Further characterization of the endogenous natriuretic and digoxin-like immunoreacting activities in human urine: effects of changes in sodium intake.

In the present study natriuretic activity and digoxin-like immunoreacting activity (DLIA) were determined in small molecular weight (MW) fractions of urine from healthy subjects during low (35 mmol/day) and high (greater than 400 mmol/day) sodium intake by bioassay and by a radioimmunoassay for digoxin, respectively. After gel filtration of urine on a Sephadex G-25 column the natriuretic activity appeared in the post-salt fraction SIV, whereas DLIA was present in small amounts in the salt fraction SIII and, with consistently higher activity, in the post-salt fraction SIV. Natriuretic activity significantly increased and DLIA decreased in fraction SIV with high sodium intake, but total urinary excretion of DLIA remained unaltered during changes in sodium intake. In addition, anion-exchange and reverse-phase chromatography revealed that DLIA is not specifically related to the natriuretic activity but also reflects unspecific binding of various urine constituents to this digoxin antibody. Although the antibody binds a natriuretic material, this radioimmunoassay is thus unsuitable to determine the endogenous natriuretic activity in urine fractions. Whereas they elute differently on reverse-phase chromatography, amino acid analyses revealed that both the natriuretic factor directly purified from the post-salt fraction SIV and the natriuretic material bound to the digoxin antibody have in common four amino acids at similar molar ratios. The physicochemical properties as evidenced by chromatographic and electrophoretic studies as well as enzymatic inactivation suggest that the low MW natriuretic factor(s) in human urine may be associated with a small peptide(s) of weak acidic nature.

Relation of endogenous digoxin-like immunoreacting activities to salt balance and renal function in man.

We have previously shown that a natriuretic factor which is present in a small molecular weight fraction (IV) of serum and urine from salt loaded animals and healthy subjects, respectively, inhibits the Na-K-ATPase enzyme in vitro and also binds to a specific digoxin antibody. In the present study digoxin-like immunoreacting activity (DLIA) was therefore determined in the serum of healthy volunteers during low (35 nmol/day) and high (greater than 400 mmol/day) sodium intake and of patients with chronic renal failure and serum creatinine concentrations ranging from 127 to 757 mumol/l. DLIA was determined with a radioimmunoassay for digoxin in native serum and in the salt (III) and post-salt (IV) serum fractions eluted from a Sephadex G-25 column. DLIA in native serum of healthy subjects was less than 0.125 ng/ml. After gel filtration DLIA eluted exclusively in the small molecular weight salt (F III) and post-salt (F IV) fractions. Whereas DLIA increased in F III and decreased in F IV, total DLIA in F III + IV slightly increased from 0.37 +/- 0.03 to 0.49 +/- 0.05 ng/ml (p less than 0.01) with the change from low to high sodium intake. DLIA in native serum of uremic patients ranged from 0 to 1.70 ng/ml and was detectable consistently only in patients with serum creatinine concentrations above 250 mumol/l. DLIA in F III which averaged 0.22 +/- 0.04 ng/ml and total activity which ranged from 0.11 to 0.88 ng/ml closely correlated with the degree of renal impairment (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)