Proton/solute cotransport in rat kidney brush-border membrane vesicles: relative importance to both d-glucose and peptide transport

Abstract

We have determined the relative importance of the transmembrane proton electrochemical gradient to the transport of d-[ 14C]glucose and [ 14C]glycylsarcosine (gly-sar) in rat kidney brush-border membrane vesicles (BBMV) from superficial renal cortex. Electrogenic [ 14C]gly-sar transport was first optimised by imposing a pH gradient (pH o = 5.7, pH i = 8.4) and an interior negative p.d. (using outwardly directed K + gradient plus valinomycin). Under identical conditions (pH o = 5.7, pH i = 8.4), an acceleration f of initial d-[ 14C]glucose (at 100 μM) transport by 2.0 ± 0.7-fold was observed compared to no proton gradient (pH o = 8.4, pH i = 8.4). This increase was due primarily to an effect of external protons, since acidic conditions (pH o = pH i = 5.7) also resulted in ann acceleration of d-glucose influx (2-fold). The increase in d-glucose transport in the presence of external acidity was reduced by the uncoupler FCCP, even in the absence of a proton gradient. Furthermore, the increased d-glucose transport with external acidity in the presence of a proton gradient was insensitive to a K + gradient-driven diffusion potential in the presence of valinomycin. In no instance was an overshoot accumulation of d-[ 14C]glucose observed in H + gradient conditions. H +-stimulated d-[ 14C]glucose transport showed a linear dependence on d-glucose concentration up to 20 mM d-glucose, unlike electrogenic Na +-dependent d-glucose transport, whose K m was 1.77 ± 0.35 mM. In contrast, the initial rate of [ 14C]gly-sar (100 μM) transport by the renal H +/di-tripeptide transporter was accelerated 15.7 ± 3.3-fold and stimulated a marked overshoot of 5.1 ± 0.4-fold over equilibrium values. Conversely, the electrogenic, Na +/glucose transporter could be readily demonstrated, whilst [ 14C]gly-sar transport could not be energised by an inward Na + gradient. The absence of electrogenic d-glucose transport in H + gradient conditions is clear evidence against H +/glucose cotransport in Na +-free conditions mediated by SGLT2 (sodium-glucose transporter, renal cortex). Furthermore, since a proton gradient does not increase brush-border membrane d-glucose uptake in Na +-rich media, it is unlikely that in vivo renal d-glucose transport mediated via SGLT2 may be energised by the transmembrane proton gradient.