Digoxin-like natriuretic activity in the urine of salt loaded healthy subjects

In previous studies we have demonstrated a natriuretic factor of small molecular weight (less than 1,000 Daltons) in the serum and urine of salt loaded subjects. This factor isolated from salt loaded animals inhibits the Na-K-ATPase enzyme system. In addition, the natriuretic material isolated from plasma of salt-loaded dogs was shown to bind to specific digoxin antibodies. It was therefore suggested that a digitalis-like endogenous natriuretic factor (endoxin) is released in response to saline loading. In the present study we therefore investigated the presence of such an endogenous natriuretic digitalis-like activity in the urine of healthy volunteers during high salt intake. Using Sephadex G-25 for chromatographic separation of urine a material elutes as a single peak in the natriuretic post-salt fraction IV which is specifically bound to digoxin antiserum complex. Mean peak activity amounted to 1.55 +/- 0.48 ng/ml digoxin equivalents. We further purified the natriuretic material by immunoprecipitation with the digoxin antiserum complex. This purification procedure resulted in a more than 10-fold increase in specific natriuretic activity from 2.7 +/- 0.4 to 30.4 +/- 5.8 muEq Na+ x min-1 x mg-1. Thus the digitalis-like natriuretic activity previously observed in the plasma of saline loaded dogs is also present in the urine of healthy subjects during high dietary salt intake. Immunoprecipitation may offer a meaningful tool for further isolation and identification of the natriuretic hormone(s).