Na(+)-H+ exchange in basolateral plasma membrane vesicles from neonatal rat liver

Basolateral plasma membrane vesicles (BLMV) were used to determine the activity of the Na(+)-H+ exchanger in neonatal compared with adult rat liver. Imposition of an outwardly directed H+ gradient (pH 5.5in/7.5out) markedly stimulated the uptake of 1 mM 22Na+ by adult and neonatal BLMV above rates observed under pH-equilibrated conditions (pH 7.5in/7.5out) and resulted in a transient uphill accumulation ("overshoot") of Na+ to levels 1.5- to 2-fold higher than at equilibrium. 22Na+ uptake via Na(+)-H+ exchange (identified as the amiloride-sensitive component of total Na+ flux) was greater in BLMV from neonatal compared with adult liver under both pH-gradient and -equilibrated conditions. These age-related differences in Na+ uptake could not be explained by membrane potential effects or by differences in membrane permeability to H+. Kinetic analysis revealed a saturable process with a similar Km for Na+ (8.6 +/- 1.6 vs. 8.1 +/- 1.5 mM) but with a 2.5-fold higher Vmax (8.06 +/- 0.67 vs. 3.20 +/- 0.40 nmol.mg protein-1.min-1; P less than 0.01) in neonatal vs. adult vesicles. We conclude from these studies that basolateral membrane vesicles from neonatal rat liver exhibit enhanced Na(+)-H+ exchange as a result of an increased number or translocation rate of carriers in the plasma membrane.