Ontogeny of Na/H Antiporter Activity in Rabbit Renal Brush Border Membrane Vesicles

The development of the Na/H antiporter was studied in renal brush border membrane vesicles (BBMV) from fetal and adult rabbits using isotopic and fluorescent techniques. The kinetics ofthe antiporter studied by 'Na ' uptake revealed that the V.,. was only 25 % of that in the adult; however, the K.'s for Na' were not significantly different. These data were confirmed by a fluorescent assay using the pH-sensitive probe, acridine or-ange: the V.. was significantly lower in the fetal BBMV. Con-ductive Na ' movement was estimated from amiloride-insensi-tive 22Na ' uptake and the rate of alkalinization induced by K+, an ion whose relative conductance was found to be similar to that of Na'. Although relative Na ' conductance was signifi-cantly greater in fetal BBMV, the lower V.,,, in fetal vesicles could not be ascribed to this factor. Maternal administration of betamethasone (50,ug/kg intramuscularly) for 2 d before deliv-ery significantly increased the V.., of the antiporter to levels observed in the adult; K. was unaffected. Na/K ATPase activ-ity increased fourfold after betamethasone, but the specific ac-tivities offour brush border marker enzymes and the kinetics of Na'-glucose cotransport were unchanged. These data indicate that there is a developmental increase in brush border Na/H exchange which is the result of an increase in the number and/ or the turnover number of the carriers. Further, these data sug-gest that the postnatal increase in antiporter activity may be related to the surge in glucocorticoid concentration that occurs perinatally. (J. Clin. Invest. 1991. 87:2067-2076.) Key words: acridine orange * amiloride * betamethasone * development. ionic conductanc