Species-specific differences and temperature-dependence of Na+/K+-ATPase in freshwater mussels Anodonta anatina and Unio tumidus (Bivalvia: Unionidae).

The predicted global warming of surface waters can be challenging to aquatic ectotherms like freshwater mussels. Especially animals in northern temperate latitudes may face and physiologically acclimate to significant stress from seasonal temperature fluctuations. Na+/K+-ATPase enzyme is one of the key mechanisms that allow mussels to cope with changing water temperatures. This enzyme plays a major role in osmoregulation, energy control, ion balance, metabolite transport and electrical excitability. Here, we experimentally studied the effects of temperature on Na+/K+-ATPase activity of gills in two freshwater mussel species, Anodonta anatina and Unio tumidus. The study animals were acclimated to three ambient temperatures (+4, +14, +24 °C) and Na+/K+-ATPase activity was measured at those temperatures for each acclimation group. Both species had their highest gill Na+/K+-ATPase activity at the highest acclimation temperature. Na+/K+-ATPase activity of gills exhibited species-specific differences, and was higher in A. anatina than U. tumidus in all test groups at all test temperatures. Temperature dependence of Na+/K+-ATPase was confirmed in both species, being highest at temperatures between +4 and + 14 °C when Q10 values in the acclimation groups varied between 5.06 and 6.71. Our results underline the importance of Na+/K+-ATPase of gills for the freshwater mussels in warming waters. Because Na+/K+-ATPase is the driving force behind ciliary motion, our results also suggest that in warming waters A. anatina may be more tolerant at sustaining vigorous ciliary action (associated with elevated respiration rates and filter-feeding) than U. tumidus. Overall, our results indicate great flexibility of the mussel's ecophysiological characteristics as response to changing conditions.

Dual effect of polyaromatic hydrocarbons on sarco(endo)plasmic reticulum calcium ATPase (SERCA) activity of a teleost fish (Oncorhynchus mykiss).

Polycyclic aromatic hydrocarbons (PAHs) are embryo- and cardiotoxic to fish that might be associated with improper intracellular Ca2+ management. Since sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) is a major regulator of intracellular Ca2+, the SERCA activity and the contractile properties of rainbow trout (Oncorhynchus mykiss) ventricle were measured in the presence of 3- and 4-cyclic PAHs. In unfractionated ventricular homogenates, acute exposure of SERCA to 0.1-1.0 µM phenanthrene (Phe), retene (Ret), fluoranthene (Flu), or pyrene (Pyr) resulted in concentration-dependent increase in SERCA activity, except for the Flu exposure, with maximal effects of 49.7-83 % at 1 µM. However, PAH mixture did not affect the contractile parameters of trout ventricular strips. Similarly, all PAHs, except Ret, increased the myotomal SERCA activity, but with lower effect (27.8-40.8 % at 1 µM). To investigate the putative chronic effects of PAHs on SERCA, the atp2a2a gene encoding trout cardiac SERCA was expressed in human embryonic kidney (HEK) cells. Culture of HEK cells in the presence of 0.3-1.0 µM Phe, Ret, Flu, and Pyr for 4 days suppressed SERCA expression in a concentration-dependent manner, with maximal inhibition of 49 %, 65 %, 39 % (P < 0.05), and 18 % (P > 0.05), respectively at 1 µM. Current findings indicate divergent effects of submicromolar PAH concentrations on SERCA: stimulation of SERCA activity in acute exposure and inhibition of SERCA expression in chronic exposure. The depressed expression of SERCA is likely to contribute to the embryo- and cardiotoxicity of PAHs by depressing muscle function and altering gene expression.

Dual effect of metals on branchial and renal Na,K-ATPase activity in thermally acclimated crucian carp (Carassius carassius) and rainbow trout (Oncorhynchus mykiss).

