Cardiovascular Responses of the Red-Blooded Antarctic Fishes Pagothenia Bernacchii and P. Borchgrevinki
Authors: Axelsson M. , Davison W. , Forster M. E. , Farrell A. P. 
Published: April 2021

ABSTRACT
The aim of this study was to investigate cardiac performance and cardiovascular control in two red-blooded nototheniid species of antarctic fishes, Pagothenia bernacchii (a benthic fish) and P. borchgrevinki (a cryopelagic fish), and to make comparisons with existing information on haemoglobin-free antarctic teleosts. In quiescent P. bernacchii at 0°C ventral aortic pressure (PVA) was 3.09kPa and cardiac output (Q̇) was 17.6 ml min−1 kg−1, with a heart rate (fH) of 10.5 beats min−1 and stroke volume of 1.56 ml kg−1. Following atropine treatment, Q̇ was maintained but heart rate increased and stroke volume decreased. Resting heart rate resulted from an inhibitory cholinergic tone of 80.4% and an excitatory adrenergic tone of 27.5%. The intrinsic heart rate was 21.7 beats min−1 at 0°C.
In quiescent P. borchgrevinki at 0°C, PVA was 3.6 kPa, Q̇ was 29.6 ml min−1 kg−1 and stroke volume was 2.16 ml kg−1. The resting heart rate in P. borchgrevinki of 11.3 beats min−1 resulted from an inhibitory cholinergic tone of 54.5% and an excitatory adrenergic tone of 3.2%. The intrinsic heart rate was 23.3 beats min−1.
P. bernacchii maintained Q̇ during a progressive decrease in water oxygen tension from 20 to 6.7 kPa, but fH was increased significantly. Thus, although there is cholinergic control of the heart, no hypoxic bradycardia was observed. Recovery from hypoxia was associated with increases in Q̇ and fH; stroke volume returned to control values. PVA declined in recovery as total vascular resistance decreased. Hypoxic exposure following atropine treatment resulted in progressive increases in PVA, Q̇ and stroke volume; fH decreased during the recovery period. Hypoxic exposure in P. borchgrevinki produced similar cardiovascular responses to those observed in P. bernacchii.
During an acute increase in water temperature from 0 to 5°C, P. bernacchii regulated Q̇ and total vascular resistance. Stroke volume decreased as fH increased. The intrinsic heart rate had a Q10 of 1.96 over this temperature range. P. bernacchii maintained chronotropic inhibition up to a temperature of 2.5–3.0°C. However, by 5°C this chronotropic inhibition of the heart rate was lost.
Infusion of adrenaline into the ventral aorta of P. bernacchii resulted in significant increases in Q̇, fH, PVA and total vascular resistance. Infusion of adrenaline after atropine treatment caused similar cardiovascular changes without the change in fH.
P. borchgrevinki could sustain swimming in a water tunnel at approximately 1 body length per second for 6-10min. During the bout of swimming there was a doubling of ventilation frequency, a 75% increase in Q̇ as fH doubled and a decrease in total vascular resistance.
Similarities exist between these two red-blooded antarctic teleosts and haemoglobin-free channichthyids. These include a low PVA, associated with a low vascular resistance, and a high cardiac stroke volume. Ventricle mass is somewhat larger than in temperate species of teleosts, especially considering the low aortic pressures developed by the heart. The absence of a sustained bradycardia in P. bernacchii during hypoxia or an adrenaline-induced increase in PVA is unusual. This may in part reflect the very high inhibitory cholinergic tone to the heart, the highest value found in resting fish.