SODIUM SPACE AND INTRAVASCULAR VOLUME - DIETARY-SODIUM EFFECTS IN CYSTIC-FIBROSIS AND HEALTHY ADOLESCENT SUBJECTS

Objective. To assess the physiologic response to salt depletion in sub jects with cystic fibrosis (CF) and control male adolescents for sodiu m balance, sodium space, and stimulation of the renin-angiotensin-aldo sterone axis. Design. Seven subjects with CF and six controls received a salt-replete (150 or 290 mmol NaCl per day) diet and then a salt-de plete (10 mmol NaCl per day) diet while in a clinical research center, Results. Space maintenance: CF subjects responded to salt depletion w ith a greater weight loss than did controls (1.9 vs 0.8 kg) and a decr ease in Na-24(+) space, whereas controls maintained Na-24(+) space. Pa ired (Na-deplete/Na-replete) blood volumes decreased in subjects with. CF, but not in controls. Renin-angiotensin-aldosterone axis stimulati on: During salt repletion, subjects with CF had significantly higher a ldosterone values than did controls in the afternoon, but not at 7:00 AM, Dining salt depletion, plasma renin activity and aldosterone incre ased significantly more in subjects with CF than in controls (renin, 3 5 vs 13 ng/mL/hour [9.7 vs 3.6 ng.L-1 s(-1)]; aldosterone: 181 vs 101 ng/dL [5021 vs 2802 pmol/L]). Furthermore, the angiotensin antagonist saralasin increased renin much more in subjects with CF (154 vs 36 ng/ mL per hour [43 vs 10 ng.L-1 s(-1)]), Vasomotor functions: Mean arteri al pressure was decreased in subjects with CF on both diets and decrea sed significantly more with low salt only in subjects with CF. During salt depletion, subjects with CF showed enhanced orthostatic tolerance (less heart rate increase with standing) compared with controls, thus obscuring their Volume loss. The blood pressure response to an acute infusion of saralasin suggested that in salt-replete subjects with CF, but not in controls, angiotensin receptors were functional in maintai ning vascular tone. During salt depletion, angiotensin was more import ant for maintenance of blood pressure in subjects with CF than in cont rols, because the saralasin-induced drop in blood pressure was 20%, ie , close to shock levels, in subjects with CF, and only 6% in controls. Conclusion. The data suggest that patients with CP are so successful in compensating for volume depletion by vigorous activation of the ren in-angiotensin system that salt depletion/dehydration cannot be recogn ized easily by routine clinical measurements, eg, capillary refill, se rum sodium levels, or tachycardia.