The activity and expression level of cytoplasmic malate dehydrogenase (MDH 1.1.1.37) gene and adenylate system state in tissues of the Mediterranean mussel Mytilus galloprovincialis Lamarck, 1819 were investigated. The highest values of adenylate energy charge (AEC) (0.66 ± 0.01) and adenylate content (ATP, ADP, and AMP) were observed in the mollusc’s foot. Close AEC values were found in the gills (0.61 ± 0.02). The lowest values were recorded in the hepatopancreas and mantle edge (AEC of 0.46–0.54) indicating moderate depression of physiological processes. A direct relationship between adenylate system state in muscle tissue and MDH activity ( R 2 above 0.7) was revealed, and thus this parameter can be recommended as a biomarker for energy status assessment in the mollusc’s tissue structures. A direct relationship between gene expression levels and MDH activity was shown for tissues of mussels in the range hepatopancreas → gills → foot. In contrast, in the mantle high levels of gene expression corresponded to a relatively low activity of the enzyme, presumably due to non-homogeneous tissue structure.

Hypoxic/ischemic brain injuries are a major medical challenge. One of the approaches to the development of therapeutic interventions is elucidating the neuronal survival pathways in O2 deficiency-tolerant vertebrates, which could suggest the ways to mitigate a hypoxia-induced catastrophe in individual nerve cells under conditions of oxygen starvation. Metabolic depression is considered a universal survival strategy for hypoxia-tolerant animals; however, the ins and outs of the mechanism that imposes limitations on brain metabolism when PO2 decreases are still unknown. Under oxygen starvation, an increase in the extracellular concentration of inhibitory neurotransmitters can be one of the significant links in the apparatus for electrical activity suppression, which makes it possible to reduce energy consumption. GABA (γ-aminobutyric acid) serves as a universal inhibitory neurotransmitter in the CNS of higher and lower vertebrates, whose functioning is attributed to metabolic suppression and leveling of energy failure consequences. GABA is found in various vertebrate taxa. This review addresses the strategies of GABA involvement in the mechanisms that ensure brain tolerance to oxygen starvation in members of various taxonomic groups of lower vertebrates (cyclostomes, cartilaginous and bony fish, amphibians, reptiles) distinguished by a most prominent ability to survive under acute and chronic hypoxia-anoxia.

The paper shows that introducing rhodium into zeolite significantly enhances the catalytic performance of the system. Rh/IK(UST), a catalyst sample distinguished by an increased concentration of strong acid sites, exhibited the highest activity and selectivity towards acetic acid (AA). In the presence of this catalyst, the yield of AA more than doubled, and the AA/methanol ratio increased by more than an order of magnitude, compared to the other Rh-modified catalysts. A synergistic effect of strong Brønsted acid sites and Rh single-atom sites was found. Their proximity plays a key role in this mechanism.

The influence of the method used for introduction of Rh into a ZSM-5 zeolite-based catalyst and on its physicochemical and catalytic properties exhibited in dimethyl ether conversion to light olefins was evaluated. Use of chitosan as a medium for rhodium dispersing affords a 10% increase in the dimethyl ether conversion while maintaining the selectivity for light olefins at a level of 75%, which is most likely due to location of rhodium predominantly on the zeolite surface, as well as to its high dispersion. The method of rhodium introduction also affects the amount of the formed coke deactivating the catalysts. Through the use of chitosan as a medium for rhodium dispersing and of a rotary evaporator in the zeolite impregnation stage the amount of the coke on the catalyst surface was reduced. The presence of chitosan as a polymer matrix during modification of the ultrasonically treated zeolite leads to a 6% decrease in the yield of the target reaction products (ethylene and propylene), which is associated with increased coke formation.

