Race, sex, and the regulation of urine osmolality: observations made during water deprivation

Abstract

A more concentrated urine is excreted by blacks than whites and by men than women. The purpose of this study was to explore the physiological bases for the race and sex effects during water deprivation when osmoregulation is challenged and differences are amplified. Drinking water was withheld from 17 blacks (10 men) and 19 whites (9 men) for 24 h. Vasopressin (VP) levels and osmolality in plasma (P(osmol)) and urine (U(osmol)) were measured basally and then every 4 h. U(osmol) was higher in blacks at baseline (P = 0.01) and during water deprivation (P = 0.046). Before and during water deprivation, no differences were seen in levels of VP, P(osmol), or the VP-U(osmol) relationship between blacks and whites. Although VP levels were initially higher in men (P < 0.02 for samples collected over the first 12 h), over the last 12 h of water deprivation, U(osmol) was higher (P = 0.027) and more responsive to the level of VP (in terms of slopes, P = 0.0001) in women than men. Our results suggest that, after a period of water deprivation, there develops a sensitivity of the collecting duct to VP that is greater in women. Although U(osmol) is higher in blacks, the race difference in water conservation did not appear to result from differences in the level of VP or the sensitivity of the collecting duct to VP. Upstream effects such as Na(+) uptake in the thick ascending limb, with its ensuing effects on water reabsorption, need to be considered in future studies of the relationship of race to water conservation.

A Consideration of Genetic Mechanisms Behind the Development of Hypertension in Blacks

“…Bankir and coworkers have now shown that, indeed, in blacks, urine volumes are typically lower and urine concentrations higher in comparison to whites [35]. Interventional studies with furosemide [36] and with water loading [37] have provided additional evidence for a more active NKCC2 in blacks.…”

Sodium Reabsorption in the Thick Ascending Limb in Relation to Blood Pressure

“…Using data collected in earlier clinical studies, Bankir and her collaborators showed that blacks indeed concentrate urine to a greater extent than whites 47–48 . Higher levels of vasopressin in blacks that could account for the race difference have been observed, but only in blacks who were hypertensive 49–51 , not in blacks who were normotensive 32, 50 , even when challenged with water deprivation 52 . If not vasopressin-dependent, the possibility remains that the renal medullary osmotic gradient that water in collecting duct is reabsorbed against is greater in blacks.…”

Physiology and pathophysiology of the renal Na-K-2Cl cotransporter (NKCC2)

“…Thus urinary osmolarity is higher and the average urine volume is smaller in individuals of African descent compared with Caucasians (145). Accordingly, Caucasians excrete a water load more rapidly than African-Americans, and this difference does not appear to be related to differences in the vasopressin levels (7,58,154). Furthermore, a reduced excretion of calcium in African-Americans compared with Caucasians may indirectly be related to increased NKCC2 activity, which is expected to enhance the driving force for paracellular calcium reabsorption in the TAL (116,155).…”