Influence of the circadian rhythmicity of some inflammatory biomarkers in the athlete horse: evaluation of the time of exercise training

Background: The physiological ability of the animal to cope with environmental changes is a crucial requirement for maintaining homeostasis. Many species have evolved a specific endogenous circadian pacemaker as an evolutionarily adapted survival mechanism. The temporal organization of all biological processes of all living organisms that follow a 24-hour scale with specific rhythmicity is defined as circadian physiology. Physical activity induces metabolic modifications that involve most of the physiological system. Many physiological and biochemical variables associated with exercise show circadian rhythms that are phased similarly, such as physical exercise, which can act as a synchronization of some physiological and biochemical variables. In light of such consideration, the purpose of the present study was to evaluate the possible involvement of circadian changes of some physiological parameters in the gain of the response to exercise of some physiological variables in horses. Methods: The study was conducted on 10 Italian Saddle horses (7-12 years old, body weight 480±30 kg), housed in the same private horse training center in Sicily, subjected to the same management routine. Daily rhythmicity of white blood cell count (WBCs), leukocyte subpopulation (neutrophils, lymphocytes, monocytes, eosinophils and basophils), CD4+, CD8+ population, electrophoretic parameters (albumin, α1-, α2-, β1-, β2- and γ-globulins; interleukin-1β (IL-1 β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) were investigated by blood sampling performed every 4 hours over a 48-hour period. The level of these parameters was also tested in the same horses performing an exercise routine kept rigorously constant for eight days, in the morning (am) and afternoon (pm) in alternate order, performing the blood sampling before, immediately after, and 1 hour after the end of the exercise. Results: Inflammatory biomarkers analyzed (WBCs, leukocyte subpopulation, CD4+, CD8+, total proteins, electrophoretic fractions, IL-1β, IL-6, and TNFα) exhibited a daily rhythmicity. Most of them (WBCs, neutrophils, total proteins, albumin, α1- and α2-globulins, IL-1β and IL-6 ) reached their acrophases during the dark phase; lymphocytes, CD4+, CD8+, and TNFα showed a diurnal acrophase. A statistically significant effect of exercise was observed on WBCs, CD4+, CD8+, IL-1β and IL-6. Statistically differences between am and pm exercise responses were observed in CD4+, CD8+, IL-1β, and IL-6. Conclusion: In horses, a daily rhythm of the investigated parameters (WBCs, leukocyte subpopulation, CD4+, CD8+, total proteins, electrophoretic fractions, IL-1β, IL-6, and TNFα) was observed. The differences recorded during the exercise sections on the CD4+, CD8+, IL-1β, and IL-6 trends could be attributed to the daily fluctuation of these parameters. Considering the role of these parameters as inflammatory biomarkers their circadian rhythms should be taken in order with the daily fluctuation of the other physiological parameters to set a daily chronoprogram in athletic horses raising a safety margin against the tissues microtraum due to exercise.