ERYTHROCYTE LI(I)-NA(0)+ COUNTERTRANSPORT IN THE NEWBORN()

The present study investigates the erythrocyte Li(i)+-Na(o)+ countertr ansport (CT) in the newborn. Erythrocytes of term infants exhibit no o r 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 pre term 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 p remature infants subjected to prenatal stress are significantly higher (0.082 +/- 0.017 mmol Li/l RBC.h). It is concluded that erythrocyte C T activity in the newborn reaches adult values within the first week o f life, apparently reflecting the postnatal maturation of the CT syste m in the cell membrane. We suggest that prenatal stress induces the sy nthesis of the CT protein in utero.