The aim of our study was to test, whether the different digoxin sensitivity of the neonates and children can be explained by the different functional properties of the Na+/K+ -ATPase enzyme. The activity, ouabain sensitivity and isoform composition of erythrocyte Na+/K+-ATPase of 53 healthy full term neonates (1-5 postnatal days) and that of 61 healthy children (6-38 months) were compared. The enzyme activity was elevated in neonates (mean±SEM: 429.2±17.1 vs 295.5±10.2 nmol ATP/mg prot*h-1, p<0.001). 150 value for ouabain inhibition was also higher (1.5±0.1*10.6 vs 0.96±0.1*10.6 mol/1, p<0.05). In neonates the expression of enzyme alpha subunits(1.16±0.1 vs 75±0.03, p<0.001) and the alpha1/alpha2 subtypes-ratio (4.14±0.21 vs 2.02±0.16, p<0.01) was higher than in children.

These results partly explain the higher digoxin tolerance of neonates.