Temperature acclimation modifies Na+ current in fish cardiac myocytes

Sodium current preserves electrical excitability in the heart of hibernating ground squirrel (Citellus undulatus)

Hibernating mammals are capable of maintaining normal cardiac function at low temperatures. Excitability of cardiac myocytes crucially depends on the fast sodium current (I_Na), which is decreased in hypothermia due to both depolarization of resting membrane potential and direct negative effect of low temperature. Therefore, I_Na in hibernating mammals should have specific features allowing to maintain excitability of myocardium at low temperatures. The current-voltage dependence of I_Na, its steady-state inactivation and activation and recovery from inactivation were studied in winter hibernating (WH) and summer active (SA) ground squirrels and in rats using whole-cell patch clamp at 10 °C and 20 °C. I_Na peak amplitude and the parameters of steady-state activation and inactivation curves did not differ between SA and WH ground squirrels at both temperatures. However, at both temperatures strong positive shift of activation and inactivation curves by 5-12 mV was observed in both WH and SA ground squirrels if compared to rats. This peculiarity of cardiac I_Na in ground squirrels helps to maintain excitability in conditions of depolarized resting membrane potential. The time course of I_Na recovery from inactivation at 10 °C was faster in WH than in SA ground squirrels, which could ensure normal activation of myocardium during hibernation.

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

Cardiac Toxicity of Cadmium Involves Complex Interactions Among Multiple Ion Currents in Rainbow Trout (Oncorhynchus mykiss) Ventricular Myocytes

Cadmium (Cd²⁺ ) is cardiotoxic to fish, but its effect on the electrical excitability of cardiac myocytes is largely unknown. To this end, we used the whole-cell patch-clamp method to investigate the effects of Cd²⁺ on ventricular action potentials (APs) and major ion currents in rainbow trout (Oncorhynchus mykiss) ventricular myocytes. Trout were acclimated to +4 °C, and APs were measured at the acclimated temperature and elevated temperature (+18 °C). Cd²⁺ (10, 20, and 100 µM) altered the shape of the ventricular AP in a complex manner. The early plateau fell to less positive membrane voltages, and the total duration of AP prolonged. These effects were obvious at both +4 °C and +18 °C. The depression of the early plateau is due to the strong Cd²⁺ -induced inhibition of the L-type calcium (Ca²⁺ ) current (I_CaL ), whereas the prolongation of the AP is an indirect consequence of the I_CaL inhibition: at low voltages of the early plateau, the delayed rectifier potassium (K⁺ ) current (I_Kr ) remains small, delaying repolarization of AP. Cd²⁺ reduced the density and slowed the kinetics of the Na⁺ current (I_Na ) but left the inward rectifier K⁺ current (I_K1 ) intact. These altered cellular and molecular functions can explain several Cd²⁺ -induced changes in impulse conduction of the fish heart, for example, slowed propagation of the AP in atrial and ventricular myocardia (inhibition of I_Na ), delayed relaxation of the ventricle (prolongation of ventricular AP duration), bradycardia, and atrioventricular block (inhibition of I_CaL ). These findings indicate that the cardiotoxicity of Cd²⁺ in fish involves multiple ion currents that are directly and indirectly altered by Cd²⁺ . Through these mechanisms, Cd²⁺ may trigger cardiac arrhythmias and impair myocardial contraction. Elevated temperature (+18 °C) slightly increases Cd²⁺ toxicity in trout ventricular myocytes. Environ Toxicol Chem 2021;40:2874-2885. © 2021 SETAC.

Effect of atrial artificial electrical stimulation on depolarization and repolarization and hemodynamics of the heart ventricle in rainbow trout Oncorhynchus mykiss

The spatial-temporal organization of the activation, repolarization and hemodynamics of the heart ventricle in rainbow trout, Oncorhynchus mykiss, adapted to a temperature of 5-7 °C, were studied from the normal sinus rhythm (21.6 ± 4.9 bpm) to the highest possible heart rhythm (HR) (60 bpm), during which deterioration of the contractile activity of the myocardium occurred. Regardless of the HR, the main pattern of excitation of the heart ventricle was the movement of the depolarization wave from the dorsal areas of the base in the base-apical and ventral directions with the capture of the entire thickness of the walls, with a slight difference in the time of activation of the subendocardium compared to the subepicardium. The increase in HR above the sinus rhythm caused significant shortening of local repolarization durations in all areas and layers (endocardial, intramural and subepicardial) of the heart ventricle. Changes in local durations of repolarization led to an increase in the heterogeneity of repolarization of the ventricular myocardium; as a result, a deterioration of its contractility was observed. In relation to the sinus rhythm, the maximal systolic pressure in the heart ventricle decreased, the diastolic and end-diastolic pressure increased, and the maximum rates of pressure rise and fall decreased. In rainbow trout adapted to a temperature of 5-7 °C at sinus rhythm, the pumping function of the heart was probably within the upper limit of the physiological norm, and a further increase in the heart rate led to a decline in myocardial contractility.

