Energy-dependence of phlorizin binding to isolated renal microvillus membranes. Evidence concerning the mechanism of coupling between the electrochemical Na+ gradient the sugar transport

Renal nerve stimulation leads to the activation of the Na+/H+ exchanger isoform 3 via angiotensin II type I receptor

Renal nerve stimulation at a low frequency (below 2 Hz) causes water and sodium reabsorption via α1-adrenoreceptor tubular activation, a process independent of changes in systemic blood pressure, renal blood flow, or glomerular filtration rate. However, the underlying mechanism of the reabsorption of sodium is not fully understood. Since the sympathetic nervous system and intrarenal ANG II appear to act synergistically to mediate the process of sodium reabsorption, we hypothesized that low-frequency acute electrical stimulation of the renal nerve (ESRN) activates NHE3-mediated sodium reabsorption via ANG II AT1 receptor activation in Wistar rats. We found that ESRN significantly increased urinary angiotensinogen excretion and renal cortical ANG II content, but not the circulating angiotensinogen levels, and also decreased urinary flow and pH and sodium excretion via mechanisms independent of alterations in creatinine clearance. Urinary cAMP excretion was reduced, as was renal cortical PKA activity. ESRN significantly increased NHE3 activity and abundance in the apical microvillar domain of the proximal tubule, decreased the ratio of phosphorylated NHE3 at serine 552/total NHE3, but did not alter total cortical NHE3 abundance. All responses mediated by ESRN were completely abolished by a losartan-mediated AT1 receptor blockade. Taken together, our results demonstrate that higher NHE3-mediated proximal tubular sodium reabsorption induced by ESRN occurs via intrarenal renin angiotensin system activation and triggering of the AT1 receptor/inhibitory G-protein signaling pathway, which leads to inhibition of cAMP formation and reduction of PKA activity.

Forces and dynamics of glucose and inhibitor binding to sodium glucose co-transporter SGLT1 studied by single molecule force spectroscopy

Single molecule force spectroscopy was employed to investigate the dynamics of the sodium glucose co-transporter (SGLT1) upon substrate and inhibitor binding on the single molecule level. CHO cells stably expressing rbSGLT1 were probed by using atomic force microscopy tips carrying either thioglucose, 2'-aminoethyl β-d-glucopyranoside, or aminophlorizin. Poly(ethylene glycol) (PEG) chains of different length and varying end groups were used as tether. Experiments were performed at 10, 25 and 37 °C to address different conformational states of SGLT1. Unbinding forces between ligands and SGLT1 were recorded at different loading rates by changing the retraction velocity, yielding binding probability, width of energy barrier of the binding pocket, and the kinetic off rate constant of the binding reaction. With increasing temperature, width of energy barrier and average life time increased for the interaction of SGLT1 with thioglucose (coupled via acrylamide to a long PEG) but decreased for aminophlorizin binding. The former indicates that in the membrane-bound SGLT1 the pathway to sugar translocation involves several steps with different temperature sensitivity. The latter suggests that also the aglucon binding sites for transport inhibitors have specific, temperature-sensitive conformations.

Increased NHE3 abundance and transport activity in renal proximal tubule of rats with heart failure

Heart failure (HF) is associated with a reduced effective circulating volume that drives sodium and water retention and extracellular volume expansion. We therefore hypothesized that Na+/H+ exchanger isoform 3 (NHE3), the major apical transcellular pathway for sodium reabsorption in the proximal tubule, is upregulated in an experimental model of HF. HF was induced in male rats by left ventricle radiofrequency ablation. Sham-operated rats (sham) were used as controls. At 6 wk after surgery, HF rats exhibited cardiac dysfunction with a dramatic increase in left ventricular end-diastolic pressure. By means of stationary in vivo microperfusion and pH-dependent sodium uptake, we demonstrated that NHE3 transport activity was significantly higher in the proximal tubule of HF compared with sham rats. Increased NHE3 activity was paralleled by increased renal cortical NHE3 expression at both protein and mRNA levels. In addition, the baseline PKA-dependent NHE3 phosphorylation at serine 552 was reduced in renal cortical membranes of rats with HF. Collectively, these results suggest that NHE3 is upregulated in the proximal tubule of HF rats by transcriptional, translational, and posttranslational mechanisms. Enhanced NHE3-mediated sodium reabsorption in the proximal tubule may contribute to extracellular volume expansion and edema, the hallmark feature of HF. Moreover, our study emphasizes the importance of undertaking a cardiorenal approach to contain progression of cardiac disease.

