Nitric oxide bioavailability and cardiovascular function in children and young adults

Abstract

Background and motivation: The global burden of cardiovascular disease (CVD) including hypertension is ever-increasing, especially in the South African context. Nitric oxide (NO) plays a vital role in normal vascular endothelial function with considerable evidence of imbalanced NO bioavailability in the elderly and diseased states such as hypertension. Endothelial dysfunction with attenuated NO bioavailability is central to the pathogenesis of CVD and has been implicated as a possible mechanism in the premature development of hypertension. Due to the prevalence of hypertension being the highest in Sub-Saharan Africa, the need for early identification and risk stratification of vascular abnormalities in arterial hypertension is paramount, especially in understudied black populations. Due to the nature of NO, measuring markers involved in its bioavailability as possible indicators of a NO profile is warranted. Moreover, further understanding the interactions of NO-related markers with endothelial function may impact the progression of CVD. Conversely, there is limited data surrounding the NO profile and the possible pathophysiological roles thereof in young healthy black and white populations respectively. Therefore, exploring whether an unfavourable NO profile plays a pivotal role in the development of CVD is warranted, especially in the black population who seem to be predisposed to CVD. Aim: The aim of this study was to explore the associations of NO-related markers with cardiovascular structure and function in apparently healthy children and young adults. The study also aimed to determine whether the NO profile differed among groups stratified by age, sex and ethnicity and if there are any associations of plasma and urinary NO-related markers with blood pressure (BP), arterial structure and endothelial function in black and white South Africans. Methodology: This thesis included data from the Arterial Stiffness in Offspring Study (ASOS) and the African Prospective study on the Early Detection and Identification of Cardiovascular Disease and Hypertension (African-PREDICT). These studies were cross-sectional and included black and white children (ASOS: n=81) and young adults (African-PREDICT n=1202). Anthropometric procedures included body height (cm), body weight (kg), waist and hip circumference (cm). Additionally, body mass index (BMI) was calculated. Biochemical analyses were performed where urinary arginine, homoarginine, asymmetric (ADMA) and symmetric dimethylarginine (SDMA) as well as ornithine/citrulline, malondialdehyde (MDA), creatinine, nitrite and nitrate were determined in both the ASOS and African-PREDICT studies (gas chromatography-mass spectrometry (GC-MS)). The urinary nitrate-to-nitrite ratio (UNOxR) was additionally calculated. Plasma arginine, homoarginine, ADMA and SDMA were determined in the African-PREDICT study only (liquid chromatography-tandem mass spectrometry (LC-MS/MS)). Additional biochemical analyses were performed in the African-PREDICT study which included the lipid profile (total cholesterol, high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and triglycerides), gamma glutamyltransferase, creatinine and high sensitivity C-reactive protein (Cobas Integra 400 plus Roche, Basel Switzerland). Serum cotinine levels were determined with a chemiluminescence method on the Immulite (Siemens, Erlangen, Germany). Creatine kinase (CK) was determined with electrochemiluminescence method on the E411 (Roche, Basel Switzerland). Sodium fluoride plasma glucose (Siemens, Erlangen, Germany) and EDTA whole blood glycated haemoglobin was determined (Cobas Integra 400 plus Roche, Basel Switzerland). Urinary albumin (mg/L) and creatinine (mmol/L) were determined (Cobras Integra® 400plus, Roche, Basel, Switzerland) and the urinary albumin-to-creatinine ratio (uACR) was calculated. Furthermore, the Chronic Kidney Disease Epidemiology (CKD-EPI) formula was utilised to calculate the estimated glomerular filtration rate (eGFR) from serum creatinine values. Cardiovascular measures in the ASOS study included brachial office BP using the Omron HEM-759-E (750IT) device (Omron Healthcare, Tokyo, Japan) and carotid intima media thickness (cIMT) using B mode ultrasonography (SonoSite MicroMaxx, Bothell, WA). Likewise, the cardiovascular measures in the African-PREDICT study included brachial office BP (Dinamap® ProCare 100 Vital Signs Monitor (GE Medical Systems, Milwaukee, USA)), 24-hour ambulatory BP (CardioXplore, Meditech, Budapest, Hungary, British Hypertension Society (BHS) validated), cIMT (General Electric Vivid E9 device; GE Vingmed Ultrasound A/S, Horten, Norway), central BP (central systolic blood pressure (cSBP)) and pulse wave velocity (PWV) (SphygmoCor XCEL device (AtCor Medical Pty. Ltd., Sydney, New South Wales, Australia). Statistical analyses included independent T-tests to compare means and chi-square to compare proportions between the groups. Pearson, partial and multiple regression analyses were performed and adjusted for potential confounders to investigate the associations of both plasma and urinary NO-related markers with cardiovascular and biochemical markers according to the specified focus of each research manuscript.