O2 consumption and heart rate in developing zebrafish (Danio rerio): influence of temperature and ambient O2

Abstract

Body mass, length, oxygen consumption (M˙o 2) and heart rate ( f H) were measured in “embryos” (prior to hatching), “larvae” ( days 10–20), “juveniles” ( days 30–70 in 10-day intervals), and “adults” ( day 100) of the zebrafish Danio rerio. Fish were chronically reared at either 25, 28, or 31°C and then acutely exposed to hypoxia at different developmental stages. We hypothesized that at any given rearing and measurement temperature, D. rerio would maintainM˙o 2at lower ambient [Formula: see text] [i.e., have a lower critical partial pressure (Pcrit)] as development progressed and that at any given developmental stage individuals reared and measured at higher temperatures would show a more pronounced hypoxic bradycardia.M˙o 2in normoxic fish at 28°C peaked at ∼40 μmol ⋅ g−1 ⋅ h−1at day 10, thereafter falling to 4–5 μmol ⋅ g−1 ⋅ h−1at day 100. The Q10 forM˙o 2was 4–5 in embryos, falling to 2–3 from day 10 to day 60 and rising again to 4–5 at day 100. Pcrit at 28°C was ∼80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supporting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient [Formula: see text] levels. Pcrit increased sharply with measurement temperature. Heart rate ( f H) at 28°C increased from about 125 beats/min in embryos to a peak of ∼175 beats/min at days 10–30and then fell to ∼130 beats/min by day 100. Unlike forM˙o 2, the Q10 for f H was more constant at 1.2–2.5 throughout development. Hypoxic exposure at any temperature had no effect on f H until ∼ day 30, after which time a hypoxic bradycardia was evident. As evident forM˙o 2, the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative of hypoxic stress, the increasing prevalence of a hypoxic bradycardia in older, warmer individuals supports the hypothesis that increasing hypoxic susceptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulateM˙o 2and f H in response to the compounding demands of increased temperature and/or decreased oxygen availability first develops after ∼20 days in D. rerio and, thereafter, the ability to maintainM˙o 2in the face of ambient hypoxia progressively builds through to adulthood. Additionally, the temperature responses of metabolism and heart rate differ substantially at different phases of development, suggesting a loose coupling between the respiratory and cardiovascular systems, at least early in development.