Sarcoplasmic reticulum: a key factor in cardiac contractility of sea bass Dicentrarchus labrax and common sole Solea solea during thermal acclimations

Cardiovascular oxygen transport and peripheral oxygen extraction capacity contribute to acute heat tolerance in European seabass

This study evaluated whether different parameters describing cardiovascular function, energy metabolism, oxygen transport and oxidative stress were related to the critical thermal maximum (CT_MAX) of European seabass (Dicentrarchus labrax) and if there were differential changes in these parameters during and after heat shock in animals with different CT_MAX in order to characterize which physiological features make seabass vulnerable to heat waves. Seabass (n = 621) were tested for CT_MAX and the physiological parameters were measured in individuals with good or poor temperature tolerance before and after a heat shock (change in temperature from 15 °C to 28 °C in 1.5 h). Fish with good thermal tolerance had larger ventricles with higher maximal heart rate during the heat shock than individuals with poor tolerance. Furthermore, they initially had a high ventricular Ca²⁺-ATPase activity, which was reduced to a similar level as in fish with poor tolerance following heat shock. The activity of heart lactate dehydrogenase increased in fish with high tolerance, when they were exposed to heat shock, while the aerobic enzyme activity did not differ between groups. The tolerant individuals had smaller red muscle fibers with higher myoglobin content than the poorly tolerant ones. The poorly tolerant individuals had higher hematocrit, which increased with heat shock in both groups. The poorly tolerant individuals had also higher activity of enzymes related to oxidative stress especially after heat shock. In general, CT_MAX was not depending on merely one physiological factor but several organ and cellular parameters were related to the CT_MAX of seabass and when working in combination they might protect the highly tolerant seabass from future heat waves.

Warm, but not hypoxic acclimation, prolongs ventricular diastole and decreases the protein level of Na+/Ca2+ exchanger to enhance cardiac thermal tolerance in European sea bass

One of the physiological mechanisms that can limit the fish's ability to face hypoxia or elevated temperature, is maximal cardiac performance. Yet, few studies have measured how cardiac electrical activity and associated calcium cycling proteins change with acclimation to those environmental stressors. To examine this, we acclimated European sea bass for 6 weeks to three experimental conditions: a seasonal average temperature in normoxia (16 °C; 100% air sat.), an elevated temperature in normoxia (25 °C; 100% air sat.) and a seasonal average temperature in hypoxia (16 °C; 50% air sat.). Following each acclimation, the electrocardiogram was measured to assess how acclimation affected the different phases of cardiac cycle, the maximal heart rate (fH_max) and cardiac thermal performance during an acute increase of temperature. Whereas warm acclimation prolonged especially the diastolic phase of the ventricular contraction, reduced the fH_max and increased the cardiac arrhythmia temperature (T_ARR), hypoxic acclimation was without effect on these functional indices. We measured the level of two key proteins involved with cellular relaxation of cardiomyocytes, i.e. sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and Na⁺/Ca²⁺ exchanger (NCX). Warm acclimation reduced protein level of both NCX and SERCA and hypoxic acclimation reduced SERCA protein levels without affecting NCX. The changes in ventricular NCX level correlated with the observed changes in diastole duration and fH_max as well as T_ARR. Our results shed new light on mechanisms of cardiac plasticity to environmental stressors and suggest that NCX might be involved with the observed functional changes, yet future studies should also measure its electrophysiological activity.

What determines systemic blood flow in vertebrates?

In the 1950s, Arthur C. Guyton removed the heart from its pedestal in cardiovascular physiology by arguing that cardiac output is primarily regulated by the peripheral vasculature. This is counterintuitive, as modulating heart rate would appear to be the most obvious means of regulating cardiac output. In this Review, we visit recent and classic advances in comparative physiology in light of this concept. Although most vertebrates increase heart rate when oxygen demands rise (e.g. during activity or warming), experimental evidence suggests that this tachycardia is neither necessary nor sufficient to drive a change in cardiac output (i.e. systemic blood flow, Q̇ _sys) under most circumstances. Instead, Q̇ _sys is determined by the interplay between vascular conductance (resistance) and capacitance (which is mainly determined by the venous circulation), with a limited and variable contribution from heart function (myocardial inotropy). This pattern prevails across vertebrates; however, we also highlight the unique adaptations that have evolved in certain vertebrate groups to regulate venous return during diving bradycardia (i.e. inferior caval sphincters in diving mammals and atrial smooth muscle in turtles). Going forward, future investigation of cardiovascular responses to altered metabolic rate should pay equal consideration to the factors influencing venous return and cardiac filling as to the factors dictating cardiac function and heart rate.

Ocean warming combined with lower omega-3 nutritional availability impairs the cardio-respiratory function of a marine fish

Highly unsaturated fatty acids of the omega-3 series (HUFA) are major constituents of cell membranes, yet are poorly synthesised de novo by consumers. Their production, mainly supported by aquatic microalgae, has been decreasing with global change. The consequences of such reductions may be profound for ectotherm consumers, as temperature tightly regulates the HUFA content in cell membranes, maintaining their functionality. Integrating individual, tissue and molecular approaches, we examined the consequences of the combined effects of temperature and HUFA depletion on the key cardio-respiratory functions of the golden grey mullet, an ectotherm grazer of high ecological importance. For 4 months, fish were exposed to two contrasting HUFA diets [4.8% eicosapentaenoic acid (EPA)+docosahexaenoic acid (DHA) on dry matter (DM) versus 0.2% EPA+DHA on DM] at 12 and 20°C. Ventricular force development coupled with gene expression profiles measured on cardiac muscle suggest that combining HUFA depletion with warmer temperatures leads to: (1) a proliferation of sarcolemmal and sarcoplasmic reticulum Ca²⁺ channels and (2) a higher force-generating ability by increasing extracellular Ca²⁺ influx via sarcolemmal channels when the heart has to sustain excessive effort due to stress and/or exercise. At the individual scale, these responses were associated with a greater aerobic scope, maximum metabolic rate and net cost of locomotion, suggesting the higher energy cost of this strategy. This impaired cardiac performance could have wider consequences for other physiological performance such as growth, reproduction or migration, all of which greatly depend on heart function.