Effects of Na+ channel isoforms and cellular environment on temperature tolerance of cardiac Na+ current in zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss)

Heat tolerance of heart rate in fish is suggested to be limited by impaired electrical excitation of the ventricle due to the antagonistic effects of high temperature on Na+ (INa) and K+ (IK1) ion currents (INa is depressed at high temperatures while IK1 is resistant to them). To examine the role of Na+ channel proteins in heat tolerance of INa, we compared temperature dependencies of zebrafish (Danio rerio, warm-dwelling subtropical species) and rainbow trout (Oncorhynchus mykiss, cold-active temperate species) ventricular INa, and INa generated by the cloned zebrafish and rainbow trout NaV1.4 and NaV1.5 Na+ channels in human embryonic kidney (HEK) cells. Whole-cell patch-clamp recordings showed that zebrafish ventricular INa has better heat tolerance and slower inactivation kinetics than rainbow trout ventricular INa. In contrast, heat tolerance and inactivation kinetics of zebrafish and rainbow trout NaV1.4 channels are similar when expressed in the identical cellular environment of HEK cells. The same applies to NaV1.5 channels. These findings indicate that thermal adaptation of ventricular INa is largely achieved by differential expression of Na+ channel alpha subunits: zebrafish that tolerate higher temperatures mainly express the slower NaV1.5 isoform, while rainbow trout that prefer cold waters mainly express the faster NaV1.4 isoform. Differences in elasticity (stiffness) of the lipid bilayer and/or accessory protein subunits of the channel assembly may also be involved in thermal adaptation of INa. The results are consistent with the hypothesis that slow Na+ channel kinetics are associated with increased heat tolerance of cardiac excitation.

Small G-protein RhoA is a potential inhibitor of cardiac fast sodium current

Small G-proteins of Rho family modulate the activity of several classes of ion channels, including K⁺ channels Kv1.2, Kir2.1, and ERG; Ca²⁺ channels; and epithelial Na⁺ channels. The present study was aimed to check the RhoA potential regulatory effects on Na⁺ current (I_Na) transferred by Na⁺ channel cardiac isoform Na_V1.5 in heterologous expression system and in native rat cardiomyocytes. Whole-cell patch-clamp experiments showed that coexpression of Na_V1.5 with the wild-type RhoA in CHO-K1 cell line caused 2.7-fold decrease of I_Na density with minimal influence on steady-state activation and inactivation. This effect was reproduced by the coexpression with a constitutively active RhoA, but not with a dominant negative RhoA. In isolated ventricular rat cardiomyocytes, a 5-h incubation with the RhoA activator narciclasine (5 × 10^-6 M) reduced the maximal I_Na density by 38.8%. The RhoA-selective inhibitor rhosin (10^-5 M) increased the maximal I_Na density by 25.3%. Experiments with sharp microelectrode recordings in isolated right ventricular wall preparations showed that 5 × 10^-6 M narciclasine induced a significant reduction of action potential upstroke velocity after 2 h of incubation. Thus, RhoA might be considered as a potential negative regulator of sodium channels cardiac isoform Na_V1.5.

Feeling the heat: source–sink mismatch as a mechanism underlying the failure of thermal tolerance

A mechanistic explanation for the tolerance limits of animals at high temperatures is still missing, but one potential target for thermal failure is the electrical signaling off cells and tissues. With this in mind, here I review the effects of high temperature on the electrical excitability of heart, muscle and nerves, and refine a hypothesis regarding high temperature-induced failure of electrical excitation and signal transfer [the temperature-dependent deterioration of electrical excitability (TDEE) hypothesis]. A central tenet of the hypothesis is temperature-dependent mismatch between the depolarizing ion current (i.e. source) of the signaling cell and the repolarizing ion current (i.e. sink) of the receiving cell, which prevents the generation of action potentials (APs) in the latter. A source–sink mismatch can develop in heart, muscles and nerves at high temperatures owing to opposite effects of temperature on source and sink currents. AP propagation is more likely to fail at the sites of structural discontinuities, including electrically coupled cells, synapses and branching points of nerves and muscle, which impose an increased demand of inward current. At these sites, temperature-induced source–sink mismatch can reduce AP frequency, resulting in low-pass filtering or a complete block of signal transmission. In principle, this hypothesis can explain a number of heat-induced effects, including reduced heart rate, reduced synaptic transmission between neurons and reduced impulse transfer from neurons to muscles. The hypothesis is equally valid for ectothermic and endothermic animals, and for both aquatic and terrestrial species. Importantly, the hypothesis is strictly mechanistic and lends itself to experimental falsification.

Polycyclic Aromatic Hydrocarbons Phenanthrene and Retene Modify the Action Potential via Multiple Ion Currents in Rainbow Trout Oncorhynchus mykiss Cardiac Myocytes

