Oxygen-dependent energetics of anoxia-tolerant and anoxia-intolerant hepatocytes

The oxygen-dependence of cellular energetics was investigated in hepatocytes from goldfish Carassius auratus (anoxia-tolerant) and rainbow trout Oncorhynchus mykiss (anoxia-intolerant). In goldfish hepatocytes, an approximately 50 % reduction in the rate of oxygen consumption was observed in response to both acute and prolonged hypoxia, the latter treatment shifting the threshold for this reduction to a higher oxygen level. A concomitant increase in the rate of lactate production did not compensate for the decreased aerobic ATP supply, resulting in an overall metabolic depression of 26 % during acute hypoxia and of 42 % during prolonged hypoxia. Trout hepatocytes showed a similar suppression of cellular respiration after prolonged hypoxia but were unresponsive to acute hypoxia. Similarly, the rate of lactate production was unaltered during acute hypoxia but was increased during prolonged hypoxia, metabolic depression amounting to 7 % during acute hypoxia and 30 % during prolonged hypoxia. In both species, the affinity of hepatocytes for oxygen decreased during hypoxia, but this alteration was not sufficient in absolute terms to account for the observed decrease in aerobic ATP supply. Protein synthesis was suppressed in both cell types under hypoxia, whereas Na(+)/K(+)-ATPase activity decreased in trout but not in goldfish hepatocytes, emphasising the importance of membrane function in these cells during conditions of limited energy supply.

Energetics of trout hepatocytes during A23187-induced disruption of Ca2+ homeostasis

The impact of an increase of intracellular Ca2+ i on the energy metabolism of trout hepatocytes was assessed by applying the Ca2+ ionophore A23187 and studying the consequences of the ensuing elevation of Ca2+ i on various metabolic parameters. After application of A23187 no loss of viability occurred for 2 h, but glutathione content decreased by 46%. A concomitant decrease of [ATP] as well as of Na,K-ATPase activity by over 50% could be prevented by incubating the cells in a Ca2+-free medium. Upon addition of the ionophore cellular oxygen consumption more than doubled in a strictly Ca2+-dependent manner, with half of this increase being sensitive to ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter. This increase in oxygen consumption was transient in nature and at its peak it was similar in magnitude to that induced by 2,4-dinitrophenol. Similarly, oxygen consumption sensitive to the protein synthesis inhibitor cycloheximide was transiently increased by A23187, but returned to control levels within 30 min of incubation. These results suggest that elevation of intracellular Ca2+ leads to an energetic imbalance not related to stimulation of ATP consuming processes, but mainly due to impairment of mitochondrial function, possibly by the decoupling of oxidative phosphorylation and by inducing dissipative Ca2+ cycling.

Loss of K+ homeostasis in trout hepatocytes during chemical anoxia: a screening study for potential causes and mechanisms

In isolated trout hepatocytes intoxication with CN- (chemical anoxia) leads to a rapid breakdown of K+ homeostasis. In the present study an attempt has been made to identify the causes and mechanisms underlying this phenomenon. Our results indicate that neither Ca2+ elevation nor cell swelling, both of which occurred during chemical anoxia and could be prevented by exposure to Ca2+ chelating agents or to hyperosmotic conditions, respectively, is solely responsible for the breakdown of K+ homeostasis. From a number of inhibitors of dissipative K+ fluxes tested, only BaCl2, an inhibitor of voltage-gated K+ channels, proved to be effective in significantly reducing K+ efflux during chemical anoxia. The KCl cotransporter known to be involved in regulatory volume decrease after hypoosmotic shock does not seem to be activated during CN(-)-induced cell swelling.

A note on interactions between temperature, viscosity, body size and swimming energetics in fish larvae

In a previous study, it was shown that at a given speed the larvae of a species of freshwater fish, the Danube bleak Chalcalburnus chalcoides, expended considerably more metabolic energy at 15 degreesC than at 20 degreesC. We applied hydromechanical arguments to our previous data in order to determine whether the higher cost of swimming at the lower temperature might be due to the effects of viscous forces. However, even under the unrealistic assumption of the larvae swimming in the viscous regime at Reynolds numbers as high as 2000, we show here that hydromechanical forces cannot explain the high energy cost of swimming at 15 degreesC. Instead, we offer a new hypothesis that the 'two-gear system' of the swimming muscles operating in juvenile and adult fish is not yet functional in the larvae, with the consequence that, when these fish are swimming at high speeds in cold water, the muscle fibres have to operate over an increasingly inefficient range of shortening velocities.

