Coronary-supplied compact shell of ventricular myocardium in salmonids: growth and enzyme pattern

Latent impacts on juvenile rainbow trout (Oncorhynchus mykiss) cardio-respiratory function and swimming performance following embryonic exposures to hydraulic fracturing flowback and produced water

Technologies associated with hydraulic fracturing continue to be prevalent in many regions worldwide. As a result, the production of flowback and produced water (FPW) - a wastewater generated once pressure is released from subterranean wellbores - continues to rise in regions experiencing fracturing activities, while waste management strategies attempt to mitigate compounding burdens of increased FPW production. The heightened production of FPW increases the potential for release to the environment. However, relatively few studies have directly investigated how ecosystems and organisms may be latently affected long after exposures occur. The current study examines rainbow trout exposed in ovo at select critical cardiac developmental time points to differing dilutions and lengths of time (acute versus chronic) to determine how FPW-mediated exposure in ovo may alter later cardiac function and development. After exposure, we allowed fish to grow for ∼ 8 months post-fertilization and measured fish swimming performance, aerobic scope, and cardiac structure of juvenile trout. Acute 48 h embryonic 5% FPW exposure at either 3 days post-fertilization (dpf) or 10 dpf significantly reduced later swimming performance and aerobic scope in juvenile trout. In ovo exposure to 2.5% FPW at 3 dpf yielded significant decreases in these metrics as well, while exposing trout to 2.5% FPW at 10 dpf did not induce as significant effects. Morphometric analyses of heart muscle tissue in all treatments decreased compact myocardium thickness. Chronic 1% FPW in ovo exposure for 28 days induced similar reductions in swimming performance, aerobic scope, and decreased compact myocardium thickness as acute exposures. Overall, our results demonstrate that FPW exposure during egg development ultimately results in persistently impaired heart morphology and resulting physiological (swimming) performance.

Cardiac hemodynamics and ventricular stiffness of sea-run cherry salmon (Oncorhynchus masou masou) differ critically from those of landlocked masu salmon

Ventricular diastolic mechanical properties are important determinants of cardiac function and are optimized by changes in cardiac structure and physical properties. Oncorhynchus masou masou is an anadromous migratory fish of the Salmonidae family, and several ecological studies on it have been conducted; however, the cardiac functions of the fish are not well known. Therefore, we investigated ventricular diastolic function in landlocked (masu salmon) and sea-run (cherry salmon) types at 29–30 months post fertilization. Pulsed-wave Doppler echocardiography showed that the atrioventricular inflow waveforms of cherry salmon were biphasic with early diastolic filling and atrial contraction, whereas those of masu salmon were monophasic with atrial contraction. In addition, end-diastolic pressure–volume relationship analysis revealed that the dilatability per unit myocardial mass of the ventricle in cherry salmon was significantly suppressed compared to that in masu salmon, suggesting that the ventricle of the cherry salmon was relatively stiffer (relative ventricular stiffness index; p = 0.0263). Contrastingly, the extensibility of cardiomyocytes, characterized by the expression pattern of Connectin isoforms in their ventricles, was similar in both types. Histological analysis showed that the percentage of the collagen accumulation area in the compact layer of cherry salmon increased compared with that of the masu salmon, which may contribute to ventricle stiffness. Although the heart mass of cherry salmon was about 11-fold greater than that of masu salmon, there was no difference in the morphology of the isolated cardiomyocytes, suggesting that the heart of the cherry salmon grows by cardiomyocyte proliferation, but not cell hypertrophy. The cardiac physiological function of the teleosts varies with differences in their developmental processes and life history. Our multidimensional analysis of the O. masou heart may provide a clue to the process by which the heart acquires a biphasic blood-filling pattern, i.e., a ventricular diastolic suction.

