Regulation Of Oxygen Consumption Research Articles

Nitric oxide synthase system in the brain development of neonatal hypothyroid rats

Thyroid hormones play an important morphogenetic role during the fetal and neonatal periods and regulate numerous metabolic processes. In the central nervous system, they control myelination and overall brain development, regional gene expression, and regulation of oxygen consumption. Their deficiency in the fetal and neonatal periods causes severe mental retardation, due to lack of thyroid function, or to iodine deficiency. At the same time, nitric oxide is an atypical neurotransmitter that also has special relevance in neuronal development and plasticity and functions as a vasodilator, regulating cerebral blood flow. Although under physiological conditions it functions as a neuroprotector, in excess it can be neurotoxic. We have studied, by immunocytochemical and Western blot techniques, the evolution of the expression of neuronal and inducible isoforms of the enzyme nitric oxide synthase, and of nitrotyrosine as a marker of protein nitration produced by the presence of nitric oxide, during the early stages of postnatal brain development. We induced hypothyroidism by administering mercaptomethylimidazole to pregnant mothers, from the seventh day of gestation until the sacrifice of the offspring. The results show a delay in the evolution of the expression of the two isoforms of the enzyme nitric oxide synthase in hypothyroid animals, followed by an anomalous overexpression in later stages. Finally, the expression of nitrotyrosine follows an evolution that is synchronized with that shown by both isoenzymes in control and hypothyroid animals.

APPL1 Is a Prognostic Biomarker and Correlated with Treg Cell Infiltration via Oxygen-Consuming Metabolism in Renal Clear Cell Carcinoma.

Kidney renal clear cell carcinoma (KIRC) is one of the most hazardous tumors in the urinary system. The regulation of oxygen consumption in renal clear cell carcinoma is a consequence of adaptive reprogramming of oxidative metabolism in tumor cells. APPL1 is a signaling adaptor involved in cell survival, oxidative stress, inflammation, and energy metabolism. However, the correlation of APPL1 with regulatory T cell (Treg) infiltration and prognostic value in KIRC remain unclear. In this study, we comprehensively predicted the potential function and prognostic value of APPL1 in KIRC. For KIRC patients, relatively low expression of APPL1 was associated with high degree of metastasis, pathological stage, and shorter overall time or poor prognosis. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses suggested that low expression of APPL1 may be adapted to the malignant progression of tumors via affecting oxygen-consuming metabolism. In addition, the expression level of APPL1 was negatively correlated with Treg cell infiltration and chemotherapy sensitivity, which indicated that APPL1 may regulate the tumor immune infiltration and chemotherapy resistance by decrease oxygen-consuming metabolic process in KIRC. Therefore, APPL1 may become one of the important prognostic factors, and it may serve as a candidate prognostic biomarker in KIRC.

Identification of the Large-Conductance Ca2+-Regulated Potassium Channel in Mitochondria of Human Bronchial Epithelial Cells

Mitochondria play a key role in energy metabolism within the cell. Potassium channels such as ATP-sensitive, voltage-gated or large-conductance Ca2+-regulated channels have been described in the inner mitochondrial membrane. Several hypotheses have been proposed to describe the important roles of mitochondrial potassium channels in cell survival and death pathways. In the current study, we identified two populations of mitochondrial large-conductance Ca2+-regulated potassium (mitoBKCa) channels in human bronchial epithelial (HBE) cells. The biophysical properties of the channels were characterized using the patch-clamp technique. We observed the activity of the channel with a mean conductance close to 285 pS in symmetric 150/150 mM KCl solution. Channel activity was increased upon application of the potassium channel opener NS11021 in the micromolar concentration range. The channel activity was completely inhibited by 1 µM paxilline and 300 nM iberiotoxin, selective inhibitors of the BKCa channels. Based on calcium and iberiotoxin modulation, we suggest that the C-terminus of the protein is localized to the mitochondrial matrix. Additionally, using RT-PCR, we confirmed the presence of α pore-forming (Slo1) and auxiliary β3-β4 subunits of BKCa channel in HBE cells. Western blot analysis of cellular fractions confirmed the mitochondrial localization of α pore-forming and predominately β3 subunits. Additionally, the regulation of oxygen consumption and membrane potential of human bronchial epithelial mitochondria in the presence of the potassium channel opener NS11021 and inhibitor paxilline were also studied. In summary, for the first time, the electrophysiological and functional properties of the mitoBKCa channel in a bronchial epithelial cell line were described.

