Salt-regulating hormones in young normotensive obese subjects. Effects of saline load

To investigate whether the response of salt-regulating hormones to volume expansion is impaired in obese subjects, we assessed the effects of saline load (0.25 mL/kg.min.120 min) in 9 young, healthy, normotensive obese subjects (body mass index, > 30 kg/m2) and in 10 lean control subjects (body mass index, < 25 kg/m2) matched for age, gender, height, and mean blood pressure. Hematocrit, plasma renin activity (PRA), plasma aldosterone (PA), atrial natriuretic factor (ANF), and urinary sodium excretion (UNaV) were evaluated. Saline load increased ANF levels significantly (P < .001) in lean subjects at both 60 and 120 minutes, whereas they decreased in obese subjects. Such decreases became significant (P < .01) at 120 minutes. Suppression of PRA and PA by saline load were more marked in lean than obese subjects. Hematocrit decreased in both groups, and UNaV increased more in lean than obese subjects during saline load. Comparisons of percent changes in ANF, PRA, and PA after saline load showed that the responses of lean and obese subjects were significantly different (P < .001 for ANF at both 60 and 120 minutes; P < .05 for PRA and PA at both 60 and 120 minutes). In conclusion, the lack of ANF response and the reduced suppression of PRA and PA to saline load indicate a dysfunction of these systems in obese subjects. This alteration may be involved in the higher susceptibility of obese subjects to developing hypertension.