Microvillus Membrane Vesicles Research Articles

Chloride/formate exchange with formic acid recycling: a mechanism of active chloride transport across epithelial membranes.

The pathways for transport of Cl- and formate in microvillus membrane vesicles isolated from rabbit renal cortex were evaluated. An outward formate gradient stimulated the uptake of Cl-, and an outward Cl- gradient stimulated the uptake of formate, indicating Cl-/formate exchange. In addition, an inside alkaline pH gradient induced the accumulation of formate, consistent with nonionic diffusion of formic acid. Although an inward Na+ gradient also stimulated uphill formate accumulation, suggesting Na+/formate cotransport, this effect was abolished when ionophores were used to prevent the generation of a transmembrane pH gradient, indicating an indirect coupling of formic acid transport to Na+/H+ exchange. An inside alkaline pH gradient only minimally stimulated the uptake of 82Br-, used as tracer for Cl-, confirming the absence of appreciable Cl-/OH- exchange. However, the same pH gradient in the presence of a physiologic formate concentration (0.2 mM) markedly stimulated 82Br- influx. These data suggest that Cl-/formate exchange with recycling of formic acid by nonionic diffusion is a potential mechanism for active Cl- absorption across the luminal membrane in the proximal tubule and perhaps in other epithelia.

Paradoxical effects of pyrazinoate and nicotinate on urate transport in dog renal microvillus membranes.

The effects of pyrazinoate and nicotinate on urate transport in microvillus membrane vesicles isolated from canine renal cortex were evaluated. An outwardly directed gradient of pyrazinoate stimulated uphill urate accumulation, suggesting urate-pyrazinoate exchange. An inside-alkaline pH gradient stimulated uphill pyrazinoate accumulation, which suggested pyrazinoate-OH- exchange. Pyrazinoate-OH- exchange and urate-OH- exchange were similarly sensitive to inhibitors, implying that both processes occur via the same transport system. In addition, an inward Na+ gradient stimulated uphill pyrazinoate accumulation, suggesting Na+-pyrazinoate cotransport. Inhibitor studies demonstrated that Na+-pyrazinoate cotransport takes place via the same pathway that mediates Na+-lactate cotransport in these membrane vesicles. Previously we found that urate does not share this Na+-dependent cotransport pathway. Nicotinate inhibited transport of pyrazinoate by the anion exchange pathway and the Na+ cotransport pathway, suggesting that it is a substrate for both transport systems. Finally, in the presence of an inward Na+ gradient, low doses of pyrazinoate or nicotinate stimulated urate uptake, and higher doses of pyrazinoate or nicotinate inhibited urate accumulation, thereby mimicking in vitro the paradoxical effects of drugs on renal urate excretion that have been observed in vivo. These findings indicate that the paradoxical effect of uricosuric drugs at low doses to cause urate retention may result at least in part from stimulation of urate reabsorption across the luminal membrane of the proximal tubular cell.

Vitamin B12 absorption in the neonatal piglet. 3. Influence of vitamin B12-binding protein from sows' milk on uptake of vitamin B12 by microvillus membrane vesicles prepared from small intestine of the piglet.

The influence of the vitamin B12-binding protein isolated from sows' milk on the uptake of vitamin B12 was investigated using microvillus membrane vesicles prepared from the small intestine of 7- and 28-d-old piglets. Uptake of radioiodinated purified binder was also examined. The binder strongly promoted uptake of vitamin B12 at both ages; in the absence of the binder there was little uptake. The uptake mechanism was specific and operative in vesicles prepared from all regions of the small intestine. Uptake was a rapid process, exhibiting saturation kinetics, with a pH optimum at about 7.0, and dependent on the presence of magnesium or calcium ions for maximum activity. Affinity constants of the binding sites for the milk binder were determined. Competition experiments using complexes of the binder with vitamin B12 and with non-cobalamin analogues (cobinamide and Co-alpha-[2-methyladenyl]cobamide) showed that the bound analogues competed with bound vitamin B12 for uptake but with lower efficiency. Intrinsic factor also promoted vitamin B12 uptake by the vesicles but it did not compete with the milk binder for the same binding sites. It promoted uptake only in microvilli isolated from the lower third of the small intestine, and was more effective with preparations from 28-d-old piglets, whereas the milk binder was more effective with the 7-d-old piglets. Porcine gastric cobalophilin competed with the milk cobalophilin, but with lower efficiency. It was concluded that a specific transport mechanism for absorption of vitamin B12, mediated by the vitamin B12-binder in milk, exists at the intestinal brush border of neonatal piglets and strongly reinforces the developing intrinsic factor-mediated mechanism during the early days or weeks of life. It is suggested that the binder in the milk has a wider physiological significance and acts also as a 'host protective' factor and as a scavenger of adventitious vitamin B12.

