Salt sensitivity of blood pressure in chronic renal failure. Evidence for renal control of body fluid distribution in man.

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Abstract
Patients (23) with different degrees of renal insufficiency were studied after equilibration on 2 levels of salt intake. Blood pressure increased after increased salt intake. This blood pressure increase, when related to Na excretion increase (salt sensitivity index), tended to be larger in patients with a greater loss of kidney function. In fact, the salt sensitivity of blood pressure rose exponentially with the decline of the kidney function, which resulted in a linear negative relationship between the log of salt sensitivity index and creatinine clearance (r = 0.89, P < 0.0001). After increased salt intake, plasma renin activity (PRA) decreased, whereas body-fluid volumes increased. Concomitantly, the products of log PRA and extracellular-fluid volume or blood volume decreased (P < 0.005). Weak interrelations were found between increases of body fluid volumes and blood pressure (r = 0.49, P < 0.05). When the patients were divided into 2 groups according to creatinine clearance, Group 2 (creatinine clearance < 22 ml/min, n = 13) showed a significantly greater increase of blood pressure for any given expansion of extracellular fluid volume than Group 1 (creatinine clearance > 32 ml/min, n = 9). For any given increase in Na excretion blood volume increased more in Group 2 than in Group 1 (P < 0.03), whereas the increase of extracellular fluid volume was similar in the 2 groups. Consequently, after increase of salt intake the plasma volume/interstitial fluid volume ratio (PV/IF) tended to decrease in Group 1 and to increase in Group 2. This difference in PV/IF ratio behavior was significant (P < 0.01). A significant interrelation was also found between the salt sensitivity index and change of PV/IF ratio (r = 0.60, P < 0.01). With a decrease in functioning renal mass, the salt sensitivity of the blood pressure increases. This increase is accompanied by an increased intra/extravascular-fluid volume ratio after salt loading, which suggests a change of tissue-capillary filtration forces in patients with renal insufficiency.