Temperature-sensitivity of electrical excitability is a potential limiting factor for performance level and thermal tolerance of excitable tissues in ectothermic animals. In order to test whether rate and rhythm of the heart acclimatize to seasonal temperature changes, thermal sensitivity of cardiac excitation in a eurythermal teleost, the roach (Rutilus rutilus), was examined. Excitability of the heart was determined from in vivo electrocardiograms and in vitro microelectrode recordings of action potentials (APs) from winter (WiR) and summer (SuR) roach acclimated to 4°C and 18°C, respectively. Under heat ramps (3°C/h), starting from the acclimatization temperatures of the fish, heart rate (HR) increased to the maximum value of 78±5 beats/min (at 19.8±0.5°C) and 150±7 beats/min (at 28.1±0.5°C) for WiR and SuR, respectively, and then declined in both groups. Below 20°C HR was significantly higher in WiR than SuR (P<0.05) indicating positive thermal compensation. Cardiac arrhythmias appeared with rising temperature as missing QRS complexes, increase in variability of HR, episodes of atrial tachycardia, ventricular bradycardia and complete cessation of the heartbeat (asystole) in both WiR and SuR. Unlike WiR, atrial APs of SuR had a distinct early repolarization phase, which appeared as shorter duration of atrial AP at 10% and 20% repolarization levels in comparison to WiR (P<0.05). In contrast, seasonal acclimatization had only subtle effects on ventricular AP characteristics. Plasticity of cardiac excitation appears to be necessary for seasonal improvements in performance level and thermal resilience of the roach heart.

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