It takes time to heal a broken heart: ventricular plasticity improves heart performance after myocardial infarction in rainbow trout, Oncorhynchus mykiss

Coronary arteriosclerosis is a common feature of both wild and farmed salmonid fishes and may be linked to stress-induced cardiac pathologies. Yet, the plasticity and capacity for long-term myocardial restructuring and recovery following a restriction in coronary blood supply are unknown. Here, we analyzed the consequences of acute (3 days) and chronic (from 33 to 62 days) coronary occlusion (i.e. coronary artery ligation) on cardiac morphological characteristics and in vivo function in juvenile rainbow trout, Oncorhynchus mykiss. Acute coronary artery occlusion resulted in elevated resting heart rate and decreased inter-beat variability, which are both markers of autonomic dysfunction following acute myocardial ischemia, along with severely reduced heart rate scope (maximum-resting heart rate) relative to sham-operated trout. We also observed a loss of myocardial interstitial collagen and compact myocardium. Following long-term coronary artery ligation, resting heart rate and heart rate scope normalized relative to sham-operated trout. Moreover, a distinct fibrous collagen layer separating the compact myocardium into two layers had formed. This may contribute to maintain ventricular integrity across the cardiac cycle or, alternatively, demark a region of the compact myocardium that continues to receive oxygen from the luminal venous blood. Taken together, we demonstrate that rainbow trout may cope with the aversive effects caused by coronary artery obstruction through plastic ventricular remodeling, which, at least in part, restores cardiac performance and myocardium oxygenation.

The Relationship between Myoglobin, Aerobic Capacity, Nitric Oxide Synthase Activity and Mitochondrial Function in Fish Hearts

The dynamic interactions between nitric oxide (NO) and myoglobin (Mb) in the cardiovascular system have received considerable attention. The loss of Mb, the principal O₂ carrier and a NO scavenger/producer, in the heart of some red-blooded fishes provides a unique opportunity for assessing this globin’s role in NO homeostasis and mitochondrial function. We measured Mb content, activities of enzymes of NO and aerobic metabolism [NO Synthase (NOS) and citrate synthase, respectively] and mitochondrial parameters [Complex-I and -I+II respiration, coupling efficiency, reactive oxygen species production/release rates and mitochondrial sensitivity to inhibition by NO (i.e., NO IC₅₀)] in the heart of three species of red-blooded fish. The expression of Mb correlated positively with NOS activity and NO IC₅₀, with low NOS activity and a reduced NO IC₅₀ in the Mb-lacking lumpfish (Cyclopterus lumpus) as compared to the Mb-expressing Atlantic salmon (Salmo salar) and short-horned sculpin (Myoxocephalus scorpius). Collectively, our data show that NO levels are fine-tuned so that NO homeostasis and mitochondrial function are preserved; indicate that compensatory mechanisms are in place to tightly regulate [NO] and mitochondrial function in a species without Mb; and strongly suggest that the NO IC₅₀ for oxidative phosphorylation is closely related to a fish’s hypoxia tolerance.

Cardiac remodeling and increased central venous pressure underlie elevated stroke volume and cardiac output of seawater-acclimated rainbow trout

Substantial increases in cardiac output (CO), stroke volume (SV), and gastrointestinal blood flow are essential for euryhaline rainbow trout ( Oncorhyncus mykiss) osmoregulation in seawater. However, the underlying hemodynamic mechanisms responsible for these changes are unknown. By examining a range of circulatory and cardiac morphological variables of seawater- and freshwater-acclimated rainbow trout, the present study revealed a significantly higher central venous pressure (CVP) in seawater-acclimated trout (~0.09 vs. −0.02 kPa). This serves to increase cardiac end-diastolic volume in seawater and explains the elevations in SV (~0.41 vs. 0.27 ml/kg) and CO (~21.5 vs. 14.2 ml·min−1·kg−1) when compared with trout in freshwater. Furthermore, these hemodynamic modifications coincided with a significant increase in the proportion of compact myocardium, which may be necessary to compensate for the increased wall tension associated with a larger stroke volume. Following a temperature increase from 10 to 16.5°C, both acclimation groups exhibited similar increases in heart rate (Q10 of ~2), but SV tended to decrease in seawater-acclimated trout despite the fact that CVP was maintained in both groups. This resulted in CO of seawater- and freshwater-acclimated trout stabilizing at a similar level after warming (~26 ml·min−1·kg−1). The consistently higher CVP of seawater-acclimated trout suggests that factors other than compromised cardiac filling constrained the SV and CO of these individuals at high temperatures. The present study highlights, for the first time, the complex interacting effects of temperature and water salinity on cardiovascular responses in a euryhaline fish species.