Heavy metals are harmful to aquatic animals by disrupting their ionic balance. Here, we compare the effects of three metals, zinc (Zn), nickel (Ni) and manganese (Mn) on Na,K-ATPase activity in gills and kidneys in fish species with different ecophysiological characteristics. Crucian carp (Carassius carassius), a cold-dormant species, and rainbow trout (Oncorhynchus mykiss), a cold-active species, were acclimated to 2 °C and 18 °C, and branchial and renal Na,K-ATPase activities were measure in the presence of Zn, Ni and Mn. Under basal conditions, species-, tissues- and temperature-dependent differences appeared in Na,K-ATPase activity. Renal Na,K-ATPase activity was higher in trout than carp, and cold-acclimation increased Na,K-ATPase activity in both species. Cold-acclimation reduced branchial Na,K-ATPase activity in carp, but no acclimation effect was found in trout. In both species and tissues, Zn stimulated Na,K-ATPase in concentration-dependent manner at 0.1 to 3 µM. At 30 µM, Zn strongly inhibited both branchial and renal Na,K-ATPase in both species. Inhibition by Zn was stronger in trout than carp, but no differences existed between acclimation groups in either species. Ni (0.1-3.0 µM) stimulated renal Na,K-ATPase in crucian carp but not in rainbow trout. At 30 µM, Ni depressed the renal Na,K-ATPase of carp back to the control level. Mn had no statistically significant effect on Na,K-ATPase in either species. At low concentrations, Zn and Ni impose an energetic cost to fish by increasing ATP consumption in Na,K-ATPase activity. At higher concentrations, Zn, but not Ni and Mn, strongly inhibit renal and branchial Na,K-ATPase. Due to differences in baseline activity level and acclimation-induced changes in renal and branchial Na,K-ATPase, metal pollution may impair ion regulation of fish in species-specific manner and depending on season.

High temperature and hyperkalemia cause exit block of action potentials at the atrioventricular junction of rainbow trout (Oncorhynchus mykiss) heart.

At critically high temperatures, atrioventricular (AV) block causes ventricular bradycardia and collapse of cardiac output in fish. Here, the possible role of the AV canal in high temperature-induced heart failure was examined. To this end, optical mapping was used to measure action potential (AP) conduction in isolated AV junction preparations of the rainbow trout (Oncorhynchus mykiss) heart during acute warming/cooling in the presence of 4 or 8 mM external K+ concentration. The preparation included the AV canal and some atrial and ventricular tissue at its edges, and it was paced either from atrial or ventricular side at a frequency of 0.67 Hz (40 beats min-1) to trigger forward (anterograde) and backward (retrograde) conduction, respectively. The propagation of AP was fast in atrial and ventricular tissues, but much slower in the AV canal, causing an AV delay. Acute warming from 15 °C to 27 °C or cooling from 15 °C to 5 °C did not impair AP conduction in the AV canal, as both anterograde and retrograde excitations propagated regularly through the AV canal. In contrast, anterograde conduction through the AV canal did not trigger ventricular excitation at the boundary zone between the AV canal and the ventricle when extracellular K+ concentration was raised from 4 mM to 8 mM at 27 °C. Also, the retrograde conduction was blocked at the border between the AV canal and the atrium in high K+ at 27 °C. These findings suggest that the AV canal is resistant against high temperatures (and high K+), but the ventricular muscle cannot be excited by APs coming from the AV canal when temperature and external K+ concentration are simultaneously elevated. Therefore, bradycardia at high temperatures in fish may occur due to inability of AP of the AV canal to trigger ventricular AP at the junctional zone between the AV canal and the proximal part of the ventricle.

Spatial uniformity of action potentials indicates base-to-apex depolarization and repolarization of rainbow trout (Oncorhynchus mykiss) ventricle.

The spatial pattern of electrical activation is crucial for a full understanding of fish heart function. However, it remains unclear whether there is regional variation in action potential (AP) morphologies and underlying ion currents. Because the direction of depolarization and spatial differences in the durations of ventricular APs set limits to potential patterns of ventricular repolarization, we determined AP morphologies, underlying ion currents and ion channel expression in four different ventricular regions (spongy myocardium; and apex, base and middle of the compact myocardium), and correlated them with in vivo electrocardiograms (ECGs) in rainbow trout (Oncorhynchus mykiss). ECGs recorded from three leads indicated that the depolarization and repolarization of APs propagate from base to apex, and the main depolarization axis of the ventricle is between +90 and +120 deg. AP shape was uniform across the whole ventricle, and little regional differences were found in the density of repolarizing K+ currents or depolarizing Ca2+ and Na+ currents and the underlying transcripts of ion channels, providing compelling evidence for the suggested excitation pattern. The spatial uniformity of AP durations and base-to-apex propagation of activation with a relatively slow velocity of propagation indicates no special ventricular conduction pathway in the trout ventricle such as the His-Purkinje system of mammalian hearts. The sequence of repolarization is solely determined by activation time without being affected by regional differences in AP duration.

Effect of Channel Assembly (KCNQ1 or KCNQ1 + KCNE1) on the Response of Zebrafish IKs Current to IKs Inhibitors and Activators.

