Characteristics of CO₂-independent pH equilibration in human red blood cells

Summary

The rate of dissipation of a pH gradient across the red cell membrane has been measured at pH 5.7–7.5 in a medium free of CO₂ and other penetrating acids or bases. The measured rates and extents of pH movements are influenced only slightly by valinomycin-induced changes in the membrane potential. This indicates that the primary process involved is electrically silent OH⁻/Cl⁻ exchange or H⁺/Cl⁻ cotransport. This electrically silent pH equilibration has several characteristics which suggest the involvement of the red cell anion exchange protein.

• 1. It is strongly inhibited by phloretin and DIDS (4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid).

• 2. The rates of pH equilibration depend on the halide present in the medium, the relative rates being 100, 18, and 2 in NaCl, NaBr, and NaI media, respectively.

• 3. The pH equilibration has apparent activation energies of 27 kcal/mol atT<13°C and 16 kcal/mol atT>13°C.

The pH dependence of the equilibration rate, however, is much more consistent with H⁺/Cl⁻ cotransport than with OH⁻/Cl⁻ exchange; the rate increases steeply with the H⁺, rather than the OH⁻ concentration. It is suggested therefore that the transport event is H⁺/Cl⁻ cotransport, but that this transport is mediated by the membrane protein that catalyzes anion exchange.