Relevance and significance
The multifaceted antioxidant system consists of numerous components that
collectively provide individuals with a tool-box of ‘antioxidant
strategies’ they can use to protect against oxidative damage (Halliwell
and Gutteridge 2007, Costantini 2014). Migratory birds can use a
combination of non-enzymatic antioxidants gained from their diets and a
suite of endogenous antioxidant enzymes and sacrificial molecules to
protect against excess reactive species produced by relevant oxidative
challenges, including repeated bouts of flight and high-PUFA diets
(Skrip and McWilliams 2016, McWilliams et al. 2021). This study
demonstrated the ability of a songbird to use OXY during an acute flight
and to recover OXY levels within 2 days of rest (Fig. 2), revealing the
recovery timeline of antioxidant capacity for the first time (McWilliams
et al. 2021). We also found that such rapid adjustments to the
antioxidant system allowed birds to maintain overall low levels of
circulating damage, although the extent was tissue-dependent.
Researchers should remain mindful of the tissue-dependency of
antioxidant capacity and oxidative damage demonstrated here and
elsewhere (Frawley et al. 2021a) and that future studies that assess
oxidative status select biologically relevant tissues to measure that
directly relate to the hypotheses of interest.
Individual variation in the ability to utilize antioxidants must exist
for phenotypic flexibility of the antioxidant system to persist (Piersma
and Van Gils 2011) and we found evidence of such individual-level,
condition-dependent modulation of OXY and uric acid during long-duration
flight (Fig. S1). A subset of birds that expended energy in the upper
50% range (>0.51 kJ/min) seemed to either deplete OXY or
increase levels of uric acid, a potent antioxidant (Fig. 3B and Fig.
S1C), whereas the remaining birds did not change these circulating
measures (OXY, uric acid) from the averages. Studies, such as those done
with mosquitofish (Loughland and Seebacher 2020), that elucidate the
antioxidant strategies used by individuals to maintain their oxidative
balance under oxidatively challenging conditions and its effects on
performance (e.g. temperature acclimation, running speed, breeding
success, flight efficiency) would be particularly revealing for
migratory songbirds.
The study described here was part of a larger integrated research
project that also compared metabolic and antioxidant gene expression and
enzyme activities in multiple tissues within the same individuals
(citations redacted for review). This allows us to compare how flight
training, dietary antioxidants, and dietary PUFA affect the antioxidant
system across the traditional scales of the Central Dogma (DNA, to RNA,
to protein). All measures except oxidative status in tissues responded
strongly to flight training. In the plasma, proteins are subject to
posttranslational modifications and so better able to optimal to detect
individual strategies to combat oxidative damage in response to flight
exercise. In contrast, diet effects on oxidative status were most
evident in tissues (i.e., relative gene expression and antioxidant
assays) compared to functional assays in the plasma (i.e., markers for
antioxidant capacity and oxidative damage) perhaps because the former is
more tightly regulated. Our studies also suggest that plasma oxidative
capacity is not a good predictor of oxidative damage in tissues, whereas
antioxidant and metabolic gene expression and antioxidant enzyme
activities are significant predictors of oxidative damage in the
pectoralis and liver (citation redacted for initial review). Thus,
future studies must carefully consider the appropriate scale of
measurement(s) to best reveal how the antioxidant system of wild animals
responds to ecologically-relevant oxidative challenges such as
migration.