Thermal acclimation and seasonal acclimatization: a comparative study of cardiac response to prolonged temperature change in shorthorn sculpin

Abstract

Seasonal thermal remodelling (acclimatization) and laboratory thermal remodelling (acclimation) can induce different physiological changes in ectothermic animals. As global temperatures are changing at an increasing rate, there is urgency to understand the compensatory abilities of key organs like the heart to adjust under natural conditions. Thus, the aim of the present study was to directly compare the acclimatization and acclimatory response within a single eurythermal fish species, the European shorthorn sculpin (Myoxocephalus scorpio L.). We used current- and voltage-clamp to measure ionic current densities in both isolated atrial and ventricular myocytes at a common temperature of 7.5°C from three groups of fish: 1) summer-acclimated (SumAcl, 12°C); 2) cold-acclimated (ColdAcl, 3°C); and 3) winter-acclimatized fish (WinAcz, 3°C). At the common test temperature of 7.5°C, action potential (AP) was shortened by both WinAcz and ColdAcl, compared with SumAcl, however, WinAcz caused a greater shortening than did ColdAcl. Shortening of AP was achieved mostly by a significant increase in repolarizing current density (IKr and IK1) following WinAcz, with ColdAcl having only minor effects. Compared with SumAcl, the depolarizing L-type calcium current (ICa) was larger following WinAcz, but again there was no effect of ColdAcl on ICa. Interestingly, the other depolarizing current, INa was downregulated at low temperatures. Our analysis further shows that ionic current remodelling is primarily due to changes ion channel density rather than currents kinetics. In summary, acclimatization profoundly modified the electrical activity of the sculpin heart while acclimation to the same temperature for >1.5 month produced very limited remodelling effects.