Seasonal is between 0 and 4°C, the

Seasonal thermal acclimation of electrical excitationis vital for convenient heart function under widely range of temperatures betweenwinter and summer waters. Roach are eurythermic fish with the ultimate upperlethal temperature of 33.5°C (Cocking 1959).

The ultimate upper lethaltemperature of roach acclimated at 4 and 18°C is consistent with recentfindings of TBPs for in vivo fH(Cocking 1959; Badr et al. 2016). These findings show that the upper thermaltolerance limit, below which the majority of the fish are able to survive”indefinitely”, and the temperature above which in vivo fH startsto decrease almost coincide. Pumping capacity of the roach heart increased by maximizingfH in both seasonal acclimatization groups withoutcompromising the stability of cardiac excitation in response to seasonalacclimatization. Even though fH is seasonally optimized, bothwinter and summer roach hearts retain the safety margin of approximately 10°Cfor cardiac arrhythmias. Considering that water temperature in winter inice-covered lakes is between 0 and 4°C, the upper thermal tolerance limits of fHand INa of winter roach heart are more than sufficient to allowactive life in winter and there is plenty of rooms for temperature increases withoutdanger of heat intolerance. The situation is slightly different for summerroach, since maximum summer temperatures of surface water (25-26°C) in lakes innorthern Europe can be within a few degrees from the TBPs of fHand arrhythmia (TARR) , but above the TBPof INa (Badr et al., 2016; Badr et al.

, 2017b).Thermaltolerance of the cardiac IK1, IKr and ICaL aremarkedly higher in winter- and summer-acclimatized roach than the lethaltemperatures of the fish and the TBPs of fH.This suggests that IK1, IKr and ICaL are notcritical factors in thermal tolerance of the heart or the fish. Contrary, INais much more sensitive to high temperatures than either fH orintact fish in both seasonal acclimatized roach. Similar to the brown trout heart,INa is clearly the most heat-sensitive ionic current of cardiacmyocytes in roach, and therefore the weakest link in cardiac excitation(Vornanen et al., 2014; Badr et al., 2017b).

These present findings areconsistent with the assumptions of the TDEE hypothesis in that a mismatch intemperature dependence between inward INa and outward K+ currents(IK1) is causative to high temperature-induced arrhythmias andbradycardia in fish hearts in vivo (Vornanen, 2016). The match between TBPsof fH and INa was not quantitatively perfect insummer-acclimatized roach as in winter-acclimatized fishes suggesting thatother factors may be involved. Densities of INa and IK1 invivo are affected by the shifts in intra- and extracellular Na+ andK+ ion concentrations, which are likely to occur as an outcome fromchanges in fH (Kunze 1977; Kline & Morad 1978; Cohen etal.

1982). Increased fH and shortened diastolic interval at hightemperature may limit recovery of INa from inactivation (Haverinen &Vornanen 2006). Furthermore, atrial myocytes have an acetylcholine-inducedinward rectifier K+ current, IKACh, which is seasonallyprimed, and could therefore antagonize INa together with thebackground IK1 (Abramochkin & Vornanen, 2016).

High temperaturelimitation of fH does not occur at single cell level: beatingrate of isolated pacemaker cells is not limited by high temperatures andventricular APs can be triggered much above the TBP of fHin vivo (Vornanen et al. 2014; Haverinen et al. 2017; Badr et al. 2017a).

Thermal limitation of fH is an emergent property that appearsat tissue level as an outcome of interaction between cardiac myocytes.Smallchanges in K+o have a major impact on electricalexcitability of roach ventricular myocytes. Effects of temperature and high K+oon excitability of roach ventricular myocytes are partly antagonistic and insome respects synergistic. High temperature hyperpolarizes RMP, increases Vmaxand elevates excitation threshold, while high K+odepolarizes RMP, depresses Vmax and reduces excitation threshold.APD50 is strongly shortened and densities of IK1 and IKrare increased by both high temperature and high K+o.However, the depressing effects of high K+o are sostrong that they override the positive effects of high temperature on RMP and Vmax.

Indeed, electrophysiological properties of roach ventricular AP are verysensitive to small changes in K+o. High concentrationof extracellular K+ is most probably cardiotoxic to roach, since Vmaxis severely depressed and some ventricular myocytes become unexcitable andcannot generate propagating APs. Because basic features of electrical excitabilityare common to all excitable cells, the present findings are probably valid forneurons and muscle cells. Therefore, future studies should examine the combinedeffects of K+o and temperature on muscular and neuronalexcitability.

Those studies could reveal the impact of environmental andphysiological stresses on locomotion, sensory function, behavior and fitness ofectotherms.