Exhaustive exercise and vitamins C and E modulate thyroid hormone levels at low and high altitudes.

Thyroid hormones play an important role in cell growth and differentiation and regulation of oxygen consumption and thermogenesis. The effect of altitude and vitamin supplementation on thyroid hormone levels in animals or humans performing acute exhaustive exercise have not been investigated before. Therefore, we thought to test whether exhaustive exercise-induced stress with antioxidant supplementation was capable of modulating the level of thyroid hormones at different altitudes. Serum levels of T4 (Thyroxin), T3 (Triiodothyronine), and TSH (Thyroid Stimulating Hormone) were measured in rats (N=36) born and bred in low altitude (600 m above sea level) and high altitude (2200 m above sea level) following forced swimming with or without vitamins C and E (25 mg/kg) pre-treatments. Thyroid levels were significantly decreased in resting rats at high altitude compared to low altitude, and swimming exercise moderately increased T3 and TSH at both high and low altitudes, whereas T4 was markedly increased (62 %) at low altitude compared to a moderate high altitude increase (28 %). Co-administration of vitamins C and E augmented the observed forced swimming-induced thyroid release. However, the conversion of T4 to T3 was reduced in both altitude areas following swimming exercise and vitamin pre-treatment had no effect. We conclude that acute stress induced thyroidal hormones in rats, which was augmented by antioxidant drugs in both high and low altitude areas. These findings may play an important role in the human pathophysiology of thyroid gland at different altitudes.

Endogenous angiotensin II and the regulation of oxygen consumption and colonic temperature in rats

Previous studies have suggested that angiotensin II, a hormone known to regulate water and salt balance and blood pressure, may also regulate oxygen consumption and body temperature. In this study we investigated the role of endogenous angiotensin in the regulation of oxygen consumption and colonic temperature in rats under low (control) and high (water deprivation, administration of isoproterenol and hemorrhage) peripheral angiotensin conditions. Peripheral administration of losartan, an AT 1 receptor antagonist or enalapril, an angiotensin converting enzyme inhibitor, did not alter oxygen consumption or colonic temperature in control or water deprived rats. Peripheral administration of losartan did not alter the oxygen consumption and colonic temperature responses to the administration of isoproterenol or hemorrhage. Endogenous peripherally generated angiotensin II does not play an important role in regulating either oxygen consumption or colonic temperature in rats under either low or high angiotensin II levels. The reductions in oxygen consumption and colonic temperature that accompany hemorrhage in rats are not mediated by angiotensin II.

Control and Regulation of Mitochondrial Energetics in an Integrated Model of Cardiomyocyte Function

Understanding the regulation and control of complex networks of reactions requires analytical tools that take into account the interactions between individual network components controlling global network function. Here, we apply a generalized matrix method of control analysis to calculate flux and concentration control coefficients, as well as response coefficients, in an integrated model of excitation-contraction (EC) coupling and mitochondrial energetics (ECME model) in the cardiac ventricular myocyte. Control and regulation of oxygen consumption ( V O2) was first assessed in a mitochondrion model, and then in the integrated cardiac myocyte model under resting and working conditions. The results demonstrate that in the ECME model, control of respiration is distributed among cytoplasmic ATPases and mitochondrial processes. The magnitude of control by cytoplasmic ATPases increases under working conditions. The model prediction that the respiratory chain exerts strong positive control on V O2 (control coefficient 0.89) was corroborated experimentally in cardiac trabeculae utilizing the inhibitor titration method. In the model, mitochondrial respiration displayed the highest response coefficients with respect to the concentration of cytoplasmic ATP. This was due to the high elasticity of ANT flux toward ATP in the cytoplasm. The analysis reveals the complex interdependence of sarcolemmal, cytoplasmic, and mitochondrial processes that contribute to the control of energy supply and demand in the heart. Moreover, by visualizing the structure of control of the metabolic network of the myocyte, we provide support for the emerging concept of control by diffuse loops, in which action on the network (e.g., by a pharmacological agent) may bring about changes in processes without obvious direct mechanistic links between them.

