1. Exchangeable sodium (NaE), plasma electrolytes and arterial pressure were measured in 121 normal subjects and 91 patients with untreated essential hypertension (diastolic >100 mmHg), 21 of whom had low-renin hypertension. Plasma concentrations of renin, angiotensin II and aldosterone were measured in all hypertensive patients, total body sodium, total body potassium and exchangeable potassium (KE) in some patients.

2. Mean NaE was not different in normal and hypertensive subjects provided the two groups were matched for leanness index. In the subgroup of young hypertensive patients aged 35 years or less mean NaE was below normal. NaE was not related to arterial pressure in normal subjects but in hypertensive patients there were positive and significant correlations of arterial pressure with NaE and with total body sodium.

3. NaE and total body sodium increased with age in hypertensive but not in normal subjects. Partial regression analysis suggested that the correlation of NaE with arterial pressure was not explained by an influence of age.

4. Mean NaE was not increased and mean KE was not decreased in patients with low-renin hypertension.

5. Plasma potassium concentration, KE and total body potassium correlated inversely and significantly with blood pressure in hypertensive patients. These correlations were more marked in young than in old patients.

6. Multiple regression analysis showed that the combination of NaE and plasma potassium concentration ‘explained’ more of the variation of systolic blood pressure in hypertensive patients than it did in normal subjects. Plasma potassium concentration ‘explained’ more of the variation in young hypertensives and NaE ‘explained’ more in older patients.

7. Our findings suggest that changes of plasma and body potassium are important in the earlier stages of essential hypertension and that changes of body sodium become important later.

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