Intramembranous particles are clustered on microvillus membrane vesicles

Many intramembranous particles in pig jejunal microvillus membranes cluster during cell disruption and membrane vesiculation with the MgCl 2 aggregation technique (Hauser, H., Howell, K., Dawson, R.M.C. and Bowyer, D.E. (1980) Biochim. Biophys. Acta 602, 567–577). Isolated brush borders and purified microvillus membrane vesicles were jet-frozen and examined by freeze-fracture electron microscopy. From 30 to 60% of purified vesicles exhibited no intramembranous particles on their fracture face and 22–39% exhibited clustered or aggregated intramembranous particles. Only 6–15% of the vesicles exhibited the random distribution of intramembranous particles that is characteristic of intact enterocytes. Aggregation was not reversed after dialysis to remove divalent cations. Prior freezing of tissue or vesicles (−70°C) gave the same results as fresh unfrozen material. Heterogeneity of microvillus vesicles may occur among the vesicles generated from a single microvillus.

Water and urea transport in renal microvillus membrane vesicles.

Light scattering was used to measure the water and urea permeability of brush border membrane vesicles (BBMV) isolated from rabbit renal cortex. In stop-flow experiments, exposure of BBMV to a 200 mM inwardly directed mannitol gradient gave a monophasic time course of decreasing BBMV volume corresponding to an osmotic water permeability (Pf) of 1.1 +/- 0.1 X 10(-2) cm/s at 37 degrees C. The temperature dependence of Pf was biphasic with delta H = 2 kcal/mol for T less than 33 degrees C and delta H = 14 kcal/mol for T greater than 33 degrees C. A 200 mM inwardly directed urea gradient gave a biphasic time course of BBMV volume due to rapid water efflux (approximately 50 ms) followed by slower urea influx (1-5 s) with urea permeability (Purea) of 2.4 +/- 0.2 X 10(-6) cm/s. Preincubation of BBMV with increasing [urea] reversibly inhibited both urea flux (Kd = 1,200 mM) and thiourea flux (Kd = 370 mM) according to a single-site inhibition model, suggesting a saturable urea carrier. Comparison of BBMV Pf and Purea with proximal tubule transepithelial water and urea transport rates suggests that the permeability of the tubular apical membrane (BBMV) is high enough to support a transcellular route for both osmotic water and urea transport.

Protein kinase C activity in renal microvillus membranes

Protein kinase C activity was found in rabbit renal microvillus membrane vesicles. C-kinase activity was assayed by examining H 1 histone phosphorylation using microvillus membrane vesicles dispersed with Triton X. Calcium-activated protein kinase activity was only demonstrable in the presence of phosphatidylserine (PS). With PS (15 μg/ml) the Ka for activation by calcium was 1.04 μM. This was reduced to 0.38 μM by addition of diolein (3.75 μg/ml). These activations were dose-dependent and their combined synergistic activation could be reproduced by the combination of PS (15 μg/ml) and the phorbol ester, TPA (1.17 ng/ml). During microvillus membrane purification, protein kinase C activity enriched 5-fold relative to its activity in the homogenates. The activity was not due to trapped cytosol or adventitious association with microvillus membranes during homogenization. During further purification on sucrose gradients, the C-kinase activity coenriched with brush border and not with basolateral enzyme markers. We conclude that protein kinase C is a normal component of the renal microvillus membrane.

