Photoperiod-dependent changes in oxidative stress markers in the blood of Shetland pony mares and stallions involved in recreational horseback riding.

The objective of the current study was to determine the photoperiod-induced variations and the impact of exercise on oxidative stress biomarkers [2-thiobarbituric acid reactive substances (TBARS), aldehydic (AD) and ketonic (KD) derivatives of oxidatively modified proteins (OMP), total antioxidant capacity (TAC), and activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)] and biomarkers of metabolic alterations [glucose, urea, and uric acid and the activity of lactate dehydrogenase (LDH)] in the blood of Shetland pony mares and stallions involved in recreational horseback riding. Twenty-one healthy adult Shetland ponies (11 mares and 10 stallions) aged 6.5 ± 1.4 years old from the central Pomeranian region in Poland were used in this study. Blood samples were taken once per season for one year: spring (3 April), summer (5 July), autumn (1 October) and winter (25 January). A MANOVA analysis revealed that the photoperiod factor had a leading role in alterations of these biomarkers, while the exercise and sex of the ponies exerted a lower impact. The lipid peroxidation biomarkers, for example, the plasma TBARS level, indicated the maximum adjusted coefficient of determination R ² _ad  = 0.77. Before exercise (at rest), the plasma of the stallions and mares exhibited minimum values of TBARS levels in the spring and summer photoperiods and maximum levels in autumn and winter. A statistically significant reduction in the levels of both aldehydic and ketonic derivatives of OMP in the blood of ponies was observed during the autumn and winter periods; additionally, the level of ketonic derivatives of OMP declined after exercise in spring. TAC was statistically significant in the spring and winter photoperiods both before and after exercise. SOD activity did not have a pronounced photoperiod-induced pattern but was dependent on the sex and exercise. CAT activity varied and was statistically significant only in the plasma of the mares after exercise in the spring, summer, and winter photoperiods. The minimum GPx activity in the blood of the mares before exercise (at rest) was observed in autumn, while the maximum was noted in winter and summer. Photoperiod- and exercise-induced alterations in markers of oxidative stress and antioxidant defences may contribute to the adaptation of animals to exercise, depending on sex. The seasonal variations in the antioxidant defences demonstrated in our study, as well as substrates of energy metabolism in the blood of mares and stallions, depending on exercise capacity, could be an important aspect in the ability of endogenous adaptive mechanisms of animals to react in advance to environmental changes associated with seasons.