Functional role of ecto-ATPase activity in goldfish hepatocytes

Extracellular [gamma-32P]ATP added to a suspension of goldfish hepatocytes can be hydrolyzed to ADP plus gamma-32Pi due to the presence of an ecto-ATPase located in the plasma membrane. Ecto-ATPase activity was a hyperbolic function of ATP concentration ([ATP]), with apparent maximal activity of 8.3 +/- 0.4 nmol P(i).(10(6) cells)-1.min-1 and substrate concentration at which a half-maximal hydrolysis rate is obtained of 667 +/- 123 microM. Ecto-ATPase activity was inhibited 70% by suramin but was insensitive to inhibitors of transport ATPases. Addition of 5 microM [alpha-32P]ATP to the hepatocyte suspension induced the extracellular release of alpha-32P(i) [8.2 pmol.(10(6) cells)-1.min-1] and adenosine, suggesting the presence of other ectonucleotidase(s). Exposure of cell suspensions to 5 microM [2,8-3H]ATP resulted in uptake of [2,8-3H]adenosine at 7.9 pmol.(10(6) cells)-1.min-1. Addition of low micromolar [ATP] strongly increased cytosolic free Ca2+ (Ca2+i). This effect could be partially mimicked by adenosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analog of ATP. The blockage of both glycolysis and oxidative phosphorylation led to a sixfold increase of Ca2+i and an 80% decrease of intracellular ATP, but ecto-ATPase activity was insensitive to these metabolic changes. Ecto-ATPase activity represents the first step leading to the complete hydrolysis of extracellular ATP, which allows 1) termination of the action of ATP on specific purinoceptors and 2) the resulting adenosine to be taken up by the cells.

Effects of energy limitation on Ca2+ and K+ homeostasis in anoxia-tolerant and anoxia-intolerant hepatocytes

To gain more insight into the mechanistic basis of anoxia tolerance and intolerance, a comparative study was conducted on calcium homeostasis in goldfish and trout hepatocytes subjected to different forms of energy limitation. Using the fluorescent Ca2+ indicator fura 2, we observed that both chemical anoxia and true anoxia led to an increase of the concentration of cytosolic free calcium (Ca2+i) in the anoxia-sensitive hepatocytes of rainbow trout, whereas Ca2+i was maintained at control levels in the anoxia-tolerant hepatocytes of goldfish. Various lines of evidence suggest an intracellular origin of the Ca2+ increase observed in trout cells. Cyclosporin A, a specific inhibitor of the mitochondrial permeability transition pore in mammalian cells, was ineffective in preventing the Ca2+ increase, whereas a high dose of fructose depressed the Ca2+ surge by approximately 50%. The latter effect was not accompanied by improvement of the energetic state of the cells. A comparison of chemical anoxia with true (physiological) anoxia revealed that both treatments affected energy metabolism to a similar degree in trout hepatocytes, whereas the decrease of ATP seen in goldfish hepatocytes during chemical anoxia was absent during true anoxia. Elevation of Ca2+i with the calcium ionophore A-23187 led to a decoupling of unidirectional K+ fluxes in both normoxic and anoxic trout cells, whereas in goldfish hepatocytes the coupling of K+ fluxes was not affected by the rise of Ca2+i.

Acute and chronic effects of temperature, and of nutritional state, on ion homeostasis and energy metabolism in teleost hepatocytes