The Dynamic Nature of Hypertrophic and Fibrotic Remodeling of the Fish Ventricle

Chronic pressure or volume overload can cause the vertebrate heart to remodel. The hearts of fish remodel in response to seasonal temperature change. Here we focus on the passive properties of the fish heart. Building upon our previous work on thermal-remodeling of the rainbow trout ventricle, we hypothesized that chronic cooling would initiate fibrotic cardiac remodeling, with increased myocardial stiffness, similar to that seen with pathological hypertrophy in mammals. We hypothesized that, in contrast to pathological hypertrophy in mammals, the remodeling response in fish would be plastic and the opposite response would occur following chronic warming. Rainbow trout held at 10°C (control group) were chronically (>8 weeks) exposed to cooling (5°C) or warming (18°C). Chronic cold induced hypertrophy in the highly trabeculated inner layer of the fish heart, with a 41% increase in myocyte bundle cross-sectional area, and an up-regulation of hypertrophic marker genes. Cold acclimation also increased collagen deposition by 1.7-fold and caused an up-regulation of collagen promoting genes. In contrast, chronic warming reduced myocyte bundle cross-sectional area, expression of hypertrophic markers and collagen deposition. Functionally, the cold-induced fibrosis and hypertrophy were associated with increased passive stiffness of the whole ventricle and with increased micromechanical stiffness of tissue sections. The opposite occurred with chronic warming. These findings suggest chronic cooling in the trout heart invokes a hypertrophic phenotype with increased cardiac stiffness and fibrosis that are associated with pathological hypertrophy in the mammalian heart. The loss of collagen and increased compliance following warming is particularly interesting as it suggests fibrosis may oscillate seasonally in the fish heart, revealing a more dynamic nature than the fibrosis associated with dysfunction in mammals.

Very low embryonic crude oil exposures cause lasting cardiac defects in salmon and herring

The 1989 Exxon Valdez disaster exposed embryos of pink salmon and Pacific herring to crude oil in shoreline spawning habitats throughout Prince William Sound, Alaska. The herring fishery collapsed four years later. The role of the spill, if any, in this decline remains one of the most controversial unanswered questions in modern natural resource injury assessment. Crude oil disrupts excitation-contraction coupling in fish heart muscle cells, and we show here that salmon and herring exposed as embryos to trace levels of crude oil grow into juveniles with abnormal hearts and reduced cardiorespiratory function, the latter a key determinant of individual survival and population recruitment. Oil exposure during cardiogenesis led to specific defects in the outflow tract and compact myocardium, and a hypertrophic response in spongy myocardium, evident in juveniles 7 to 9 months after exposure. The thresholds for developmental cardiotoxicity were remarkably low, suggesting the scale of the Exxon Valdez impact in shoreline spawning habitats was much greater than previously appreciated. Moreover, an irreversible loss of cardiac fitness and consequent increases in delayed mortality in oil-exposed cohorts may have been important contributors to the delayed decline of pink salmon and herring stocks in Prince William Sound.

A mathematical model of the carp heart ventricle during the cardiac cycle

The poikilothermic heart has been suggested as a model for studying some of the mechanisms of early postnatal mammalian heart adaptations. We assessed morphological parameters of the carp heart (Cyprinus carpio L.) with diastolic dimensions: heart radius (5.73mm), thickness of the compact (0.50mm) and spongy myocardium (4.34mm), in two conditions (systole, diastole): volume fraction of the compact myocardium (20.7% systole, 19.6% diastole), spongy myocardium (58.9% systole, 62.8% diastole), trabeculae (37.8% systole, 28.6% diastole), and cavities (41.5% systole, 51.9% diastole) within the ventricle; volume fraction of the trabeculae (64.1% systole, 45.5% diastole) and sinuses (35.9% systole, 54.5% diastole) within the spongy myocardium; ratio between the volume of compact and spongy myocardium (0.35 systole, 0.31 diastole); ratio between compact myocardium and trabeculae (0.55 systole, 0.69 diastole); and surface density of the trabeculae (0.095μm(-1) systole, 0.147μm(-1) diastole). We created a mathematical model of the carp heart based on actual morphometric data to simulate how the compact/spongy myocardium ratio, the permeability of the spongy myocardium, and sinus-trabeculae volume fractions within the spongy myocardium influence stroke volume, stroke work, ejection fraction and p-V diagram. Increasing permeability led to increasing and then decreasing stroke volume and work, and increasing ejection fraction. An increased amount of spongy myocardium led to an increased stroke volume, work, and ejection fraction. Varying sinus-trabeculae volume fractions within the spongy myocardium showed that an increased sinus volume fraction led to an increased stroke volume and work, and a decreased ejection fraction.