Adaptation of Na+-H+ exchange in renal microvillus membrane vesicles. Role of dietary protein and uninephrectomy.

The ablation of renal mass and institution of a high protein diet both lead to renal cortical hypertrophy and increased glomerular filtration rate (GFR). We studied Na+ transport in rat microvillus membrane vesicles isolated from uninephrectomized or sham operated rats fed 6% (low), 24% (standard), or 40% (high) protein diets. The feeding of high protein, as compared with low protein, was associated with a 50% increase in rates of pH-stimulated 22Na+ transport in isolated vesicles from sham and uninephrectomized animals. Values for the standard protein diet were intermediate to values for high and low protein. At each level of dietary protein intake, vesicular Na+ transport was greater in the uninephrectomized than in sham rats. The high protein diet was also associated with increased vesicular 22Na+ flux inhibitable by 1 mM amiloride. Increases in total and amiloride sensitive flux were also noted in the absence of a pH gradient. Conductive Na+ and H+ transport were not altered, nor were sodium-glucose and sodium-alanine cotransport. Kinetic studies revealed evidence for an increased Vmax of Na+-H+ exchange in uninephrectomized animals fed a 40 vs. a 6% protein diet whereas Km was unchanged. Supplements of NaHCO3 in the 40% protein diet, to adjust for an increased rate of net acid excretion, did not prevent the increased rates of Na+-H+ exchange. However, treatment with actinomycin D (0.12 mg/kg) prevented the increased Na+-H+ activity as well as the increased renal mass and GFR noted 24 h after unilateral nephrectomy. Na+-H+ exchange rate was closely correlated with GFR (r = 0.961; P less than 0.005) and renal mass (r = .986; P less than 0.001). These observations provide evidence for modification of the luminal membrane Na+-H+ exchanger in response to changes in dietary protein content and nephron number.

Cl- transport via anion exchange in Necturus renal microvillus membranes.

We investigated the mechanism of Cl- transport in microvillus membrane vesicles isolated from Necturus kidneys. Cl- influx was insensitive to changes in membrane potential induced by K+ gradients and the K+ ionophore valinomycin, arguing against conductive Cl- transport. Inward gradients of Na+ or Na+ + K+ did not stimulate initial Cl- influx, arguing against direct Na+-Cl- or Na+-K+-Cl- cotransport. External Cl-, HCO3-, and NO3- each stimulated 36Cl efflux and inhibited 36Cl uptake, indicating anion exchange. Outward HCO3- gradients but not OH- gradients stimulated 36Cl influx, consistent with Cl- -HCO3- exchange. Cl- transport via anion exchange was inhibited by furosemide, bumetanide, and disulfonic stilbenes, but not by acetazolamide. External halides stimulated 36Cl efflux (Cl- = Br- greater than I- greater than F-) but the organic anions lactate, p-aminohippurate, and urate did not. Amiloride-sensitive Na+-H+ exchange was demonstrated. Finally, in the presence of a CO2/HCO3 buffer system, imposing an inward Na+ gradient caused a time-delayed stimulation of 36Cl uptake, consistent with indirect coupling of Na+-H+ and Cl- -HCO3- exchangers. We conclude that the parallel operation of Na+-H+ and Cl- -HCO3- exchangers rather than direct cotransport may account for the Na+-coupled uphill Cl- entry previously observed in the intact proximal tubular cell of Necturus.

Absence of Cl-OH exchange and NaCl cotransport in rabbit renal microvillus membrane vesicles.

Cl-transport was studied in microvillus membrane vesicles isolated from the rabbit renal cortex. Inwardly directed K+ gradients in the presence of the K+ ionophore valinomycin (Val) enhanced 10 mM 36Cl uptake 2.5-fold, confirming a Cl- conductive pathway. An inwardly directed H+ gradient (pHin 7.5, pHout 6.0) stimulated 10 mM Cl- uptake 1.5-fold compared with pHin = pHout = 6.0. However, this H+ gradient stimulation of Cl- uptake appeared secondary to the H+ diffusion potential rather than to Cl-OH exchange, as it was abolished by Val and K+in = K+out. Additional evidence against Cl- transport via anion exchange was the failure of an inwardly directed Cl- gradient to generate an inside-acid pH gradient as monitored by quenching of acridine orange fluorescence. Cl- influx was the same in the presence of inwardly directed gradients of Na+, K+, Cs+, Li+, and Rb+, arguing against NaCl cotransport. Finally, conductive Cl- transport was reduced by the inhibitors furosemide, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. These data indicate the presence of an inhibitor-sensitive, conductive mode of Cl- transport but fail to demonstrate significant pathways for Cl- OH exchange or NaCl cotransport in rabbit renal microvillus membrane vesicles.