Effects of exercise training on excitation-contraction coupling, calcium dynamics and protein expression in the heart of the Neotropical fish Brycon amazonicus

Matrinxã (Brycon amazonicus) is a great swimming performance teleost fish from the Amazon basin. However, the possible cardiac adaptations of this ability are still unknown. Therefore, the aim of the present work was to investigate the effects of prolonged exercise (EX group - 60days under 0.4BL·s^-1) on ventricular contractility by (i) in-vitro analysis of contractility comparing the relative roles of sodium/calcium exchanger (NCX) and sarcoplasmic reticulum (SR) in the excitation-contraction (E-C) coupling and (ii) molecular analysis of NCX, sarcoplasmic reticulum Ca²⁺ ATPase (SERCA2) and phospholamban (PLB) expression and quantification. The exercise training significantly improved twitch tension, cardiac pumping capacity and the contraction rate when compared to controls (CT). Inhibition of the NCX function, replacing Na⁺ by Li⁺ in the physiological solutions, diminished cardiac contractility in the EX group, reduced all analyzed parameters under both high and low stimulation frequencies. The SR blockage, using 10μM ryanodine, caused ~50% tension reduction in CT at most analyzed frequencies while in EX, reductions (34-54%) were only found at higher frequencies. SR inhibition also decreased contraction and relaxation rates in both groups. Additionally, higher post-rest contraction values were recorded for EX, indicating an increase in SR Ca²⁺ loading. Higher NCX and PLB expression rates and lower SERCA2 rates were found in EX. Our data indicate that matrinxã presents a modulation in E-C coupling after exercise-training, enhancing the SR function under higher frequencies. This was the first study to functionally analyze the effects of swimming-induced exercise on fish cardiac E-C coupling.

Intraspecific individual variation of temperature tolerance associated with oxygen demand in the European sea bass (Dicentrarchus labrax)

The European sea bass (Dicentrarchus labrax) is an economically important fish native to the Mediterranean and Northern Atlantic. Its complex life cycle involves many migrations through temperature gradients that affect the energetic demands of swimming. Previous studies have shown large intraspecific variation in swimming performance and temperature tolerance, which could include deleterious and advantageous traits under the evolutionary pressure of climate change. However, little is known of the underlying determinants of this individual variation. We investigated individual variation in temperature tolerance in 30 sea bass by exposing them to a warm temperature challenge test. The eight most temperature-tolerant and eight most temperature-sensitive fish were then studied further to determine maximal swimming speed (U CAT), aerobic scope and post-exercise oxygen consumption. Finally, ventricular contractility in each group was determined using isometric muscle preparations. The temperature-tolerant fish showed lower resting oxygen consumption rates, possessed larger hearts and initially recovered from exhaustive exercise faster than the temperature-sensitive fish. Thus, whole-animal temperature tolerance was associated with important performance traits. However, the temperature-tolerant fish also demonstrated poorer maximal swimming capacity (i.e. lower U CAT) than their temperature-sensitive counterparts, which may indicate a trade-off between temperature tolerance and swimming performance. Interestingly, the larger relative ventricular mass of the temperature-tolerant fish did not equate to greater ventricular contractility, suggesting that larger stroke volumes, rather than greater contractile strength, may be associated with thermal tolerance in this species.

Calcium response of KCl-excited populations of ventricular myocytes from the European sea bass (Dicentrarchus labrax): a promising approach to integrate cell-to-cell heterogeneity in studying the cellular basis of fish cardiac performance

Climate change challenges the capacity of fishes to thrive in their habitat. However, through phenotypic diversity, they demonstrate remarkable resilience to deteriorating conditions. In fish populations, inter-individual variation in a number of fitness-determining physiological traits, including cardiac performance, is classically observed. Information about the cellular bases of inter-individual variability in cardiac performance is scarce including the possible contribution of excitation-contraction (EC) coupling. This study aimed at providing insight into EC coupling-related Ca(2+) response and thermal plasticity in the European sea bass (Dicentrarchus labrax). A cell population approach was used to lay the methodological basis for identifying the cellular determinants of cardiac performance. Fish were acclimated at 12 and 22 °C and changes in intracellular calcium concentration ([Ca(2+)]i) following KCl stimulation were measured using Fura-2, at 12 or 22 °C-test. The increase in [Ca(2+)]i resulted primarily from extracellular Ca(2+) entry but sarcoplasmic reticulum stores were also shown to be involved. As previously reported in sea bass, a modest effect of adrenaline was observed. Moreover, although the response appeared relatively insensitive to an acute temperature change, a difference in Ca(2+) response was observed between 12- and 22 °C-acclimated fish. In particular, a greater increase in [Ca(2+)]i at a high level of adrenaline was observed in 22 °C-acclimated fish that may be related to an improved efficiency of adrenaline under these conditions. In conclusion, this method allows a rapid screening of cellular characteristics. It represents a promising tool to identify the cellular determinants of inter-individual variability in fishes' capacity for environmental adaptation.