DOI: 10.5061/dryad.n2z34tn3v
title: Mechanisms of enhanced cardiorespiratory performance under hyperoxia differ with exposure duration in yellowtail kingfish

description: Hyperoxia has been shown to expand the aerobic capacity of some fishes,
 although there have been very few studies examining the underlying
 mechanisms and how they vary across different exposure durations. Here, we
 investigated cardiorespiratory function of yellowtail kingfish (Seriola
 lalandi) acutely (~20 hours) and chronically (3-5 weeks) acclimated to
 hyperoxia (~200 % air saturation). Our results show that aerobic
 performance of kingfish is limited in normoxia and increases with
 environmental hyperoxia. Aerobic scope was elevated in both hyperoxia
 treatments driven by a ~33% increase in maximum O2 uptake (MO2max),
 although the mechanisms differed across treatments. Fish acutely
 transferred to hyperoxia primarily elevated tissue O2 extraction, while
 increased stroke volume-mediated maximum cardiac output was the main
 driving factor in chronically acclimated fish. Still an improved O2
 delivery to the heart in chronic hyperoxia was not the only explanatory
 factor as such. Here, maximum cardiac output only increased in chronic
 hyperoxia compared to normoxia when plastic ventricular growth occurred,
 as increased stroke volume was partly enabled by an ~8-12% larger relative
 ventricular mass. Our findings suggest that hyperoxia may be used
 long-term to boost cardiorespiratory function potentially rendering fish
 more resilient to metabolically challenging events and stages in their
 life-cycle.
description: We recorded the rate of whole-animal O<sub>2</sub>
 uptake (MO<sub>2</sub>) using intermittent-flow respirometry
 where the % air saturation inside the respirometer was continuously
 measured using an O<sub>2</sub> optode connected to a
 Firesting O<sub>2</sub> system. Automated flush pumps, were
 set to flush the respirometers for 5 min every 7 min (i.e., 2 min
 measurement cycles). MO<sub>2</sub> was calculated from the
 slope of the decline in % air saturation between flushes using the
 following formula: MO<sub>2</sub> = (Vr – Vf) × (Δ%Sat/t) × α;
 where Vr is the volume of the respirometer, Vf is the volume of the fish
 assuming that 1 g of tissue equals 1 ml of water, Δ%Sat/t is the change in
 % O<sub>2</sub> saturation per time and α is the temperature-,
 salinity- and atmospheric pressure-dependent solubility coefficient of
 O<sub>2</sub>. The first ~30 s of each measurement cycle was
 excluded from the slope determination to ensure the inclusion of only the
 linear section of the decline in O<sub>2</sub>. SMR (standard
 metabolic rate) was calculated as the mean of the lowest 20% of all
 MO<sub>2</sub> values obtained throughout the whole 20+ hours
 of recordings, with measurements 2 standard deviations below the mean of
 the lowest 20% removed as outliers. MO<sub>2max</sub> (maximum
 O<sub>2</sub> uptake) was calculated as the highest
 MO<sub>2</sub> value obtained at any point following exercise.
 The lowest 20% were chosen instead of the more commonly used lowest 10% to
 maximize the number of fish that achieve EPOC repayment for each
 treatment. Aerobic scope was then calculated as the difference between SMR
 and MO<sub>2max</sub><sub>⁠</sub>. EPOC (excess
 post-exercise O<sub>2 </sub>consumption) was calculated as the
 area between the MO<sub>2</sub> curve following the stress
 protocol and SMR + 10% using GraphPad Prism 9.1.2. Briefly, before
 analysis, individual MO<sub>2</sub> traces were smoothed by
 removing routine MO<sub>2</sub> values that were 10% larger
 than the previous value. EPOC duration was defined as the time in hours
 between the exhaustive protocol and the intersection of the
 MO<sub>2</sub> trace with the individual SMR + 10%. The rate
 of EPOC repayment was defined as EPOC/EPOC duration. We cleaned the
 respirometers thoroughly after each trial, and measured background
 respiration before and after each individual experiment and was negligible
 throughout the study (&lt; 0.2% of the MO<sub>2</sub>
 slope). Heart rate was calculated from the pulsating
 blood flow signal, and stroke volume was calculated as cardiac
 output/heart rate. The arterial-venous O<sub>2</sub>
 difference was estimated as MO<sub>2</sub>/cardiac output
 (Fick´s principle´s equation). All cardiovascular variables were measured