3H]ethylpropylamiloride, a radio-labelled diuretic for the analysis of the Na+/H+ exchange system. Its use with kidney cell membranes.

The interaction of amiloride and several amiloride derivatives with the Na+/H+ exchange system in Madin-Darby canine kidney cells and in rabbit renal microvillus membrane vesicles was studied from 22Na+ uptake experiments. On both types of preparation, the order of potency of the different molecules tested is: ethylisopropylamiloride greater than ethylpropylamiloride (EPA) greater than amiloride greater than benzamil. 3H-labelled EPA was prepared and used to titrate amiloride binding sites in solubilized microvillus membranes. Kinetics experiments, equilibrium binding studies and competition experiments between [3H]EPA and unlabelled EPA indicate that EPA recognizes a single family of binding sites with a Kd value of 45 nM and a maximum binding capacity of 2 pmol/mg of protein. The order of potency of different amiloride analogs tested in [3H]EPA competition experiments is identical to that found for the inhibition of 22Na+ uptake by the Na+/H+ exchange system, suggesting that [3H]EPA binding sites are associated with the Na+/H+ exchange system. [3H]EPA binding sites are pharmacologically distinct from those of [3H]benzamil and [3H]bumetanide in kidney membranes.

Ca2+ in the control of active intestinal Na and Cl transport: involvement in neurohumoral action.

Intracellular Ca2+ is a regulator of active intestinal Na and Cl transport. Most studies have been done with rabbit ileum. Increasing intracellular Ca2+ decreases active Na and Cl absorption and/or stimulates active Cl secretion; lowering intracellular Ca2+ stimulates Na and Cl absorption. Based on studies with microvillus membrane vesicles from rabbit ileum, a direct effect of Ca2+ and calmodulin on linked Na and Cl uptake is established. Intracellular Ca2+ and cAMP affect the same transport processes and act in a nonadditive manner. Intracellular Ca2+ does not act by changing intestinal cAMP or cGMP contents, and increasing cAMP mobilizes intracellular Ca2+. Whether this Ca2+ is involved in regulation of ion transport is not known. The aspects of Ca2+ handling identified as involved in regulation of active intestinal Na and Cl transport include entry of Ca2+ across the basolateral membrane, mobilization of Ca2+ from intracellular stores, and involvement of the Ca2+-binding protein calmodulin. Several neurohumoral substances alter intestinal transport by Ca2+-dependent mechanisms and appear to act primarily by increasing (serotonin, carbachol, substance P, and neurotensin) or decreasing (dopamine) Ca2+ entry across the basolateral membrane of intestinal epithelial cells.

Interaction of external H+ with the Na+-H+ exchanger in renal microvillus membrane vesicles.

We examined the effects of external H+ on the kinetics of Na+-H+ exchange in microvillus membrane vesicles isolated from the rabbit renal cortex. The initial rate of Na+ influx into vesicles with internal pH 6.0 was optimal at external pH 8.5 and was progressively inhibited as external pH was reduced to 6.0. A plot of 1/V versus [H+]o was linear and yielded apparent KH = 35 nM (apparent pK 7.5). In vesicles with internal pH 6.0 studied at external pH 7.5 or 6.6, apparent KNa was 13 or 54 mM, Ki for inhibition of Na+ influx by external Li+ was 1.2 or 5.2 mM, Ki for inhibition by external NH4+ was 11 or 50 mM, and Ki for inhibition by external amiloride was 7 or 25 microM, respectively. These findings were consistent with competition between each cation and H+ at a site with apparent pK 7.3-7.5. Lastly, stimulation of 22Na efflux by external Na+ (i.e. Na+-Na+ exchange) was inhibited as external pH was reduced from 7.5 to 6.0, also consistent with competition between external H+ and external Na+. Thus, in contrast with internal H+, which interacts at both transport and activator sites, external H+ interacts with the renal microvillus membrane Na+-H+ exchanger at a single site, namely the external transport site, where H+, Na+, Li+, NH4+, and amiloride all compete for binding.

