In the present study first we investigated the effect of a diet high in fat, sugar and salt on spleen oxidative/antioxidative status and spleen histomorphometry of Wistar rats. Our findings revealed that a HCSD diet could increase oxidants (MDA) and decrease antioxidants (SOD, GPx, and TAC) in spleen tissue of rats. It is believed that a diet high in saturated fat, glucose, and salt can lead to neuro-hormonal disorders, inflammation, and oxidative stress [24], and some previous studies have reported that a high-fat diet or high-fat high-salt diet could increase oxidative stress and result in reduced antioxidant capacity in different bodily organs [25–27]. Increased ROS production and resulting oxidative stress is supposed to be a key player in fibrosis [28]; accordingly, our findings showed that HCSD diet led to significant changes in splenic morphology. We found that 10 weeks of HCSD diet led to an increase in the red pulp/white pulp ratio. Moreover, it led to a significant decrease in the number of lymphocytes, and the central trabecular volume, which all suggest that there has been a considerable structural change. Concomitant changes in oxidative stress and morphological parameters supports the notion that ROS are involved in splenic enlargement.
However, the main purpose of this study was to investigate the therapeutic effect of exercise on reducing the diet-induced oxidative stress and deleterious alterations in spleen structure. Our findings indicated that SIT training prevented the diet-induced changes in oxidative/antioxidative status in spleen, as was assessed by MDA, SOD, GPx and TAC level alterations. In line with the findings of the present study swimming training has been shown to increase the SOD and GPx antioxidant enzymes in rat spleen tissue [29]. Moreover, Feriani et al., (2021) showed that chronic immobility stress led to elevation of pro-oxidant and inflammatory markers, but the exercise training improved the antioxidant mechanisms and decreased inflammatory cytokine content in in the spleen of mice [30]. They concluded that exercise training was effective in mitigating the stress induced damages in the spleen and it provides a non-pharmacological therapeutic strategy for stress-evoked pathologies. additionally, Senna et al., (2016) reported that in low-protein fed rats which received lipopolysaccharide (LPS) shot, exercise training (8 weeks, 5 days a week, 60 minutes a day, with 70% VO2max) led to attenuation of splenic lymphocytes apoptosis and TNF-α plasma concentrations [31]. Inflammation and oxidative stress are interrelated phenomena; just as inflammation leads to oxidative stress, and vice versa [32]. Hence, although not assessed in the present study, exercise-induced favourable changes in oxidative/antioxidative status in the spleen may have been in part due to anti-inflammatory properties of exercise, as exercise has been shown to modulate the function of immune cells that are abundant in the spleen [6]. Even light exercise has been shown to be sufficient to reduce circulating proinflammatory cytokines like TNF secreted by lymphocytes and macrophages [33, 34]. Moreover, it has been reported that splenic volume decreases during exercise in intensity dependent manner [6, 35]. This decrease, at least in part, may be related to increased catecholamine levels [35], and resulting vasoconstriction in spleen during sprinting intervals, which may lead to some ischemia in the spleen. The subsequent reperfusion during low intensity phase between sprinting intervals can increase ROS production. This repeated physiologic stress may stimulate the antioxidant enzyme expression, namely SOD and GPx, through the Nrf-2 accumulation and nuclear translocation [36]. However, considering the paucity of information about the spleen response and adaptation to various exercise intensities and modalities, further studies are required to confirm these findings.
Our results also show in rats under HFSD there was significant changes in splenic morphology. Rats fed a HFSD showed a significant increase in the red to white pulp ratio, and a significant decrease in the number of lymph cells, and splenic trabecular volume. This indicates that HFSD has led to expansion of the red pulp or abridgment of the white pulp. However, considering the significant decrease in the splenic lymph cells, it seems that HFSD has led to specific changes in white pulp rather than morphological alterations in whole spleen. Altukkaynak et al., (2007), and Buchan et al., (2018) reported that high-fat diet led to expansion of sinusoids and surrounding tissue in both the red and white pulps rather that specific changes in on pulp [6, 37]. The decrease in the white pulp may be related to diet induced changes in immune cell number and apoptosis in the spleen. As a previous study reported that HFD can increase in the apoptotic rate of splenic lymphocytes [37]. Moreover, a diet high in salt has been shown to decrease the percentage and absolute number of natural killer cells in spleen [38].
Interestingly, our result showed that there was no significant difference between the C and HCSD + SIT group in none of the assessed morphometric parameters which indicates SIT has prevented HCSD-induced disturbances in spleen microstructure. This finding is in line with the findings of Buchan et al., (2018) who reported regular moderate-intensity exercise reduced high-fat high-sugar diet induced increment in spleen weight and IL-6 levels, and reversed diet-induced splenic enlargement [6]. The preventative effect of exercise may be related to exercise induced improvement in oxidative/antioxidative status in the spleen. As it has been reported that high-fat diet may induce oxidative stress in different organs including spleen [39], and high-salt diet may increase the production of reactive oxygen species (ROS) by NK cells in the spleen [38], which in turn they can cause inflammation and microstructural abnormalities in the spleen tissue.