Adaptations in pectoralis muscle, heart mass, and energy metabolism during premigratory fattening in semipalmated sandpipers (Calidris pusilla)

In late summer, semipalmated sandpipers (Calidris pusilla) migrate nonstop from eastern Canada to wintering sites on the northwest coast of South America. Before their transoceanic flight, the birds feed intensively for about 15 days during which time lipids are stored. The fat-free dry mass of the pectoralis muscle increases during the fattening period, probably increasing the maximal power output of the muscle. Plasma free fatty acids and triglycerides, pectoralis muscle lipid content, and the activity of carnitine oleoyl coenzyme A transferase are higher in heavy (fat) than in light (lean) birds. These alterations imply an enhanced capacity to utilize fatty acids as a metabolic fuel during migration. Total pectoralis muscle glycogen levels and the activity of pyruvate kinase increase, suggesting a higher capacity for glycogenolysis, which may be important during intense levels of energy demand. Heart size and protein content per gram of tissue increase in association with an increase in body mass. However, total levels of key mitochondrial enzymes, citrate synthase and carnitine oleoyl coenzyme A transferase, do not change, suggesting that in the heart an increase in total protein content occurs without an increase in mitochondrial proteins.

Myoglobin supported oxygen consumption in isolated rat hearts under dysoxic conditions

Oxygen consumption was assessed in contracting, isolated rat hearts subjected to Langendorff perfusion. Initially, hearts were perfused with Krebs-Henseleit bicarbonate medium (KHB). Some hearts were treated with a 10 min pulse of medium containing 0.05 mM phenylhydrazine to oxidize approximately 78% of myoglobin to a state incapable of binding oxygen. Stepwise reduction in input PO2 resulted in a decline in oxygen consumption (MO2) in control and treated hearts. Phenylhydrazine treatment had no effect upon MO2 in hearts perfused with medium having a PO2 of about 585 mmHg or higher. However, at an input PO2 of approximately 370 mmHg, MO2 was decreased to 60% of the level at an input PO2 of 710 mmHg in untreated hearts and significantly lower to 32% of initial level in myoglobin blocked hearts. In subsequent experiments, hearts were perfused with KHB containing human red blood cells (RBCs) to elevate the oxygen content of the perfusate. The addition of RBCs to medium having a PO2 of approximately 140 mmHg resulted in enhancement of MO2 and maintenance of performance. But the preparations were considered to be dysoxic since MO2 with RBCs in the medium (PO2 approximately 140 mmHg) was lower than under perfusion with KHB (PO2 approximately 710 mmHg). This should however not detract from the utility of the model in elucidating myoglobin function under oxygen limiting conditions. At an input PO2 of 140 mmHg hearts treated with phenylhydrazine to impede myoglobin function had a significantly lower MO2 and viability than untreated hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty-acid-binding protein facilitates the diffusion of oleate in a model cytosol system

The proposed function of fatty-acid-binding proteins in facilitating the diffusion of their ligand has been examined in a cytosolic model using a steady-state diaphragm cell apparatus. The white muscle of ocean pout (Macrozoarces americanus) does not have a fatty-acid-binding protein: nor does it exhibit detectable beta-oxidative capacity, and as such is a good source of cytosol preparation. After determining the diffusion coefficient of oleate in this medium, fatty-acid-binding protein, isolated from the ventricle of this animal, was added and the apparent diffusion coefficient was again determined. The presence of the fatty-acid-binding protein increased the apparent diffusion coefficient of this long-chain fatty acid about 6-fold, from 0.087 x 10(-5) cm2.s-1 to 0.58 x 10(-5)cm2.s-1. This confirms the facilitated diffusion hypothesis as one role of fatty-acid-binding protein. The diffusion coefficients of oleate in 50 mM-sodium phosphate buffer, pH 7.4, at 10 degrees C and 25 degrees C (0.15 x 10(-5) cm2.s-1 and 0.28 x 10(-5) cm2.s-1 respectively) were also measured.