Specificity and modes of the anion exchanger in dog renal microvillus membranes.

The transport of various organic anions via the pathway that mediates the exchange of urate or p-aminohippurate (PAH) for OH- or Cl- in dog renal microvillus membrane vesicles was investigated. The pH gradient-stimulated uptakes of tracer urate and PAH were significantly inhibited by 5 mM PAH, n-valerate, lactate, beta-hydroxybutyrate, pyruvate, acetoacetate, maleate, succinate, alpha-ketoglutarate, oxaloacetate, and cis-aconitate but not by 5 mM acetate, malate, oxalate, or citrate. the pH dependence of inhibition suggested that it was in their monovalent forms that these acid anions interacted with the urate exchange pathway. Outwardly directed gradients of succinate, lactate, and PAH stimulated uphill urate accumulation. Imposition of an inside-alkaline pH gradient stimulated the uphill accumulation of lactate and succinate. Na+ cotransport pathways for lactate and succinate were also present. In the presence of an inwardly directed Na+ gradient, lactate stimulated the uphill accumulation of urate, indicating that the pathways mediating Na+-lactate cotransport and lactate-urate exchange coexisted in at least some membrane vesicles. We conclude that the anion exchange pathway for urate in dog renal microvillus membrane vesicles has affinity for additional organic anions and can function in multiple exchange modes. Exchange of luminal urate or Cl- for intracellular organic anions or OH- is a possible mechanism for effecting uphill anion reabsorption in the proximal tubule.

Purification of microvillus membrane vesicles from pig small intestine by adsorption chromatography on sepharose

A simple, rapid method for the preparation of pure microvillus membrane vesicles from pig small intestine is described. The method is based on the ability of agarose beads to adsorb selectively the impurities, mainly basolateral membrane fragments, from a microvillus vesicle preparation isolated by hypotonic lysis, Mg 2+ aggregation of contaminants and differential centrifugation.

Urate transport via anion exchange in dog renal microvillus membrane vesicles.

The transport of urate was evaluated in brush border membrane vesicles isolated from the dog renal cortex. It was previously shown that uphill urate and p-aminohippurate (PAH) uptake into these membrane vesicles results from imposing an outwardly directed OH- gradient. In the present study, the OH- gradient-stimulated uptake of urate was inhibited by external Cl-. In the absence of OH- gradients, imposing an outwardly directed Cl- gradient induced the transient accumulation of urate against its concentration gradient (overshoot), whereas imposing an inwardly directed Cl- gradient induced the transient uphill efflux of urate (undershoot). The effects of Cl- gradients on urate transport persisted when the diffusion potentials caused by the Cl- gradients were negated by superimposing K+ gradients in the presence of the K+ ionophore valinomycin. The dose-response curves for three different inhibitors were identical for the OH- and Cl- gradient-stimulated modes of urate uptake. The uptake of PAH was also stimulated by imposing an outwardly directed Cl- gradient. The dose-response curves for probenecid inhibition were identical for the OH- and Cl- gradient-stimulated modes of PAH uptake. Finally, the existence of a Na+ cotransport pathway for urate or PAH could not be demonstrated. We conclude that the principal mechanism for urate transport in dog renal microvillus membrane vesicles is via an anion exchanger with affinity for urate, PAH, OH-, and Cl-. This anion exchanger may play an important role in mediating organic anion reabsorption and secretion in the proximal tubule.

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

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.

