Proton/solute cotransport in rat kidney brush-border membrane vesicles: relative importance to both D-glucose and...

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 (pHo = 5.7, pHi = 8.4) and an interior negative p.d. (using outwardly directed K+ gradient plus valinomycin). Under identical conditions (pHo = 5.7, pHi = 8.4), an acceleration of initial D-[14C]glucose (at 100 microM) transport by 2.0 +/- 0.7-fold was observed compared to no proton gradient (pHo = 8.4, pHi = 8.4). This increase was due primarily to an effect of external protons, since acidic conditions (pHo = pHi = 5.7) also resulted in 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 Km was 1.77 +/- 0.35 mM. In contrast, the initial rate of [14C]gly-sar (100 microM) 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.