Acute and chronic cold exposure differentially affect cardiac control, but not cardiorespiratory function, in resting Atlantic salmon (Salmo salar)

No studies have examined the effects of cold temperatures (∼0–1 °C) on in vivo cardiac function and control, and metabolism, in salmonids. Thus, we examined: 1) how acclimation to 8 °C vs. acclimation (>3 weeks) or acute exposure (8-1 °C at 1 °C h−1) to 1 °C influenced cardiorespiratory parameters in resting Atlantic salmon; and 2) if/how the control of cardiac function was affected. Oxygen consumption (M˙O2) and cardiac function [i.e., heart rate (fH) and cardiac output (Q˙)] were 50% lower in the acutely cooled and 1oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (VS) was unchanged. Intrinsic fH was not affected by whether the fish were acutely exposed or acclimated to 1 °C (values ∼51, 24 and 21 beats min−1 in 8 and 1 °C-acclimated fish, and 8-1 °C fish, respectively), and in all groups fH was primarily under adrenergic control/tone (cholinergic tone 13–18%; adrenergic tone 37–70%). However, β-adrenergic blockade resulted in a 50% increase in VS in the 1oC-acclimated group, and this was surprising as circulating catecholamine levels were ∼1–3 nM in all groups. Overall, the data suggest that this species has a limited capacity to acclimate to temperatures approaching 0 °C. However, we cannot exclude the possibility that cardiac and metabolic responses are evoked when salmon are cooled to ∼ 0–1 °C, and that this prevented further declines in these parameters (i.e., they ‘reset’ quickly). Our data also provide further evidence that VS is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C. •No data exist on in vivo salmonid cardiorespiratory function at ∼ 0–1 °C.•Acute exposure vs. acclimation to 1 °C generally had similar effects.•Heart rate, cardiac output and oxygen consumption at 1 °C were ∼50% of those at 8 °C.•Stroke volume and tissue oxygen extraction did not change between 8 and 1 °C.•Salmon are stressed, and rely primarily on cardiac adrenergic tone at 1 °C.