Intracellular regulation of oxygen consumption in the isolated crayfish stretch receptor neuron

Morphological correlations of the functional regulation of oxygen consumption have been studied on single isolated crayfish mechanoreceptor neurons. An enhancement of oxygen consumption is promoted by the following: (1) redistribution of mitochondria and an increase in cytochrome oxidase (CO) activity in mitochondria near the plasmatic membrane, (2) coordination of mitochondria aggregation rhythms with pO2 rhythms in the medium external for a cell, (3) a decrease in the area of high CO activity and mitochondria and a shortening of the oxygen diffusion pathway, (4) an increase of the CO activity gradient from the neuron body periphery to its center, (5) a transfer of oxygen with the water flow during neuron body hydration and cytoplasm dilution during the transfer of a portion of the gel into sol, (6) cyclic changes in the ratio of the neuron body and hillock sizes at which there is a transfer of oxygen with the water flow into the neuron body, its mitochondrial uptake in the neuron body, and transfer of the oxygen-free water from the neuron body into the axonal hillock and further into the external medium.

Myocardial oxygen consumption regulation in isolated mouse heart: assessment by intracoronary administration of exogenous nitric oxide

In our previous work we have shown that in mouse heart basal level of endothelial produced nitrite, as a marker of nitric oxide (NO) formation, was 9.7 nmol l(-1). Bradykinin (10 microl l(-1)) induced a 5-fold rise in nitrite release, the coronary venous effluent concentration being 58 nmol l(-1), but there was no effect on myocardial oxygen consumption (MVO2). The aim of this study was to assess the levels of authentic nitric oxide solution, exogenously applied, on myocardial oxygen consumption. Isolated mouse hearts (n=36) were paced (500 imp./min) and perfused at constant flow (16.0 +/- 0.3 ml g(-1) min(-1)). When coronary vasculature resistance was carefully controlled by adenosine (1 micromol l(-1)), authentic nitric oxide solution, in a concentration less than 5 micromol l(-1) did not alter myocardial oxygen consumption. Only concentrations of nitric oxide higher than 5 micromol l(-1) induced reduction in myocardial oxygen consumption. Thus in the saline perfused mouse heart, with carefully controlled vasodilatation, modulating myocardial nitric oxide levels using an arterial application of authentic nitric oxide, concentrations higher than 5 micromol l(-1) of nitric oxide were required to induce a decrease in myocardial oxygen consumption.

HIF-1 mediates adaptation to hypoxia by actively downregulating mitochondrial oxygen consumption

The HIF-1 transcription factor drives hypoxic gene expression changes that are thought to be adaptive for cells exposed to a reduced-oxygen environment. For example, HIF-1 induces the expression of glycolytic genes. It is presumed that increased glycolysis is necessary to produce energy when low oxygen will not support oxidative phosphorylation at the mitochondria. However, we find that while HIF-1 stimulates glycolysis, it also actively represses mitochondrial function and oxygen consumption by inducing pyruvate dehydrogenase kinase 1 (PDK1). PDK1 phosphorylates and inhibits pyruvate dehydrogenase from using pyruvate to fuel the mitochondrial TCA cycle. This causes a drop in mitochondrial oxygen consumption and results in a relative increase in intracellular oxygen tension. We show by genetic means that HIF-1-dependent block to oxygen utilization results in increased oxygen availability, decreased cell death when total oxygen is limiting, and reduced cell death in response to the hypoxic cytotoxin tirapazamine.

Intraretinal Oxygen Distribution in the Monkey Retina and the Response to Systemic Hyperoxia

To measure the intraretinal oxygen distribution and consumption in the fovea, the parafovea, and the inferior retina in the monkey eye and determine the influence of graded systemic hyperoxia. Oxygen sensitive microelectrodes were used to measure the oxygen tension as a function of depth through the retina in anesthetized monkeys (n = 8) under normoxic and hyperoxic conditions. Oxygen consumption rates in the avascular regions of retina were determined by fitting the oxygen profiles to established oxygen consumption models. Under normoxic conditions, in the foveal area, the intraretinal oxygen distribution reflected the absence of retinal capillaries and the predominantly choroidal origin of retinal oxygenation. A similar shape of oxygen distribution was seen in the parafoveal retina with the addition of local perturbations in the inner retina attributed to the presence of retinal capillaries. In the inferior retina the same general shape was found. Oxygen consumption in the outer retina was higher in the parafoveal region, and the minimum oxygen tension was lower. During hyperoxia, choroidal oxygen levels in all areas increased dramatically, but the increase in oxygen tension in the inner retina was much less. The avascular nature of the foveal area allowed oxygen consumption analysis of both the inner and outer retina and showed that inner retinal oxygen consumption increased significantly during hyperoxic ventilation to a level equivalent to that of the outer retina. In the outer retina of the monkey the P(O2) minimum is lower, and the oxygen consumption rate is higher in the parafoveal region. During systemic hyperoxia, outer retinal oxygen consumption is unaffected, but in the foveal area, total oxygen consumption increases. This regulation of oxygen consumption in the monkey retina is comparable to that reported in lower mammals and may represent an important mechanism in retinal homeostasis.