Humoral control of cardiac remodeling in fish: role of Angiotensin II

Angiotensin II (AngII), the principal effector of the Renin-Angiotensin-System (RAS), is a multipotent hormone whose biological actions include short-term modulation as well as long-term adjustments. In the eel heart, AngII elicits short-term inotropic and chronotropic effects. However, information regarding the influence of AngII on cardiac remodeling, expressed as morphological and hemodynamic changes, is lacking. To clarify the putative actions of AngII on eel cardiac remodeling, we used freshwater eels (Anguilla anguilla) intraperitoneally injected for 4 weeks with saline or AngII (0.4 or 1.2 nmol g BW(-1)) or AngII (1.2 nmol g BW(-1)) plus the AT₂ receptor antagonist CGP42112. Using an in vitro working heart preparation, the cardiac response (stroke volume changes) to preload and afterload increases has been evaluated. Hearts of all groups showed similar Frank-Starling responses. However, in response to afterload increases, stroke volume rapidly decreased in control hearts, while it was better maintained in AngII-treated counterparts. These effects were abolished by an antagonist of the AT₂ receptor, whose cardiac expression was revealed by western blotting analysis. We also found by immunolocalization and immunoblotting that AngII influences both expression and localization of molecules which regulate cell growth [such as c-kit, heat shock protein 90 (Hsp-90), endothelial Nitric Oxide Synthase "(eNOS)-like" isoform] and apoptosis [i.e. apoptosis repressor with CARD domain (ARC)], thus playing a role in cardiac long-term adjustments. These results point to a role of AngII in eel heart remodeling, providing new insights regarding the modulation of cardiac plasticity in fish.

Cardiac myocyte diversity and a fibroblast network in the junctional region of the zebrafish heart revealed by transmission and serial block-face scanning electron microscopy

The zebrafish has emerged as an important model of heart development and regeneration. While the structural characteristics of the developing and adult zebrafish ventricle have been previously studied, little attention has been paid to the nature of the interface between the compact and spongy myocardium. Here we describe how these two distinct layers are structurally and functionally integrated. We demonstrate by transmission electron microscopy that this interface is complex and composed primarily of a junctional region occupied by collagen, as well as a population of fibroblasts that form a highly complex network. We also describe a continuum of uniquely flattened transitional cardiac myocytes that form a circumferential plate upon which the radially-oriented luminal trabeculae are anchored. In addition, we have uncovered within the transitional ring a subpopulation of markedly electron dense cardiac myocytes. At discrete intervals the transitional cardiac myocytes form contact bridges across the junctional space that are stabilized through localized desmosomes and fascia adherentes junctions with adjacent compact cardiac myocytes. Finally using serial block-face scanning electron microscopy, segmentation and volume reconstruction, we confirm the three-dimensional nature of the junctional region as well as the presence of the sheet-like fibroblast network. These ultrastructural studies demonstrate the previously unrecognized complexity with which the compact and spongy layers are structurally integrated, and provide a new basis for understanding development and regeneration in the zebrafish heart.

Morpho-functional characterization of the goldfish (Carassius auratus L.) heart

Using morphological and physiological approaches we provided, for the first time, a structural and functional characterization of Carassius auratus L. heart. Besides to the classical four chambers, i.e. sinus venosus, atrium, ventricle, bulbus, we described two distinct structures corresponding to the atrio-ventricular (AV) region and the conus arteriosus. The atrium is very large and highly trabeculated; the ventricle shows an outer compacta, vascularized by coronary vessels, and an inner spongiosa; the bulbus wall is characterized by a high elastin/collagen ratio, which makes it extremely compliant. Immunolocalization revealed a strong expression of activated "eNOS-like" isoforms both at coronary endothelium and, to a lesser extent, in the myocardiocytes and the endocardial endothelium (EE). The structural design of the heart appears to comply with its mechanical function. Using an in vitro working heart preparation, cardiac performance was evaluated at different filling and afterload pressures. The hearts were very sensitive to filling pressure increases. Maximum Stroke volume (SV=1.08 ± 0.09 mL/kg body mass) was obtained with an input pressure of 0.4 kPa. The heart was not able to sustain afterload increases, values higher than 1.5 kPa impairing its performance. These morpho-functional features are consistent with a volume pump mechanical performance.

