Is the decrease of central venous compliance the factor responsible for salt sensitivity?

To detect the possible mechanisms of salt sensitivity, blood pressure (BP), renal biological and red blood cell (RBC) ionic fluxes, responses were studied in 20 normotensive males (<30 years old), after acute isotonie saline infusion (1 litre in less than 30 min). Salt sensitivity was considered to be present if mean supine BP increased by more than 9 mmHg when the salt diet, which was applied each time for 1 week, increased from less than 20 mmol/l NaCI per day to more than 340 mmol/l. Three subgroups were defined: eight salt-sensitives (SS), 10 salt-resistants (SR) and two patients with only a very slight BP increase. ln each group (SS or SR), the same percentage of family history of hypertension was noted (60%). During the normal diet (± 170 mmol/l NaCl per day), vascular resistances were lower , and creatinine clearance higher in SS (26 ± 8 units, and 122 ± 6 ml/min, respectively) than in SR (48 ± 1 units and 102 ± 1 ml/min). As other markers of greater salt overloading could not be detected in SS, these differences could be due to higher cardiac volume ejection in SS than in SR (heart rate was the same in the two populations. During acute salt loading, mean BP rase in SS (tram 88 ± 6 to 93 ± 7 mmHg), while it decreased in SR (from 85 ± 10 to 81 ± 10 mmHg, P < 0.05). During the same period RBC Na+K+-ATPase activity decreased more in SS (from 3970 ± 715 to 3312 ± 452 umol/l cell per h) than in SR (from 4430 ± 550 to 4300 ± 520). Moreover, during chronic oral salt loading (> 340 mmol/l NaCl per day) , SS showed a greater BP increase during noradrenaline (NE) infusion (10 ± 6 mmHg for 60 ng/min per kg NA) than in SR (5 ± 3 mmHg, P < 0.02). In conclusion, salt sensitivity is already present in young normotensives. It could be due to a higher sympathetic venous tone , which in turn would increase cardiac output and secretio n of a plasma Na+K+-pump inhibitor.