ABSTRACT: In cardiac myocytes, the slow component of the delayed rectifier K+ current (IKs) ensures repolarization of action potential during beta-adrenergic activation or when other repolarizing K+ currents fail. As a key factor of cardiac repolarization, IKs should be present in model species used for cardiovascular drug screening, preferably with pharmacological characteristics similar to those of the human IKs. To this end, we investigated the effects of inhibitors and activators of the IKs on KCNQ1 and KCNQ1 + KCNE1 channels of the zebrafish, an important model species, in Chinese hamster ovary cells. Inhibitors of IKs, chromanol 293B and HMR-1556, inhibited zebrafish IKs channels with approximately similar potency as that of mammalian IKs. Chromanol 293B concentration for half-maximal inhibition (IC50) of zebrafish IKs was at 13.1 ± 5.8 and 13.4 ± 2.8 µM for KCNQ1 and KCNQ1+KCNE1 channels, respectively. HMR-1556 was a more potent inhibitor of zebrafish IKs channels with IC50 = 0.1 ± 0.1 µM and 1.5 ± 0.8 µM for KCNQ1 and KCNQ1 + KCNE1 channels, respectively. R-L3 and mefenamic acid, generally identified as IKs activators, both inhibited zebrafish IKs. R-L3 almost completely inhibited the current generated by KCNQ1 and KCNQ1 + KCNE1 channels with similar potency (IC50 1.1 ± 0.4 and 1.0 ± 0.4 µM, respectively). Mefenamic acid partially blocked zebrafish KCNQ1 (IC50 = 9.5 ± 4.8 µM) and completely blocked KCNQ1 + KCNE1 channels (IC50 = 3.3 ± 1.8 µM). Although zebrafish IKs channels respond to IKs inhibitors in the same way as mammalian IKs channels, their response to activators is atypical, probably because of the differences in the binding domain of KCNE1 to KCNQ1. Therefore, care must be taken when translating the results from zebrafish to humans.

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 (ICaL ), whereas the prolongation of the AP is an indirect consequence of the ICaL inhibition: at low voltages of the early plateau, the delayed rectifier potassium (K+ ) current (IKr ) remains small, delaying repolarization of AP. Cd2+ reduced the density and slowed the kinetics of the Na+ current (INa ) but left the inward rectifier K+ current (IK1 ) 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 INa ), delayed relaxation of the ventricle (prolongation of ventricular AP duration), bradycardia, and atrioventricular block (inhibition of ICaL ). 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.

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.

Ionic basis of atrioventricular conduction: ion channel expression and sarcolemmal ion currents of the atrioventricular canal of the rainbow trout (Oncorhynchus mykiss) heart.

Atrioventricular (AV) nodal tissue synchronizes activities of atria and ventricles of the vertebrate heart and is also a potential site of cardiac arrhythmia, e.g., under acute heat stress. Since ion channel composition and ion currents of the fish AV canal have not been previously studied, we measured major cation currents and transcript expression of ion channels in rainbow trout (Oncorhynchus mykiss) AV tissue. Both ion current densities and expression of ion channel transcripts indicate that the fish AV canal has a characteristic electrophysiological phenotype that differs from those of sinoatrial tissue, atrium and ventricle. Two types of cardiomyocytes were distinguished electrophysiologically in trout AV nodal tissue: the one (transitional cell) is functionally intermediate between working atrial/ventricular myocytes and the other (AV nodal cell) has a less negative resting membrane potential than atrial and ventricular myocytes and is a more similar to the sinoatrial nodal cells in ion channel composition. The AV nodal cells are characterized by a small or non-existent inward rectifier potassium current (IK1), low density of fast sodium current (INa) and relatively high expression of T-type calcium channels (CACNA3.1). Pacemaker channel (HCN4 and HCN2) transcripts were expressed in the AV nodal tissue but If current was not found in enzymatically isolated nodal myocytes. The electrophysiological properties of the rainbow trout nodal cells are appropriate for a slow rate of action potential conduction (small INa) and a moderate propensity for pacemaking activity (absence of IK1).

Effects of acute warming on cardiac and myotomal sarco(endo)plasmic reticulum ATPase (SERCA) of thermally acclimated brown trout (Salmo trutta).