The activities of energy metabolism oxidoreductases—malate dehydrogenase (MDH; EC 1.1.1.37) and lactate dehydrogenase (LDH; EC 1.1.1.27)—were compared in the medulla oblongata (MB), forebrain, diencephalon, midbrain (FDMB), heart atrium and ventricle of the Black Sea scorpionfish Scorpaena porcus Linnaeus, 1758 under acute hypoxia (0.9–1.2 mg O 2 ·L –1 , 90 min). In the brain and heart, MDH activity was significantly higher compared to that of LDH act ( p < 0.05). In the MB and heart atrium, MDH activity was higher compared to the FDMB and heart ventricle, respectively. Under acute hypoxia, there were observed specific shifts in energy metabolism of the brain divisions and heart compartments, manifested as two patterns of changes in oxidoreductase activity. In the MB and heart atrium, MDH activity significantly decreased ( p < 0.05) after hypoxic exposure. In the FDMB and heart ventricle, there was a pronounced activation of LDH ( p < 0.05). It is assumed that energy metabolism synchronization in the MB and heart atrium is due to the operation of the integrated respiratory and heart rhythm generators associated with cAMP-regulated HCN-channels. Under normoxia, electrical oscillations in the MB and atrium require high energy costs, which are provided by aerobic glycolysis. Under hypoxia, a drop in ATP synthesis promotes the inactivation of HCN-channels and the switchover of respiratory and cardiac functions to a regime of functional suppression favorable for scorpionfish survival.

In the production of light olefins from dimethyl ether, a nanozeolite catalyst modified with lanthanum and zirconium exhibited lower activity and selectivity than a magnesium-loaded zeolite catalyst. This is due to a higher percentage of strong acid sites in La–Zr/HZSM-5, which promoted secondary reactions and decreased the selectivity towards light olefins. Furthermore, the difference in the catalytic properties of the samples modified with Mg and La–Zr can be explained by the different concentrations of the methanol product: the methanol content was markedly higher in the presence of La–Zr/HZSM-5 than with Mg/HZSM-5. To gain better insight into the role of methanol in the synthesis of light olefins from dimethyl ether, the relative activity of Brønsted acid sites on the catalyst surface in the conversion of methanol and dimethyl ether was comparatively assessed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was demonstrated that at high temperatures (above 260°C) the conversion of both DME and methanol occurs by the oxonium–ylide mechanism. However, there are differences in the formation routes of primary intermediates: formaldehyde from methanol, and ketene from dimethyl ether.

The activity of oxidoreductases, malate dehydrogenase and lactate dehydrogenase (MDH, 1.1.1.37; LDH, 1.1.1.27), as well as parameters of adenylate system—[ATP], [ADP], [AMP], total adenylate pool (AP), and adenylate energy charge (AEC) in medulla oblongata (MB) and forebrain, midbrain, and diencephalon (FDMB)—were studied in the scorpionfish under acute hypoxia (0.9–1.2 mg O2·L−1, 90 min). A higher MDH activity level was observed in MB and FDMB, as compared to LDH (p < 0.05). At the same time, MB showed a higher adenylate content and increased AP (p < 0.05). AEC did not exceed ~ 0.7 (vs. the maximum of this index ~ 0.9–1.0) in the brain of the scorpionfish indicating adaptation of the tissue energy status to hypoxia. A rapid decrease in MDH activity (p < 0.05) was observed in MB under acute hypoxia. These changes were accompanied by insignificant LDH activation. A pronounced LDH activation (p < 0.05), a decrease in MDH activity, and the highest AP raise (p < 0.05) were observed in FDMB, suggesting activation of glycolysis and simultaneous decrease in the rate of ATP consumption. MB and FDMB demonstrated the ability to a relative retention of AEC during hypoxia. The unidirectional metabolic adaptation was based on the intensification of glycolysis, a decrease of ATP consumption, and a subsequent increase in adenylate concentration that allowed the scorpionfish brain structures to maintain the energy status under acute hypoxia.

