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 ($\dot{M}{O}_2$) and cardiac function [i.e., heart rate (f_H) and cardiac output ($\dot{Q})$] were 50% lower in the acutely cooled and 1^oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (V_S) was unchanged. Intrinsic f_H 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 f_H was primarily under adrenergic control/tone (cholinergic tone 13–18%; adrenergic tone 37–70%). However, β-adrenergic blockade resulted in a 50% increase in V_S in the 1^oC-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 V_S is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C.