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants in aqueous environments. They affect cardiovascular development and function in fishes. The 3-ring PAH phenanthrene has recently been shown to impair cardiac excitation-contraction coupling by inhibiting Ca²⁺ and K⁺ currents in marine warm-water scombrid fishes. To see if similar events take place in a boreal freshwater fish, we studied whether the PAHs phenanthrene and retene (an alkylated phenanthrene) modify the action potential (AP) via effects on Na⁺ (I_Na ), Ca²⁺ (I_CaL ), or K⁺ (I_Kr , I_K1 ) currents in the ventricular myocytes of the rainbow trout (Oncorhynchus mykiss) heart. Electrophysiological characteristics of myocytes were measured using whole-cell patch clamp. Micromolar concentrations of phenanthrene and retene modified the shape of the ventricular AP, and retene profoundly shortened the AP at low micromolar concentrations. Both PAHs increased I_Na and reduced I_CaL and I_Kr , but retene was more potent. Neither of the PAHs had an effect on I_K1 . Our results show that phenanthrene and retene affect cardiac function in rainbow trout by a mechanism that involves multiple cardiac ion channels, and the final outcome of these changes (shortening of AP) is opposite to that observed in scombrid fishes (prolongation of AP). The results also show that retene and aryl hydrocarbon receptor (AhR) agonist have an additional mechanism of toxicity besides the previously known AhR-mediated, transcription-dependent one. Environ Toxicol Chem 2019;38:2145-2153. © 2019 SETAC.

Thermal acclimation and seasonal acclimatization: a comparative study of cardiac response to prolonged temperature change in shorthorn sculpin

Seasonal thermal remodelling (acclimatization) and laboratory thermal remodelling (acclimation) can induce different physiological changes in ectothermic animals. As global temperatures are changing at an increasing rate, there is urgency to understand the compensatory abilities of key organs such as the heart to adjust under natural conditions. Thus, the aim of the present study was to directly compare the acclimatization and acclimatory response within a single eurythermal fish species, the European shorthorn sculpin (Myoxocephalus scorpio). We used current- and voltage-clamp to measure ionic current densities in both isolated atrial and ventricular myocytes from three groups of fish: (1) summer-caught fish kept at 12°C ('summer-acclimated'); (2) summer-caught fish kept at 3°C ('cold acclimated'); and (3) fish caught in March ('winter-acclimatized'). At a common test temperature of 7.5°C, action potential (AP) was shortened by both winter acclimatization and cold acclimation compared with summer acclimation; however, winter acclimatization caused a greater shortening than did cold acclimation. Shortening of AP was achieved mostly by a significant increase in repolarizing current density (I _Kr and I _K1) following winter acclimatization, with cold acclimation having only minor effects. Compared with summer acclimation, the depolarizing L-type calcium current (I _Ca) was larger following winter acclimatization, but again, there was no effect of cold acclimation on I _Ca Interestingly, the other depolarizing current, I _Na, was downregulated at low temperatures. Our further analysis shows that ionic current remodelling is primarily due to changes in ion channel density rather than current kinetics. In summary, acclimatization profoundly modified the electrical activity of the sculpin heart while acclimation to the same temperature for >1.5 months produced very limited remodelling effects.

Temperature- and external K+-dependence of electrical excitation in ventricular myocytes of cod-like fishes

Electrical excitability (EE) is vital for cardiac function and strongly modulated by temperature and external K+ concentration ([K+]o) as formulated in the hypothesis of temperature-dependent deterioration of electrical excitability (TDEE). Since little is known about EE of arctic stenothermic fishes, we tested the TDEE hypothesis on ventricular myocytes of polar cod (Boreogadus saida) and navaga cod (Eleginus navaga) of the Arctic Ocean and those of temperate freshwater burbot (Lota lota). Ventricular action potentials (APs) were elicited in current-clamp experiments at 3, 9 and 15°C, and AP characteristics and the current needed to elicit AP were examined. At 3°C, ventricular APs of polar and navaga cod were similar but differed from that of burbot in having lower rate of AP upstroke and higher rate of repolarization. EE of ventricular myocytes - defined as the ease with which all-or-none APs are triggered - was little affected by acute temperature changes between 3 and 15°C in any species. However, AP duration (APD50) was drastically reduced at higher temperatures. Elevation of [K+]o from 3 to 5.4 and further to 8 mM at 3, 9 and 15°C strongly affected EE and AP characteristics in polar and navaga cod, but less in burbot. In all species, ventricular excitation was resistant to acute temperature elevations, while small increases in [K+]o severely compromised EE, in particular in the marine stenotherms. This suggests that EE of the heart in these Gadiformes species is well equipped against acute warming, but less so against the simultaneous temperature and exercise stresses.

Electrical excitability of roach (Rutilus rutilus) ventricular myocytes: effects of extracellular K+, temperature, and pacing frequency

Exercise, capture, and handling stress in fish can elevate extracellular K+ concentration ([K+]o) with potential impact on heart function in a temperature- and frequency-dependent manner. To this end, the effects of [K+]o on the excitability of ventricular myocytes of winter-acclimatized roach ( Rutilus rutilus) (4 ± 0.5°C) were examined at different test temperatures and varying pacing rates. Frequencies corresponding to in vivo heart rates at 4°C (0.37 Hz), 14°C (1.16 Hz), and 24°C (1.96 Hz) had no significant effect on the excitability of ventricular myocytes. Acute increase of temperature from 4 to 14°C did not affect excitability, but a further rise to 24 markedly decreased excitability: stimulus current and critical depolarization needed to elicit an action potential (AP) were ~25 and 14% higher, respectively, at 24°C than at 4°C and 14°C ( P < 0.05). This depression could be due to temperature-related mismatch between inward Na+ and outward K+ currents. In contrast, an increase of [K+]o from 3 to 5.4 or 8 mM at 24°C reduced the stimulus current needed to trigger AP. However, other aspects of excitability were strongly depressed by high [K+]o: maximum rate of AP upstroke and AP duration were drastically (89 and 50%, respectively) reduced at 8 mM [K+]o in comparison with 3 mM ( P < 0.05). As an extreme case, some myocytes completely failed to elicit all-or-none AP at 8 mM [K+]o at 24°C. Also, amplitude and overshoot of AP were reduced by elevation of [K+]o ( P < 0.05). Although high [K+]o antagonizes the negative effects of high temperature on excitation threshold, the precipitous depression of the rate of AP upstroke and complete loss of excitability in some myocytes suggest that the combination of high temperature and high [K+]o will severely impair ventricular excitability in roach.

