O-2 consumption and heart rate in developing zebrafish (Danio rerio): influence of temperature and ambient O-2

Body mass, length, oxygen consumption ((M) over dot 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 t he zebrafish Danio rerio. Fish were chronically reared at either 25, 28, or 31 degrees C and then acutely exposed to hypoxia at different developmenta l stages. We hypothesized that at any given rearing and measurement tempera ture, D. rerio would maintain Mot at lower ambient Po-2 [i.e., have a lower critical partial pressure (P-crit)] as development progressed and that at ally given developmental stage individuals reared and measured at higher te mperatures would show a more pronounced hypoxic bradycardia. (M) over dot o (2) in normoxic fish at 28 degrees C peaked at similar to 40 mu mol . g(-1) . h(-1) at day 10, thereafter falling to 4-5 mu mol . g(-1) . h(-1) at day 100. The Q(10) for Mo-2 was 4-5 in embryos, falling to 2-3 from day 10 to day 60 and rising again to 4-5 at day 100. P-crit at 28 degrees C was simil ar to 80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supp orting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient Po-2 levels. P-crit increased sharply with measur ement temperature. Heart rate (f(H)) at 28 degrees C increased from about 1 25 beats/min in embryos to a peak of similar to 175 beats/min at days 10-30 and then fell to similar to 130 beats/min by day 100. Unlike for (M) over dot o(2), the Q(10) for f(H) was more constant at 1.2-2.5 throughout develo pment. Hypoxic exposure at any temperature had no effect on f(H) until simi lar to day 30, after which time a hypoxic bradycardia was evident. As evide nt for (M) over dot o(2), the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative o f hypoxic stress, the increasing prevalence of a hypoxic bradycardia in old er, warmer individuals supports the hypothesis that increasing hypoxic susc eptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulate (M) over do t o(2) and f(H) in response to the compounding demands of increased tempera ture and/or decreased oxygen availability first develops after similar to 2 0 days in D. rerio and, thereafter, the ability to maintain (M) over dot o( 2) in the face of ambient hypoxia progressively builds through to adulthood . Additionally, the temperature responses of metabolism and heart rate diff er substantially at different phases of development, suggesting a loose cou pling between the respiratory and cardiovascular systems, at least early in development.