William R. Driedzic

8 – Metabolism in Fish During Exercise

This chapter describes the biochemical events involved in the transition from a resting to an actively swimming condition. The chapter deals with metabolism in myotomal muscle, but there are other organs, integrally important for the maintenance of homeostasis, that may be extremely expensive to operate during sustained swimming. For instance, the cost of osmoregulation, in freshwater, for actively swimming rainbow trout ( Salmo gairdneri ), may be as high as 30% of the total metabolism of the fish. Much of this energy expenditure must be borne by the gills. The cost of maintaining the cardiac pump may also be quite substantial. In fact, at high oxygen consumption rates, this may approach 15% of the total metabolism of the animal. Thus, the myotomal muscle per se may account for only about 50% of the total energy output. In light of this, it is somewhat surprising that tissues other than skeletal muscle are rarely studied by fish physiologists interested in metabolic alterations during swimming. The chapter describes the control of metabolism in these support tissues and skeletal muscle; however, the understanding of metabolism in fish tissues is at best fragmentary. Hence, where appropriate, the better-studied mammalian systems are called upon to complement and supplement the discussion.

Ca2+ protection from the negative inotropic effect of contraction frequency on teleost hearts

SummaryIsometric tension development by ventricular strips of 9 species of teleosts, a frog and a turtle was assessed at varying contraction frequencies and Cao (external calcium concentration). With teleost hearts an increase in contraction frequency at constant Cao was always associated with a decrease in tension development; however, under comparable conditions a positive staircase was exhibited by the frog and turtle heart preparations. The reaction of the teleost heart was thus very different from the well established response of the hearts of higher vertebrates. Elevations in Cao always resulted in an increase in tension development such that the positive inotropic effect of Cao could compensate for the negative effect of a high contraction frequency.Perfused isolated cod hearts exhibited an increase in cardiac output and pressure development as a result of increases in Cao. At 30 contractions min−1 a transition from 1–2 mM Cao led to a 68% increase in performance defined as the product of cardiac output times pressure development. The response was in excess of that of ventricular strips. At low Cao increases in rate from in situ resting levels to the high end of the physiological range resulted in a decrease in performance. Increases in Cao were able to ameliorate the detrimental effect of high imposed contraction frequency.In conclusion, both ventricular strip and perfused heart experiments show that a positive inotropic effect of increased Cao can compensate for or even surpass the negative effect of high contraction frequency when both variables are at physiological levels. This finding could have relevance to the maintenance of cardiac performance during/or following intense swimming when both heart rate and plasma calcium may be elevated.

CARDIAC ADAPTATIONS TO LOW TEMPERATURE IN NON-POLAR TELEOST FISH

Acclimation of teleost fish to low temperature results in a number of well-characterized adaptive responses in skeletal muscle. Our understanding of the response of cardiac muscle to such a challenge though is still poorly understood. It is considered of interest to learn more about the impact of chronic low temperature exposure on the cardiac response since the heart is required to support other tissues through the delivery of oxygen and metabolic fuels and this system may be a useful model in which to identify loci of cardiac plasticity. An overview of the available data reveals a number of fundamental responses. These include an increase in heart mass, an increase in maximal contraction frequency, an increase in complex lipid biosynthesis, and an increase in aerobic-based fatty acid metabolism. Two different strategies seem to emerge and lead to working hypotheses. As one example, rainbow trout develop larger hearts which may lead to enhanced stroke volume. The net accretion of protein is associated with either a more effective use of the available protein synthetic machinery and/or a decrease in protein degradation. The net accretion of lipid is associated with an increased rate of biosynthesis. Energy metabolism at low temperature appears to be much more dependent upon the oxidation of fatty acids. As another example, perch acclimated to low temperature show enhanced maximal rates of contraction which may be associated with increase in the volume of sarcoplasmic reticulum. The metabolic processes necessary to supply ATP appear quite adequate and require no further expansion as evidenced by constant mitochondrial volume density and activity of mitochondrial marker enzymes. The data however, in both cases are fragmentary and call for comprehensive studies on selected species.

