Antioxidants, Vol. 11, Pages 2326: L-Citrulline Supplementation Reduces Blood Pressure and Myocardial Infarct Size under Chronic Intermittent Hypoxia, a Major Feature of Sleep Apnea Syndrome

Oxidative stress generated by IH is a key mechanism triggering BP elevation and endothelial dysfunction in OSA patients [53] and in rodents exposed to IH [27,48,54,55]. Superoxide anion production was reported to increase in the aorta and mesenteric arteries of mice exposed to 8 days of hypercapnic hypoxia [55] or to 14 days of IH (60 cycles/h; 21–5%) [54], while SOD activity decreased in rats exposed to 14 days of IH (60 cycles/h; 21–5%) [27]. Finally, NOX, XOX, and catalase activities were increased, whereas GPX activity was decreased in the aorta from rats exposed to 7 days of IH (10 cycles/h; 21–7%) [48]. These studies demonstrate different oxidative stress responses to IH among vascular beds that depend on the duration of total IH exposure as well as characteristics (number of cycles per hour, depth) of IH [56]. In recent meta-analyses performed on rodents, we demonstrated that the duration (acute exposure of few hours vs. chronic exposure of several days) and the depth of hypoxia (10% vs. 5%) impact the response to ischemia-reperfusion [57]. Since IH did not impact superoxide anion content or pro/anti-oxidant balance in the aorta, we cannot say that vascular oxidative stress is the main driver of an increase in BP under IH. However, IH tends to increase aortic and plasmatic nitrotyrosine levels, as previously demonstrated by the same stimulus in the aorta from hypoxic mice [54]. Citrulline supplementation significantly decreased the nitrotyrosine levels in the aorta and in plasma, solely in IH rats. Intimal eNOS-derived NO plays a major role in maintaining the vascular function, and eNOS uncoupling is associated with oxidative stress production [58,59,60]. However, the eNOS expression has been demonstrated to be increased after 3 days and drastically decreased after 8 weeks of IH, suggesting a compensatory followed by a deleterious mechanism associated with oxidative stress [61]. In our study, neither eNOS nor its inhibiting or activating phosphorylation on the serine1177 (S1177) and threonin495 (T495) residue were modified after 2 weeks of IH (Figure S3a,b). Accordingly, Siques et al. reported that chronic IH increased nitrotyrosine levels in pulmonary arteries without any modification in the eNOS expression [62]. Under IH, we observed a significant decrease in nitrite content, suggesting a partial decrease in NO availability. This could be explained by a decrease in eNOS activity triggered by ADMA production and/or by a decrease in arginine availability due to an increase in arginase activity as previously described under IH [30,63]. Regarding citrulline protective mechanisms, it was demonstrated to impact eNOS dynamics by increasing the total eNOS protein expression, S1177 phosphorylation, and NO production [52,64]. However, in the small pulmonary arteries of piglets, the vasorelaxation induced by citrulline was maintained in the presence of N(G)-Nitro-L-arginine methyl ester (L-NAME), an indication of factors other than eNOS in the beneficial impact of citrulline on vascular tone [52]. This could explain why after 2 weeks of IH, citrulline decreased the MBP without affecting the eNOS dynamics. The decrease in NO availability can also be explained by an increase in oxidative stress (i.e., O2−), leading to an increase in ONOO- and subsequent nitrosative stress. In our study, we reported that citrulline increased SOD activity in N and IH groups (Figure 4e). Citrulline especially decreased aortic and plasmatic nitrotyrosine levels only under IH and abolished the IH-decreased nitrites content (Figure 4g–i). This demonstrates that under IH, citrulline targets (1) oxidative stress and (2) the combination of oxidative stress with NO imbalance. Regarding the myocardium, we demonstrated that IH increased superoxide anion content and XOX activity. This is in agreement with previous studies demonstrating that chronic IH is a potent oxidative stress generator, particularly via XOX activation [65]. Previous studies demonstrated that a myocardial superoxide anion production increase is associated with an infarct size increase since the anti-oxidative agent use decreased cardiomyocyte death after an ischemia-reperfusion protocol [18,27,28]. Our results showed that citrulline prevented the IH-increase in O2− and XOX activity and also decreased infarct size without any modification in the eNOS activity and expression (Figure S3c,d). This is in agreement with other studies highlighting the potential anti-oxidant effects of citrulline: an increase in SOD activity and a decrease in lipid peroxidation in the heart tissue in a rat model of sepsis [66]; a blunt in H2O2− induced ROS production in retinal epithelial cells while limiting lipid peroxidation and preventing cell death [67]; a renal decrease in oxidative-DNA damage without affecting the eNOS expression in spontaneously hypertensive rats [35]. This suggests that citrulline can preserve the heart against oxidative stress without any modification of the eNOS expression. This contributes to explaining how, in the context of IH, citrulline decreased infarct size after an ischemia-reperfusion protocol. Finally, during ischemia-reperfusion, the decrease in ATP/ADP ratio slowed reactions requiring high energy levels. By thermodynamic action, previously described as a decrease in the energy activation levels of one or several ATP-consuming reactions [68,69], citrulline may protect against cardiomyocyte death.

Very interestingly, we observed that chronic IH altered both vascular and myocardial responses and that citrulline prevented these effects by exclusively reducing nitrosative stress in the aorta under IH and by significantly reducing superoxide anion content increased by IH in the heart. After 14 days of IH, we showed a photo (see graphical abstract) of a redox imbalance that deserves to be deeply investigated in further studies.

留言 (0)

沒有登入
gif