Cardiac voltage-gated sodium channel expression and electrophysiological characterization of the sodium current in the zebrafish (Danio rerio) ventricle

Na⁺ channel α-subunit composition of the zebrafish heart and electrophysiological properties of Na⁺ current (I_Na) of zebrafish ventricular myocytes were examined. Eight Na⁺ channel α-subunits were expressed in both atrium and ventricle of the zebrafish heart. Na_v1.5Lb, an orthologue to the human Na_v1.5, was clearly the predominant isoform in both chambers representing 65.2 ± 4.1% and 83.1 ± 2.1% of all Na⁺ channel transcripts in atrium and ventricle, respectively. Na_v1.4b, an orthologue to human Na_v1.4, formed 34.1 ± 4.1 and 16.2 ± 2.0% of the Na⁺ channel transcripts in atrium and ventricle, respectively. The density of I_Na and the rate of action potential upstroke in zebrafish ventricular myocytes at 28 °C were similar to those of human ventricles at the comparable temperature. Na⁺ channel isoforms and the main electrophysiological characteristics of the I_Na are largely similar in zebrafish and human hearts indicating evolutionary conservation of Na⁺ channel composition and function. The zebrafish I_Na differs from the human cardiac I_Na in terms of higher tetrodotoxin sensitivity (IC₅₀-value = 5.3 ± 0.1 nM) and slower inactivation kinetics. The zebrafish I_Na was inhibited with tricaine (MS-222) with an IC₅₀-value of 1.2 ± 0.18 mM (336 mg l^-1), suggesting some care in the use of MS-222 as an anesthetic.

The temperature dependence of electrical excitability in fish hearts

Environmental temperature has pervasive effects on the rate of life processes in ectothermic animals. Animal performance is affected by temperature, but there are finite thermal limits for vital body functions, including contraction of the heart. This Review discusses the electrical excitation that initiates and controls the rate and rhythm of fish cardiac contraction and is therefore a central factor in the temperature-dependent modulation of fish cardiac function. The control of cardiac electrical excitability should be sensitive enough to respond to temperature changes but simultaneously robust enough to protect against cardiac arrhythmia; therefore, the thermal resilience and plasticity of electrical excitation are physiological qualities that may affect the ability of fishes to adjust to climate change. Acute changes in temperature alter the frequency of the heartbeat and the duration of atrial and ventricular action potentials (APs). Prolonged exposure to new thermal conditions induces compensatory changes in ion channel expression and function, which usually partially alleviate the direct effects of temperature on cardiac APs and heart rate. The most heat-sensitive molecular components contributing to the electrical excitation of the fish heart seem to be Na+ channels, which may set the upper thermal limit for the cardiac excitability by compromising the initiation of the cardiac AP at high temperatures. In cardiac and other excitable cells, the different temperature dependencies of the outward K+ current and inward Na+ current may compromise electrical excitability at temperature extremes, a hypothesis termed the temperature-dependent depression of electrical excitation.

Deltamethrin is toxic to the fish (crucian carp, Carassius carassius) heart

Pyrethroids are extensively used for the control of insect pests and disease vectors. Pyrethroids are regarded safe due to their selective toxicity: they are effective against insects but relatively harmless to mammals and birds. Unfortunately, pyrethroids are very toxic to fishes. The high toxicity of pyrethroids to fishes is only partly explained by slow metabolic elimination of pyrethroids, suggesting that some molecular targets in vital organs of the fish body are sensitive to pyrethroids. To this end we tested the effect of deltamethrin (DM) on fish (crucian carp, Carassius carassius) heart function in vitro. In sinoatrial preparations of the crucian carp heart DM (10 μM) caused irregularities in rate and rhythm of atrial beating and strong reductions in force of atrial contraction, thus indicating that DM is arrhythmogenic to the fish heart. Consistent with this, DM (10.0 μM) induced irregularities in electrical activity (surface electrocardiogram) of spontaneous beating hearts in vitro. In isolated ventricular myocytes, DM (0.1-30.0 μM) modified Na(+) current by slowing channel closing and shifting reversal potential and steady-state activation of the current to more negative voltages. Maximally about 48% of the cardiac Na(+) channels were affected by DM with a half-maximal effect occurring at the concentration of 1.3 μM. These findings indicate that DM can be cardiotoxic to the crucian carp and that these effects could be due to DM related changes in Na(+) channel function. These findings indicate that in addition to their neurotoxicity effects pyrethroid could also be cardiotoxic to fishes.

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.