Species with a high resistance to hypoxia are usually characterized by an increased H 2 S tolerance of hydrogen sulfide; however, high anaerobic potential cannot be the only explanation for survival in an environment with elevated concentrations of sulfides. The activity of oxidoreductases, as well as parameters of adenylate system were studied in the tissues of hypoxia/anoxia-tolerant clam Anadara kagoshimensis (Tokunaga, 1906) under conditions of experimental H 2 S loading (HSL). Adult specimens with a shell height of 26–38 mm are used. The control group of clams is kept in an aquarium with an oxygen concentration of 7.0–7.1 mg/L (normoxia). The experimental group is exposed to the effect of HSL created by dissolving sodium sulfide (H 2 S donor) in water to a final concentration of 6 mg S 2– /L; exposure time is 24 h. After the first day of the experiment, the level of O 2 in water is 1.8 mg/L and there is no hydrogen sulfide. Some of the clams are exposed to repeated hydrogen sulfide loading (second day of the experiment), and Na 2 S is introduced to a final concentration of 9 mg S 2– /L; by the end of the second day, 1.9 mg S 2– /L and trace concentration of O 2 (0.03 mg/L) are registered. In the first days of HSL, a high activity of malate dehydrogenase (MDH) against the background of a significant suppression of the activity of lactate dehydrogenase (LDH) and an increase in the values of MDH/LDG index persists; this reflects a strengthening of anaerobic processes in the tissues of anadara with relatively high concentrations of О 2 in water (1.8 mg/L). After the second day of HSL, the activity of oxidoreductases in the clam tissues does not change when compared with the first day; however, the value of adenylate energy charge (AEC) persists against the background of a relative decrease in [ATP]. The retention of AEC indicates the ability of the anadara to exist under conditions of hydrogen sulfide contamination and acute forms of hypoxia/anoxia.

Teleosts serve as experimental models for the physiological and pathophysiological processes investigation, in particular those related to the heart work. Methods allowing to analyze the frequency parameters of the heart functioning over a long period of time require taking into account the peculiarities of fish behavioral reactions, that can affect the results of the experiment. The aim of this study was to examine the effect of the simplest test loading (sound stimulus) on the frequency parameters of the heart activity recorded by the fiber-optic method. The objects of the study were adults of Scorpaena porcus (12–15 cm long, 80–120 g in weight). In the course of experiments, each scorpionfish was kept in a separate aquarium with seawater (400×400×350 mm), with constant temperature (21 ± 0.5) °C and given oxygen content (5.5–6.7 mg·L−1, normoxia). The heart rate (HR) recording was carried out by an invasive fiber-optic method, the essence of which is to transmit the radiation of the infrared semiconductor laser of the photoplethysmograph through a thin fiber-optic cable to the pericardial membrane of the heart and then to fix the signal reflected from the contracting myocardium in the photodetector. During implantation of the photoplethysmograph light guides, the fish were anesthetized by placing them in an anesthetic solution (urethane, 2.4 g·L−1 of seawater). In the fornix of the opercular cavity above the area of the conditional heart projection, a minimal dissection of the lining epithelium was performed, through which the underlying tissues were sequentially separated by a blunt method until the pericardial membrane was reached without breaking it. Through the lumen formed in the tissues, two optical light guide sensors were introduced to the surface of the pericardial membrane. Further, free-swimming scorpionfish participated in the experiment after a day of rehabilitation after the surgery. Additionally, we assessed the functional state of the animals by visual fixation of respiratory activity by the quantity of movements of the opercular covers per minute. During studying the test loading effect on the correct registration of the scorpionfish HR, the phenomenon of temporary complete suppression of cardiac activity was revealed, which manifested itself upon presentation of sound stimuli (alertness, “freezing” reaction). The duration of cardiac arrest was 31 to 50 seconds; it was accompanied by the cessation of movement of the opercular covers (respiratory arrest, apnea). During the restoration of cardiac activity, two types of physiological reactions were noted. The first type of recovery reaction was characterized by a simultaneous 1.5-fold increase in the HR and a 2-fold enhancement in the photoplethysmograph signal amplitude. The second type of reaction was accompanied by a rise in the HR by 22 % (p < 0.05) against the backdrop of a decrement in the signal amplitude of the photoplethysmograph sensors by 28 % (p < 0.05); within 120 seconds, the scorpionfish HR returned to baseline. It is assumed that the short-term delay in the scorpionfish cardiac activity is based on the phenomenon of cardiorespiratory coupling and synchronization. The behavioral reaction in the form of suppression for the cardiac and simultaneously respiratory activity generation ensures the complete absence of acoustic and electrical signals, which unmask an ambush predator location, and contributes to the scorpionfish survival.

