Cold acclimation increases basal heart rate but decreases its thermal tolerance in rainbow trout (Oncorhynchus mykiss)

Abstract.

Rainbow trout (Oncorhynchus mykiss, Walbaum) were acclimated to 4°C and 17°C for more than 4 weeks and heart rate was determined in the absence and presence of adrenaline to see how thermal adaptation influences basal heart rate and its β-adrenergic control in a eurythermal fish species. The basal heart rate in vitro was higher in cold-acclimated than warm-acclimated rainbow trout at temperatures below 17°C. On the other hand, adaptation to cold decreased thermal tolerance of heart rate so that the maximal heart rates were achieved at 17°C (75±4 bpm) and 24°C (88±2 bpm) in cold-acclimated and warm-acclimated trout, respectively. β-Adrenergic response of the heart was enhanced by cold-adaptation, since adrenaline (100 nmol l^–1) caused stronger stimulation of heart rate in cold-acclimated (29±14%) than in warm-acclimated fish (10±1%; P=0.03). Furthermore, adrenaline strongly opposed the temperature-dependent deterioration of force production in cold-acclimated trout but not in warm-acclimated trout. The results indicate that adaptation to cold increases basal heart rate but decreases its thermal tolerance in rainbow trout. Cold acclimation up-regulates the β-adrenergic system, and β-adrenoceptor activation seems to provide cardioprotection against high temperatures in the cold-adapted rainbow trout.