Optical mapping of the electrical activity of isolated adult zebrafish hearts: acute effects of temperature

The zebrafish (Danio rerio) has emerged as an important model for developmental cardiovascular (CV) biology; however, little is known about the cardiac function of the adult zebrafish enabling it to be used as a model of teleost CV biology. Here, we describe electrophysiological parameters, such as heart rate (HR), action potential duration (APD), and atrioventricular (AV) delay, in the zebrafish heart over a range of physiological temperatures (18-28°C). Hearts were isolated and incubated in a potentiometric dye, RH-237, enabling electrical activity assessment in several distinct regions of the heart simultaneously. Integration of a rapid thermoelectric cooling system facilitated the investigation of acute changes in temperature on critical electrophysiological parameters in the zebrafish heart. While intrinsic HR varied considerably between fish, the ex vivo preparation exhibited impressively stable HRs and sinus rhythm for more than 5 h, with a mean HR of 158 ± 9 bpm (means ± SE; n = 20) at 28°C. Atrial and ventricular APDs at 50% repolarization (APD50) were 33 ± 1 ms and 98 ± 2 ms, respectively. Excitation originated in the atrium, and there was an AV delay of 61 ± 3 ms prior to activation of the ventricle at 28°C. APD and AV delay varied between hearts beating at unique HRs; however, APD and AV delay did not appear to be statistically dependent on intrinsic basal HR, likely due to the innate beat-to-beat variability within each heart. As hearts were cooled to 18°C (by 1°C increments), HR decreased by ~40%, and atrial and ventricular APD50 increased by a factor of ~3 and 2, respectively. The increase in APD with cooling was disproportionate at different levels of repolarization, indicating unique temperature sensitivities for ion currents at different phases of the action potential. The effect of temperature was more apparent at lower levels of repolarization and, as a whole, the atrial APD was the cardiac parameter most affected by acute temperature change. In conclusion, this study describes a preparation enabling the in-depth analysis of transmembrane potential dynamics in whole zebrafish hearts. Because the zebrafish offers some critical advantages over the murine model for cardiac electrophysiology, optical mapping studies utilizing zebrafish offer insightful information into the understanding and treatment of human cardiac arrhythmias, as well as serving as a model for other teleosts.

Seasonal acclimatization of the cardiac action potential in the Arctic navaga cod (Eleginus navaga, Gadidae)

Freshwater fishes of north-temperate latitudes adjust electrical excitability of the heart to seasonal temperature changes by changing expression levels of ion channel isoforms. However, little is known about thermal responses of action potential (AP) in the hearts of marine polar fishes. To this end, we examined cardiac AP in the atrial myocardium of the Arctic navaga cod (Eleginus navaga) from the White Sea (Russia) acclimatized to winter (March) and summer (September) seasons. Acute increases in temperature from 4 to 10 °C were associated with increases in heart rate, maximum velocity of AP upstroke and negative resting membrane potential, while duration of AP was shortened in both winter-acclimatized and summer-acclimatized navaga hearts. In winter, there was a compensatory shortening (41.1%) of atrial AP duration and this was associated with a strong increase in transcript expression of Erg K(+) channels, known to produce the rapid component of the delayed rectifier K(+) current, I(Kr). Smaller increases were found in the expression of Kir2.1 channels that produce the inward rectifier K(+) current, I(K1). These findings indicate that the heart of navaga cod has a good acclimatory capacity in electrical excitation of cardiac myocytes, which enables cardiac function in the cold-eurythermal waters of the subarctic White Sea.

Effects of deltamethrin on excitability and contractility of the rainbow trout (Oncorhynchus mykiss) heart

Pyrethroids are extensively used for the control of pest insects and disease vectors. Pyrethroid use is regarded safe due to their selective toxicity: they are effective against insects but relatively harmless to mammals and birds. Unfortunately, pyrethroids are very toxic to fishes. The high toxicity of pyrethroids to fishes is only partly explained by slow elimination rate of toxins, suggesting that high affinity binding to their molecular targets, the Na(+) channels, is involved. This study tests the hypothesis that Na(+) channels of the fish heart are targets to a type II pyrethroid, deltamethrin (DM), and therefore pyrethroids are cardiotoxic to fishes. In ventricular myocytes of the rainbow trout (Oncorhynchus mykiss) heart DM (10(-7)-3·10(-5) M) modified Na(+) current by slowing inactivation and shifting the reversal potential of the current to the left. Maximally 31±2% of the cardiac Na(+) channels were modified by DM and the half-maximal effect occurred at the concentration of 2.1 μM. The effect of DM on trout cardiac Na(+) channels is stronger and occurs about an order of magnitude lower in concentration in comparison to the orthologous mammalian Na(+) channels. In sinoatrial preparations of the trout heart DM (10 μM) caused irregularities in rate, rhythm and force of the heartbeat indicating that DM can be arrhythmogenic for the trout heart. Consistent with this, DM (>0.1 μM) induced spontaneous action potentials in otherwise quiescent ventricular myocytes. DM (10 μM) did not affect calcium current or inward rectifier and delayed rectifier potassium currents. Collectively, these findings indicate that DM exerts cardiotoxic effects in trout, and suggest that the high sensitivity of fishes to pyrethroid toxicity might be partially due to the high affinity of fish Na(+) channels to pyrethroids.

