The cardiac APs are triggered by aharmonious co-operation between depolarization (inward) and repolarization(outward) ion currents. Depolarization is mainly achieved by the inward flow ofNa+ and Ca2+ ions, while repolarization achieved mainly bythe outward flow of K+ ions (Hodgkin and Huxley, 1952; Opie, 1998;Bers, 2001). Ion channels in cardiac myocytes of fishes should be flexible and stronglyrespond to temperature changes (Haverinen and Vornanen, 2004; Hassinen et al.

,2007; Hassinen et al., 2008; Haverinen and Vornanen, 2009; Galli et al., 2009;Abramochkin and Vornanen, 2015), thence they have a profound role in thermaltolerance and temperature acclimation/acclimatization of cardiac function. Channelcomposition and underline subunits are different between species-specific andmay set the upper thermal tolerance of electrical excitability in fish heartand thereby the ability of fishes to accommodate to the predicted globalheating (Hassinen et al. 2007; Hassinen et al.

2008; Somero 2010; Hassinen etal. 2014).The density ofINa is the center factor in determining the rate of AP propagationover the heart and activates other ion channels to produce chamber-specific AP.(Fozzard and Hanck, 1996; Schram et al.

, 2002; Kleber and Rudy, 2004). Thermal acclimationeffect on INa density is variable between species which propably dueto differences in fishes activity of lifestyle. The density of INawas larger in cold-active (rainbow trout) than cold-dormant (crucian carp)fishes. INa is higher in cold-acclimated rainbow trout (4°C) than in warm acclimated fishes (18°C ).Contrary, INa is higher in warm acclimated crucian carp than coldacclimated fishes (Haverinen and Vornanen, 2004). Also, the molecularcomposition shifts in temperature-dependent manner, where the predominance isoformin cold-active rainbow trout is Nav1.4 that was higher in coldacclimated (4°C) than warm acclimated (18°C) fishes, while Nav1.5was the predominance in cold-dormant crucian carp with slightly equal expressedin winter and summer acclimatized fishes (Haverinen and Vornanen 2007, Tikkanenet al.

2017).ICaLis the primary path for the majority of Ca2+ influx for activationof the myofilaments during EC coupling in most of fishes and maintains the longplateau duration of the cardiac AP through the balance with outward K+currents (Hove-Madsen and Tort, 1998; Vornanen, 1998). Decrease in temperature declinesthe Ca2+ sensitivity of the myofilaments and declines the density ofICa (Cavalié et al., 1985; Harrison and Bers, 1990). ICadensity increased with acute increasing in temperature in most of fish species(Shiels et al., 2000; Shiels et al.

, 2006; Galli et al., 2011; Shiels and Galli,2014; Vornanen et al., 2014; Kubly and Stecyk, 2015). Thermal acclimation hasno effect on the density of ICa in rainbow trout and crucian carp ventricularmyocytes when measured at room temperature (Vornanen, 1998). Contrary, seasonalacclimatized has a striking effect on ICa density in ventricularmyocytes of crucian carp and the ICa density in summer acclimatizedcarp was higher than that in winter acclimatized carp at 11°C (Vornanen andPaajanen, 2004). The temperature sensitivity of ICa in fishes iswithin the lower range of those found in most of mammals (Kim et al., 2000;Shiels et al., 2000; Shiels et al.

, 2006).in contrast with INa, transcriptsof ICaL channels are slightly affected by seasons in ventricularmyocytes of crucian carp (Tiitu and Vornanen, 2003; Vornanen and Paajanen, 2004;Tikkanen et al., 2017). Fishcardiomyocytes have two major K+ currents, the background inwardrectifier K+ current (IK1) and the rapid component of therapid delayed rectifier K+ current (IKr) (Vornanen et al.,2002; Hassinen et al., 2007; Haverinen and Vornanen, 2009). IK1 isresponsible for maintaining the negative resting membrane potential RMP and setthe rate of AP repolarization (Vornanen et al., 2002), while IKrmodulates the AP duration (APD) (Haverinen and Vornanen, 2009).

Acute changesin temperature have strongly effect on the density of IK1 and IKrin thermal acclimated and seasonal acclimatized fishes (Paajanen andVornanen, 2004; Haverinen and Vornanen, 2009; Galli et al., 2009; Abramochkinand Vornanen, 2015). In the most of studied fishes, IKr isupregulated by cold-acclimtion where the density of IKr is higher incold acclimated fishes than warm acclimated fishes (Haverinen and Vornanen,2009; Galli et al., 2009; Hassinen et al., 2014; Abramochkin and Vornanen, 2015).Moreover, there is a close correlation between the density of IKr andfH in cold- and warm-acclimated fishes (Vornanen, 2016).

Contrary,temperature-dependent of IK1 density varies from species to other,where IK1 density was higher in cold acclimated crucian carp, roachand perch than warm acclimated fishes in atrial and ventricular myocytes, whileit was higher in warm acclimated perch than cold acclimated fish. Also, there aredifferences between atrial and ventricular IK1 in the same fish, e.g.IK1 density was higher in atrial myocytes of warm acclimated burbotand Pacific bluefin tuna (Thunnus thynnus) than cold acclimated fishes,while it was higher in ventricular myocytes of cold acclimated fishes than warmacclimated (Haverinen and Vornanen, 2009; Galli et al.,  2009).

In seasonal acclimatized navaga andthermal acclimated roach, IK1 density in ventricular myocytes washigher in warm- than cold-acclimated fishes (Haverinen and Vornanen, 2009; Abramochkinand Vornanen, 2015).Temperature-dependentchanges in K+ current density in fish cardiomyocytes are associatedwith changes in gene expression either at the transcript or protein level. IK1and IKr are produced by Kir2 and Erg isoforms channels, respectivelyHassinen et al., 2007; Hassinen et al., 2008; Hassinen et al.

, 2014). In allstudied fish species, ERG2 is the main isoform in IKr production andalso resposible for thermal regulation which upregulated in cold/winter-acclimatedfishes (Hassinen et al., 2008; Hassinen et al., 2014; Vornanen and Hassinen,2016). Kir2.2a and Kir2.2b were the main isoforms for IK1 productionin warm and cold acclimated crucian carp in atrial and ventricular myocytes,respectively (Hassinen et al.

2008). In thermal acclimated rainbow trout andseasonal acclimated navaga, Kir2.1 considered the up-regulated isoform incold/winter fishes (Hassinen et al., 2007; Hassinen et al., 2014).

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