Ethnic differences in urine concentration: possible relationship to blood pressure.

The mechanisms that account for the susceptibility of black individuals to hypertension and their reduced ability to excrete sodium are poorly understood. Vasopressin administration has been shown in healthy humans to delay sodium excretion along with its antidiuretic action. Black individuals have been reported to have higher vasopressin levels than white individuals. Therefore, this study investigated retrospectively 24-h urine volume (V) and urine concentration index (urine-to-plasma ratio of creatinine concentration), as well as their possible relationships with BP, in a cohort of 141 healthy young black and white individuals (18 to 40 y). Black individuals were found to have a significantly lower V and higher urine concentration than white individuals, especially during daytime. In addition, they exhibited a blunted nocturnal fall in fluid and electrolyte excretion and a higher pulse pressure than white individuals. Higher urine concentration and lower V were associated significantly with higher PP (but not with systolic or diastolic BP) in men. These relations remained significant after adjustment for age, body mass index, and sodium and potassium excretion. These results suggest that an enhanced tendency to concentrate urine may delay the excretion of the daily ingested fluid and sodium and may increase pulse pressure in young normotensive individuals. The higher urine concentration that is observed in black individuals (which could represent an adaptation to better water conservation) may participate in their enhanced susceptibility to hypertension. If these results are confirmed in further studies, then vasopressin V2 receptor antagonists might offer a novel antihypertensive strategy, especially in the black population.