The activities of cytosolic oxidoreductases (malate and lactate dehydrogenases) and the level of ATP production in the hypoxic resistive tissues of Scorpaena porcus Linnaeus, 1758 were studied. It was found that “oxyphilic” tissues (structures of the brain, gills) are pre-adapted to hypoxia, since under normal conditions they had high MDH activity and an increased MDH/LDH ratio, the value of which was 10 to 20 times higher than that in the liver and white muscles. Moreover, in the relatively “young” brain divisions (forebrain, diencephalon, midbrain) the aerobic pathway of carbohydrate metabolism predominated. The ATP content decreased in a line of examined tissues as follows: white muscles → liver → medulla oblongata → gills → forebrain, diencephalon and midbrain. The ATP level in white muscles was an order of magnitude higher than in “oxyphilic” tissues and it probably served for the provision of throwing strategy for hunting of bottom predator.

The fish heart is a unique model to compare the resistance to hypoxia of its two chambers (atrium and ventricle), which are different in the structure and functional loading. The activity of oxydoreductases malate dehydrogenase (MDH, 1.1.1.37) and lactate dehydrogenase (LDH, 1.1.1.27), as well as the parameters of the adenylate system in the heart chambers of a Black Sea scorpaena, were studied under acute hypoxia (0.9–1.2 mg O2·L–1, 90 min). Despite the leading functional role of the ventricle, MDH activity in this heart compartment tended to decrease compared to the atrium in the absence of differences in LDH activity. At the same time, the difference in the level of adenylates (ATP, ADP, AMP), total adenylate pool (AP), and adenylate energy charge (AEС) between the atrium and ventricle was statistically nonsignificant, although the absolute value of the ventricular AP was almost twice as large as the atrial AP. The AEC values of the atrium and ventricle perfused only with venous blood did not exceed ~0.7 (vs. the maximum of this parameter ~0.9–1.0), apparently reflecting the energy status of tissues initially adapted to hypoxia. Under acute hypoxia, there were found two strategies for energy metabolism transformation in the heart chambers in the form of a 2.4-fold drop in MDH activity (р < 0.05) in the atrium and a 2.2-fold increment in LDH activity (р < 0.05) in the ventricle. Probably, the decline in MDH activity in the atrial tissue was determined by a more passive function of this heart chamber in providing the blood flow. The exposure to acute hypoxia led to a decrease in the level of adenylate nucleotides and an AEC decline in the heart chambers, as pronounced most distinctly in the ventricular myocardium. When decreasing PO2, the AEC in the heart chambers shifted within quite a narrow range (from 0.7 to 0.6), indicating the retention of a certain stationary energy status achieved by inhibition of ATP consumption or demand. The putative mechanism for retaining the AEC may be based on the negative chronotropic effect of hypoxia.

