DIFFERENTIAL REGULATION OF NA+ H+ EXCHANGE AND H+-ATPASE BY PH AND HCO3- IN KIDNEY PROXIMAL TUBULES/

This study examines the effects of acute in vitro acid-base disorders on Na+/H+ and H+-ATPase transporters in rabbit kidney proximal tubules (PT). PT suspensions were incubated in solutions with varying acid ba se conditions for 45 min and utilized for brush border membrane (BBM) vesicles preparation. BBM vesicles were studied for Na+/H+ exchange ac tivity (assayed by Na-22(+) influx) or abundance (using NHE-3 specific antibody) and H+-ATPase transporter abundance (using antibody against the 31 kDa subunit). The Na+/H+ exchanger activity increased by 55% i n metabolic acidosis (pH 6.5, HCO3- 3 mM) and decreased by 41% in meta bolic alkalosis (pH 8.0, HCO3- 90 mM). The abundance of NHE-3 remained constant in acidic, control, and alkalotic groups. H+-ATPase abundanc e, however, decreased in metabolic acidosis and increased in metabolic alkalosis by 57% and 42%, respectively. In PT suspensions incubated i n isohydric conditions (pH 7.4), Na+/H+ exchanger activity increased b y 29% in high HCO3- group (HCO3- 96 mM) and decreased by 16% in the lo w HCO3- groups (HCO3- 7 mM. The NHE-3 abundance remained constant in h igh, normal, and low [HCO3-] tubules, The abundance of H+-ATPase, howe ver, increased by 82% in high [HCO3-] and decreased by 77% in the low [HCO3-] tubules. In PT suspensions incubated in varying pCO(2) and con stant [HCO3-], Na+/H+ exchanger activity increased by 35% in high pCO( 2) (20% pCO(2), respiratory acidosis) and decreased by 32% in low pCO( 2) (1.5% pCO(2), respiratory alkalosis) tubules. The NHE-3 abundance r emained unchanged in high, normal, and low pCO(2) tubules. However, th e H+-ATPase abundance increased by 74% in high pCO(2) and decreased by 69% in low pCO(2) tubules. The results of these studies suggest that the luminal Na+/H+ exchanger is predominantly regulated by pH whereas H+-ATPase is mainly regulated by [HCO3-] and/ or pCO(2). They further suggest that the adaptive changes in H+-ATPase transporter are likely mediated via endocytic/exocytic pathway whereas the adaptive changes i n Na+/H+ exchanger are via the nonendocytic/exocytic pathway.