Absence of increased electroneutral Na +-H + exchange in renal cortical brush-border membranes from hyperthyroid rats

Abstract

The fluorescence quenching of acridine orange was used to compare Na +-H + exchange and ion conductances in renal cortical brush-border membrane vesicles (BBMV) isolated from euthyroid and hyperthyroid rats. In BBMV from euthyroid animals, Na +-H + exchange was entirely electroneutral. In BBMV from hyperthyroid rats, the total rates of Na +-H + exchange were about 30% higher than in BBMV from euthyroid animals. However, the electroneutral exchange in these membranes was similar to that in BBMV from euthyroid rats; the observed increase in exchange was due to electrically coupled Na + and H + movements through conductive pathways in the membranes. Ion conductances in isolated BBMV were tested with outwardly directed K + gradients in the presence of carbonyl cyanide m-chlorophenylhydrazone (K + conductance) or valinomycin (H + conductance). The K + conductance was negligible and similar in BBMV from both groups of rats. A significant H + conductance was present in both kinds of membrane preparations and was by 37% higher in BBMV from hyperthyroid animals. Therefore, our experiments failed to demonstrate an increased electroneutral Na +-H + exchange in BBMV from hyperthyroid rats. Instead, a finding of a significant electrically coupled Na +-H + antiport in the presence of increased H + conductance in BBMV from hyperthyroid rats indicates that these membranes may also have increased Na + conductance.