Activity-induced tissue oxygenation changes in rat cerebellar cortex: Interplay of postsynaptic activity and cerebral blood flow

Neuronal activity, cerebral blood flow, and oxygen consumption are all strongly coupled, although the mechanisms behind remain unclear. We examined this coupling in rat cerebellar cortex by simultaneously recording local field potentials, tissue oxygen partial pressure (tpO2), and CBF using laser-Doppler flowmetry. Stimulation of the monosynaptic, excitatory climbing fiber system evoked activity-dependent increases in synaptic activity and blood flow, and biphasic tissue pO2 responses. Relating parameters of the initial negative tpO2 response to changes in synaptic activity revealed that the disappearance rate of oxygen in the tissue varied as a linear function of synaptic activity, while the area of the negative tpO2 response saturated at high levels of synaptic activity. Following the initial oxygen dip, tpO2 increased as a non-linear function of synaptic activity, paralleling the steep rise in blood flow at high levels of synaptic activity. Attenuation of the activity-dependent rise in blood flow by nNOS inhibition enlarged the negative tpO2 response and decreased the positive tpO2 response. These findings imply that the activity-induced reduction in tpO2 is counteracted by an increase in oxygen supply due to the rise in CBF. They also support the hypothesis that the positive tpO2 response is an overshoot caused by the evoked increase in CBF. Blockade of glutamatergic AMPA receptors with CNQX abolished synaptic activity, and CBF and tpO2 responses. We suggest, that in the cerebellum activity-dependent increaes in oxygen consumption can be recorded as the disapperance rate of oxygen in the tissue, and that oxygen consumption increases as a linear function of activity in postsynaptic cellular elements without evidence for a threshold. Our results provide new fundamental insights into the regulation of oxygen consumption during activation, showing direct coupling of activity in postsynaptic AMPA receptors to oxygen consumption.

Regulation of oxygen consumption by vasomotion

Objective: To describe a stochastic model of the variability and heterogeneity of blood flow through the microcirculation, and to show the ability of vasomotion to vary oxygen consumption at a steady blood flow. Methods: The description of vasomotion is based on whether each microvessel is open for blood flow or closed. Over a unit time period, let α be the probability that a given vessel is open and will remain open, β be the probability that an open vessel will close, ν be the probability that a closed vessel will remain closed, and μ be the probability that a closed vessel will become open. Two main parameters that characterize such a scheme are: the fraction of open microvessels [ n o = μ/( β + μ)], and the rate of vasomotion defined as the rate of switching between open and closed microvessels ( R = β + μ). A model of O 2 transport to tissues is based on the following assumptions: (a) the flux of O 2 is due to passive diffusion, (b) the amount of O 2 dissolved in tissue is negligible as compared with that contained in arterial blood, and (c) aerobic metabolism is proportional to the delivery of O 2. Results: The stochastic model substantiates the possibility that vasomotion can control O 2 consumption. The rate of vasomotion activity can change O 2 consumption by 2–8-fold, depending on the fraction of open microvessels. Conclusion: A stochastic description of blood flow through the microcirculation system demonstrates that vasomotion rate could be a factor influencing O 2 consumption.

Modeling the regulation of oxygen consumption by nitric oxide.

Nitric oxide (NO) is not only a potent gaseous vasodilator regulating blood flow, but also a reversible inhibitor of mitochondrial 02 consumption. Mathematical models for NO biotransport were recently reviewed by Buerk, who pointed out that previous models for 02 transport to tissue must be revised to include multiple effects of NO. To address this issue, we developed and solved coupled mass transport models including O2-dependent properties of NO synthases, Michaelis-Menten kinetics, and known inhibitory effects of NO on mitochondrial 02 consumption, recently reviewed by Brown.

Regulation of oxygen consumption at exercise onset: is it really controversial?

The conflicting hypotheses on the limiting factors for skeletal muscle VO2 on-kinetics might be reconciled in a unifying scenario. Under "normal" conditions, during transitions to moderate intensity exercise, the limiting factor appears to be an inertia of oxidative metabolism. During transitions to exercise of higher metabolic intensity, O2 delivery could play a relatively minor but significant role as a limiting factor.