Enzymes of cardiac energy metabolism in Amazonian teleosts and the fresh‐water stingray (Potamotrygon hystrix)

The maximal in vitro activity of key enzymes of energy metabolism was determined in heart from three Amazonian teleosts, matrincha (Brycon cephalus), acara acu (Astronaotus ocellatus) and tambaqui (Collossoma macropomum), as well as an elasmobranch, the fresh-water stingray (Potamotrygon hystrix). All species are obligate water breathers. Hearts of Amazonian teleosts have activity levels of the glycolytic enzymes hexokinase (HK), phosphofructokinase (PFG), pyruvate kinase (PK), and lactate dehydrogenase (LDH) similar to north temperate and Antarctic species when comparisons are made within the usual body temperature range. In contrast, activity level of enzymes required for aerobic oxidation of fatty acids, citrate synthase (CS), carnitine palmitoyl transferase (CPT), and 3-hydroxyacyl CoA dehydrogenase (HOAD) were all substantially lower in the Amazonian teleosts compared to other teleosts. The enzyme profile suggests that 1) activity levels of enzymes of carbohydrate metabolism are conserved over a wide range of body temperatures, and 2) Amazonian teleosts have a much greater reliance upon anaerobic metabolism from glucose than aerobic metabolism to sustain energy production. The heart of fresh-water stingray has high levels of CS, HK, PFK, and PK, implying an aerobic metabolism which is glucose based. In contrast to marine elasmobranchs, the fresh-water stingray has detectable levels of CPT and HOAD, suggestive of a capacity for low-level fatty acid catabolism. As such, the inability of muscle of marine elasmobranchs to utilize fatty acids as an energy source may not be common feature of all elasmobranchs.

Myoglobin content and the activities of enzymes of energy metabolism in red and white fish hearts

SummaryThe myoglobin content of representative red and white coloured fish hearts was quantitated. It was confirmed that the macroscopic difference in appcarance is due to the presence or absence of myoglobin. Thereafter, the cytochrome c content as well as the maximal activities of key enzymes of energy metabolism were assessed in myoglobin-rich sea raven (Hemitripterus americanus) and myoglobin-poor ocean pout (Macrozoarces americanus) hearts. Both species are sluggish benthic dwellers that occur in similar habitats in the North Atlantic Ocean. The activities of enzymes associated with aerobic metabolism were similar in sea raven and ocean pout hearts, but far in excess of activities observed from white skeletal muscle. The two hearts also displayed comparable activities of enzymes associated with anaerobic energy metabolism. It therefore appears that the capacity to produce reducing equivalents for the electron transport system is similar in two selected fish hearts despite great differences in myoglobin content.

5 - Cardiac Energy Metabolism

This chapter discusses the cardiac energy metabolism of fishes. Sustained heart performance requires an ongoing match between energy demand and supply. Teleosts and elasmobranchs exhibit similar levels of cardiac power output/g heart and by extension, similar metabolic supplies. There appears to be about a two- to fivefold scope for increases in power output, which must be tracked by energy production mechanisms. Maximal in vivo power development by commonly studied species at 10°C–20°C is about 7 mW/g, which is about the resting power development of the heart from a 2.5-kg mammal operating at 37°C and about one half of the maximal power output of the fish white muscle performing oscillatory work cycles. The maximal in vitro activity of enzymes expressed in tissue homogenates is often used to assess the organization of energy metabolism in vivo . There is a sound theoretical and experimentally supported basis for the use of enzymes that catalyze rate-limiting and nonequilibrium reactions as the predictors of metabolic flux rates in vivo . The metabolism in epicardial and endocardial layers is also described in the chapter.

Heart performance, Ca2+ regulation and energy metabolism at high temperatures in Bathygobius soporator, a tropical marine teleost

Experiments were conducted with Bathygobius soporator (Gobbiidae), a small tropical marine teleost which lives in tide pools along the east coast of South America. n nIn whole animals, VO2 remained constant from 25 to 30°C and then increased until it reached a maximum value at 40°C of about 160 ml·kg−1·h−1. The fH increased progressively and significantly from 25 to 35°C, at which fH reached its maximum value of about 225 beats·min−1. At 40°C, however, the fH decreased to a value similar to that recorded at 25°C. n nTwitch force and resting tension were determined for isolated ventricle strips. At an extracellular Ca2+ level of 1.25 mM a transition from 25 to 40°C resulted in a decrease in twitch force which was restored upon a return to 25°C. This restoration of twitch force did not occur at an extracellular Ca2+ concentration of 9.25 mM. n nAt 25°C, increments in extracellular Ca2+ from 1.25 to 7.25 mM resulted in increases in twitch force development However, at 40°C only resting tension increased in concert with elevations in Ca2+. n nAt 25°C, twitch force declined as frequency was increased above 30 contractions·min−1 and became irregular above 120 contractions·min−1. At 40°C, twitch force development remained constant at frequencies up to about 150 contractions·min−1 and declined thereafter. Preparations were able to maintain rhythmic response up to about 240 contractions·min−1. n nAn increase in in vitro assay temperature from 25 to 40°C resulted in an elevation of total ATPase activity. Citrate synthase was present in high activities with hexokinase and 3-hydroxyacyl coenzyme A (CoA) being detectable but at lower activities. n nHeart performance is fragile at high temperature and under conditions which lead to high intracellular Ca2+. A controlled decrease in heart rate at high temperature may have a protective effect in maintaining low levels of intracellular Ca2+.