The fish heart is a unique model to compare the resistanceto hypoxia of its two chambers (atrium and ventricle), which aredifferent in the structure and functional loads. The activity of oxydoreductasesmalate dehydrogenase (MDH, 1.1.1.37) and lactate dehydrogenase (LDH, 1.1.1.27),as well as the parameters of the adenylate system in the heart chambersof a Black Sea scorpaena, were studied under acute hypoxia (0.9–1.2mg O2·L–1, 90min). Despite the leading functional role of the ventricle, MDHactivity in this heart compartment tended to decrease compared tothe atrium in the absence of differences in LDH activity. At thesame time, the difference in the level of adenylates (ATP, ADP,AMP), total adenylate pool (AP), and adenylate energy charge (AEС)between the atrium and ventricle was statistically nonsignificant,although the absolute value of the ventricular AP was almost twiceas large as the atrial AP. The AEC values of the atrium and ventricleperfused only with venous blood did not exceed ~0.7 (vs. the maximumof this parameter ~0.9–1.0), apparently reflecting the energy statusof tissues initially adapted to hypoxia. Under acute hypoxia, therewere found two strategies for energy metabolism transformation inthe heart chambers in the form of a 2.4-fold drop in MDH activity(р < 0.05) in the atriumand a 2.2-fold increment in LDH activity (р <0.05) in the ventricle. Probably, the decline in MDH activity inthe atrial tissue was determined by a more passive function of thisheart chamber in providing the blood flow. The exposure to acutehypoxia led to a decrease in the level of adenylate nucleotidesand an AEC decline in the heart chambers, as pronounced most distinctlyin the ventricular myocardium. When decreasing PO2,the AEC in the heart chambers shifted within quite a narrow range(from 0.7 to 0.6), indicating the retention of a certain stationaryenergy status achieved by inhibition of ATP consumption or demand.The putative mechanism for retaining the AEC may be based on thenegative chronotropic effect of hypoxia.

The heart of jawless fish ( Cyclostomata ; lamprey, hagfish) and jawed fish ( Teleostei ) is homologous to the heart of higher vertebrates. A study of this organ in archaic Cyclostomata and Teleostei , which are different in their evolutionary “ages”, genetic characteristics and hypoxia tolerance, is of particular interest in the search for the factors that determine myocardial resistance to oxygen deficiency. Cyclostomata and Teleostei share the same branchial type of respiration and the presence of only one circle of blood circulation. The principal contractile organ that provides blood circulation, the branchial heart, consists of two chambers. Hagfish make up the oldest class of extant vertebrates whose circulation is maintained by the non-innervated (aneural) branchial heart and three sets of accessory “hearts”. Lampreys are the first vertebrates whose heart receives innervation from the vagus nerve. In turn, Teleostei are the first to receive sympathetic innervation of the heart from the vagosympathetic trunks. In the heart of Cyclostomata and Teleostei , no signs of the cardiac conduction system similar to that in higher vertebrates were found, which does not negate the existence of a well-coordinated mechanism for the propagation of myocardial excitation–contraction coupling. The mechanism of heart rhythm generation links the electrical processes that arise in the myocardium with the expression of hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels). In the heart of hagfish and teleosts, six isoforms of HCN channels are expressed. The regulated distribution density of HCN channels in the myocardium may be a precursor of the cardiac conduction system which characterizes higher vertebrates. The expression of the three cardiac HCN isoforms (HCN2, HCN3 and HCN4) in such a relict taxon as hagfish suggests their presence in the myocardium of the common ancestor of vertebrates before the divergence with Myxiniformes . This may also suggest a particular significance of HCN2, HCN3 and HCN4 in the formation of cardiac activity at the time of the emergence of a multi-chambered myogenic heart. It is assumed that the evolutionary progress in the archaic groups of primitive vertebrates was aimed at the “creation” of a faster effector system for the regulation of cardiac activity and dual (excitatory/inhibitory) control of myocardial functions.

