Whites Excrete a Water Load More Rapidly Than Blacks

Abstract

Abstract-A recent report demonstrated a racial difference in response to furosemide compatible with increased ion reabsorption in the thick ascending limb of the loop of Henle in blacks. Urinary dilution is another function of the loop-diuretic-sensitive Na,K,2Cl cotransporter in the thick ascending limb, and racial differences in urinary diluting capacity have not been reported previously. We assessed diluting segment (cortical thick ascending limb and distal convoluted tubule) function in black and white normotensives in 2 studies using a water-loading approach. In both studies, we found that whites excreted a water load more rapidly than blacks. In the first study, the final free water clearance rates (meanϮSD) were 7.3Ϯ4.7 mL/min in whites (nϭ17, 7 females and 10 males) and 3.8Ϯ3.6 mL/min in blacks (nϭ14, 9 females and 5 males; PϽ0.03). In the second study, final free water clearance rates were 8.3Ϯ2.6 mL/min in whites (nϭ17, 8 females and 9 males) and 6.4Ϯ1.8 mL/min in blacks (nϭ11, 8 females and 3 males; PϽ0.01). We found no evidence of a racial difference in renal proximal tubular fluid reabsorption as assessed by renal endogenous lithium clearance or in plasma vasopressin level that could explain the difference in free water excretion. We conclude that our observations are most consistent with a lower capacity of ion reabsorption in the renal diluting segment in blacks. Key Words: kidney Ⅲ water Ⅲ race Ⅲ renal tubule Ⅲ ion Ⅲ vasopressin C hun et al 1 recently demonstrated a racial difference in response to furosemide compatible with increased activity of the loop-diuretic-sensitive Na,K,2Cl (NKCC2) cotransporter in the thick ascending limb (TAL) of the loop of Henle in blacks compared with whites. In addition to helping to concentrate urine, the cortical TAL NKCC2 (in concert with the Na,Cl cotransporter in the water-impermeable segments of the distal convoluted tubule) generates free water by the net reabsorption of ions from tubular fluid, and a racial difference in NKCC2 activity could, therefore, affect the ability to dilute urine. Tubular diluting capacity can be characterized by measuring free water generation during water loading, 2 and we report here the results of 2 experimental studies of renal responses to water loading in blacks and whites. MethodsBoth studies were approved by the University of Michigan Institutional Review Board, and all of the subjects gave written informed consent. For both studies, we recruited subjects by public advertisement from Ann Arbor, Michigan, and the surrounding area. Subjects were healthy normotensive black and white (non-Hispanic) men and women, aged 18 to 50 years. As per National Institutes of Health guidelines, race was self-determined. Normotensive status was established during a screening visit as a systolic blood pressure of Ͻ140 mm Hg and a diastolic blood pressure of Ͻ90 mm Hg for the average of 2 seated auscultatory blood pressure measurements performed by an experienced observer. A general health history was obtained and a physical examination perfor...

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