OXYGEN UPTAKE BY ISOLATED PERFUSED FISH HEARTS WITH DIFFERING MYOGLOBIN CONCENTRATIONS UNDER HYPOXIC CONDITIONS

Hearts from three species of fish with varying myoglobin content were perfused with stepwise changes in input perfusate PO2 from approximately 160 to 10 mmHg. Flow through the heart, rate of contraction, and afterload were kept constant. This standardized stroke volume and bulk flow of perfusate to the myocytes since these hearts are nourished by the fluid in the ventricular lumen. In some cases NaNO2 was added to the perfusion medium to decrease existing levels of functional myoglobin. Myoglobin-rich hearts were able to extract a constant amount of oxygen until perfusate PO2 had fallen below 80 mmHg. At this point oxygen uptake began to decline. These hearts consumed oxygen until input PO2 was 10 mmHg or less. When normoxic conditions were restored the myoglobin-rich hearts showed complete recovery. Performance was maintained at a constant level over the entire range of input PO2. Myoglobin-poor hearts and nitrite-treated hearts were unable to sustain constant levels of oxygen consumption in the face of a declining perfusate PO2. These hearts were unable to extract oxygen from the medium and failed at perfusate PO2s of 40 mmHg for naturally myoglobin-poor hearts and 30 mmHg for nitrite-treated hearts. Half-maximal oxygen consumptions were attained by myoglobin-rich hearts at lower input PO2s than either myoglobin-poor or nitrite-treated hearts. The impact of myoglobin in intact heart is apparent at relatively high extracellular PO2s (40-80 mmHg) in this model system.

Amino Acids Are a Source of Glycerol in Cold-Acclimatized Rainbow Smelt

Abstract Rainbow smelt ( Osmerus mordax ) produce high concentrations of glycerol in winter to protect against freezing. The glycerol is lost to the environment and so must be continually replaced. Glycogen has previously been shown to be a source of glycerol in this species, but glycerol levels are maintained even when glycogen levels have been depleted. In the present study, using radiolabeled substrates, it is qualitatively shown that the carbon skeletons of alanine and glutamate are largely converted to glycerol and glucose in −1°C-acclimatized smelt, suggesting that protein is another important source of glycerol. Bicarbonate carbon was also largely converted to glycerol and glucose, apparently through its involvement in the conversion of amino acids to oxaloacetate. Together, these results indicate an active gluconeogenesis in winter-acclimatized rainbow smelt.

Neuropeptide Y-like immunoreactivity in the cardiovascular nerve plexus of the elasmobranchs Raja erinacea and Raja radiata

SummaryThe distribution of nerves showing neuropeptide Y (NPY)-like immunoreactivity in the cardiovascular system of elasmobranchs and teleosts has been investigated. Two species of teleosts, the rainbow trout (Salmo gairdneri) and the Atlantic cod (Gadus morhua), and three species of elasmobranchs, the spiny dogfish (Squalus acanthias), the little skate (Raja erinacea) and the starry ray (Raja radiata), were used in this study. An innervation of the cardiovascular system by an NPY-like substance was found only in the two species of Raja. A rich innervation was encountered in these skates, with the highest density of fibres in the wall of the ventricle, the conus arteriosus, the coeliac artery and smaller mesenterial vessels. In the vessels, the fibres formed a plexus at the adventitio-mediol border. Few fibres were found in the walls of the dorsal aorta, the sinus venosus and the atrium, and no fibres were observed in the walls of the ventral aorta. Falck-Hillarp fluorescence histochemistry showed the presence of a rich innervation of arteries and arterioles of the gut by catecholamine-containing nerve fibres.