Acute heat tolerance of cardiac excitation in the brown trout (Salmo trutta fario)

The upper thermal tolerance and mechanisms of heat-induced cardiac failure in the brown trout (Salmo trutta fario) was examined. The point above which ion channel function and sinoatrial contractility in vitro, and electrocardiogram (ECG) in vivo, started to fail (break point temperature, BPT) was determined by acute temperature increases. In general, electrical excitation of the heart was most sensitive to heat in the intact animal (electrocardiogram, ECG) and least sensitive in isolated cardiac myocytes (ion currents). BPTs of Ca(2+) and K(+) currents of cardiac myocytes were much higher (>28°C) than BPT of in vivo heart rate (23.5 ± 0.6°C) (P<0.05). A striking exception among sarcolemmal ion conductances was the Na(+) current (INa), which was the most heat-sensitive molecular function, with a BPT of 20.9 ± 0.5°C. The low heat tolerance of INa was reflected as a low BPT for the rate of action potential upstroke in vitro (21.7 ± 1.2°C) and the velocity of impulse transmission in vivo (21.9 ± 2.2°C). These findings from different levels of biological organization strongly suggest that heat-dependent deterioration of Na(+) channel function disturbs normal spread of electrical excitation over the heart, leading to progressive variability of cardiac rhythmicity (missed beats, bursts of fast beating), reduction of heart rate and finally cessation of the normal heartbeat. Among the cardiac ion currents INa is 'the weakest link' and possibly a limiting factor for upper thermal tolerance of electrical excitation in the brown trout heart. Heat sensitivity of INa may result from functional requirements for very high flux rates and fast gating kinetics of the Na(+) channels, i.e. a trade-off between high catalytic activity and thermal stability.

Atlantic cod (Gadus morhua L.) in situ cardiac performance at cold temperatures: long-term acclimation, acute thermal challenge and the role of adrenaline

The resting and maximum in situ cardiac performance of Newfoundland Atlantic cod (Gadus morhua L.) acclimated to 10, 4 and 0°C were measured at their respective acclimation temperatures, and when acutely exposed to temperature changes: i.e. hearts from 10°C fish cooled to 4°C, and hearts from 4°C fish measured at 10°C and 0°C. Intrinsic heart rate (fH) decreased from 41 beats min-1 (bpm) at 10°C to 33 bpm at 4°C and to 25 bpm at 0°C. However, this degree of thermal dependency was not reflected in maximal cardiac output. Qmax values were ~44, ~37 and ~34 ml min-1 kg-1 at 10, 4 and 0°C, respectively. Further, cardiac scope showed a slight positive compensation between 4 and 0°C (Q10 = 1.7), and full, if not a slight over compensation between 10 and 4°C (Q10 = 0.9). The maximal performance of hearts exposed to an acute decrease in temperature (i.e. from 10°C to 4°C and 4°C to 0°C) was comparable to that measured for hearts from 4 and 0°C acclimated fish, respectively. In contrast, 4°C acclimated hearts significantly out-performed 10°C acclimated hearts when tested at a common temperature of 10°C (in terms of both Qmax and power output). Only minimal differences in cardiac function were seen between hearts stimulated with basal (5 nM) vs. maximal (200 nM) levels of adrenaline, the effects of which were not temperature dependant. These results: 1) show that maximum performance of the isolated cod heart is not compromised by exposure to cold temperatures; and 2) support data from other studies which show that, in contrast to salmonids, cod cardiac performance/myocardial contractility is not dependent upon humoral adrenergic stimulation.

Tetrodotoxin sensitivity of the vertebrate cardiac Na+ current

Evolutionary origin and physiological significance of the tetrodotoxin (TTX) resistance of the vertebrate cardiac Na⁺ current (I_Na) is still unresolved. To this end, TTX sensitivity of the cardiac I_Na was examined in cardiac myocytes of a cyclostome (lamprey), three teleost fishes (crucian carp, burbot and rainbow trout), a clawed frog, a snake (viper) and a bird (quail). In lamprey, teleost fishes, frog and bird the cardiac I_Na was highly TTX-sensitive with EC₅₀-values between 1.4 and 6.6 nmol·L⁻¹. In the snake heart, about 80% of the I_Na was TTX-resistant with EC₅₀ value of 0.65 μmol·L⁻¹, the rest being TTX-sensitive (EC₅₀ = 0.5 nmol·L⁻¹). Although TTX-resistance of the cardiac I_Na appears to be limited to mammals and reptiles, the presence of TTX-resistant isoform of Na⁺ channel in the lamprey heart suggest an early evolutionary origin of the TTX-resistance, perhaps in the common ancestor of all vertebrates.

