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 su bstance (EOLS). The role of EOLS in the adaptation of premature infant s to alterations of sodium balance was investigated by measuring urina ry ouabain excretion serially in 9 low birth weight premature infants with (group S, mean birth weight 1,578 g, mean gestational age 30.4 we eks) and without (group NS, mean birth weight 1,537 g, mean gestationa l age 30.8 weeks) NaCl supplementation. The study was performed on the 7th day and weekly thereafter until the 5th week of life. NaCl supple mentation was given in a dose of 3-5 and 1.5-2.5 mmol/kg/day at the po stnatal ages of 8-21 and 22-35 days, respectively. Prior to NaCl suppl ementation, urinary ouabain excretion was similar in the two groups (1 46.2 +/- 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 su pplemental NaCl was provided. In infants of group NS, urinary ouabain excretion increased significantly by the 3rd week (p < 0.01) and no co nsistent change occurred later on. As a result, the differences in uri nary 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 positi ve correlation between urinary sodium and ouabain excretion. It is con cluded that premature infants receiving low sodium intake have elevate d EOLS excretion by the 3rd week of life. Although the relationship be tween ouabain and sodium excretion in supplemented premature infants s uggests some physiological significance for sodium excretion, ouabain does not appear to be regulated by extracellular volume.