The heart of jawless fish (Cyclostomata; lamprey, hagfish) and jawed fish (Teleostei) is homologous to the heart of higher vertebrates. A study of this organ in archaic Cyclostomata and Teleostei, which are different in their evolutionary “ages”, genetic characteristics and hypoxia tolerance, is of particular interest in the search for the factors that determine myocardial resistance to oxygen deficiency. Cyclostomata and Teleostei share the same branchial type of respiration and the presence of only one circle of blood circulation. The principal contractile organ that provides blood circulation, the branchial heart, consists of two chambers. Hagfish make up the oldest class of extant vertebrates whose circulation is maintained by the non-innervated (aneural) branchial heart and three sets of accessory “hearts”. Lampreys are the first vertebrates whose heart receives innervation from the vagus nerve. In turn, Teleostei are the first to receive sympathetic innervation of the heart from the vagosympathetic trunks. In the heart of Cyclostomata and Teleostei, no signs of the cardiac conduction system similar to that in higher vertebrates were found, which does not negate the existence of a well-coordinated mechanism for the propagation of myocardial excitation−contraction coupling. The mechanism of cardiac rhythm generation links the electrical processes that arise in the myocardium with the expression of hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels). In the heart of hagfish and teleosts, six isoforms of HCN channels are expressed. The regulated distribution density of HCN channels in the myocardium may be a precursor of the cardiac conduction system which characterizes higher vertebrates. The expression of the three cardiac HCN isoforms (HCN2, HCN3 and HCN4) in such a relict taxon as hagfish suggests their presence in the myocardium of the common ancestor of vertebrates before the divergence with Myxiniformes. This may also suggest a particular significance of HCN2, HCN3 and HCN4 in the formation of cardiac activity at the time of the emergence of a multi-chambered myogenic heart. It is assumed that the evolutionary progress in the archaic groups of primitive vertebrates was aimed at the “creation” of a faster effector system for the regulation of cardiac activity and dual (excitatory/inhibitory) control of myocardial functions.

A comparative analysis is performed between the oxidoreductase activity (malate and lactate dehydrogenase: MDH, LDH) in the gills (the lamellae of first gill arch) and brain structures (medulla oblongata, middle brain, forebrain, and diencephalon) of Scorpaena porcus L, 1758 under short-term separated exposure to hypoxia (90 min, 1.7–3.7 mg О2/L and 0.3–1.0 mg О2/L) and hydrogen sulfide (5 min, 37 µМ and 74 µМ Na2S). Under experimental conditions, the increase in hypoxic and hydrogen sulfide loading promotes an increase in the interaction in the MDH ↔ LDH activity system in the tissues under study: the highest value of the correlation coefficient is found in the gills (r = 0.87, p < 0.05) and medulla oblongata (r = 0.96, p < 0.01) This paper further considers the functional relationship between the oxidoreductase activities in the tissues and discusses the metabolic effects of hypoxic and H2S loading on the activity of oxidoreductases and the possible mechanisms of the effects.

Hydrogen sulfide can exert a toxic effect and also function as a signaling molecule in various physiological processes. Under conditions of short-term experimental hydrogen sulfide loading (HSL), activities of oxidoreductases (enzymes of energy metabolism and antioxidant defense) - malate dehydrogenase (MDH, 1.1.1.37), lactate dehydrogenase (LDH, 1.1.1.27), catalase (1.11.1.6) – were analyzed in oxyphilic tissues (brain, heart, gills) of the Black Sea scorpionfish Scorpaena porcus Linnaeus, 1758. Two groups of fish were exposed for 5 min to different concentrations of sodium sulfide (37 and 75 μM Na2S) used as a hydrogen sulfide donor. High MDH and LDH activities were detected in the brain structures and heart chambers, reflecting a similar potential of energy metabolism in these tissues, while enhanced LDH activity in the brain indicated anaerobization of the energy metabolism pathways. Differences in oxydoreductase activities were found between the first and fourth branchialarches, heart atrium and ventricle, as well as between different brain compartments. Under HSL, the gills exhibited some signs of hypoxemia (increasing darkening of the lamellae as the Na2S concentration moved upward) and metabolic depression. At a low Na2S concentration, HSL did not cause significant changes in oxidoreductase activities in the brain and heart, while upon a 2-fold increase in the concentration these changes became more pronounced. In the key oxyphilic structures (anterior brain compartments, heart ventricle) intense HSL led to a simultaneous increase in LDH and MDH activities, which was due to the ability of MDH to participate in anaerobic processes. The less O2-sensitive structures (medulla oblongata and heart atrium) exhibited almost no changes in MDH and LDH activities even in response to high-intensity HSL, suggesting a different mode of energy metabolism in these tissues. Under intense HSL, the oxidoreductases in the heart ventricles and anterior brain compartments demonstrated responses, which were similar to those under hypoxia/anoxia.

