Cardiophysiological responses of the air-breathing Alaska blackfish to cold acclimation and chronic hypoxic submergence at 5°C.

The Alaska blackfish () remains active at cold temperatures when experiencing aquatic hypoxia without air access. To discern the cardiophysiological adjustments that permit this behaviour, we quantified the effect of acclimation from 15°C to 5°C in normoxia (15N and 5N fish), as well as chronic hypoxic submergence (6-8 weeks; ∼6.3-8.4 kPa; no air access) at 5°C (5H fish), on and spontaneous heart rate (), electrocardiogram, ventricular action potential (AP) shape and duration (APD), the background inward rectifier () and rapid delayed rectifier () K currents and ventricular gene expression of proteins involved in excitation-contraction coupling. was ∼50% slower in 5N than in 15N fish, but 5H fish did not display hypoxic bradycardia. Atypically, cold acclimation in normoxia did not induce shortening of APD or alter resting membrane potential. Rather, QT interval and APD were ∼2.6-fold longer in 5N than in 15N fish because outward and were not upregulated in 5N fish. By contrast, chronic hypoxic submergence elicited a shortening of QT interval and APD, driven by an upregulation of The altered electrophysiology of 5H fish was accompanied by increased gene expression of (3.5-fold; K11.2 of ), (7.4-fold; K2.2 of ) and (2.9-fold; K2.4 of ). 5H fish also exhibited a unique gene expression pattern that suggests modification of ventricular Ca cycling. Overall, the findings reveal that Alaska blackfish exposed to chronic hypoxic submergence prioritize the continuation of cardiac performance to support an active lifestyle over reducing cardiac ATP demand.