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

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.

Metabolic ward at the University of Greifswald (Greifswald, Germany; study 1), and Clinical Research Center (Indiana University, Indianapolis, Indiana, USA; study 2).

In addition to conventional statistics, a pharmacokinetic analysis was carried out to determine the elimination rate constant and half-life.

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.

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.