Effect of acute and chronic temperature transition on enzymes of cardiac metabolism in white perch (Morone americana), yellow perch (Perca flavescens), and smallmouth bass (Micropterus dolomieui)

White perch (Morone americana), yellow perch (Perca flavescens), and smallmouth bass (Micropterus dolomieui) were acclimated to 5 and 20 °C. There was an increase in ventricle mass relative to body mass in smallmouth bass only following acclimation to 5° C. Maximal in vitro activities of hexokinase, citrate synthase, carnitine acyl CoA transferase (with palmitoyl CoA, palmitoleoyl CoA, and oleoyl CoA as substrates), and total ATPase were assessed in crude heart homogenates. Tissues removed from warm-acclimated animals were tested at 20 and 5 °C; tissues removed from cold-acclimated animals were assessed at 5 °C. Acute temperature transitions were associated with decreases in the activities of hexokinase (Q10 ≈ 1.8), citrate synthase (Q10 ≈ 1.4), and ATPase (Q10 ≈ 1.7). The impact of temperature on carnitine acyl CoA transferases was generally less severe. This suggests that maximal fatty acid oxidation is conserved better than glucose oxidation during a warm to cold transition. Maximal enzyme activities were generally unaffected by the acclimation regime, with the exception of that of carnitine acyl CoA transferase in white perch heart. The substantial increase in carnitine acyl CoA transferase activity when unsaturated CoA derivatives were provided as substrate suggests an increased capacity to oxidize unsaturated fatty acids at low temperature following an acclimation period. Attempts to sustantiate this contention by offering labelled oleic acid to ventricle sheets were thwarted by a high rate of incorporation into the total lipid pool.

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 PO2's 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 PO2's than either myoglobin-poor or nitrite-treated hearts. The impact of myoglobin in intact heart is apparent at relatively high extracellular PO2's (40-80 mmHg) in this model system.

Enzyme activity levels underestimate lactate production rates in cod (Gadus morhua) gas gland

Maximal in vitro activities of hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase, β-hydroxyacyl-CoA dehydrogenase, citrate synthase, malate dehydrogenase, and cytochrome oxidase were assessed in cod (Gadus morhua) gas gland. The metabolic profile predicts a substantial anaerobic relative to aerobic metabolism. The effect of catecholamines, acetylcholine, and low pH on in vitro rates of lactate production by gas gland was assessed. Adrenaline and acetylcholine both increased the rate of lactate production even under aerobic incubation conditions. However, the rates of lactate production were well below the capacity suggested by the enzyme levels. It is suggested that the tissue has an abundance of enzymes that operate at submaximal rates.

Regulation of cardiac output and gut blood flow in the sea raven,Hemitripterus americanus

Coeliac artery blood flow (Fca) before and after feeding was recorded in the sea raven. To obtain basic information about the scope of cardiovascular adjustment in the sea raven, a separate series of experiments was performed, in which ventral (Pva), and dorsal (Pda) aortic blood pressure, heart rate (HR) and cardiac output (jaz) were monitored during rest and encouraged exercise.Measurements of coeliac artery flow showed that visceral blood flow is substantial, particularly after feeding, and variations in the visceral vascular conductance affect Pda directly. Simultaneous recordings of intestinal and dorsal aortic blood pressures showed no measurable difference in the two arterial pressures, refuting the idea of a vascular control at the level of the main coeliac artery. Thus, in the sea raven, the adrenergic tonus affecting the visceral vasculature presumably acts at the arteriolar level.Sea ravens encouraged to exercise increased theirjaz by 64%; 32% through HR and 25% through stroke volume. The increase injaz during encouraged exercise was sufficient to produce an elevation of both Pva and Pda, despite an increase of systemic vascular conductance, β-adrenoceptor blockade with sotalol, however, severely impaired the increase injaz during exercise, and the change in Pda was reversed.During rest there were both an adrenergic and a cholinergic tonus affecting the HR, as revealed by the effects of injected pharmacological antagonists. Swimming activity decreased the cholinergic tonus, while the adrenergic tonus increased.

