Effects of seasonal acclimatization on action potentials and sarcolemmal K + currents in roach (Rutilus rutilus) cardiac myocytes.

Temperature sensitivity of electrical excitability is a potential limiting factor for high temperature tolerance of ectotherms. The present study examines whether heat resistance of electrical excitability of cardiac myocytes is modified by seasonal thermal acclimatization in roach (Rutilus rutilus), a eurythermal teleost species. To this end, temperature dependencies of ventricular action potentials (APs), and atrial and ventricular K+ currents were measured from winter-acclimatized (WiR) and summer-acclimatized (SuR) roach. Under patch-clamp recording conditions, ventricular APs could be triggered over a wide range of temperatures (4-43°C) with prominent changes in resting membrane potential (RMP), AP duration and amplitude. In general, APs of SuR were slightly more tolerant to high temperatures than those of WiR, e.g. the break point temperature (TBP ) of RMP was 37.6±0.4°C in WiR and 41±1°C in SuR (p<0.05). Of the two major cardiac K+ currents, the inward rectifier K+ current (IK1 ) was particularly heat resistant in both SuR (TBP 39.4±0.4°C) and WiR (TBP 40.0±0.4°C) ventricular myocytes. The delayed rectifier K+ current (IKr ) was not as heat resistant as IK1 . Surprisingly, IKr of WiR tolerated heat better (TBP 31.9±0.8°C) than IKr of SuR (TBP 24.1±0.5°C) (p<0.05). IKr (Erg2) channel transcripts of both atrial and ventricular myocytes were up-regulated in WiR. IK1 (Kir2) channel transcripts were not affected by seasonal acclimatization, although ventricular IK1 current was up-regulated in summer. Collectively, these findings show that thermal tolerance limits of K+ currents in isolated myocytes between seasonally acclimatized roach are much less pronounced than the heat sensitivity of ECG variables in intact fish.