxia (FIO2 = 0.12). Marked alveolar hyperventilation occurred during all exercise conditions, suggesting that respiratory muscle motor activity was high. [ATP] was maintained at rest levels in all muscles during all normoxic and hypoxic exercise bouts, but at the expense of creatine phosphate (CP) in plantaris muscle and diaphragm muscle following brief exercise at maximum O2 uptake (VO2max) in normoxia. In normoxic exercise plantaris [glycogen] fell as exercise exceeded 60% VO2max, and was reduced to less than 50% control during exhaustive endurance exercise (68% VO2max for 54 min and 84% for 38 min). Respiratory muscle [glycogen] was unchanged at VO2max as well as during either type of endurance exercise. Glucose 6-phosphate (G6P) rose consistently during heavy exercise in diaphragm but not in plantaris. With all types of exercise greater than 84% VO2max, lactate concentration ([LA]) in all three muscles rose to the same extent as arterial [LA], except at VO2max, where respiratory muscle [LA] rose to less than half that in arterial blood or plantaris. Exhaustive exercise in hypoxia caused marked hyperventilation and reduced arterial O2 content; glycogen fell in plantaris (20% of control) and in diaphragm (58%) and intercostals (44%). We conclude that respiratory muscle glycogen stores are spared during exhaustive exercise in the face of substantial glycogen utilization in plantaris, even under conditions of extreme hyperventilation and reduced O2 transport. This sparing effect is due primarily to G6P inhibition of glycogen phosphorylase in diaphragm muscle. The presence of elevated [LA] in the absence of glycogen utilization suggests that increased lactate uptake, rather than lactate production, occurred in the respiratory muscles during exhaustive exercise.     

The influence of environmental temperature and oxygen concentration on the recovery of largemouth bass from exercise: implications for live–release angling tournaments

The impact of variation in water temperature and dissolved oxygen on recovery of largemouth bass Micropterus salmoides from exercise was examined. For this, largemouth bass were first exercised and recovered for either 1, 2 or 4 h at ambient water temperatures (25° C) in fully oxygenated water. Results showed that exercise forced fish to utilize anaerobic metabolism to meet energy demands, and resulted in reductions in anaerobic energy stores adenosine triphosphate (ATP), Phosphocreatine (PCr) and glycogen. Exercise also resulted in a seven‐fold increase in lactate within white muscle. After 2 h of recovery in oxygenated water at acclimation temperature, physiological recovery from exercise was under way, and by 4 h most variables examined had returned to control levels. Next, largemouth bass were exercised at ambient temperatures and recovered for 2 h in environments with either elevated temperature (32° C), reduced temperature (14 and 20° C), hypoxia or hyperoxia. Both elevated and reduced temperature impaired recovery of tissue lactate and tissue ATP relative to fish recovered in water at acclimation temperature, while hyperoxic water impaired recovery of tissue ATP. Moderately hypoxic waters impaired the recovery of plasma glucose, plasma lactate and tissue PCr relative to fish recovered in fully oxygenated water. Results from this study are discussed in the context of critical oxygen and temperature guidelines for largemouth bass. In addition, several recommendations are made concerning remedial treatments used in livewells (tanks) during angling tournaments when fish are recovering from exercise associated with angling.     

Estimation of effective concentrations of ATP-regenerating enzymes in cilia of Paramecium caudatum

The phosphoarginine shuttle system effectively regenerates ATP in the cilia of Paramecium caudatum. To estimate the effective concentration of ATP‐regenerating enzymes, we attempted to reconstitute certain ATP‐regenerating systems within the cilia of intact cortical sheets using exogenous enzymes and high‐energy substances. The addition of phosphoenolpyruvate, which is one of the substrates in glycolysis, did not increase the ciliary beat frequency, whereas phosphocreatine together with exogenous creatine kinase, effectively increased the ciliary beat frequency. In the presence of 0.6 mg/ml creatine kinase and 0.4 mM phosphocreatine, the ciliary beat frequency was comparable to that produced by the addition of phosphoarginine. This result indicates that the reconstituted phosphocreatine shuttle system can work as an artificial ATP‐regenerating system for ciliary movements. The effective concentration of creatine kinase in the reconstituted phosphocreatine shuttle system was estimated to be about 7.4 μM based on the molecular mass of creatine kinase (MW 81,000). Therefore, the effective concentration of arginine kinase in the cilia of live Paramecium is approximately 10 μM. This estimated concentration of intraciliary arginine kinase is sufficient to maintain a high ATP concentration throughout the cilia of P. caudatum.     

