Erythrocyte Li+i-Na+o countertransport in the newborn

The present study investigates the erythrocyte Li+i-Na+o countertransport (CT) in the newborn. Erythrocytes of term infants exhibit no or very low CT activity at birth (0.025 +/- 0.011 mmol Li/l RBC.h). CT rates increase gradually and reach adult values within the 4th-7th day of life (0.225 +/- 0.018 mmol Li/l RBC.h). CT activity in healthy preterm infants at birth is not significantly different from that in term infants (0.014 +/- 0.007 mmol Li/l RBC.h). Postnatal maturation of CT activity in preterm infants is similar to that found in term infants and is independent of gestational age. However, CT rates at birth of premature infants subjected to prenatal stress are significantly higher (0.082 +/- 0.017 mmol Li/l RBC.h). It is concluded that erythrocyte CT activity in the newborn reaches adult values within the first week of life, apparently reflecting the postnatal maturation of the CT system in the cell membrane. We suggest that prenatal stress induces the synthesis of the CT protein in utero.