Determination of the coupling ratio for Na +-H + exchange in renal microvillus membrane vesicles

We evaluated the H +:Na + coupling ratio of the Na +-H + exchanger present in microvillus membrane vesicles isolated from the rabbit renal cortex. Our approach was to impose transmembrane Na + and H + gradients of varying magnitude and then to measure the net flux of Na + over the subsequent 5-s period. The Na +-H + exchanger was observed to be at equilibrium (i.e. no significant net Na + flux) whenever [Na +] i/[Na +] o was equal to [H +] i/[H +] o. Moreover, under all conditions the magnitude and direction of net Na + flux was independent of changes in the transmembrane electrical potential difference. These results are consistent with a value of 1.0 for the coupling ratio of Na +-H + exchange in renal microvillus membrane vesicles.

Purification of microvillus membrane vesicles from pig small intestine by immunoadsorbent chromatography

Microvillus membrane vesicles from pig small intestine, isolated by hypotonic lysis, Mg 2+ aggregation of contaminants and differential centrifugation, have been further purified by immunoadsorbent chromatography. The vesicles adhere to an immunoadsorbent prepared by coupling antibodies raised against three of the principal proteins of the brush border membrane (aminopeptidase, sucrase-isomaltase and lactase) to Sepharose 4B. After the contaminants are removed by washing, the adherent vesicles are released from the immunoadsorbent by applying shear forces. The purity of the immunoadsorbed vesicles has been established by electron microscopy and by measuring the activity of marker enzymes. The enrichment factor is 1.17±0.02 for aminopeptidase and 0.70±0.05 for 5′-nucleotidase. The contamination of the preparation before immunoadsorption constitutes 10% of the membrane protein and consists mainly of basolateral membrane fragments as judged from marker enzyme determinants and the lipid composition.

Glucose and alanine inhibition of phosphate transport in renal microvillus membrane vesicles.

Transport interactions among phosphate, glucose, and alanine were evaluated in brush border membrane vesicles isolated from the rabbit renal cortex. Both glucose and alanine inhibited the Na+ gradient-stimulated uphill accumulation of phosphate. Neither glucose nor alanine inhibited the phosphate uptake measured in the absence of Na+ or in the presence of a collapsed Na+ gradient shunted with amphotericin B. Phosphate and alanine inhibited the Na+ gradient-stimulated uphill accumulation of glucose but not the glucose uptake measured in the presence of a collapsed Na+ gradient shunted with amphotericin B. Alanine, which was more potent than glucose or phosphate in inhibiting Na+ gradient-stimulated solute transport, was also more potent than glucose or phosphate in stimulating the uptake of Na+ into the vesicles. We conclude that the inhibitory interactions among phosphate, glucose, and alanine probably represent indirect effects resulting from solute-induced alterations in the transmembrane electrochemical Na+ gradient rather than direct effects resulting from competition for a polyfunctional carrier or from allosteric interactions. Such a mechanism could explain the inhibitory interactions among phosphate, glucose, and alanine that have been observed in the intact proximal tubule.

Interaction of NH4+ and Li+ with the renal microvillus membrane Na+-H+ exchanger.

We have examined the interaction of NH4+ and Li+ with the Na+-H+ exchanger in rabbit renal microvillus membrane vesicles. Alkalinization of the intravesicular space, monitored with [14C]DMO uptake by flow dialysis, resulted from the addition of Cl- salts of Na+, Li+, or NH4+ to the external medium. The 22Na+ uptake rate, assayed by a rapid filtration technique, was saturable with respect to external Na+ concentration (KT 6 mM Na+) and was competitively inhibited by external Li+ (Ki 1.9 mM) and NH4+ (Ki 4.3 mM). Efflux by 22Na+ from vesicles preloaded with Na+ was stimulated by external Na+ and NH4+ but inhibited by external Li+. We conclude that the renal microvillus membrane Na+-H+ exchanger has affinity for Li+ and NH4+ and can mediate the exchange not only of Na+ for H+ but also of Na+ for Na+, Na+ for Nh4+, Li+ for H+, possibly NH4+ for H+, and perhaps Li+ for Na+. The physiological significance of exchange modes other than Na+-H+ exchange is not certain at present, but Na+-NH4+ exchange could play a role in the proximal tubular acidification process.