HUMORAL DETERMINANTS OF NA+ EXCRETION AFTER INTRAVENOUS NACL LOADING IN NORMAL VOLUNTEERS

1. Twelve healthy volunteers maintained on a 100 mmol/day Na+ diet, we re given an intravenous infusion of 2L saline (0.9%) between 10.00 and 13.00 h on 2 study days at least 1 week apart. Urine collections (90 min) were made from 08.30 to 16.00 h. Either carbidopa 100 mg or indom ethacin 50 mg was given orally at 07.45 h on one study day and placebo was given on the other (in random order). 2. On the placebo day, sali ne infusion caused significant decreases in plasma albumin concentrati on, plasma renin activity (PRA), plasma aldosterone concentration and urinary aldosterone excretion, with 2 to 3-fold increases in plasma at rial natriuretic peptide (ANP) concentration and urinary dopamine: nor adrenaline ratio (DA:NA), whereas mean urinary kallikrein and prostagl andin E2 (PGE2) excretion rates were unchanged. Carbidopa decreased ur inary DA:NA and indomethacin decreased urinary PGE2 excretion, compare d with the placebo day. Excretion of sodium (Na+) decreased below base line in two out of six carbidopa-treated subjects and in three out of six indomethacin-treated subjects, but showed little or no change in t he remainder. 3. These preliminary observations suggest that some subj ects in the early phase of natriuresis after an intravenous Na+ load c an be identified as having prostaglandin-dependent or dopamine-depende nt mechanisms for Na+ excretion.