Regulation of Oxygen Consumption at the Onset of Exercise

Increased aerobic production of ATP at the onset of exercise could be limited by availability of metabolic substrates independent of O2, or interaction between O2 and metabolic substrates. We point out the importance of feedback control to match O2 supply to demand and discuss metabolic control at the onset of exercise.

Life history of Gonatus onyx (Cephalopoda: Teuthoidea): ontogenetic changes in habitat, behavior and physiology

In the present study 670 individuals of Gonatus onyx (Cephalopoda: Teuthoidea) were observed in Monterey Bay, California from a remotely operated vehicle. The vertical distribution of this species was bimodal, with peaks at 400 and 800 m depth during the day and 300 and 500 m during the night. The bimodal distribution reflects a life stage shift between younger, schooling juveniles living in shallower water and older, solitary adults which live deeper. Ontogenetic changes in behavior associated with this life stage shift are reflected in the physiology of the organisms as well. Both aerobic and anaerobic metabolism, as estimated using mitochondrial and glycolytic enzymes, decline with increasing body mass, suggesting reduced locomotory capacity in deeper-living adults. Oxygen consumption rates were also determined in relation to oxygen partial pressure. Oxygen consumption regulation was similar between juvenile and adult squids. The critical oxygen partial pressures (29 to 30 mmHg) correspond precisely to the oxygen concentrations found at the depth of maximal abundance for day and night populations of juveniles and adults, respectively. Behavioral and physiological changes with ontogeny of G. onyx are believed to result from reduced visual predator/prey interactions in the light-limited deep sea.

Metabolism of benthic octopods (Cephalopoda) as a function of habitat depth and oxygen concentration

The oxygen consumption rates and activities of key metabolic enzymes were measured and analyzed as a function of habitat depth for several species of benthic octopod (Cephalopoda: Octopoda) including a recently described hydrothermal vent endemic species. Oxygen consumption rates and citrate synthase activity, an indicator of aerobic metabolic potential, did not vary significantly with increasing habitat depth. Anaerobic metabolic potential, as evidenced by octopine dehydrogenase activity, declined significantly with increasing habitat depth. It is suggested that burst swimming abilities, and hence glycolytic potential, are not strongly selected for in the deep-sea, where visual predator-prey interactions are reduced because of light-limitation. Oxygen consumption rates for Octopus californicus and O. bimaculoides were analyzed as a function of oxygen partial pressure as well. O. californicus, which lives in the hypoxic Santa Barbara basin at 500 m depth, was able to regulate its oxygen consumption to the limit of detectable oxygen partial pressures. O. bimaculoides, an intertidal species, had a minimum critical oxygen partial pressure of 16 mmHg. It is also shown that oxygen consumption rates and oxygen consumption regulation are strongly affected by individual experiment duration (either handling stress or food deprivation). O. californicus appears to be much more strongly affected by experiment duration than is O. bimaculoides.

Role of nitric oxide in the control of renal oxygen consumption and the regulation of chemical work in the kidney.

Inhibition of NO synthesis has recently been shown to increase oxygen extraction in vivo, and NO has been proposed to play a significant role in the regulation of oxygen consumption by both skeletal and cardiac muscle in vivo and in vitro. It was our aim to determine whether NO also has such a role in the kidney, a tissue with a relatively low basal oxygen extraction. In chronically instrumented conscious dogs, administration of an inhibitor of NO synthase, nitro-L-arginine (NLA, 30 mg/kg i.v.), caused a maintained increase in mean arterial pressure and renal vascular resistance and a decrease in heart rate (all P<0.05). At 60 minutes, urine flow rate and glomerular flow rate decreased by 44+/-12% and 45+/-7%, respectively; moreover, the amount of sodium reabsorbed fell from 16+/-1.7 to 8.5+/-1.1 mmol/min (all P<0.05). At this time, oxygen uptake and extraction increased markedly by 115+/-37% and 102+/-34%, respectively (P<0.05). Oxygen consumption also significantly increased from 4.5+/-0.6 to 7.1+/-0.9 mL O2/min. Most important, the ratio of oxygen consumption to sodium reabsorbed increased dramatically from 0.33+/-0.07 to 0.75+/-0.11 mL O2/mmol Na+ (P<0.05), suggesting a reduction in renal efficiency for transporting sodium. In vitro, both a NO-donating agent and the NO synthase-stimulating agonist bradykinin significantly decreased both cortical and medullary renal oxygen consumption. In conclusion, NO plays a role in maintaining a balance between oxygen consumption and sodium reabsorption, the major ATP-consuming process in the kidney, in conscious dogs, and NO can inhibit mitochondrial oxygen consumption in canine renal slices in vitro.

