Relationship of sodium sensitivity and the sympathetic nervous system

To test the hypothesis that salt-sensitive hypertension reflects an abnormal response of the sympathetic nervous system (SNS) to salt, arterial pressure and plasma norepinephrine (NE) concentration were measured in 23 black hypertensive patients and 28 black normotensive control subjects during 4 days of salt depletion and 3 days of salt loading. When the sequence was salt depletion followed by salt loading, plasma NE rose with salt depletion and was suppressed by salt loading. In normotensive subjects, plasma NE with salt loading returned to control values, but in hypertensive patients it remained elevated suggesting SNS stimulation by the salt load. Plasma NE responses of hypertensive patients did not distinguish between those who became normotensive with salt depletion from those who did not. The sequence was reversed with salt loading preceding salt depletion, to investigate further these abnormal plasma NE responses to salt loading. With this sequence, plasma NE was only slightly and insignificantly reduced by salt loading and responses of hypertensive patients were similar to those of normotensive subjects. In contrast, plasma NE rose with salt depletion as before. With both protocols plasma NE correlated inversely with urinary sodium excretion as previously described. These results in black hypertensive patients are not consonant with those obtained in other studies of primarily white hypertensive patients, suggesting that salt-sensitive hypertension may have a different mechanism in blacks than in whites.