Cortisol response to stress is associated with myocardial remodeling in salmonid fishes

Cardiac disease is frequently reported in farmed animals, and stress has been implicated as a factor for myocardial dysfunction in commercial fish rearing. Cortisol is a major stress hormone in teleosts, and this hormone has adverse effects on the myocardium. Strains of rainbow trout (Oncorhynchus mykiss) selected for divergent post-stress cortisol levels [high responsive (HR) and low responsive (LR)] have been established as a comparative model to examine how fish with contrasting stress-coping styles differ in their physiological and behavioral profiles. We show that the mean cardiosomatic index (CSI) of adult HR fish was 34% higher than in LR fish, mainly because of hypertrophy of the compact myocardium. To characterize the hypertrophy as physiological or pathological, we investigated specific cardiac markers at the transcriptional level. HR hearts had higher mRNA levels of cortisol receptors (MR, GR1 and GR2), increased RCAN1 levels [suggesting enhanced pro-hypertrophic nuclear factor of activated T-cell (NFAT) signaling] and increased VEGF gene expression (reflecting increased angiogenesis). Elevated collagen (Col1a2) expression and deposition in HR hearts supported enhanced fibrosis, whereas the heart failure markers ANP and BNP were not upregulated in HR hearts. To confirm our results outside the selection model, we investigated the effect of acute confinement stress in wild-type European brown trout, Salmo trutta. A positive correlation between post-stress cortisol levels and CSI was observed, supporting an association between enhanced cortisol response and myocardial remodeling. In conclusion, post-stress cortisol production correlates with myocardial remodeling, and coincides with several indicators of heart pathology, well-known from mammalian cardiology.

Coronary vascular volume remodelling in rainbow trout Oncorhynchus mykiss

The 34% increase in relative ventricular mass (Mrv) resulting from chronic anaemia (induced by an intraperitoneal injection of phenylhydrazine hydrochloride) was accompanied by a 117% increase in coronary vascular volume of diploid rainbow trout Oncorhynchus mykiss. Coronary vascular volume of normocythemic triploid fish was similar to that of normocythemic diploid fish despite a larger Mrv. These observations, in combination with previous studies, suggest that the vascularity of compact myocardium in O. mykiss can vary independently of Mrv.

The intercellular organization of the two muscular systems in the adult salmonid heart, the compact and the spongy myocardium

The ventricle of the salmonid heart consists of an outer compact layer of circumferentially arranged cardiomyocytes encasing a spongy myocardium that spans the lumen of the ventricle with a fine arrangement of muscular trabeculae. While many studies have detailed the anatomical structure of fish hearts, few have considered how these two cardiac muscle architectures are attached to form a functional working unit. The present study considers how the spindle-like cardiomyocytes, unlike the more rectangular structure of adult mammalian cardiomyocytes, form perpendicular connections between the two muscle layers that withstand the mechanical forces generated during cardiac systole and permit a simultaneous, coordinated contraction of both ventricular components. Therefore, hearts of rainbow trout (Oncorhynchus mykiss) and sockeye salmon (Oncorhynchus nerka) were investigated in detail using scanning electron microscopy (SEM) and various light microscopic techniques. In contrast to earlier suggestions, we found no evidence for a distinct connective tissue layer between the two muscle architectures that might 'glue' together the compact and the spongy myocardium. Instead, the contact layer between the compact and the spongy myocardium was characterized by a significantly higher amount of desmosome-like (D) and fascia adhaerens-like (FA) adhering junctions compared with either region alone. In addition, we observed that the trabeculae form muscular sheets of fairly uniform thickness and variable width rather than thick cylinders of variable diameter. This sheet-like trabecular anatomy would minimize diffusion distance while maximizing the area of contact between the trabecular muscle and the venous blood as well as the muscle tension generated by a single trabecular sheet.

Tribute to P. L. Lutz: a message from the heart--why hypoxic bradycardia in fishes?