Developmental changes in oxygen consumption regulation in larvae of the South African clawed frog Xenopus laevis

Well-developed larval Xenopus laevis (NF stages 58&shy;66) are oxygen regulators, at least during mild hypoxia. When and how they change from oxygen conformers (the presumed condition of the fertilized egg) to oxygen regulators is unknown. Also unknown is how anaerobic metabolic capabilities change during development, especially in response to acute hypoxia, and to what extent, if any, anaerobiosis is used to supplement aerobic metabolism. Consequently, we have investigated resting rates of oxygen consumption (M.O2) and concentrations of whole-body lactate (lactic acid) during development in normoxia and in response to acute hypoxia in Xenopus laevis. M.O2 increased in an episodic, non-linear fashion during development. Resting, normoxic M.O2 increased about tenfold (to approximately 0.20 &micro;mol g-1 h-1) between NF stages 1&shy;39 and 40&shy;44, and then another tenfold between NF stages 45&shy;48 and 49&shy;51 (to approximately 2.0 &micro;mol g-1 h-1), remaining at about 2 &micro;mol g-1 h-1 for the remainder of larval development. M.O2 reached its highest level in newly metamorphosed frogs (nearly 4 &micro;mol g-1 h-1), before decreasing to about 1.0 &micro;mol g-1 h-1 in large adults. X. laevis embryos and larvae up to NF stage 54&shy;57 were oxygen conformers when exposed to variable levels of acute hypoxia. The only exception was NF stage 45&shy;48 (external gills present yet body mass still very small), which showed some capability of oxygen regulation. All larvae older than stage 54&shy;57 and adults were oxygen regulators and had the lowest values of Pcrit (the oxygen partial pressure at which M.O2 begins to decline). Whole-body lactate concentration in normoxia was about 1 &micro;mol g-1 for all larval groups, rising to about 12 &micro;mol g-1 in adults. Concentrations of lactic acid in NF stages 1&shy;51 were unaffected by even severe ambient hypoxia. However, whole-body lactate levels in NF stages 52&shy;66 increased in response to severe hypoxia, indicating that some anaerobic metabolism was being used to supplement diminishing aerobic metabolism. The largest increases in concentration of lactate occurred in late larvae and adults.

The effect of sublethal levels of copper on oxygen consumption and ammonia excretion in the common carp, Cyprinus carpio

Common carp ( Cyprinus carpio) of 15–30 g body weight were exposed to copper levels of 0.22 ± 0.07, 0.34 ± 0.12 and 0.84 ± 0.35 μmol · l −1. Oxygen consumption and nitrogen excretion were determined repeatedly for up to 2 weeks of exposure to copper. Critical oxygen concentrations for oxygen consumption as well as for ammonia excretion were determined after 1 week of exposure to copper. Oxygen consumption dropped significantly immediately after exposure to 0.34 and 0.84 μmol · l −1 of copper whereas nitrogen excretion remained stable. After 1 week of continuous exposure to 0.34 μmol · l −1 of copper the oxygen consumption showed an apparent recovery, while the ammonia quotient (AQ = mole to mole ratio of ammonia excreted to oxygen consumed) did not. At a copper concentration of 0.84 μmol · l −1, no recovery was observed. The critical oxygen concentration for oxygen consumption shifted from 45 μmol · l −1 (1.4 mg · l −1) in copper-free water to 126 μmol. l −1 (3.9 mg · l −1) at a copper concentration of 0.34 μmol · l −1. At 0.84 μmol · 1 −1, regulation of oxygen consumption was lost. Also ammonia excretion showed a decline at lower oxygen concentrations and a critical oxygen concentration for ammonia excretion was determined. For the nitrogen excretion, loss of regulation already occurred at copper concentrations of 0.34 μmol · l −1. For the AQ, no critical oxygen concentration was found. The results obtained in this study suggest that measurements of oxygen consumption in combination with measurements of nitrogen excretion can be useful indicators of stress. Furthermore, it is shown that a critical oxygen concentration for ammonia exists in carp and that the critical oxygen concentrations for oxygen consumption and for ammonia excretion are affected by exposure to copper.