Urinary excretion of endogenous ouabain-like substance is reduced in NaCl supplemented premature infants.

Ouabain or an isomer has been identified as endogenous ouabain-like substance (EOLS). The role of EOLS in the adaptation of premature infants to alterations of sodium balance was investigated by measuring urinary ouabain excretion serially in 9 low birth weight premature infants with (group S, mean birth weight 1,578 g, mean gestational age 30.4 weeks) and without (group NS, mean birth weight 1,537 g, mean gestational age 30.8 weeks) NaCl supplementation. The study was performed on the 7th day and weekly thereafter until the 5th week of life. NaCl supplementation was given in a dose of 3-5 and 1.5-2.5 mmol/kg/day at the postnatal ages of 8-21 and 22-35 days, respectively. Prior to NaCl supplementation, urinary ouabain excretion was similar in the two groups (146.2 +/- 16.8 pg/kg/h in group S versus 180.0 +/- 9.6 pg/kg/h in group NS) and remained at about the same level throughout the study when supplemental NaCl was provided. In infants of group NS, urinary ouabain excretion increased significantly by the 3rd week (p < 0.01) and no consistent change occurred later on. As a result, the differences in urinary ouabain excretion between the two groups proved to be significant during weeks 2-5 (p < 0.001). Essentially the same pattern of ouabain excretion was seen when it was expressed in terms of pg/mg creatinine. In infants receiving high sodium diet there was a significant positive correlation between urinary sodium and ouabain excretion. It is concluded that premature infants receiving low sodium intake have elevated EOLS excretion by the 3rd week of life. Although the relationship between ouabain and sodium excretion in supplemented premature infants suggests some physiological significance for sodium excretion, ouabain does not appear to be regulated by extracellular volume.