Sublethal effects of pentachlorophenol in the abalone (haliotis rufescens) as measured by in vivo 31P NMRSpectroscopy

The sublethal biochemical effects of pentachlorophenol (PCP) were investigated in live, intact red abalones (Haliotis rufescens), using a flow-through exposure system, by in vivo ³¹P NMR spectroscopy. Based on rangefinding tests (6-hr LC₅₀ = 1.6 mg/L; 6-hr no-observable-effect-level (NOEL) = 0.8 mg/L), three abalones were separately exposed to a sublethal concentration (1.2 mg/L) for 5 hr, followed by a 13 hr recovery period. Effects in foot muscle included both a decrease in phosphoarginine and an increase in inorganic monophosphate concentrations ([PA] and [P_i], respectively); both foot muscle concentrations of adenosine triphosphate [ATP] and intracellular pH (pH_i) also declined. Parallel in vitro experiments revealed that concentrations of glycerol 3-phosphate, lactate, citrate, succinate, malate, and alanine (Ala) all increased, while those of glyceraldehyde 3-phosphate and glutamine (Gln) remained stable. Also, these effects were not evident until 2 hr into exposure, possibly the time required for PCP to attain an effective concentration in foot muscle. During recovery, while P_i declined to pre-exposure levels, [PA] completely recovered in only one individual. Also, realkalinization of pH_i was similar to recovery of [P_i], and ATP returned to near-initial levels, as did glycerol 3-phosphate, lactate, succinate, malate, and Ala; glyceraldehyde 3-phosphate, citrate, and Gln levels declined. Recovery responses corresponded to the time for PCP clearance from foot muscle. The effects of PCP were similar to those of hypoxia, fatigue, hypersalinity, and arginine kinase inhibitors, and so sublethal PCP concentrations may also inhibit electron transport and arginine kinase as well as uncouple mitochondrial oxidative phosphorylation in intact molluscs. Thus, the effects of pollutants on key biochemical processes may now be measured in intact aquatic organisms as they occur, improving our ability to accurately assess the environmental effects of pollutants in the laboratory.     

Metabolic adjustments in the common carp during prolonged hypoxia

Biochemical and respiratory changes in the common carp Cyprinus carpio , were studied 6, 24, 96 and 168 h upon exposure to hypoxia (0·5 mgO₂ l⁻¹). Modification of kinetic properties of phosphofructokinase (PFK-1), coupled with a decreased in PFK-1 activities, were evident in muscle. No changes in kinetics and activities could be observed in muscle pyruvate kinase (PK) and lactate dehydrogenase (LDH). A decrease in muscle citrate synthase (CS) and an increase in muscle cytochrome c oxidase (CCO) were found. The common carp was able to maintain a constant level of muscle glycogen, muscle ATP, and liver CS throughout the 168-h experimental period. Changes in activities of liver LDH and muscle CCO were observed only at 168 h, which indicates that common carp may switch to alternative metabolic pathway to deal with prolonged hypoxia. A severe decrease in liver glycogen was accompanied by increases in lactate levels in both the muscle and liver. Oxygen consumption rate was reduced under hypoxia, but resumed to normoxic levels within 2 h upon return to normoxic condition. Overall, these results indicate that carp adopt different strategies in an attempt to deal with short term and long term hypoxia in the natural environment.     

S6 kinase deletion suppresses muscle growth adaptations to nutrient availability by activating AMP kinase

S6 kinase (S6K) deletion in metazoans causes small cell size, insulin hypersensitivity, and metabolic adaptations; however, the underlying molecular mechanisms are unclear. Here we show that S6K-deficient skeletal muscle cells have increased AMP and inorganic phosphate levels relative to ATP and phosphocreatine, causing AMP-activated protein kinase (AMPK) upregulation. Energy stress and muscle cell atrophy are specifically triggered by the S6K1 deletion, independent of S6K2 activity. Two known AMPK-dependent functions, mitochondrial biogenesis and fatty acid β-oxidation, are upregulated in S6K-deficient muscle cells, leading to a sharp depletion of lipid content, while glycogen stores are spared. Strikingly, AMPK inhibition in S6K-deficient cells restores cell growth and sensitivity to nutrient signals. These data indicate that S6K1 controls the energy state of the cell and the AMPK-dependent metabolic program, providing a mechanism for cell mass accumulation under high-calorie diet.