Thermal acclimation and acute temperature effects on cardiac barostatic reflexes in rainbow trout (Oncorhynchus mykiss)

Abstract

Acute warming raises the metabolism of fish, which is matched by increased heart rate. However, thermal acclimation may reduce heart rate through combinations of lowered intrinsic pacemaker rate and increased inhibitory vagal tone. This could affect the baroreflex, which regulates arterial blood pressure through heart rate changes via altered vagal tone. Using pharmacological tools, we assessed autonomic tones and baroreflex regulation of heart rate in rainbow trout (Oncorhynchus mykiss) at 11 °C, and after acute (24 h, 17°Cacute) or chronic (>7 weeks, 17°Cchronic) warming to 17 °C. We hypothesised that warm acclimation would manifest as reduced heart rate and elevated vagal tone in 17°Cchronic trout relative to 11 °C and 17°Cacute trout, which would increase baroreflex gain and the scope for fH increase through vagal release during hypotension. Compared to 11 °C, the 17°Cacute group exhibited slightly higher heart rate (Q10 = 1.5) and a strong trend for elevated vagal tone (54%). Surprisingly, however, routine heart rate was unaltered by warm acclimation (Q10 = 1.6), while intrinsic heart rate and vagal tone (22%) declined. Consequently, baroreflex sensitivity to reduced blood pressure was elevated in the 17°Cacute group but returned towards 11 °C conditions in the 17°Cchronic group. Atropine abolished nearly all chronotropic changes. Bradycardic responses to hypertension and cardiac adrenergic tone were unaltered across temperature treatments. The lack of a clear acclimation effect on routine heart rate in the present study is likely explained by a seasonal effect as the experiments were performed in early winter. Nonetheless, we conclude that baroreflex sensitivity in trout is thermally plastic; the heightened baroreflex sensitivity to hypotension following acute warming likely serves to safeguard tissue oxygen delivery as metabolism is elevated, while the reduced baroreflex sensitivity observed with warm acclimation may be linked to a pronounced metabolic down-regulation.