The effect of short-term hypoxia on the activity of oxidoreductases,malate dehydrogenase (MDH, 1.1.1.37) and lactate dehydrogenase (LDH,1.1.1.27) responsible for urgent adaptation to oxygen deficiency,was studied in the brain and gills (first branchial arch) of theBlack Sea scorpionfish Scorpaena porcus.The control group of fish was kept at 4.5–6.7 mg O2 L–1 (normoxia),experimental groups were exposed to 1.7–3.7 mg O2 L–1 (mildhypoxia) and 0.3–1.0 mg O2 L–1 (acutehypoxia); the exposure time was 90 min, water temperature 21–22°C.The dissolved oxygen level was reduced by saturating water withnitrogen. Hypoxia had no significant impact on the brain structures.Under acute hypoxia, MDH and LDH activities, the MDH/LDH index andATP level in the forebrain, diencephalon and midbrain (FDMB), aswell as in the medulla oblongata (MB), remained at the level ofcontrol values. Mild hypoxia caused a proportional rise in MDH andLDH activities and an increase in the ATP level in FDMB, most likely,due to a reduced demand for ATP in this brain region. This phenomenonis supposed to be based on the GABAergic mechanism of brain activityregulation, which is able to reduce energy demands of nervous tissuedue to increasing the GABAA receptor density.The gills were distinguished by minimum MDH and LDH activities atthe background of high MDH/LDH index values. Acute hypoxia led todecrease LDH activity and increase the MDH/LDH index in the gills, reflectingthereby a transition of this organ to an anaerobic operational mode.Under acute hypoxia, there was detected an increase in the relationshipwithin the “MDH activity ↔ LDH activity” system in all types oftissues (r = 0.81–0.94, p < 0.05–0.01), which is typicalof the species tolerant to oxygen deficiency. Apparently, this effectis based on MDH coupling with glycolytic substrates under conditionsof acute oxygen deficiency, which rules out excessive lactate accumulationunder metabolic depression.

The effect of short-term hypoxia on the activity of oxidoreductases – malate dehydrogenase (MDH, 1.1.1.37) and lactate dehydrogenase (LDH, 1.1.1.27) in the scorpaena organs determining an urgent adaptation to oxygen deficiency – brain and gills (first branchial arch) was studied. The control group of fish was exposed to 4.5–6.7 mg O2 L –1 (normoxia), experimental groups were kept under 1.7–3.7 mg O2 L –1 (mild hypoxia) and to 0.3–1.0 mg O2 L –1 (acute hypoxia). The oxygen content in the water was reduced by saturating it with nitrogen. All groups of fish were ex- posed to examined condition during 90 min under 21–22 ° С. It was shown that hypoxia had not a significant im- pact on the brain structures of scorpaena. Under acute hypoxia MDH, LDH activity, MDH/LDH index and ATP content in forebrain, diencephalon, midbrain (FDMB) and medulla (MB) remained at the level of control values. Mild hypoxia caused a proportional rise of MDH, LDH activity and increased ATP level in FDMB like a consequence of the reducing need in ATP in these brain area. Above-mentioned phenomenon could be based on the GABA-ergic mechanism of the regulation of brain tissue activity that is able to reduce the energetic needs of the nervous tissue through the increased density in GABA A receptor apparatus. The gill tissue had a minimal activity of MDH, LDH on the background of high MDH/LDH index value. Acute hypoxia was accompanied by the drop in LDH activity and increased MDH/LDH which was related to anaerobic mode of operation in gill tissue. Under acute hypoxia there was an increase in the relationship in “MDH activity ↔ LDH activity” system (r = 0.81–0.94, p < 0.05–0.01) in all types of tissues, which is typical for the species tolerant to oxygen deficiency. Probably, this effect is achieved by the conjugation of MDH with glycolysis substrates in the conditions of acute O 2 deficiency, which excludes the accumulation of lactate during metabolic depression.