Short- and long-term effects of temperature on ion flux and energy turnover were studied in hepatocytes from thermally acclimated trout and roach. In trout hepatocytes K+ efflux was insensitive towards acute exposure to low temperature but was downregulated during cold acclimation of the fish so as to balance the uncompensated decreased K+ (Rb+) uptake of the cells. In contrast, both K+ (Rb+) uptake and K+ efflux of roach hepatocytes were temperature sensitive in the short term. These acute effects, however, were offset during cold acclimation by a near perfect compensation of both fluxes leading to re-establishment of ion flux homeostasis at the original level. Our findings, based on a new method permitting the simultaneous monitoring of K+ efflux and uptake in the same cell population, provide experimental verification of two of the three possible strategies, recently discussed by Cossins et al. (1995), by which the ionic steady state of fish cells may adjust to acute and chronic temperature change. By comparing hepatocytes from two groups of trout, one kept on a maintenance diet (ration I), the other fed ad libitum (ration II), we discovered striking effects of nutritional state on the absolute levels as well as on the temperature relationships of K+ uptake and protein synthetic activity. Both of these functions in the hepatocytes increased in the ration II fed as compared to the ration I fed trouts, but the increase of protein synthetic activity was greater and more uniform at the three experimental temperatures than that of K+ uptake. Moreover, protein synthetic activity proved to be considerably more temperature sensitive than K+ uptake and, in contrast to the latter, showed a compensatory response after cold acclimation.

Membrane-metabolic coupling and ion homeostasis in anoxia-tolerant and anoxia-intolerant hepatocytes

The relationship between membrane function and energy metabolism was studied in rainbow trout hepatocytes, an anoxia-intolerant cell system, and compared with the situation in hepatocytes from the goldfish, a typical anoxia-tolerant species. In trout hepatocytes, under normoxia and under chemical anoxia, inhibition of ATP consumption by the Na+ pump induced a decrease in ATP production of the same magnitude. In response to chemical anoxia, total ATP production was reduced to 15% and Na+ pump activity to 22% of the control rate under normoxia. Measurement of the cellular ATP content under these conditions revealed that, despite the reduction in Na+ pump activity, the cells became rapidly depleted of ATP, with the time course of this process resembling that observed in the anoxic rat hepatocyte. This is in contrast to the responses of goldfish hepatocytes, where, during chemical anoxia, 1) inhibition of the Na+ pump did not lead to a corresponding reduction in ATP production and 2) ATP levels, after a transient decrease, stabilized at a new steady state. To investigate the consequences of chemical anoxia on ion homeostasis, efflux and uptake rates of K+ were determined simultaneously. In the trout cells, chemical anoxia led to a decoupling of influx and efflux rates, the latter exceeding the former three- to eightfold. In contrast, goldfish hepatocytes were able to preserve ion homeostasis by a concerted decrease in Rb+ uptake and K+ efflux, so that the net flux of K+ was always close to zero. In neither species did chemical anoxia induce a change in pump density. Other potential control mechanisms are briefly discussed.

Energetics of fish larvae, the smallest vertebrates

In this review recent findings on the energetics of fish larvae are presented, highlighting some of the physiological problems linked to small body size. The existence of a mass-independent phase of specific metabolic rate is confirmed but it is pointed out that in young fish ontogenetic transitions of metabolic scaling have so far been documented only for the routine level of activity. Maximum metabolic rate is limited by mitochondrial density in the swimming muscles which scales with a mass exponent of approximately 0.9. Mitochondrial density in the swimming muscles of a species of fish, from larva to adult, covers about the same range as mitochondrial density in the skeletal muscles of mammals. However, the aerobic capacity (power density) of mitochondria is one order of magnitude lower in fish than in mammals. Energy metabolism in embryos and early larvae of fish is almost entirely aerobic. Anaerobic power in the fast muscle fibres is low after hatching but increases during the transition from larva to juvenile with a mass exponent greater than one. In hypoxic water fish larvae swim more economically (i.e. their cost of transport is lower) than in normoxic water. If the rate of growth exceeds a critical threshold (about 10% d-1) fish larvae are capable of increasing the apparent efficiency of growth, probably by reducing the costs of other energy-consuming functions of maintenance.