At high temperatures, ventricular beating rate collapses and depresses cardiac output in fish. The role of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in thermal tolerance of ventricular function was examined in brown trout (Salmo trutta) by measuring heart SERCA and comparing it to that of the dorsolateral myotomal muscle. Activity of SERCA was measured from crude homogenates of cold-acclimated (+ 3 °C, c.a.) and warm-acclimated (+ 13 °C, w.a.) brown trout as cyclopiazonic acid (20 µM) sensitive Ca2+-ATPase between + 3 and + 33 °C. Activity of the heart SERCA was significantly higher in c.a. than w.a. trout and increased strongly between + 3 and + 23 °C with linear Arrhenius plots but started to plateau between + 23 and + 33 °C in both acclimation groups. The rate of thermal inactivation of the heart SERCA at + 35 °C was similar in c.a. and w.a. fish. Activity of the muscle SERCA was less temperature dependent and more heat resistant than that of the heart SERCA and showed linear Arrhenius plots between + 3 and + 33 °C in both c.a. and w.a. fish. SERCA activity of the c.a. muscle was slightly higher than that of w.a. muscle. The rate of thermal inactivation at + 40 °C was similar for both c.a. and w.a. muscle SERCA at + 40 °C. Although the heart SERCA is more sensitive to high temperatures than the muscle SERCA, it is unlikely to be a limiting factor for heart rate, because its heat tolerance, unlike that of the ventricular beating rate, was not changed by temperature acclimation.

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.

Reduced ventricular excitability causes atrioventricular block and depression of heart rate in fish at critically high temperatures.

At critically high temperature, cardiac output in fish collapses as a result of depression of heart rate (bradycardia). However, the cause of bradycardia remains unresolved. To investigate this, rainbow trout (Oncorhynchus mykiss; acclimated at 12°C) were exposed to acute warming while electrocardiograms were recorded. From 12°C to 25.3°C, electrical excitation between different parts of the heart was coordinated, but above 25.3°C, atrial and ventricular beating rates became partly dissociated because of 2:1 atrioventricular (AV) block. With further warming, atrial rate increased to a peak value of 188±22 beats min-1 at 27°C, whereas the ventricle rate peaked at 124±10 beats min-1 at 25.3°C and thereafter dropped to 111±15 beats min-1 at 27°C. In single ventricular myocytes, warming from 12°C to 25°C attenuated electrical excitability as evidenced by increases in rheobase current and the size of critical depolarization required to trigger action potential. Depression of excitability was caused by temperature-induced decrease in input resistance (sarcolemmal K+ leak via the outward IK1 current) of resting myocytes and decrease in inward charge transfer by the Na+ current (INa) of active myocytes. Collectively, these findings show that at critically high temperatures AV block causes ventricular bradycardia owing to the increased excitation threshold of the ventricle, which is due to changes in the passive (resting ion leak) and active (inward charge movement) electrical properties of ventricular myocytes. The sequence of events from the level of ion channels to cardiac function in vivo provides a mechanistic explanation for the depression of cardiac output in fish at critically high temperature.

Transcript expression of inward rectifier potassium channels of Kir2 subfamily in Arctic marine and freshwater fish species.

Inward rectifier K+ (Kir2) channels are critical for electrical excitability of cardiac myocytes. Here, we examine expression of Kir2 channels in the heart of three Gadiformes species, polar cod (Boreogadus saida) and navaga (Eleginus nawaga) of the Arctic Ocean and burbot (Lota lota) of the temperate lakes to find out the role of Kir2 channels in cardiac adaptation to cold. Five boreal freshwater species: brown trout (Salmo trutta fario), arctic char (Salvelinus alpinus), roach (Rutilus rutilus), perch (Perca fluviatilis) and pike (Esox lucius), and zebrafish (Danio rerio), were included for comparison. Transcript expression of genes encoding Kir2.1a, - 2.1b, - 2.2a, - 2.2b and - 2.4 was studied from atrium and ventricle of thermally acclimated or acclimatized fish by quantitative PCR. Kir2 composition in the polar cod was more diverse than in other species in that all Kir2 isoforms were relatively highly expressed. Kir2 composition of navaga and burbot differed from that of the polar cod as well as from those of other species. The relative expression of Kir2.2 transcripts, especially Kir2.2b, was higher in both atrium and ventricle of navaga and burbot (56-89% from the total Kir2 pool) than in other species (0.1-11%). Thermal acclimation induced only small changes in cardiac Kir2 transcript expression in Gadiformes species. However, Kir2.2b transcripts were upregulated in cold-acclimated navaga and burbot hearts. All in all, the cardiac Kir2 composition seems to be dependent on both phylogenetic position and thermal preference of the fish.

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+ (INa ), Ca2+ (ICaL ), or K+ (IKr , IK1 ) 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 INa and reduced ICaL and IKr , but retene was more potent. Neither of the PAHs had an effect on IK1 . 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.

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). As little is known about EE of arctic stenothermic fishes, we tested the TDEE hypothesis on ventricular myocytes of polar cod (Boreogadus saida) and navaga (Eleginus nawaga) 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 APs were examined. At 3°C, ventricular APs of polar cod and navaga were similar but differed from those of burbot in having a lower rate of AP upstroke and a 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 mmol l-1 and further to 8 mmol l-1 at 3, 9 and 15°C strongly affected EE and AP characteristics in polar cod and navaga, but had a lesser effect 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 resistant against acute warming, but less so against the simultaneous temperature and exercise stresses.

