Different patterns of chronic hypoxia lead to hierarchical adaptive mechanisms in goldfish metabolism

Abstract

Some hypoxia-tolerant species, such as goldfish, experience intermittent and severe hypoxia in their natural habitat causing them to develop multiple physiological adaptations. However, in fish, the metabolic impact of regular hypoxic exposure on swimming performance in normoxia is less well understood. Therefore, we experimentally tested whether chronic exposure to constant (30 days at 10% air saturation) or intermittent hypoxia (3hrs in normoxia and 21hrs in hypoxia, 5 days a week) would result in similar metabolic and swimming performance benefits after reoxygenation. Moreover, half of the normoxic and intermittent hypoxic fish were put on a 20-day normoxic training regime. After these treatments, metabolic rate (standard and maximum metabolic rates: SMR and MMR) and swimming performance (critical swimming speed [Ucrit] and cost of transport [COT]) were assessed. In addition, enzyme activities (citrate synthase CS, cytochrome c oxidase COX and lactate dehydrogenase LDH) and mitochondrial respiration were examined in red muscle fibres. We found that acclimation to constant hypoxia resulted in (1) metabolic suppression (-45% SMR, and -27% MMR), (2) increased anaerobic capacity (+117% LDH), (3) improved swimming performance (+80% Ucrit, -71% COT) and (4) no changes at the mitochondrial level. Conversely, the enhancement of swimming performance was reduced following acclimation to intermittent hypoxia (+45% Ucrit, -41% COT), with a 55% decrease in aerobic scope, despite a significant increase in oxidative metabolism (+201% COX, +49% CS). This study demonstrates that constant hypoxia leads to the greatest benefit in swimming performance and that mitochondrial metabolic adjustments only provide minor help in coping with hypoxia.

Non-lethal sampling for assessment of mitochondrial function does not affect metabolic rate and swimming performance

“…After removing potential white muscle fibres under a binocular magnifying glass, the red muscle sample was gently dried on aluminium foil and then weighed before being placed in the respirometer chamber containing 2.1 ml of MiR05 without undergoing chemical or mechanical permeabilization. We have shown in previous studies that similar tissue preparation in fish allows for stimulation of respiratory fluxes by exogenous substrates [41,43]; we can thus assert that the plasma membranes in our experimental conditions were permeabilized.…”

“…The next morning, the U crit protocol was performed as in [41]. Individuals were exposed to a gradual increase in the current (0.5 BL/s every 30 min) until the fish was exhausted (resting on the bottom grid of the swimming area for more than 10 s; [41]). During the U crit protocol, the M˙O2 was also measured, and the MMR was calculated as the maximum oxygen consumption.…”

“…These fish were anaesthetized and biopsied as above and on the next day they were anaesthetized and euthanized (cervical dislocation) and dissected to collect red muscle. Collection of red muscle was performed by cutting a square of skin and removing a piece of red muscle with a scalpel, as done in [41]. Sampling technique was not found to have any effect on the mitochondrial metabolic traits.…”

“…A sequential substrate and inhibitor titration protocol, adapted from [44] and [41], was immediately run for each fish. First, the basal respiration was obtained following the addition of pyruvate (final concentration in the respirometry chamber: 5 mM), malate (2.5 mM) and succinate (5 mM).…”

Intermittent hypoxia differentially affects metabolic and oxidative stress responses in two species of cyprinid fish

“…Our study showed that in gibel carp, CCO activity was upregulated in both the gill and liver during hypoxia, and further elevated in the gills during recovery. Similarly, another hypoxia-tolerant cyprinid, the goldfish Carassius auratus , exhibited increased CCO activity in its muscle after short-term intermittent hypoxia exposure, which was not sustained after long-term (30 days) hypoxia ( Thoral et al, 2022 ). However, our study found that the CCO activity in the liver of gibel carp was suppressed during reoxygenation, which is in line with our previous conclusion that reoxygenation is more stressful for this species than hypoxia.…”