Matching of cardiac oxygen delivery and fuel supply to energy demand in teleosts and cephalopods

Both fish and cephalopods have a single systemic ventricle which performs mechanical work at similar levels in the two groups of organisms. This example of convergent evolution is used to identify common features of cardiac oxygen delivery and fuel supply in relation to energy demand. Foremost, both groups of animals exhibit morphological alterations to enhance oxygen delivery to the myocardium. In cephalopods, the heart is positioned to directly receive oxygenated blood, and coronary arteries can be present in both fish and cephalopods. At the cellular level, oxygen extraction is facilitated by elevated concentrations of myoglobin in the hearts of fish, which face particularly acute low levels of extracellular oxygen. These features represent adaptations to meet the challenge of oxygen delivery. In temperate-zone teleosts and cephalopods, maximal in vitro activity of ATPase reflects maximal in situ oxygen demand. In both groups of animals, there is a linear relationship between maximal in vitro hexokinase and ATPase activities, and in temperate-zone teleosts a similar relationship exists between carnitine palmitoyl transferase and ATPase activities. These enzyme activities are interpreted to reflect adaptations in maximal capacities to utilize glucose and fatty acids in response to increased energy demand.

Perfusion-independent oxygen extraction in myoglobin-rich hearts

Cardiac myoglobin plays a role in oxygen consumption and has a protective effect during periods of hypoxia, but little is known about the role of myoglobin during periods of ischaemia. Myoglobin-rich sea raven hearts and myoglobin-poor ocean pout hearts were isolated and perfused at varying flow rates and under conditions of low and high oxygen demand to assess the role of myoglobin in oxygen extraction. In the myoglobin-rich hearts, oxygen extraction remained constant over the flow range. In the myoglobin-poor hearts, oxygen extraction was significantly elevated, relative to controls, at the lower flow rates but decreased as the flow rate increased. In hearts where myoglobin was inactivated by an oxidizing agent, oxygen extraction was similar to that observed in myoglobin-poor hearts. Under conditions of high oxygen demand, myoglobin-rich hearts again showed a constant oxygen extraction over the flow range. Myoglobin-inactivated hearts had a significantly elevated oxygen extraction at low flows, and this decreased as flow rate increased. These data suggest that myoglobin renders oxygen extraction by fish hearts independent of the rate of perfusion.

Matching of vertebrate cardiac energy demand to energy metabolism

Concentrations of high-energy phosphates and activities of key enzymes of energy metabolism were assessed in hearts from species with differing levels of cardiac power output. Positive correlations were found between resting power output and the total adenylate pool and between citrate synthase activity and the total adenylate pool. Maximum in vitro activity levels of enzymes from energy metabolism were compared with calculated resting cardiac power output and maximal cardiac power output (as reflected by total oligomycin-insensitive adenosine-triphosphatase activity). Three indexes of carbohydrate metabolism (hexokinase, pyruvate kinase, and L-lactate dehydrogenase) all plateau at relatively low levels of energy demand. In contrast, enzymes required for aerobic fatty acid metabolism, (carnitine palmitoyltransferase and 3-hydroxyacyl-CoA dehydrogenase) and for tricarboxylic acid and electron transport (citrate synthase and cytochrome-c oxidase) show consistent increases as ATP demand is elevated. It appears that as capacity for power development by vertebrate hearts, increases across taxa, the elevated demand for ATP is met by expansion of fatty acid based aerobic metabolism and not carbohydrate metabolism.

