Association of filtered sodium load with medullary volumes and medullary hypoxia in hypertensive African Americans as compared with whites.

BACKGROUND: African Americans develop hypertension earlier with more target manifestations than whites despite having a higher glomerular filtration rate (GFR) for any level of serum creatinine. STUDY DESIGN & PARTICIPANTS: This study tested the hypothesis that increased GFR and sodium reabsorption in African Americans is associated with increased metabolic work and medullary hypoxia in 49 nondiabetic patients with essential hypertension (29 whites and 20 African Americans) following a constant-sodium diet (150 mEq/d) and renin-angiotensin system blockade. PREDICTORS: Ethnicity, age, measured GFR, sodium excretion, and body mass index. OUTCOMES: We examined cortical and medullary volumes and blood flows using multidetector computed tomography and intrarenal deoxyhemoglobin (R2*) using blood oxygen level-dependent magnetic resonance.
RESULTS: Blood pressure and sodium excretion were similar, whereas African Americans were more obese and had higher iothalamate GFRs. Renal cortical volumes did not differ, but medullary volumes adjusted for body size and age were higher in African Americans (32.3 ± 11.2 vs 25.1 ± 7.4 cm(3)/m(2) body surface area; P < 0.001). Sodium reabsorption and blood flows were higher in African Americans. Basal cortical deoxyhemoglobin values were similar between ethnic groups, whereas medullary R2* was higher in African Americans (39.7 ± 5.1 vs 36.3 ± 6.5/s; P = 0.02), but decreased to levels similar to whites after furosemide treatment. Levels of the circulating isoprostane prostaglandin F(2α) were higher in African Americans and daily urinary prostaglandin F(2α) excretion in African Americans correlated directly with renal blood flow (R = 0.71; P < 0.01). LIMITATIONS: Studies were limited to treated volunteers with normal kidney function without knowledge of prior nutrient intake.
CONCLUSIONS: These data show for the first time that increased sodium reabsorption in obese African American patients with hypertension was associated with enlarged medullary volumes, functional hypoxia related to solute reabsorption, and a direct relationship between blood flows and urinary isoprostane levels. Our results support a model of increased oxygen consumption and oxidative stress in African Americans that may accelerate hypertension and target-organ injury compared with white patients with essential hypertension.