Co-expression of an anion conductance pathway with Na(+)-glucose cotransport in rat renal brush-border membrane vesicles

Cl- and membrane potential dependence of amino acid transport across the rat renal brush border membrane

The relative roles of the anion present and the membrane potential in the operation of each of the seven amino acid transport systems in the renal tubular brush border membrane were explored by manipulating transmembrane potential and chemical gradients across the membrane. The effect of various external anions with different permeabilities of the membrane and of valinomycin-generated K+ diffusion potential on Na+-coupled amino acid accumulation by rat renal brush border membrane vesicles was examined. Accumulation of all amino acids examined, except for cystine, was membrane potential dependent. The highest voltage dependence was observed for taurine (equivalent to glucose) and l-methionine. Addition of taurine uptake values obtained under each electrical gradient (inside negative) and a chemical gradient (100 mM NaCl out) condition yielded markedly lower values than under conditions where there was a combined electrochemical gradient. Cl- gradient rather than merely imposing a voltage gradient was a specific mediator of Na+-coupled transport of l-proline, taurine, l-glutamic acid, and glycine across the brush border membrane. Cl- gradient alone under Na+-equilibrated conditions could energize an overshoot of taurine accumulation by vesicles providing evidence that taurine is energetically activated by and coupled to Cl- transport. These data suggest that Na+-linked transport of most amino acids across the tubular luminal membrane is an electrogenic positive process and for proline, taurine, glutamic acid, and glycine, a Cl--requiring process. A negative intracellular potential combined with luminal chloride is required for optimal Na+-coupled transport of these amino acids across the luminal membrane of the proximal tubule. The coupling of Cl- to the transport of these osmoprotective amino acids may enhance their volume regulatory effect in kidney cells and other mammalian cells.

Properties of chloride-conductive pathways in rat kidney cortical and outer-medulla brush-border membranes--inhibition by anti-(cystic fibrosis transmembrane regulator) mAbs

The activity of the Cl(-)-conductive pathways, their regulation by protein kinase A (PKA) and their relationship to the cystic fibrosis transmembrane regulator (CFTR) protein were assessed in rat kidney cortical brush-border-membrane vesicles (cBBMV) and outer medullary vesicles (OMV) by measuring the rate of valinomycin-induced microsomal swelling by light scattering in the presence of an inward Cl- gradient. Valinomycin increased the rate of swelling of cBBMV and OMV, which is consistent with the presence of a Cl(-)-conductive pathway. PKA further increased these rates. This effect was blocked by the inhibitor of protein kinase A, suggesting that phosphorylation by PKA activates these pathways. Four anion-transport inhibitors were tested ¿N-phenylanthranilic acid (PhNHPhCOOH), 5-nitro-2-(3-phenylpropylamino)benzoic acid [N(PhPrNH2)BzOH], glybenclamide and 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid¿. Ph2COOH and 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid inhibited the basal Cl(-)-conductive pathways, while PKA-treated microsomes were sensitive also to N(PhPrNH2)BzOH and glybenclamide, suggesting that additional Cl- pathways were activated by phosphorylation. The pharmacological properties of these pathways were similar to those of the CFTR Cl- channel. Two anti-CFTR mAbs inhibited PKA-activated valinomycin-induced swelling in cBBMV and OMV, while immunoblot analysis of the corresponding proteins with the same antibodies indicated the presence of a 170-kDa protein. The results thus indicate the presence of a PKA-activated Cl(-)-conductive pathway in cBBMV and OMV, and suggest that CFTR protein is involved in PKA-activated Cl- fluxes in these vesicles.

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

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.