Enzymes of energy metabolism in salmonid hearts: spongy versus cortical myocardia

The maximal in vitro activity of key enzymes of energy metabolism from the spongy and cortical layers of adult Atlantic salmon (Salmo salar) hearts was determined. Enzymes were also measured in hearts from salmon parr and smolt, and from juvenile and adult trout (Salvelinus fontinalis). Indices of carbohydrate metabolism, hexokinase, and lactate dehydrogenase were significantly higher in the spongy than in the cortical layer of the salmon heart. Markers of aerobically based fatty acid metabolism (carnitine palmitoyl transferase, β-hydroxyacyl CoA dehydrogenase, and cytochrome oxidase) were higher in the cortical region. Although there were clear differences in metabolic organization between the two tissue layers, the absolute magnitude of the enzyme activities suggest that the large variance in oxygen delivery to the two regions does not present a major constraint on aerobic energy metabolism. In both salmon and trout, the hearts from large animals exhibited much higher enzyme activities at many loci than the hearts from small animals. The data imply that there is an increase in metabolic fuel supply on a per gram weight basis in large hearts. This is contrary to the accepted paradigm of scaling of aerobic metabolism. Myoglobin content was highly variable amongst experimental groups. High levels of myoglobin were associated with potential conditions of low extracellular oxygen availability.

Function of myoglobin in oxygen consumption by isolated perfused fish hearts

Myoglobin, an intracellular O2-binding protein, plays a protective role in maintaining performance of isolated fish hearts under hypoxic conditions. This study was designed to test the hypothesis that the protein contributes to O2 consumption under conditions of increased O2 demand or hypoxia. Isolated myoglobin-rich sea raven (Hemitripterus americanus) hearts and myoglobin-poor ocean pout (Macrozoarces americanus) hearts were perfused under conditions of changing partial pressure of O2 (PO2) and afterload. Sea raven hearts maintained O2 consumption and cardiac performance at low PO2 and high afterload, whereas ocean pout hearts did not. In other cases sea raven and ocean pout hearts were treated with hydroxylamine, which renders myoglobin incapable of binding O2, and subjected to changing PO2 and afterload. Sea raven hearts could not maintain O2 consumption and cardiac performance, whereas hydroxylamine treatment had no effect on O2 consumption in ocean pout hearts under these conditions. These data provide the first evidence to support the concept that myoglobin plays a role in O2 consumption of hearts.

Kinetics of lactate dehydrogenase from heart and white muscle of ocean pout: a single isozyme system

Heart and white skeletal muscle of ocean pout (Macrozoarces americanus) express only a single lactate dehydrogenase isozyme based on electrophoretic and immunological analysis. The enzyme has been partially purified and its kinetic properties elucidated in both the pyruvate reductase and lactate oxidase directions. Km values and responses to changing pH catagorize the enzyme as a classical skeletal muscle type lactate dehydrogenase. The kinetic parameters are assessed with respect to known in situ carbon flux rates through this locus. It is concluded that the enzyme data per se provide little insight into either the dominant direction or the net maximal rate of carbon flow.

Force-velocity characteristics and metabolism of carp muscle fibres following temperature acclimation

Common carp (Cyprinus carpio L.), 1 kg body weight, were acclimated for 1–2 months to water temperatures of either 7–8 degrees C (cold-acclimated group) or 23–24 degrees C (warm-acclimated group). Single fast fibres and small bundles of slow fibres were isolated from the myotomal muscles and chemically skinned. Force-velocity (P-V) characteristics were determined at 7 degrees C and 23 degrees C. The contractile properties of carp muscle fibres are dependent on acclimation temperature. In the warm-acclimated group maximum isometric tensions (P0, kN m-2) are 47 +/− 6 and 64 +/− 5 for slow muscle fibres and 76 +/− 10 and 209 +/− 21 for fast muscle fibres at 7 degrees C and 23 degrees C, respectively. Maximum contraction velocities (Vmax, muscle lengths-1), are 0.4 +/− 0.05 and 1.5 +/− 0.1 at 7 degrees C (slow fibres) and 0.6 +/− 0.04 and 1.9 +/− 0.4 at 23 degrees C (fast fibres). All values represent mean +/− S.E. P0 and Vmax at 7 degrees C are around 1.5-2.0 times higher for slow and fast muscle fibres isolated from the cold-acclimated group. Fibres from 7 degrees C-acclimated carp fail to relax completely following maximal activations at 23 degrees C. The resulting Ca-insensitive force component (50–70% P0) is associated with the development of abnormal crossbridge linkages and very slow contraction velocities. Activities of enzymes associated with energy metabolism were determined at a common temperature of 15 degrees C. Marker enzymes of the electron transport system (cytochrome oxidase), citric acid cycle (citrate synthase), fatty acid metabolism (carnitine palmitoyl transferase, beta-hydroxyacyl CoA dehydrogenase) and aerobic glucose utilization (hexokinase) have 30–60% higher activities in slow muscle from cold-acclimated than from warm-acclimated fish. Activities of cytochrome oxidase and citrate synthase in fast muscle are also elevated following acclimation to low temperature. It is concluded that thermal compensation of mechanical power output by carp skeletal muscle is matched by a concomitant increase in the potential to supply aerobically-generated ATP at low temperatures.

