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A new member of the inward-rectifier K+ channel subfamily Kir2 was isolated and characterised from the crucian carp (Carassius carassius) heart. When expressed in COS-1 cells this 422 amino acid protein produced an inward-rectifying channel with distinct single-channel conductance, mean open time and open probability. Phylogenetic sequence comparisons indicate that it is not homologous to any known vertebrate Kir channel, yet belongs to the Kir2 subfamily. This novel crucian carp channel increases the number of vertebrate Kir2 channels to five, and has therefore been designated as ccKir2.5 (cc for Carassius carassius). In addition to the ccKir2.5 channel, the ccKir2.2 and ccKir2.1 channels were expressed in the crucian carp heart, ccKir2.1 being present only in trace amounts (<0.8% of all Kir2 transcripts). Whole-cell patch clamp in COS-1 cells demonstrated that ccKir2.5 is a stronger rectifier than ccKir2.2 or ccKir2.1, and therefore passes weakly outward current. Single-channel conductance, mean open time and open probability of ccKir2.5 were, respectively, 1.6, 4.96 and 4.17 times as large as that of ccKir2.2. ccKir2.5 was abundantly expressed in atrium and ventricle of the heart and in skeletal muscle, but was a minor component of Kir2 in brain, liver, gill and kidney. Noticeably, ccKir2.5 was strongly responsive to chronic cold exposure. In fish reared at 4°C for 4 weeks, ccKir2.5 mRNA formed 59.1±2.1% and 65.6±3.2% of all cckir2 transcripts in atrium and ventricle, respectively, while in fish maintained at 18°C the corresponding transcript levels were only 16.2±1.7% and 23.3±1.7%. The increased expression of ccKir2.5 at 4°C occurred at the expense of ccKir2.2, which was the main Kir2 isoform in 18°C acclimated fish. A cold-induced increase in the slope conductance of the ventricular lx1 from 707±49 to 1001±59 pSpF-1 (P<0.05) was thus associated with an isoform shift from ccKir2.2 towards ccKir2.5, suggesting that ccKir2.5 is a cold-adapted and ccKir2.2 a warm-adapted isoform of the inward-rectifying K+ channel. [ABSTRACT FROM AUTHOR]

Hypoxia-inducible transcription factor-1 (HIF-1) is a master regulator of hypoxia-induced gene responses. To find out whether HIF-1 function is involved in gene expression changes associated with temperature acclimation as well as in hypoxia adaptation in poikilotherms, we studied HIF-1 DNA binding activity and HIF-1α expression in normoxia and during hypoxia (0.7 mg l-1 O2) in crucian carp at temperatures of 26, 18 and 8°C. Temperature had a marked influence on HIF-1 in normoxia. Although HIF-1α mRNA levels remained unaltered, cold acclimation (8°C) increased HIF-1α protein amounts in the liver, gills and heart and HIF-1 DNA binding activity in the heart, gills and kidney of crucian carp by two- to threefold compared to warm acclimated fish (26°C). In the heart and kidney HIF-1 activity was already significantly increased in the 18°C acclimated fish. Temperature also affected hypoxic regulation of HIF-1. Although hypoxia initially increased amounts of HIF-1α protein in all studied tissues at every temperature, except for liver at 18°C, HIF-1 activity increased only in the heart of 8°C acclimated and in the gills of 18°C acclimated fish. At 8°C HIF-1α mRNA levels increased transiently in the gills after 6 h of hypoxia and in the kidney after 48 h of hypoxia. In the gills at 26°C HIF-1α mRNA levels increased after 6 h of hypoxia and remained above normoxic levels for up to 48 h of hypoxia. These results show that HIF-1 is involved in controlling gene responses to both oxygen and temperature in crucian carp. No overall transcriptional control mechanism has been described for low temperature acclimation in poikilotherms, but the present results suggest that HIF-1 could have a role in such regulation. Moreover, this study highlights interaction of the two prime factors defining metabolism, temperature and oxygen, in the transcriptional control of metabolic homeostasis in animals. [ABSTRACT FROM AUTHOR]

The activation of ATP-sensitive potassium (KATP) ion channels in the heart is thought to exert a cardioprotective effect under low oxygen conditions, possibly enhancing tolerance of environmental hypoxia in aquatic vertebrates. The purpose of this study was to examine the possibility that hypoxia-induced activation of cardiac KATP channels, whether in the sarcolemma (sarcKATP) or mitochondria (mitoKATP), enhances viability in cardiac muscle cells from a species highly tolerant of low oxygen environments, the goldfish Carassius auratus. During moderate hypoxia (6-7 kPa), the activation of sarcKATP channels was indicated by a reduction in transmembrane action potential duration (APD). This response to hypoxia was mimicked by the NO-donor SNAP (100 µmol l-1) and the stable cGMP analog 8-Br-cGMP, but abolished by glibenclamide or L-NAME, an inhibitor of NO synthesis. The mitoKATP channel opener diazoxide did not affect APD. Isolated ventricular muscle cells were then incubated under normoxic and hypoxic conditions. Cell viability was decreased in hypoxia; however, the negative effects of low oxygen were reduced during simultaneous exposure to SNAP, 8-Br-cGMP, and diazoxide. The cardioprotective effect of diazoxide, but not 8-Br-cGMP, was reduced by the mitoKATP channel blocker 5-HD. These data suggest that hypoxia-induced activation of sarcKATP or mitoKATP channels could enhance tolerance of low-oxygen environments in this species, and that sarcKATP activity is increased through a NO and cGMP-dependent pathway. [ABSTRACT FROM AUTHOR]

