CONTRASTING ENDOCRINE RESPONSES TO ACUTE ORAL COMPARED WITH INTRAVENOUS-SODIUM LOADING IN NORMAL HUMANS

There is evidence in animals and in humans for accelerated natriuresis after oral compared with intravenous sodium loading. To assess the ro le of atrial natriuretic peptide (ANP) as a contributory mechanism, we compared the hormonal responses to an intravenous sodium load and to the same sodium load taken orally in three separate groups of healthy subjects in balance on low, normal, or high sodium intake. On each die t, there was a trend for an early delay in sodium excretion, followed by increased natriuresis after the oral compared with intravenous sodi um load. On all levels of dietary sodium intake, there was a significa nt (similar to 2-fold) increase in plasma ANP levels after intravenous saline infusion. There was a significant suppression of the renin sys tem both after oral and intravenous sodium loading. However, there was no acute increase in plasma ANP levels after the oral sodium load, ex cept on the very low sodium intake. This striking and unexpected obser vation suggests that changes in plasma ANP levels appear to play littl e role in the early response to an acute oral sodium load in subjects with sodium intake in the range of 150-350 mmol/day. Endocrine mechani sms for the accelerated increase in sodium excretion after oral compar ed with intravenous sodium loading remain to be elucidated.