Thermal adaptation of the crucian carp (Carassius carassius) cardiac delayed rectifier current, IKs, by homomeric assembly of Kv7.1 subunits without MinK

Ectothermic vertebrates experience acute and chronic temperature changes which affect cardiac excitability and may threaten electrical stability of the heart. Nevertheless, ectothermic hearts function over wide range of temperatures without cardiac arrhythmias, probably due to special molecular adaptations. We examine function and molecular basis of the slow delayed rectifier K+ current ( IKs) in cardiac myocytes of a eurythermic fish ( Carassius carassius L.). IKs is an important repolarizing current that prevents excessive prolongation of cardiac action potential, but it is extremely slowly activating when expressed in typical molecular composition of the endothermic animals. Comparison of the IKs of the crucian carp atrial myocytes with the currents produced by homomeric Kv7.1 and heteromeric Kv7.1/MinK channels in Chinese hamster ovary cells indicates that activation kinetics and pharmacological properties of the IKs are similar to those of the homomeric Kv7.1 channels. Consistently with electrophysiological properties and homomeric Kv7.1 channel composition, atrial transcript expression of the MinK subunit is only 1.6–1.9% of the expression level of the Kv7.1 subunit. Since activation kinetics of the homomeric Kv7.1 channels is much faster than activation of the heteromeric Kv7.1/MinK channels, the homomeric Kv7.1 composition of the crucian carp cardiac IKs is thermally adaptive: the slow delayed rectifier channels can open despite low body temperatures and curtail the duration of cardiac action potential in ectothermic crucian carp. We suggest that the homomeric Kv7.1 channel assembly is an evolutionary thermal adaptation of ectothermic hearts and the heteromeric Kv7.1/MinK channels evolved later to adapt IKs to high body temperature of endotherms.

Sinoatrial tissue of crucian carp heart has only negative contractile responses to autonomic agonists

Background

In the anoxia-tolerant crucian carp (Carassius carassius) cardiac activity varies according to the seasons. To clarify the role of autonomic nervous control in modulation of cardiac activity, responses of atrial contraction and heart rate (HR) to carbacholine (CCh) and isoprenaline (Iso) were determined in fish acclimatized to winter (4°C, cold-acclimated, CA) and summer (18°C, warm-acclimated, WA) temperatures.

Results

Inhibitory action of CCh was much stronger on atrial contractility than HR. CCh reduced force of atrial contraction at an order of magnitude lower concentrations (EC₅₀ 2.75-3.5·10^-8 M) in comparison to its depressive effect on HR (EC₅₀ 1.23-2.02·10^-7 M) (P < 0.05) without differences between winter and summer acclimatized fish. Inhibition of nitric oxide synthase with 100 μM L-NMMA did not change the response of the sinoatrial tissue to CCh. Reduction of atrial force was associated with a strong shortening of action potential (AP) duration to ~50% (48 ± 10 and 50 ± 6% for CA and WA fish, respectively) and 11% (11 ± 3 and 11 ± 2% for CA and WA fish, respectively) of the control value at 3·10^-8 M and 10^-7 M CCh, respectively (P < 0.05). In atrial myocytes, CCh induced an inwardly rectifying K⁺ current, I_K,CCh, with an EC₅₀ value of 3-4.5·10^-7 M and inhibited Ca²⁺ current (I_Ca) by 28 ± 8% and 51 ± 6% at 10^-7 M and 10^-6 M, respectively. These currents can explain the shortening of AP. Iso did not elicit any responses in crucian carp sinoatrial preparations nor did it have any effect on atrial I_Ca, probably due to the saturation of the β-adrenergic cascade in the basal state.

Conclusion

In the crucian carp, HR and force of atrial contraction show cardio-depressive responses to the cholinergic agonist, but do not have any responses to the β-adrenergic agonist. The scope of inhibitory regulation by CCh is increased by the high basal tone of the adenylate cyclase-cAMP cascade. Higher concentrations of CCh were required to induce I_K,CCh and inhibit I_Ca than was needed for CCh's negative inotropic effect on atrial muscle suggesting that neither I_K,CCh nor I_Ca alone can mediate CCh's actions but they might synergistically reduce AP duration and atrial force production. Autonomic responses were similar in CA winter fish and WA summer fish indicating that cardiac sensitivity to external modulation by the autonomic nervous system is not involved in seasonal acclimatization of the crucian carp heart to cold and anoxic winter conditions.

Cardiac survival in anoxia-tolerant vertebrates: An electrophysiological perspective

Certain vertebrates, such as freshwater turtles of the genus Chrysemys and Trachemys and crucian carp (Carassius carassius), have anoxia-tolerant hearts that continue to function throughout prolonged periods of anoxia (up to many months) due to successful balancing of cellular ATP supply and demand. In the present review, we summarize the current and limited understanding of the cellular mechanisms underlying this cardiac anoxia tolerance. What emerges is that cold temperature substantially modifies cardiac electrophysiology to precondition the heart for winter anoxia. Intrinsic heart rate is slowed and density of sarcolemmal ion currents substantially modified to alter cardiac action potential (AP) characteristics. These changes depress cardiac activity and reduce the energetic costs associated with ion pumping. In contrast, anoxia per se results in limited changes to cardiac AP shape or ion current densities in turtle and crucian carp, suggesting that anoxic modifications of cardiac electrophysiology to reduce ATP demand are not extensive. Additionally, as knowledge of cellular physiology in non-mammalian vertebrates is still in its infancy, we briefly discuss the cellular defense mechanisms towards the acidosis that accompanies anoxia as well as mammalian cardiac models of hypoxia/ischemia tolerance. By examining if fundamental cellular mechanisms have been conserved during the evolution of anoxia tolerance we hope to have provided a framework for the design of future experiments investigating cardiac cellular mechanisms of anoxia survival.

