Objective: The relationships between blood pressure and the urinary excretion rates of sodium and potassium during the day and at night were investigated.
Methods: A total of 160 participants (135 normotensive subjects and 25 untreated patients with essential hypertension) were examined using ambulatory blood pressure monitoring and timed urine collections during waking and sleeping hours.
Results: Blood pressure averaged 126/79 mmHg during waking hours and 107/62 mmHg during sleep. More sodium, potassium and aldosterone were excreted during the daytime, but the natriuretic substance kallikrein was excreted at a fixed rate throughout the 24 h. During waking hours there was poor correlation between blood pressure and urinary sodium and potassium excretion. By contrast, at night when the aldosterone: kallikrein ratio fell, the sodium and potassium excretion rates were positively correlated with blood pressure.
Conclusions: Pressure natriuresis, not apparent during waking hours, may be unmasked at night when the balance between sodium-retaining and sodium-losing mechanisms favours natriuresis. Thus, the relationship between blood pressure and 24-h sodium excretion, usually considered to show the influence of salt intake on blood pressure, may also reflect pressure-induced natriuresis, if urine is more completely collected at night than during the day, and in circumstances favouring sodium retention during the day and sodium loss during sleep.