AIM To study the relation between erythrocyte Na⁺,K⁺-ATPase subunit isoform composition, Na⁺,K⁺-ATPase activity, and cation pump function in preterm and term neonates.

DESIGN Erythrocyte Na⁺,K⁺-ATPase subunit isoform abundance, Na⁺,K⁺-ATPase activity, and cation pump function were studied in blood samples obtained from 56 preterm neonates of 28–32 weeks gestation (group 1), 58 preterm neonates of 33–36 weeks gestation (group 2), and 122 term neonates (group 3) during the first two postnatal days.

RESULTS α₁isoform abundance was higher and β₂ isoform abundance was lower in group 1 than in group 3 (p = 0.0002). α₂ and β₁ isoform abundance did not change with maturation and there was no evidence for the presence of the α₃ isoform. Gestational age was inversely related to Na⁺,K⁺-ATPase activity (p = 0.0001) and directly related to intracellular Na⁺ concentration (p = 0.0025).

CONCLUSIONS Expression of the α₁ and β₂Na⁺,K⁺-ATPase subunit isoforms is developmentally regulated. The increased abundance of α₁isoforms of immature neonates translates to increased ATPase activity. The lower intracellular Na⁺ concentration of immature neonates suggests that their erythrocyte Na⁺,K⁺-ATPase cation pump function may also be increased.