 simultaneously with MO<sub>2</sub> recordings, and
 cardiovascular variables measured concomitantly to SMR,
 MO<sub>2max</sub> and aerobic scope are referred to henceforth
 as resting, maximum and scope. Additionally, cardiorespiratory dynamics
 were assessed immediately prior to and at six time points following the
 exhaustive protocol and thus comprised: pre-exhaustion values (average of
 two last cycles prior to exhaustive protocol), immediately after the
 exhaustive protocol (0 h), and 0.5, 1, 2, 3 and 5 h following the
 exhaustive protocol. All measurements were derived from the average of two
 MO<sub>2</sub> cycles, except for the 0 h value, which was
 taken during the first measurement immediately after the exhaustive
 protocol. Additionally, peak cardiorespiratory responses (i.e., the
 highest arterial-venous O<sub>2</sub> content difference,
 cardiac output, stroke volume and heart rate measured at any time point
 throughout the recovery period independently from
 MO<sub>2max</sub>) and time to peak cardiorespiratory
 responses (i.e., the time elapsed from the beginning of the
 cardiorespiratory measurements following exhaustive exercise to the peak
 responses) were determined for each fish. The relative
 ventricular mass was calculated as wet mass of the ventricle/body mass ×
 100. To determine the relative % of ventricular compact myocardium, the
 spongy and compact layers were separated, dried and weighed⁠. The
 percentage compact myocardium was calculated as dry mass of compact
 myocardium/dry mass of ventricle × 100. The relative spleen mass was
 calculated as wet mass of the spleen/body mass × 100.
description: # Mechanisms of enhanced cardiorespiratory performance under hyperoxia
 differ with exposure duration in yellowtail kingfish
 [https://doi.org/10.5061/dryad.n2z34tn3v](https://doi.org/10.5061/dryad.n2z34tn3v) ## Description of the data and file structure The sheet named "rest, max and scope cardiorespiratory" contains information on the resting, maximum, and scope cardiorespiratory parameters, i.e., MO2 (SMR, maximum MO2 and aerobic scope, mg O2 h-1), cardiac output (ml min-1), stroke volume (ml), heart rate (beats min-1) and arterial venous O2 content difference (A-VO2, mg O2 ml-1). MO2 stands for O2 uptake and SMR for standard metabolic rate. It also includes, fish ID, weight (g), length (cm), EPOC (excess post-exercise O2 consumption, mg O2), EPOC duration (EPOC duration, h) and EPOC repayment rate (mg O2 h-1), ventricular mass (g), relative ventricular mass (%), relative spleen mass (%), haematocrit (%), haemoglobin concentration (g l-1), MCHC (mean corpuscular haemoglobin concentration, g l-1), pH and % compact myocardium (%). The treatments are normoxia, hyperoxia acute, and hyperoxia chronic. Null values for fish 21 is due to severe injury of the fish during the chase protocol, following which the fish was immediately euthanized and sampled. Thus, only resting values are included for this fish. Null values for EPOC, time to EPOC and EPOC repayment are due to the fish not achieving EPOC repayment during the experimental protocol. The sheet named "continuous cardiorespiratory" contains the continuous cardiorespiratory measurements prior to and immediately following the exhaustive protocol to elicit maximum cardiorespiratory responses. Again, treatments are normoxia, hyperoxia acute and hyperoxia chronic. Cardiorespiratory dynamics were assessed immediately prior to and at six timepoints following the exhaustive protocol and thus comprised: pre-exhaustion values (time 1), immediately after the exhaustive protocol (time 2), and 0.5 (time 3), 1 (Time 4), 2 (Time 5), 3 (Time 6) and 5 h (Time 8) following the exhaustive protocol. Units are MO2 (mg O2 h-1), cardiac output (ml min-1), stroke volume (ml), heart rate (beats min-1) and arterial venous O2 content difference (A-VO2, mg O2 ml-1). The sheet named "peak responses" contains information on peak and time to peak MO2, cardiac output, stroke volume, heart rate and arterial venous O2 content difference (A-VO2). Again, null values for fish 21 are due the fish being removed before the end of the stress protocol. Units are MO2 (mg O2 h-1), cardiac output (ml min-1), stroke volume (ml), heart rate (beats min-1), and arterial venous O2 content difference (A-VO2, mg O2 ml-1). The unit for time to peak responses is hours (h). ## Sharing/Access information n/a ## Code/Software n/a