Dynamic changes in scope for heart rate and cardiac autonomic control during warm acclimation in rainbow trout

Abstract

Time course studies are critical for understanding regulatory mechanisms and temporal constraints in ectothermic animals acclimating to warmer temperatures. Therefore, we investigated the dynamics of heart rate and its neuro-humoral control in rainbow trout (Onchorhynchus mykiss L.) acclimating to 16°C for 39 days after being acutely warmed from 9°C. Resting heart rate was 39 beats min −1 at 9°C, and increased significantly when fish were acutely warmed to 16°C (Q 10 =1.9), but then declined during acclimation (Q 10 =1.2 at day 39), mainly due to increased cholinergic inhibition while the intrinsic heart rate and adrenergic tone were little affected. Maximum heart rate also increased with warming, although a partial modest decrease occurred during the acclimation period. Consequently, heart rate scope exhibited a complex pattern with an initial increase with acute warming, followed by a steep decline and then a subsequent increase, which was primarily explained by cholinergic inhibition of resting heart rate.

The effect of acute warming and thermal acclimation on maximum heart rate of the common killifish Fundulus heteroclitus

“…Consequently, the relative contributions of other mechanisms to increases in tissue oxygen delivery ( i.e ., increases in cardiac stroke volume and tissue oxygen extraction) must then increase with the declining contribution of f H . In fact, when measured in vivo , neither cardiac stroke volume nor tissue oxygen extraction increase appreciably during acute warming until a fish reaches a temperature close to when it collapses (Clark et al ., ; Ekström et al ., ; Eliason et al ., , ,b,c; Farrell, ; Gamperl et al ., ; Sandblom et al ., ; Steinhausen et al ., ). Lastly, the temperature when 50% of the fish first developed cardiac arrhythmia was shifted a full 10°C higher with acclimation to 33°C compared with 5°C (41 v .…”

“…The primary cardiac response in this regard is an increase in heart rate ( f H ), a response seen in almost all fish studied to date (Farrell & Smith, ). Nevertheless, the response of f H to acute warming is plastic in that, with thermal acclimation, fish can reset the cardiac pacemaker to adjust the intrinsic frequency of the heartbeat (Vornanen, ) and adjust the autonomic regulation of the heartbeat (Ekstrom et al ., , ; Sandblom & Axelsson, ). Consequently, when trying to understand the upper thermal limits of f H in fishes, the response of f H to temperature acclimation is just as important as the response of f H to acute warming (Farrell, ).…”

Physiological and genomic signatures of evolutionary thermal adaptation in redband trout from extreme climates

“…Note that f h,max is pharmacologically stimulated without any external control while CT MAX is measured in conscious animals. Under acute situations a conscious fish has control of cholinergic inhibition on heart rate (Ekström, Hellgren, Gräns, Pichaud, & Sandblom, ), and maximum cardiac capacity can be reserved for higher temperatures. Indeed, measurement of the heart rate in conscious trout rarely exceeds 120 bpm, independent of body size and acclimation conditions (Aho & Vornanen, ; Altimiras & Larsen, ).…”

Temperature-induced cardiac remodeling in fish

“…We acknowledge that other aspects of fish heart function change with thermal acclimation, most notably the electrical properties. Pacemaker output can be reset, partly as a result of temperature-related changes in electrical excitability ( Aho and Vornanen, 2001 ; Ekström et al, 2016 ). Electrical excitability itself is modulated by temperature-dependent shifts in ion channel densities and/or isoform switches which can vary between species and life histories ( Vornanen, 2016 ; Badr et al, 2016 ).…”