Sodium kinetics in salt-sensitive and salt-resistant normotensive and hypertensive subjects

OBJECTIVE: To test the hypotheses that sodium kinetics are not affected by blood pressure, salt sensitivity, salt resistance or race, and that the kinetics of sodium balance are not a first-order process. DESIGN, PARTICIPANTS AND INTERVENTIONS: Two studies were conducted. In the first, 18 normotensive and 36 hypertensive men and women were given sodium at 120 mmol/day for 6 days, followed by 10 mmol/day for 8 days, then 400 mmol/day for 8 more days. Salt sensitivity was defined as an increase in diastolic blood pressure from the 10 to the 400 mmol/day intake. Salt resistance was defined as no increase, or a decrease in diastolic blood pressure with the increased sodium intake. In the second study, 12 white and 12 black normotensive men ingested sodium at 10, 200 or 400 mmol/day in random order, each for 7 days. All urine was collected in both protocols. SETTING: Metabolic ward at the University of Greifswald (Greifswald, Germany; study 1), and Clinical Research Center (Indiana University, Indianapolis, Indiana, USA; study 2). MAIN OUTCOME MEASURE: In addition to conventional statistics, a pharmacokinetic analysis was carried out to determine the elimination rate constant and half-life. RESULTS: In the Greifswald study, when the sodium intake was decreased, a longer half-life was determined for the salt-sensitive than the salt-resistant hypertensive subjects. The half-life for the normotensive salt-sensitive and salt-resistant subjects did not differ. When the sodium intake was decreased, a monoexponential equation fitted the data for all subjects; when the sodium intake was increased, only data for half the subjects could be fitted to the same equation. In the Indianapolis study, black race had a significant influence upon urinary sodium excretion. Furthermore, the half-life for sodium elimination was dependent upon sodium intake; namely, the greater the intake, the longer the elimination half-life. CONCLUSIONS: The time required to reach sodium balance may increase following salt-sensitive increases in blood pressure rather than precede them. Race influences the time required to achieve salt balance. Sodium kinetics are not a first-order process.