The relationship between oxidative stress-related markers and retinal microvascular structure and function in a bi-ethnic population : the SABPA study

Abstract

Reactive oxygen species (ROS) play a role in regulating endothelial function and vascular tone. Under normal conditions, there is a balance between ROS and antioxidant defense capacity, but any disturbance in this equilibrium may result in oxidative stress which in turn may disrupt redox signaling and lead to molecular damage. This may further disrupt the functioning of the endothelium, contributing to the development of cardiovascular disease (CVD). Since the risk of developing CVD is higher in urban black populations, previous SABPA sub-studies investigated the link between oxidative stress-related markers and components of the macrovasculature. These studies reported associations of higher levels of ROS (serum peroxides) in black men and lower glutathione peroxidase (GPx) activity in black women together with increased blood pressure. Furthermore, reduced glutathione (GSH) levels and up-regulated glutathione reductase (GR) activity were associated with increased carotid intima-media thickness (c-IMT) in black men. These results indicate that blacks are indeed at a higher risk of developing CVD compared to whites. As microvascular functional changes are thought to occur prior to the changes in the macrovasculature, it is valuable to evaluate associations of oxidative stress-related markers and microvascular parameters. The retina allows easy access and direct visualization of the microvasculature. The Dynamic Vessel Analyzer (DVA) can be used to obtain retinal photographs whereby caliber equivalents of central retinal artery (CRAE), central retinal vein (CRVE) and the subsequent calculation of the ratio between CRAE and CRVE (arterio-venous ratio (AVR)) can be determined. Furthermore, functional responses of retinal blood vessels to a flicker light-induced provocation (FLIP) can be assessed and include maximum artery dilation (Art_maxdil) and maximum vein dilation (Vein_maxdil). Retinal vascular changes are associated with both clinical and subclinical CVD. Similarly, reduced retinal vessel responses to FLIP have been associated with diabetes, hyperlipidemia, hypertension and coronary artery disease. Both retinal vessel calibers and reduced retinal vessel responses to FLIP are possible early indicators of systemic vulnerability and CVD risk. Detection of these microvascular changes may be useful in predicting future cardiovascular events. Additionally, relevant therapies may be implemented to lower the risk of developing CVD. Some results on the relationship of retinal vessel calibers and retinal vessel responses to FLIP with oxidative stress-related markers have been reported. Results from these studies suggested that antioxidant defense mechanisms play a cardio-protective role in the retinal microvasculature and regulation of retinal vascular function which may lower the CVD risk. Previous SABPA sub-studies evaluated oxidative stress-related markers in relation to the macrovasculature (as already indicated above). However, there is no available data on the associations of retinal vessel calibers and retinal vessel responses to FLIP with oxidative stress-related markers in Sub-Saharan Africa. The aims of this study were firstly, to compare oxidative stress-related markers in a cohort of black and white men from South Africa and secondly, to explore the relationships of retinal vessel calibers and retinal vessel responses to FLIP with oxidative stress-related markers in this group. Methods This cross-sectional study was performed using the data obtained from the follow-up phase of the SABPA study that was conducted between February and May of 2011/2012. Participants were informed about the purpose of the study as well as the procedure of the study and they all signed an informed consent form. A total of 359 black and white teachers from the Kenneth Kaunda Education district in the North-West Province of South Africa took part in the follow-up phase of this study. Thirty three participants were excluded as a result of no available or poor functional retinal vessel analyses data, antioxidant intake and outliers for superoxide dismutase (SOD). Women were also excluded from this study as their status of menopause and/or phase of fluctuating hormone levels during their menstrual cycle were unknown. After these exclusions, a total of 159 participants were considered eligible for our sub-study of which 82 were black men and 77 were white men. Anthropometric measurements were taken and lifestyle factors were recorded in the form of questionnaires. Standardized procedures were used for biochemical analyses of plasma/serum samples. The following oxidative stress-related markers were measured: ROS (in the form of total peroxides), thiobarbituric acid reactive substances (TBARS), SOD, GR, GPx, total glutathione (tGSH) and the ferric reducing ability of plasma (FRAP). Cardiovascular measurements included: 24h ambulatory blood pressure monitoring. Regarding the retinal vessel analyses, functional responses of retinal vessels to a FLIP were recorded as percentages of baseline and retinal vessel calibers were determined from retinal fundus images using the DVA. Oxidative stress-related markers between black and white men were compared by means of independent t-tests. Chi square tests were used to compare proportions between black and white men. The correlations between retinal vessel calibers and vessel responses to FLIP with oxidative stress-related markers were investigated by means of Pearson and partial analyses. Furthermore, independent associations of oxidative stress-related markers with retinal vessel calibers and responses of retinal vessels to a FLIP were investigated in forward stepwise multiple regression analyses. For the final multiple regressions (which provided the best fit models), variables that entered the stepwise models were used in a separate multiple regression analysis and are presented in tables. Sensitivity analyses was also performed, excluding human immune deficiency virus (HIV) infected participants and also testing the effect of smoking, hypertensive status and using anti-hypertensive medication by adding these factors individually to the models. Results Despite higher GPx activity (p=0.001) and a tendency of SOD (p=0.062) to be higher in black men, they also had higher oxidative damage (TBARS) (p=0.011) with lower levels of non-enzymatic antioxidant markers (FRAP and tGSH) (both p<0.001) compared to white men. Regarding the functional retinal vessel measurements, Art_maxdil (p=0.036) was higher in black men compared to white men. Retinal vessel caliber markers showed a smaller AVR and a wider CRVE (both p<0.001) in black men when compared to white men. An independent association between Art_maxdil and SOD (Adjusted R2=0.19; β=-0.36; p=0.002) was seen in black men only. In the black group, AVR (Adjusted R2=0.12; β=-0.32; p=0.007) and CRAE (Adjusted R2=0.27; β=-0.23; p=0.040) inversely associated with ROS. Conclusion Art_maxdil in response to FLIP was negatively associated SOD in black men. Since SOD is known to be defensive against the effects of O2∙- by increasing the dismutation of O2∙-, the up-regulation of SOD may also be needed in the black group to prevent oxidative damage to macromolecules. However, the up-regulated SOD activity may be inadequate to prevent oxidative damage to lipids (reflected by higher TBARS) and also inadequate for the preservation of NO- bioavailability but not to a point where Art_maxdil is reduced. Although the retinal vascular function is still maintained, continued exposure to increased oxidative stress may impair Art_maxdil. Retinal vessel calibers (AVR and CRAE) were independently associated with ROS in black men only. These results indicate that the narrowing of retinal arteries is mediated by ROS suggesting that oxidative stress may play a role in retinal microvascular changes.