Effects of hypoxic adaptation on myocardial performance and metabolism of Zoarces viviparous

Zoarces viviparous were maintained in either normoxic or hypoxic ([Formula: see text], 4–4.7 kPa) water for 4–6 weeks. The hypothesis that adaptation to hypoxia results in an increase in the potential for anaerobic energy production in heart was tested. There was no difference in the activities of key enzymes of energy metabolism or in the content of myoglobin between the hearts from control or experimental fish. However, ventricular strips from animals adapted to hypoxic conditions were better able to sustain tension development than hearts from control animals during anoxia in the presence of high levels of external Ca2+. A combination of high Ca2+ and glucose was particularly effective in improving performance. The data suggest that hypoxic adaptation leads to an enhancement of Ca2+-activated carbohydrate mobilization but that the enzyme complement required to process the additional glycolytic flux is already in place.

Control of lactate oxidation in fish hearts by lactate oxidase activity

Isolated hearts of ocean pout (Macrozoarces americanus) and sea raven (Hemitripterus americanus) were perfused with media containing [14C]lactate or pyruvate and the rate of 14CO2 production was monitored. Increases in exogenous lactate concentration resulted in increases in the rate of lactate metabolism. Under comparable perfusion conditions the rate of decarboxylation of pyruvate was three- to four-fold higher than that of lactate. This finding suggests that lactate oxidation was being limited by lactate oxidase. LDH was purified and the Km values for lactate and pyruvate assessed under conditions of saturating cofactor concentration. Both hearts had a muscle type LDH on the basis of Km (pyruvate). Lactate oxidase from ocean pout and sea raven heart displayed Km values of 25 and 20 mM for lactate, respectively. The Km values were well above the presumptive intracellular level of lactate in the perfused hearts. Considered together, the perfusion and isolated enzyme studies show that the catabolism of exogenous lactate is limited by the reaction catalyzed by lactate oxidase.

Cardiac performance in the in situ perfused fish heart during extracellular acidosis: interactive effects of adrenaline

The physiological integrity of the in situ perfused heart of the ocean pout was established by its ability to maintain cardiac output (Q) over a range of work loads, and by the dependence of Q upon the filling pressure of the heart. Similar observations have been reported previously for the in situ perfused heart of the sea raven. Physiological levels of extracellular acidosis (pH 7.6/1% CO2 and pH 7.4/2% CO2) significantly depressed cardiac performance in sea raven and ocean pout hearts in situ. Negative chronotropic and inotropic responses were observed. Adrenaline (AD; 10(−7) M) under control conditions (pH 7.9/0.5% CO2) produced a sustained tachycardia. The tachycardia reduced filling time of the ventricle and stroke volume was compromised because of the constant preload to the heart. Consequently, AD produced only an initial, transient increase in stroke volume and Q. Thereafter, stroke volume was reduced in proportion with the increase in heart rate, and Q remained unchanged. The combined challenge of extracellular acidosis and AD demonstrated interactive effects between AD and acidosis in situ. Q and power output were maintained in both species at both levels of extracellular acidosis during the combined challenge. Thus AD alone can maintain (but not improve upon) basal Q during extracellular acidosis. The effects of extracellular acidosis, circulating catecholamines and venous return pressure to the heart are discussed in relation to the regulation of Q following exhaustive exercise.