Modifier role of internal H+ in activating the Na+–H+ exchanger in renal microvillus membrane vesicles

Abstract

The intracellular pH in animal cells in generally maintained at a higher level than would be expected if H+ were passively distributed across the plasma membrane. In a wide variety of cells including sea urchin eggs, skeletal muscle, renal and intestinal epithelial cells, and neuroblastoma cells, plasma membrane Na+-H+ exchangers mediate the uphill extrusion of H+ coupled to, and thus energized by, the downhill entry of Na+. Plasma membrane vesicles isolated from the luminal (microvillus, brush border) surface of renal proximal tubular cells possess a Na+-H+ exchanger that seems to be representative of the Na+-H+ exchangers found in other tissues. For example, the renal microvillus membrane Na+-H+ exchanger, like other Na+-H+ exchangers, mediates electroneutral cation exchange, is sensitive to inhibition by the diuretic drug amiloride, and has affinity for Li+ in addition to Na+ and H+ (refs 5, 9). Here we have examined the effect of internal H+ on the activity of the Na+-H+ exchanger in renal microvillus membrane vesicles. Our results suggest that internal H+, independent of its role as a substrate for exchange with external independent of its role as a substrate for exchange with external independent of its role as a substrate for exchange with external Na+, has an important modifier role as an allosteric activator of the Na+-H+ exchanger. Allosteric behaviour with respect to internal H+ is a property that would enhance the ability of plasma membrane Na+-H+ exchangers to extrude intracellular acid loads and thereby contribute to the regulation of intracellular pH.

Regulation of intracellular pH in eukaryotic cells

“…Aronson and coworkers proposed that cytoplasmic H+ acts as an allosteric activator of the Na+/H+ exchanger (Aronson et al, 1982). It has been suggested that the antiport molecule has on its cytoplasmic face two proton-binding sites that are separate and functionally independent.…”

“…When this site is occupied, a conformational change is triggered, activating the exchanger. A distinct H+-transport site mediates the net extrusion of H+ once the exchanger is activated (Aronson et al, 1982;Aronson, 1985).…”

Na+/H+ Exchangers of Mammalian Cells

“…The exchangers are allosterically activated by cytosolic H ϩ , promoting the rapid extrusion of acid once intracellular pH drops below a threshold level (33). This feature is conserved in the NHE isoforms examined to date (NHE1-NHE3), although the apparent H ϩ sensitivity, which determines the "set point" for activation, varies between isoforms (30, 31).…”

The chronic administration of drugs that inhibit the regulation of intracellular pH: in vitro and anti-tumour effects

“…The viability of cells in an acidic microenvironment also depends on the activity of membrane-based exchangers that regulate pHi . Two major exchangers known to be involved in the regulation of pH, under acidic conditions are the Na+/H+ antiport (Johnson and Epel, 1976;Aronson et al, 1982;Moolenaar et al, 1984) and the Na+-dependent HCO3 /Cl1 exchanger (Thomas, 1977;L'Allemain et al, 1985;Cassel et al, 1988). The former is inhibited by amiloride and its substituted analogues, and the latter is inhibited by stilbene derivatives such as DIDS (4,4-diisothiocyanstilbene 2,2-disulphonic acid).…”