Comparing markers of the nitric oxide cycle and their association with ambulatory blood pressure and end organ damage in a bi–ethnic population : a SABPA–study
Aims There is a high prevalence of hypertension in the African population and it is known that vascular dysfunction (including nitric oxide (NO) bio-availability markers) play an important role in the development of cardiovascular diseases. Since very little is known regarding the role of markers of NO bio-availability in Africans, the aim of this study was to compare markers of NO bio-availability (namely L-arginine, L-citrulline, asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA)), ambulatory blood pressure (BP) and markers of end organ damage between African and Caucasian school teachers. Additionally, we also aimed to determine whether these markers of NO bio-availability are associated with ambulatory BP and markers of end organ damage in both ethnic groups. Methods The SABPA (Sympathetic activity and Ambulatory Blood Pressure in Africans) study was a cross-sectional study, including urbanised African (N=181) and Caucasian (N=209) men and women, between the ages of 25 and 65 years. Cardiovascular measurements included ambulatory blood pressure, pulse wave velocity (PWV), electrocardiographic Cornell product and carotid intima media thickness (cIMT). Anthropometric measurements included height, weight and waist circumference. Various bio-markers were analysed, including glucose, L-arginine, ADMA, SDMA, Lcitrulline, reactive oxygen species, albumin-to-creatinine ratio (ACR) and estimated creatinine clearance (eCCR). Characteristics of groups were compared with independent T-tests and Chi-square tests. Single and partial analyses were used to investigate associations between NO bioavailability markers with ambulatory BP measurements and markers of end organ damage. Analyses of covariance (ANCOVA) were used for comparison of variables between groups to determine significant differences, while adjusting for age, body mass index and antihypertensive medication. Forward stepwise multiple regression analyses were performed to determine if independent associations exist between ambulatory BP measurements or markers of end organ damage with either- L-arginine, L-citrulline, ADMA or SDMA as the main independent variable. Results and conclusion The Africans and Caucasians were of similar ages. However, the Africans had higher blood pressure therefore their cardiovascular profile was unfavourable compared to that of the Caucasians. The inhibitors of NO biosynthesis, ADMA and SDMA, were significantly lower in the Africans (p=0.046; p<0.001, respectively). However, the NO bio-availability markers, L-arginine and L-citrulline, were higher in the African compared to the Caucasian participants (all p values <0.05) regarded as significant. When performing unadjusted analyses, we found significant negative associations between eCCR and L-citrulline in all four subgroups: African men (r=-0.27; p=0.013), African women (r=-0.24; p=0.021), Caucasian men (r=-0.21; p=0.044) and Caucasian women (r=-0.28; p=0.003). The association of eCCR with L-citrulline was confirmed to be independent of confounders in all groups: African men (R2=0.46; β=-0.23; p=0.006), African women (R2=0.68; β= -0.12; p=0.046), Caucasian men (R2=0.62; β= -0.24; p<0.001) and Caucasian women (R2=0.72; β= -0.13; p=0.029). This implicates that renal function may be detrimentally affected by L-citrulline concentrations. In the Caucasian men and women negative correlations between eCCR and SDMA were found before adjustments (r=-0.33; p=0.003 and r=-0.26; p=0.006, respectively). This phenomenon was confirmed in the forward stepwise multiple regression analysis in Caucasian men (R2=0.75; β= -0.27; p<0.001) and women (R2=0.73; β= -0.21; p<0.001), while no associations were found in the Africans. This result is not unexpected, since SDMA can only be eliminated by the kidneys and is therefore an important risk marker for the early detection of renal dysfunction. In Caucasian men we found that ADMA correlated with ACR (r=0.36; p=0.001), night-time SBP (r=0.34; p=0.002) and night-time DBP (r=0.25; p=0.023) with single linear regression analyses. A similar trend was shown in African men with night-time SBP (r= 0.20; p=0.089) and night-time DBP (r= 0.21; p=0.078) respectively, but this association was absent in the Caucasian and African women. After adjustments for age and body mass index, the associations with ADMA, ACR and SBP in the Caucasian men remained. However, a negative association between eCCR and ADMA also became evident in the African men (r=- 0.24; p=0.025) and remained significant in the forward stepwise multiple regression analysis (R2=0.44; β= -0.18; p=0.034). It is, however, not clear why our results were gender specific, but we could speculate that the female sex hormones may play a part in protecting the vascular endothelium. Apart from the associations described above, there were no significant independent associations between the markers of the NO cycle (such as L-arginine) and PWV, cIMT, eCCR, ACR or Cornell product. In conclusion, although Africans presented a more vulnerable cardiovascular profile, we found a consistent negative association between renal function and L-citrulline in all participants, which has only been reported previously in patients with chronic renal disease. Additionally we found a gender-specific link between renal function and ADMA in African and Caucasian men. Our results may indicate that in the general population, markers of NO bioavailability may be associated with early changes in renal function, accompanying elevated blood pressure.
- Health Sciences