The aim of our study was to describe the possible relationship between the maturity of preterm infants and the enzyme Na+/K+-ATPase, central regulator of Na+, K+ homeostasis of the body.

During the first postnatal week, the enzyme activity, intracellular Na+, K+ and ATP content ([Na+]ic,[K+]ic, [ATP]ic) were measured in red cells of 46 preterm neonates (28-36 gestational weeks). In the case of twelwe infants prenatal steroid prophylaxis was present. Fifteen healthy neonates (gest. age: 39-40. week) served as controls.

The Na+/K+-ATPase activity of infants without steroid prophylaxis decreased by the gestational age (28-40. week) (preterm infant: 473±60 vs healthy neonates: 403±77 nmolATP/mg prot*h-1, p<0.01) and this correlation is significant (gest.week=790-10.8 Na+/K+-ATPase, r=0.42, p<0.01). [Na+]ic was lower in younger neonates (6.4±1.4 vs 8.3±0.6 mmol/l, p<0.05,[Na+]ic=3.49±0.11 gest.week, p<0.05).[K+]ic and [ATP]ic remained unaltered.

The Na+/K+-ATPase activity of preterm infants with steroid prophylaxis was higher, compared to age-matched preterm neonates(625±127 vs 473±60, p<0.01), while the [ATP]ic was lower (1537+318 vs 2078+327 nmol ATP/100 g Hb, p<0.05) and the[Na-]ic was higher (7.4±0.6 vs 6.5±1.0, p<0.05).

These results provide new details for the better understanding of perinatal regulation of Na+ and K+ homeostasis.