The sensing and processing of hypoxic signals, the responses to these signals and the modulation of these responses by other physical and physiological factors are an immense topic filled with numerous novel and exciting discoveries. Nestled among these discoveries, and in contrast to mammals, is the unusual cardiac response of many fish to environmental hypoxia - a reflex slowing of heart rate. The afferent and efferent arms of this reflex have been characterised, but the benefits of the hypoxic bradycardia remain enigmatic since equivocal results have emerged from experiments examining the benefit to oxygen transfer across the gills. The main thesis developed here is that hypoxic bradycardia could afford a number of direct benefits to the fish heart, largely because the oxygen supply to the spongy myocardium is precarious (i.e. it is determined primarily by the partial pressure of oxygen in venous blood, Pv(O(2))) and, secondarily, because the fish heart has an unusual ability to produce large increases in cardiac stroke volume (V(SH)) that allow cardiac output to be maintained during hypoxic bradycardia. Among the putative benefits of hypoxic bradycardia is an increase in the diastolic residence time of blood in the lumen of the heart, which offers an advantage of increased time for diffusion, and improved cardiac contractility through the negative force-frequency effect. The increase in V(SH) will stretch the cardiac chambers, potentially reducing the diffusion distance for oxygen. Hypoxic bradycardia could also reduce cardiac oxygen demand by reducing cardiac dP/dt and cardiac power output, something that could be masked at cold temperature because of a reduced myocardial work load. While the presence of a coronary circulation in certain fishes decreases the reliance of the heart on Pv(O(2)), hypoxic bradycardia could still benefit oxygen delivery via an extended diastolic period during which peak coronary blood flow occurs. The notable absence of hypoxic bradycardia among fishes that breathe air during aquatic hypoxia and thereby raise their Pv(O(2)), opens the possibility that that the evolutionary loss of hypoxic bradycardia may have coincided with some forms of air breathing in fishes. Experiments are needed to test some of these possibilities. Ultimately, any potential benefit of hypoxic bradycardia must be placed in the proper context of myocardial oxygen supply and demand, and must consider the ability of the fish heart to support its routine cardiac power output through glycolysis.

Cardiac remodelling in rainbow troutOncorhynchus mykissWalbaum in response to phenylhydrazine-induced anaemia

We examined the nature, extent and timing of cardiac ventricular remodelling in response to chronic, chemically induced anaemia in warm- and cold-acclimated rainbow trout Oncorhynchus mykiss. Chronic anaemia was induced by bi-weekly injections of phenylhydrazine hydrochloride (PHZ) and resulted in transient but large decreases in haematocrit (Hct) and haemoglobin concentration. After 2 weeks of anaemia, relative ventricular mass(rMV) in warm-acclimated rainbow trout had already increased significantly and, by the eighth week of anaemia,rMV was 58% greater than in the sham-injected control fish. Temperature modulated the anaemia-induced ventricular remodelling and erythropoietic responses, as indicated by cold-acclimation reducing the extent of the cardiac remodelling and slowing erythropoietic recovery. For example,in cold-acclimated fish, PHZ reduced Hct to 8.8±1.9% (ranging from 4–16%) and increased rMV by 15% over a 4-week period, whereas the same treatment in warm-acclimated fish reduced Hct to only 17.4±2.1% (ranging from 6–29%) and yet increased rMV by 28%. Cold-acclimated fish also recovered more slowly from anaemia. In addition, warm-acclimated fish maintained compact myocardium between 32% and 37% during anaemia, while cold-acclimated fish responded with an increase in compact myocardium (from 29% to 37%). Routine cardiac output (Q̇) was continuously monitored following a single PHZ injection to examine the initial cardiac response to anaemia. Contrary to expectations, acute anaemia did not produce an immediate, proportionate increase in routine Q̇. In fact, Q̇ did not increase significantly until Hct had decreased to 10%, suggesting that rainbow trout may initially rely on venous oxygen stores to compensate for a reduced arterial oxygen-carrying capacity. Thus, we conclude that myocardial oxygenation, acclimation temperature and cardiac work load could all influence anaemia-induced cardiac remodelling in rainbow trout.

