DEVELOPMENTAL MATURATION OF NA+-H+ EXCHANGE IN RAT RENAL TUBULAR BRUSH-BORDER MEMBRANE

The developmental maturation of the Na+-H+ exchanger present in the proximal tubular luminal membrane of the rat was investigated. An overshoot of 1 mM Na+ uptake was evident in brush-border membrane vesicles derived from the renal cortex of 7- and 21-day-old and adult rats in the presence of an outwardly directed H+ concentration ([H+]) gradient [intravesicular pH (pH(i)) = 5.5; extravesicular pH (pH(o)) = 7.5]. Na+ uptake was amiloride sensitive at all ages examined. Significantly higher initial rate (3 s) Na+ uptake and peak accumulation (60 s) in the presence of a [H+] gradient were found in vesicles from 7-day-old rats compared with adult animals. Significantly enhanced initial rate Na+ uptake by neonatal vesicles was also evident under pH-equilibrated conditions (pH(i) = pH(o) = 7.5). An age-related decrease in amiloride-sensitive Na+ accumulation by vesicles was found. Kinetic analysis of Na+-H+ exchange in voltage-clamped vesicles, in the presence of dimethylamiloride (DMA), and calculating 5-s Na+ uptake values showed a maturational decrease in capacity (decreasing V(max)) coupled with a maturational increase in affinity (decreasing K(m)) of Na+-H+ antiport. These data suggest that an enhanced amiloride-inhibitable Na+-H+ exchange activity due to increased capacity of exchange exists in the proximal tubular luminal membrane of the neonatal rat. This increased Na+-H+ exchange may potentially contribute to positive Na+ balance in the growing organism and may rapidly dissipate the electrochemical Na+ gradient across the luminal membrane necessary for Na+-solute cotransport, thereby contributing to glycosuria and aminoaciduria of early life.