Cost of growth in cells and organisms: general rules and comparative aspects

In a crude fashion it can be said that metabolizable energy (M) is partitioned into metabolic work, paid for by 'oxidations' (R), and 'assimilation', i.e. production (P), so that M = R+P. However, a fraction of R is required to meet the expenses of production and if these expenses represent, Joule for Joule, a constant proportion of the amount produced, then Rt = Rm+cP, where Rt = total metabolic expenditures, Rm = metabolic expenditures for maintaining the non-producing organism, and cP = Rp = metabolic expenditures connected with the processes of production. The partitioning of metabolizable energy into R and P as well as into Rm and Rp may vary depending on the phylogeny and life-history of the species concerned and on ecological circumstances. Thus selection is expected to act on both ratios, R/P and Rm/Rp. By comparing the ratios P/(P+Rp) (the apparent efficiency of production) and Rp/P (the apparent metabolic cost of production) in different types of organisms, one finds that a value of P/(P+Rp) = 0.75, equal to 75% efficiency, 10 mgdbm/mmol ATP, and 16 mumolO2/mg dbm (when I mg identical to 22 J), can be used as a 'consensus value' for the average efficiency, or cost, of the transformation of metabolizable energy into production in a wide range of organisms, from bacteria to mammals. This value corresponds to about three times the theoretical cost of synthesizing the same amount of tissue on the basis of known biochemical principles. The reasons why the empirical costs of production are higher than the theoretical costs of synthesis by what appears to be a common factor may be quite different in bacteria, small ectothermic and large endothermic organisms. Deviations from the consensus value may be due to differences in energy density of the nutrients assimilated and the tissues synthesized. Further complications arise because of interactions between P, Rp, and Rm. In microorganisms the existence of a constant and a variable component of maintenance metabolism has been postulated, the latter decreasing with increasing rate of production. In small ectothermic metazoans, on the other hand, the nonlinear relationship between growth metabolism and growth rate has led to the speculation that above a critical value of Pg certain energy consuming functions of maintenance are suppressed and the energy thus gained used for fuelling growth processes. There is some evidence that, at least in ectothermic metazoans, the apparent cost of growth decreases with the rate of growth, reaching a low plateau of about 10 mumolO2/mgdbm at growth rates exceeding about 8 mgdbm/g/h.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo protein synthesis rates in larval nase (Chondrostoma nasus L.)

In vivo protein synthesis rates were determined in larvae of nase (Chondrostoma nasus (L.)) by bathing the fish in [3H]phenylalanine. After a delay of about 1 h the rate of labelling of the larval protein was linear. A significant relationship was found between specific growth rate (kg) and fractional rate of protein synthesis (ks) when animals from low and high ration groups and starved animals are included, which can be described by ks = 8.5 + 1.45 kg (both variables in %∙day−1). The efficiency of retention of synthesised protein (kg∙100/ks) was 45% at a growth rate of 10%∙day−1 and 50% at 15%∙day−1. The metabolic costs of total protein synthesis were estimated by comparing the protein synthesis rates of C. nasus with levels of oxygen consumption in similarly treated larvae of the related species Rutilus rutilus. At growth rates below 10%∙day−1, oxygen consumption above maintenance (FIT, food induced thermogenesis, μmol O2∙g fresh weight−1∙h−1) is related to protein synthesis (Ps, mg protein synthesised∙g fresh weight−1∙h−1) by FIT = 24.7 Ps − 3.3. This relationship provides an estimate of the cost of protein synthesis of 24.7 μmol O2 ∙ mg protein synthesised−1 which is 3 times higher than minimal theoretical costs.

Effect of species differences and dietary vitamin C on the concentration of ascorbate- and acid-soluble thiol in fish eye

Data presented confirm the essentiality of modification of the dinitrophenylhydrazine (DNPH) method to analyze the total ascorbic acid and dehydroascorbic acid in ocular tissues and stress the need of corrections for the interfering substances. Variations in ascorbate and thiol concentrations in the lens, retina and aqueous humour of freshwater fish belonging to the Cyprinidae family were examined. The interspecific variability of ascorbate concentration was highest in the aqueous humour and lowest in the retina. The high ascorbate concentration in the retina seems to reflect the importance of the sense of vision in fish life-style as compared to chemo- and acoustico lateralis senses. The regional distribution of the total ascorbate is in the order of decreasing concentrations: retina, lens and aqueous humour. However, the retinal ascorbate is almost exclusively in the oxidized form, and the lenticular ascorbate is almost exclusively in the reduced form. Thiol concentration in the lens is five- to tenfold that in the retina and aqueous humour. This explains the oxidation status of ascorbate in different eye compartments of the eye. After 30 days on diets containing various levels of ascorbic acid or ascorbic acid sulphate, the ascorbate concentration in the eye compartments of common carp (Cyprinus carpio L.) was determined. Ocular tissue can be used to monitor the development of the ascorbate status in fish, and the retina is the most responsive tissue to the enhanced or depleted ascorbate levels.