Sex differences in energy metabolism and performance of teleost cardiac tissue

This study examined the effects of different oxygenation levels and substrate availability on cardiac performance, metabolism, and biochemistry in sexually immature male and female rainbow trout (Oncorhynchus mykiss). Ventricle strips were electrically paced (0.5 Hz, 14 degrees C) in hyperoxic or hypoxic Ringer solution. Our results demonstrate that 1) males sustain isometric force production (F) longer than females under hyperoxia (P O2 = 640 mmHg) with exogenous glucose present; 2) contractility is not maintained under moderate (P O2 = 130 mmHg) or severe hypoxia (P O2 = 10-20 mmHg) with glucose in either sex; however, following reoxygenation, F is higher in females compared with males; and 3) female tissue has higher lactate levels, net lactate efflux, and lactate dehydrogenase activity than males, whereas males have higher glycogen, citrate synthase, and beta-hydroxy acyl-CoA dehydrogenase activities, and greater inotropic responses to exogenous glucose and octanoate. No sex differences were detected in responsiveness to epinephrine and inhibitors of glucose transport or activities of hexokinase and pyruvate kinase. We conclude that sex differences exist in rainbow trout cardiac tissue: females appear to prefer glycolysis for ATP production, whereas males have a higher capacity for aerobic and lipid metabolism.

Cardiac morphodynamic remodelling in the growing eel (Anguilla anguilla L.)

The morphodynamic changes occurring during growth were evaluated in the eel(Anguilla anguilla L.) heart. Using an in vitro working heart preparation, cardiac performance of small (body mass 96.76±27.49 g; mean ± s.d.) and large (body mass 656±12 g; mean± s.d.) eels was compared under basal conditions and under loading (i.e. preload and afterload) challenges. A parallel morphometric evaluation of the ventricle was made using light and transmission electron microscope images.

The small eel hearts show a basal cardiac output lower than their large counterparts (heart rate fh, 38.93±2.82 and 52.7±1.8 beats min–1, respectively; stroke volume Vs, 0.27±0.017 and 0.37±0.016 ml kg–1, respectively; means ± s.e.m.). The two groups show similar responses at increasing preload, but differ remarkably at increasing afterload. Small eel hearts decreased Vs at afterload greater than 3 kPa, in contrast to larger hearts, which maintained constant Vs up to 6 kPa. These changes in mechanical performance are related to structural differences.

Compared with the small eels, the large eels show an increase in the compacta thickness and in the diameter of the trabeculae in the spongiosa,together with reduction of the lacunary spaces. The increased compacta thickness is attained by enlargements of both the muscular and vascular compartments and reduction of the interstitium; consequently, this layer appears more compacted. Both compacta and spongiosa show higher number of myocytes together with reduced cross-sectional area and myofibrillar compartment. The compacta also shows an increased mitochondrial compartment. Our results document a cardiac morphodynamic remodelling in the growing eel.

Myoarchitecture and vasculature of the heart ventricle in some freshwater teleosts

The morphological characteristics of the ventricular myocardium and of coronary vascularization were studied in three freshwater teleost species, Piaractus mesopotamicus, Colossoma macropomum and Clarias gariepinus (African catfish), by correlating their ventricular shapes and swimming habits. In Piaractus mesopotamicus and Colossoma macropomum, species with highly active swimming habits, the cardiac ventricle showed a pyramidal shape and a richly vascularized myocardium consisting of an outer compact layer and inner spongy layer. In Clarias gariepinus, a less active species, we observed a saccular ventricle with a mixed myocardium and coronary arteries, in contrast to the ventricular structure of other species described in the literature.

Juvenile coho salmon locomotion and mosaic muscle are modified by 3′,3′,5′-tri-iodo-l-thyronine (T(3))

Studies of maximum aerobic swimming performance in smolting juvenile salmonids indicate that these animals may be aerobically compromised during downstream migration. To test our hypothesis that hyperthyroid status contributes to decreased swimming performance through modification of muscle contractility in juvenile (112 mm mean total length) coho salmon (Oncorhynchus kisutch), we measured swimming performance and isolated muscle bundle contractility of fish implanted with 3′,3′,5′-tri-iodo-l-thyronine (T(3)) pellets, of fish implanted with sham pellets and of fish with no pellet implantation (control group). After 3 weeks (N=12-13), critical swimming speeds (maximum aerobic swimming speed or U(crit)) were measured. Muscle bundles (N=15-16) were dissected from the hypaxial musculature and stimulated to measure the force and velocity of an isometric twitch and tetani. T(3)-treated fish demonstrated visible morphological changes associated with smoltification. Mean values of U(crit) were significantly decreased and the prolonged contraction (tetani) and twitch rates of contraction, relaxation and maximum force were significantly increased by T(3) treatment compared with both the sham and control fish. Hematocrit, body mass and body length were not significantly affected by T(3) treatment. In conclusion, we suggest that the reported decrease in U(crit) during salmonid smoltification may be mediated by endogenous T(3)-induced contractile modification of mosaic muscle fibers.