Especially in aquatic habitats, hypoxia can be an important evolutionary driving force resulting in both convergent and divergent physiological strategies for hypoxic survival. Examining adaptations to anoxic/hypoxic survival in hypoxia-tolerant animals may offer fresh ideas for the treatment of hypoxia-related diseases. Here, we summarise our present knowledge of two fishes that have evolved to survive hypoxia under very different circumstances. The crucian carp (Carassius carassius) is of particular interest because of its extreme anoxia tolerance. During the long North European winter, it survives for months in completely oxygen-deprived freshwater habitats. The crucian carp also tolerates a few days of anoxia at room temperature and, unlike anoxia-tolerant freshwater turtles, it is still physically active in anoxia. Moreover, the crucian carp does not appear to reduce neuronal ion permeability during anoxia and may primarily rely on more subtle neuromodulatory mechanisms for anoxic metabolic depression. The epaulette shark (Hemiscyllium ocellatum) is a tropical marine vertebrate. It lives on shallow reef platforms that repeatedly become cut off from the ocean during periods of low tides. During nocturnal low tides, the water [O2] can fall by 80% due to respiration of the coral and associated organisms. Since the tides become lower and lower over a period of a few days, the hypoxic exposure during subsequent low tides will become progressively longer and more severe. Thus, this shark is under a natural hypoxic preconditioning regimen. Interestingly, hypoxic preconditioning lowers its metabolic rate and its critical PO2. Moreover, repeated anoxia appears to stimulate metabolic depression in an adenosine-dependent way. [ABSTRACT FROM AUTHOR]

The present study was designed to test the hypothesis that temperature acclimation modifies sarcolemmal Na+ current (INa) of the fish cardiac myocytes differently depending on the animal's lifestyle in the cold. Two eurytherm4al fish species with different physiological strategies for surviving in the cold, a cold-dormant crucian carp (Carassius carassius L.) and a cold-active rainbow trout (Oncorhynchus mykiss), were used in acclimation experiments. The INa of carp and trout were also compared with INa of a cold stenothermal burbot (Lota lota). In accordance with the hypothesis, cold-acclimation decreased the density of INa in crucian carp and increased it in rainbow trout, suggesting depression of impulse conduction in cold-acclimated carp and positive compensation of impulse propagation in cold-acclimated trout. The steady-state activation curve of trout INa was shifted by 6 mV to more negative voltages by cold acclimation, which probably lowers the stimulus threshold for action potentials and further improves cardiac excitability in the cold. In burbot myocytes, the INa density was high and the position of the steady-state activation curve on the voltage axis was even more negative than in trout or carp myocytes, suggesting that the burbot INa is adapted to maintain high excitability and conductivity in the cold. The INa of the burbot heart differed from those of carp and trout in causing four times larger charge influx per excitation, which suggests that INa may also have a significant role in cardiac excitation-contraction coupling of the burbot heart. In summary, INa of fish cardiac myocytes shows thermal plasticity that is different in several respects in cold-dormant and cold-active species and thus has a physiologically meaningful role in supporting the variable life styles and habitat conditions of each species. [ABSTRACT FROM AUTHOR]

Goldfish swimming was analysed quantitatively to determine if it exhibits distinctive individual spatiotemporal patterns. Due to the inherent variability in fish locomotion, this hypothesis was tested using five nonlinear measures, complemented by mean velocity. A library was constructed of 75 trajectories, each of 5 min duration, acquired from five fish swimming in a constant and relatively homogeneous environment. Three nonlinear measures, the 'characteristic fractal dimension' and 'Richardson dimension', both quantifying the degree to which a trajectory departs from a straight line, and 'relative dispersion', characterizing the variance as a function of the duration, have coefficients of variation less than 7%, in contrast to mean velocity (30%). A discriminant analysis, or classification system, based on all six measures revealed that trajectories are indeed highly individualistic, with the probability that any two trajectories generated from different fish are equivalent being less than 1%. That is, the combination of these measures allows a given trajectory to be assigned to its source with a high degree of confidence. The Richardson dimension and the 'Hurst exponent', which quantifies persistence, were the most effective measures. [ABSTRACT FROM AUTHOR]

The article discusses the findings of a study conducted by Norwegian scientists Göran Nilsson and Stian Ellefsen which suggest that certain members of the Carassius could withstand long periods with little to no oxygen at low temperatures during winter beneath frozen lakes. Nilsson and Ellefsen found that Carassius muscle, the major ethanol-producing tissue during anoxia, expressed more of the E1α and E1β genes responsible for driving pyruvate towards ethanol synthesis.

Explores a study by M. Vornanen and V. Paajanen about the seasonality of dihydropyridine receptor binding in the heart of an anoxia-tolerant vertebrate, the crucian carp. Testing of the channel arrest hypothesis in the crucian carp; Role of L-type calcium channels in regulating the force of heart muscle contraction through changes in the amount of free intracellular calcium; Comparison between winter-captured, anoxic carp and summer-captured, normoxic animals.