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

The Alaska blackfish (Dallia pectoralis) 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 in vivo and spontaneous heart rate (fH), electrocardiogram, ventricular action potential (AP) shape and duration (APD), the background inward rectifier (IK1) and rapid delayed rectifier (IKr) K+ currents and ventricular gene expression of proteins involved in excitation-contraction coupling. In vivo fH 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 IK1 and IKr were not upregulated in 5N fish. By contrast, chronic hypoxic submergence elicited a shortening of QT interval and APD, driven by an upregulation of IKr The altered electrophysiology of 5H fish was accompanied by increased gene expression of kcnh6 (3.5-fold; Kv11.2 of IKr), kcnj12 (7.4-fold; Kir2.2 of IK1) and kcnj14 (2.9-fold; Kir2.4 of IK1). 5H fish also exhibited a unique gene expression pattern that suggests modification of ventricular Ca2+ 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.

Contractile performance of the Alaska blackfish (Dallia pectoralis) ventricle: Assessment of the effects of temperature, pacing frequency, the role of the sarcoplasmic reticulum in contraction and adrenergic stimulation.

The air-breathing Alaska blackfish (Dallia pectoralis) experiences aquatic hypoxia, but restricted air-access in winter due to ice-cover. To lend insight into its overwintering strategy, we examined the effects of thermal acclimation (15 °C vs. 5 °C), acute temperature change (to 10 °C), increased pacing frequency, inhibition of sarcoplasmic reticulum (SR) Ca2+ release and uptake and adrenaline (1000 nmol l-1) on the contractile performance of isometrically-contracting, electrically-paced ventricular strips. At routine pacing frequencies, maximal developed force (Fmax) was equivalent at 5 °C (2.1 ±â€¯0.2 mN mm-2) and 15 °C (2.2 ±â€¯0.3 mN mm-2), whereas contraction durations were 2.2- to 2.4-times longer and contraction rates 2.4- to 3.5-times slower at 5 °C. Maximum contraction frequency was reduced by decreased temperature, being 0.91 ±â€¯0.04 Hz at 15 °C, 0.35 ±â€¯0.02 Hz at 5 °C and equivalent between acclimation groups at 10 °C (~0.8 Hz). 15 °C and 5 °C strips were insensitive to SR inhibition at routine stimulation frequencies, but SR function supported high contraction rates at 10 °C and 15 °C. Adrenaline shortened T0.5R and increased relaxation rate by 18-40% at 15 °C, whereas at 5 °C, adrenaline augmented Fmax by 15-25%, in addition to increasing contraction kinetics by 22-82% and decreasing contraction duration by 20%. Overall, the results reveal that ventricular contractility is suppressed in cold-acclimated Alaska blackfish largely by acute and perhaps direct effects of decreased temperature, which effectively preconditions the tissue for low energy supply during winter hypoxia. Additionally, the level of cardiac performance associated with maintained activity in winter is supported by enhanced inotropic responsiveness to adrenaline at 5 °C.

Temperature-dependence of L-type Ca(2+) current in ventricular cardiomyocytes of the Alaska blackfish (Dallia pectoralis).

To lend insight into the overwintering strategy of the Alaska blackfish (Dallia pectoralis), we acclimated fish to 15 or 5 °C and then utilized whole-cell patch clamp to characterize the effects of thermal acclimation and acute temperature change on the density and kinetics of ventricular L-type Ca(2+) current (I Ca). Peak I Ca density at 5 °C (-1.1 ± 0.1 pA pF(-1)) was 1/8th that at 15 °C (-8.8 ± 0.6 pA pF(-1)). However, alterations of the Ca(2+)- and voltage-dependent inactivation properties of L-type Ca(2+) channels partially compensated against the decrease. The time constant tau (τ) for the kinetics of inactivation of I Ca was ~4.5 times greater at 5 °C than at 15 °C, and the voltage for half-maximal inactivation was shifted from -23.3 ± 1.0 mV at 15 °C to -19.8 ± 1.2 mV at 5 °C. These modifications increase the open probability of the channel and culminate in an approximate doubling of the L-type Ca(2+) window current, which contributes to approximately 15% of the maximal Ca(2+) conductance at 5 °C. Consequently, the charge density of I Ca (Q Ca) and the total Ca(2+) transferred through the L-type Ca(2+) channels (Δ[Ca(2+)]) were not as severely reduced at 5 °C as compared to peak I Ca density. In combination, the results suggest that while the Alaska blackfish substantially down-regulates I Ca with acclimation to low temperature, there is sufficient compensation in the kinetics of the L-type Ca(2+) channel to support the level of cardiac performance required for the fish to remain active throughout the winter.