The efficiency of systematic sampling in stereology and its prediction

The superior efficiency of systematic sampling at all levels in stereological studies is emphasized and various commonly used ways of implementing it are briefly described. Summarizing recent theoretical and experimental studies a set of very simple estimators of efficiency are presented and illustrated with a variety of biological examples. In particular, a nomogram for predicting the necessary number of points when performing point counting is provided. The very efficient and simple unbiased estimator of the volume of an arbitrary object based on Cavalieri's principle is dealt with in some detail. The efficiency of the systematic fractionating of an object is also illustrated.

Ventricular myocardial architecture in marine fishes

The fiber architecture of the ventricular myocardium has been studied in elasmobranch (Isurus oxyrhinchus, Galeorhinus galeus, Prionace glauca) and teleost (Xiphias gladius, Thunnus thynnus, Thunnus alalunga) fish species with hearts displaying mixed types of ventricular musculature (compact and trabecular). In all cases, the compact myocardium is organized in layers of fiber bundles with an orderly arrangement within the ventricular walls. The number of these layers appears to be dependent on the relative thickness of the compact myocardium. Differences in the pattern of myocardial fiber arrangement were observed among the different fish species. In elasmobranchs the compact myocardium at the level of the atrioventricular orifice is continuous with the trabeculated myocardium. Furthermore, in elasmobranchs the trabeculated myocardium displays a precise arrangement in arcuate trabeculae running from the auriculoventricular to the conoventricular orifices. In teleosts, the compact myocardium is independent of the trabeculated myocardium and a large number of fibers insert into the bulboventricular fibrous ring. The trabeculated myocardium in these species displays an anarchic arrangement except at the level of the bulboventricular orifice, where the fibers tend to be aligned longitudinally, also being inserted into the fibrous ring. Minor differences, consisting mainly of the presence of extra bundles of fibers, were also observed among different individuals of the same species. The possible relationship between myocardial fiber architecture and ventricular shape is discussed.

Comparative and scaling aspects of heart and body weights with reference to blood supply of cardiac fibers

Relative heart weight (RHW) differs in vertebrates with the ratio 1:20 between extremes (bottom bound fishes--Pleuronectidae--and birds). When plotting heart weight (HW) against body weight (BW) one obtains channels which contain not only vertebrates of the same classes (poikilotherms, small and big mammals and birds) but also animals belonging to different classes: tuna fish data are located in the "small mammalian channel" together with data of large tropical snakes while large mammals (upwards 4000 g) belong to the "bird channel". Reasons for such groupings are not clear and physical activity seems not to be the only reason. When comparing active and non active vertebrates one finds that the RHW is as a rule greater in physically more active poikilotherms and homoiotherms. The RHW is also higher in wild than in domesticated forms the differences appearing after weaning (wild vs laboratory rat). In spongy type of myocardium the growth of cardiac fibers results in restriction of the blood flow through lacunae and the contact between endothelial cells lining growing strands of musculature probably provokes formation of capillaries. The appearance of mixed type of myocardium (outer compact and inner spongy compartments) is not bound to the water to land transition since it occurs also in some fishes; it does not occur or is rare in amphibia and is frequent in reptiles. The compact outer layer comprises a different proportion of the cardiac wall volume (5-73%). Metabolic differences were described between cardiac cells in compact and spongy compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of atrial and ventricular endocardium and cardiac capillary endothelium of the pike

The endocardium of the pike Esox lucius L. consists of an attenuated squamous endothelium, a sparse population of subendothelial cells, and varying amounts of collagen. Fibroblasts are not found in the endocardium, and the collagen is probably produced by the endothelium. Capillary and endocardial endothelium differ structurally in several aspects. Plasmalemmal vesicles are far more abundant in capillary endothelium, while the amount of ribosomes and endoplasmic reticulum is much smaller. Golgi complexes are well developed in endocardial cells, but not in capillary endothelia mainly have a transport function, while the endocardial endothelium is also largely concerned in protein synthesis.