Transcripts of Kv7.1 and MinK channels and slow delayed rectifier K+ current (IKs) are expressed in zebrafish (Danio rerio) heart.

Zebrafish are increasingly used as a model for human cardiac electrophysiology, arrhythmias, and drug screening. However, K+ ion channels of the zebrafish heart, which determine the rate of repolarization and duration of cardiac action potential (AP) are still incompletely known and characterized. Here, we provide the first evidence for the presence of the slow component of the delayed rectifier K+channels in the zebrafish heart and characterize electrophysiological properties of the slow component of the delayed rectifier K+current, IKs. Zebrafish atrium and ventricle showed strong transcript expression of the kcnq1 gene, which encodes the Kv7.1 α-subunit of the slow delayed rectifier K+ channel. In contrast, the kcne1 gene, encoding the MinK ß-subunit of the delayed rectifier, was expressed at 21 and 17 times lower level in ventricle and atrium, respectively, in comparison to the kcnq1. IKs was observed in 62% of ventricular myocytes with mean (± SEM) density of 1.23 ± 0.37 pA/pF at + 30 mV. Activation rate of IKs was 38% faster (τ50 = 1248 ± 215 ms) than kcnq1:kcne1 channels (1725 ± 792 ms) expressed in 3:1 ratio in Chinese hamster ovary cells. Microelectrode experiments demonstrated the functional relevance of IKs in the zebrafish heart, since 100 µM chromanol 293B produced a significant prolongation of AP in zebrafish ventricle. We conclude that AP repolarization in zebrafish ventricle is contributed by IKs, which is mainly generated by homotetrameric Kv7.1 channels not coupled to MinK ancillary ß-subunits. This is a clear difference to the human heart, where MinK is an essential component of the slow delayed rectifier K+channel.

Expression of calcium channel transcripts in the zebrafish heart: dominance of T-type channels.

Calcium channels are necessary for cardiac excitation-contraction (E-C) coupling, but Ca2+ channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca2+ channels in the heart of zebrafish (Danio rerio), a popular model species. Altogether, 18 Ca2+ channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Cav1.1a, Cav1.1b, Cav1.2, Cav1.3a, Cav1.3b, Cav1.4a, Cav1.4b), 5 T-type (Cav3.1, Cav3.2a, Cav3.2b, Cav3.3a, Cav3.3b) and 6 P/Q-, N- and R-type (Cav2.1a, Cav2.1b, Cav2.2a, Cav2.2b, Cav2.3a, Cav2.3b) Ca2+ channels were expressed. In the ventricle, T-type channels formed 54.9%, L-type channels 41.1% and P/Q-, N- and R-type channels 4.0% of the Ca2+ channel transcripts. In the atrium, the relative expression of T-type and L-type Ca2+ channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca2+ channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type (ICaT) and L-type (ICaL) Ca2+ current were 6.3±0.8 and 7.7±0.8 pA pF-1, respectively. ICaT mediated a sizeable sarcolemmal Ca2+ influx into ventricular myocytes: the increment in total cellular Ca2+ content via ICaT was 41.2±7.3 µmol l-1, which was 31.7% of the combined Ca2+ influx (129 µmol l-1) via ICaT and ICaL (88.5±20.5 µmol l-1). The diversity of expressed Ca2+ channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular ICaT is likely to play a significant role in E-C coupling.

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 (INa) of zebrafish ventricular myocytes were examined. Eight Na+ channel α-subunits were expressed in both atrium and ventricle of the zebrafish heart. Nav1.5Lb, an orthologue to the human Nav1.5, was clearly the predominant isoform in both chambers representing 65.2 ±â€¯4.1% and 83.1 ±â€¯2.1% of all Na+ channel transcripts in atrium and ventricle, respectively. Nav1.4b, an orthologue to human Nav1.4, formed 34.1 ±â€¯4.1 and 16.2 ±â€¯2.0% of the Na+ channel transcripts in atrium and ventricle, respectively. The density of INa 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 INa are largely similar in zebrafish and human hearts indicating evolutionary conservation of Na+ channel composition and function. The zebrafish INa differs from the human cardiac INa in terms of higher tetrodotoxin sensitivity (IC50-value = 5.3 ±â€¯0.1 nM) and slower inactivation kinetics. The zebrafish INa was inhibited with tricaine (MS-222) with an IC50-value of 1.2 ±â€¯0.18 mM (336 mg l-1), suggesting some care in the use of MS-222 as an anesthetic.