[Endogenous digoxin-like substance (nonspecific interference or a new hormone?)]

The authors assessed changes of the serum level and urinary excretion of an endogenous digoxin-like substance in healthy neonates and parturient women who did not have digitalis therapy. Higher concentrations of the endogenous digoxin-like substance (ESPD) found in the umbilical blood, as compared with parallel samples of venous blood of the mothers (n = 13; p less than 0.01), provide indirect evidence of the secretory autonomy of the neonatal organism. The highest ESPD concentrations in umbilical blood were found in neonates born before the 34th week of gestation (0.5-1.0 ng/ml), whereby with advancing gestational age the ESPD level declined significantly (n = 21; r = 0.62; p less than 0.01). This fact along with the positive correlation between the excretory fraction of sodium and renal ESPD excretion in the group of healthy 1-5-week-old premature babies (n = 10, r = 0.53; p less than 0.01) is consistent with the view on the identity of ESPD and the assumed natriuretic hormone, the circulating endogenous inhibitor of Na+, K+-ATPase.