Fish cardiac sodium channels are tetrodotoxin sensitive

AIM

Sodium current (I(Na)) of the mammalian heart is resistant to tetrodotoxin (TTX) due to low TTX affinity of the cardiac sodium channel (Na(v)) isoform Na(v)1.5. To test applicability of this finding to other vertebrates, TTX sensitivity of the fish cardiac I(Na) and its molecular identity were examined.

METHODS

Molecular cloning and whole-cell patch-clamp were used to examine alpha-subunit composition and TTX inhibition of the rainbow trout (Oncorhynchus mykiss) cardiac Na(v) respectively.

RESULTS

I(Na) of the trout heart is about 1000 times more sensitive to TTX (IC50 = 1.8-2 nm) than the mammalian cardiac I(Na) and it is produced by three Na(v)alpha-subunits which are orthologs to mammalian skeletal muscle Na(v)1.4, cardiac Na(v)1.5 and peripheral nervous system Na(v)1.6 isoforms respectively. Oncorhynchus mykiss (om) omNa(v)1.4a is the predominant isoform of the trout heart accounting for over 80% of the Na(v) transcripts, while omNa(v)1.5a forms about 18% and omNa(v)1.6a only 0.1% of the transcripts. OmNa(v)1.4a and omNa(v)1.6a have aromatic amino acids, phenylalanine and tyrosine, respectively, in the critical position 401 of the TTX binding site of the domain I, which confers their high TTX sensitivity. More surprisingly, omNa(v)1.5a also has an aromatic tyrosine in this position, instead of the cysteine of the mammalian TTX-resistant Na(v)1.5.

CONCLUSIONS

The ortholog of the mammalian skeletal muscle isoform, omNa(v)1.4a, is the predominant Na(v)alpha-subunit in the trout heart, and all trout cardiac isoforms have an aromatic residue in position 401 rendering the fish cardiac I(Na) highly sensitive to TTX.

Effect of temperature and prolonged anoxia exposure on electrophysiological properties of the turtle (Trachemys scripta) heart

Cardiac activity of the turtle (Trachemys scripta) is greatly depressed with cold acclimation and anoxia. We examined what electrophysiological modifications accompany and perhaps facilitate this depression of cardiac activity. Turtles were first acclimated to 21 degrees C or 5 degrees C and held under either normoxic or anoxic (6 h at 21 degrees C; 14 days at 5 degrees C) conditions. We then measured cardiac action potentials (APs) using spontaneously contracting whole heart preparations and whole cell current densities of sarcolemmal ion channels using isolated ventricular myocytes under appropriate normoxic and anoxic conditions. Compared with 21 degrees C-acclimated turtles, 5 degrees C-acclimated turtles exhibited a less negative resting membrane potential (by 18-29 mV), a 4.7- to 6.8-fold slower AP upstroke rate, and a 4.2- to 4.9-fold greater AP duration. Correspondingly, peak densities of ventricular voltage-gated Na(+) (I(Na)) and L-type Ca(2+) currents and inward slope conductances of inward rectifier K(+) (I(K1)) channel current were approximately 1/7th (Q(10) = 3.4), 1/13th (Q(10) = 5.0), and one-half (Q(10) = 1.4) of those of 21 degrees C-acclimated ventricular myocytes, respectively. With anoxia at 21 degrees C, peak I(Na) density doubled and ventricular AP duration increased by 47%, a change proportional to the reported approximately 30% reduction of intrinsic heart rate. In contrast, with anoxia at 5 degrees C, ventricular AP characteristics were unaffected; of the ion currents investigated, only the inward conductance via I(K1) changed significantly (reduced by 46%). The present findings indicate that cold temperature, more so than prolonged anoxia, results in substantial modifications of cardiac APs and reduction of ventricular ion current densities. These changes likely prepare cardiac muscle for winter anoxia conditions.

Significance of Na+ current in the excitability of atrial and ventricular myocardium of the fish heart

The present study examines the importance of the Na+ current (INa) in the excitability of atrial and ventricular myocardium of the rainbow trout heart. Whole-cell patch-clamp under reduced sarcolemmal Na+ gradient showed that the density of INa is similar in atrial and ventricular myocytes of the trout heart, and the same result was obtained when INa was elicited by chamber-specific action potentials (AP) in normal physiological saline solution. However, the maximum rate (Vmax) of AP upstroke, measured with microelectrodes in intact trout heart, was 21% larger in atrium than ventricle, and thus in variance with the similar INa density of the two myocyte types. Furthermore, Vmax calculated from the INa was 2.1 and 3.2 times larger for atrium and ventricle, respectively, than the values obtained from the APs. The discrepancy between INa of isolated myocytes and Vmax of intact muscle is only partly explained by the inward rectifier K+ current (IK1), which overlaps INa and decreases the net depolarising current. Clear differences exist in the voltage dependence of steady-state activation and inactivation as well as in the inactivation kinetics of INa between atrial and ventricular myocytes. As a result of a more negative voltage dependence of INa activation, smaller IK1 and higher input resistance of atrial myocytes, the voltage threshold for AP generation is more negative in atrium than ventricle of the trout heart. These findings suggest that atrial muscle is more readily excitable than ventricular muscle, and this difference is partly due to the properties of the atrial INa.