Seasonal acclimatization of the cardiac action potential in the Arctic navaga cod (Eleginus navaga, Gadidae)
Freshwater fishes of north-temperate latitudes adjust electrical excitability of the heart to seasonal temperature changes by changing expression levels of ion channel isoforms. However, little is known about thermal responses of action potential (AP) in the hearts of marine polar fishes. To this end, we examined cardiac AP in the atrial myocardium of the Arctic navaga cod (Eleginus navaga) from the White Sea (Russia) acclimatized to winter (March) and summer (September) seasons. Acute increases in temperature from 4 to 10 °C were associated with increases in heart rate, maximum velocity of AP upstroke and negative resting membrane potential, while duration of AP was shortened in both winter-acclimatized and summer-acclimatized navaga hearts. In winter, there was a compensatory shortening (41.1 %) of atrial AP duration and this was associated with a strong increase in transcript expression of Erg K+ channels, known to produce the rapid component of the delayed rectifier K+ current, IKr. Smaller increases were found in the expression of Kir2.1 channels that produce the inward rectifier K+ current, IK1. These findings indicate that the heart of navaga cod has a good acclimatory capacity in electrical excitation of cardiac myocytes, which enables cardiac function in the cold-eurythermal waters of the subarctic White Sea.