A disintegrin and metalloprotease 10 activity sheds the ectodomain of the amyloid precursor-like protein 2 and regulates protein expression in proximal tubule cells

A disintegrin and metalloprotease 10 (ADAM10) is a zinc protease that mediates ectodomain shedding of numerous receptors including Notch and members of the amyloid precursor protein family (APP, APLP1, and APLP2). Ectodomain shedding frequently activates a process called regulated intramembrane proteolysis (RIP) that links cellular events with gene regulation. To characterize ADAM10 in kidney and in opossum kidney proximal tubule (OKP) cells, we performed indirect immunofluorescence microscopy and immunoblotting of renal membrane fractions using specific antibodies. These studies show that ADAM10 and APLP2 are coexpressed in the proximal tubule and in OKP cells. To study the role of ADAM10 activity in the proximal tubule, we stably overexpressed wild-type ADAM10 or an inactive mutant ADAM10 in OKP cells. We found a direct correlation between the amount of active ADAM10 expressed and 1) the amount of APLP2 ectodomain shed into the culture supernatant and 2) the amount of Na(+)/H(+) exchanger 3 (NHE3) and megalin mRNA and protein expressed compared with control proteins. To establish a link between ADAM10-mediated shedding of APLP2 and the effect on NHE3 and megalin mRNA expression we performed RNA interference experiments using APLP2-specific short hairpin RNA (shRNA) in OKP cells. Cells expressing the APLP2 shRNA showed >80% knock down of APLP2 protein and mRNA as well as 60-70% reduction in NHE3 protein and mRNA. Levels of megalin and Na-K-ATPase protein and mRNA were not changed. These studies show 1) ADAM10 and APLP2 are expressed in proximal tubule cells and, 2) ADAM10 activity has a pronounced effect on expression of specific brush-border proteins. We postulate that ADAM10 and APLP2 may represent elements of a here-to-fore unknown signaling pathway in proximal tubule that link events at the brush border with control of gene expression.

Mechanisms mediating the diuretic and natriuretic actions of the incretin hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone considered a promising therapeutic agent for type 2 diabetes because it stimulates beta cell proliferation and insulin secretion in a glucose-dependent manner. Cumulative evidence supports a role for GLP-1 in modulating renal function; however, the mechanisms by which GLP-1 induces diuresis and natriuresis have not been completely established. This study aimed to define the cellular and molecular mechanisms mediating the renal effects of GLP-1. GLP-1 (1 μg·kg(-1)·min(-1)) was intravenously administered in rats for the period of 60 min. GLP-1-infused rats displayed increased urine flow, fractional excretion of sodium, potassium, and bicarbonate compared with those rats that received vehicle (1% BSA/saline). GLP-1-induced diuresis and natriuresis were also accompanied by increases in renal plasma flow and glomerular filtration rate. Real-time RT-PCR in microdissected rat nephron segments revealed that GLP-1 receptor-mRNA expression was restricted to glomerulus and proximal convoluted tubule. In rat renal proximal tubule, GLP-1 significantly reduced Na(+)/H(+) exchanger isoform 3 (NHE3)-mediated bicarbonate reabsorption via a protein kinase A (PKA)-dependent mechanism. Reduced proximal tubular bicarbonate flux rate was associated with a significant increase of NHE3 phosphorylation at the PKA consensus sites in microvillus membrane vesicles. Taken together, these data suggest that GLP-1 has diuretic and natriuretic effects that are mediated by changes in renal hemodynamics and by downregulation of NHE3 activity in the renal proximal tubule. Moreover, our findings support the view that GLP-1-based agents may have a potential therapeutic use not only as antidiabetic drugs but also in hypertension and other disorders of sodium retention.

Posttranslational mechanisms associated with reduced NHE3 activity in adult vs. young prehypertensive SHR

Abnormalities in renal proximal tubular (PT) sodium transport play an important role in the pathophysiology of essential hypertension. The Na+/H+ exchanger isoform 3 (NHE3) represents the major route for sodium entry across the apical membrane of renal PT cells. We therefore aimed to assess in vivo NHE3 transport activity and to define the molecular mechanisms underlying NHE3 regulation before and after development of hypertension in the spontaneously hypertensive rat (SHR). NHE3 function was measured as the rate of bicarbonate reabsorption by means of in vivo stationary microperfusion in PT from young prehypertensive SHR (Y-SHR; 5-wk-old), adult SHR (A-SHR; 14-wk-old), and age-matched Wistar Kyoto (WKY) rats. We found that NHE3-mediated PT bicarbonate reabsorption was reduced with age in the SHR (1.08 ± 0.10 vs. 0.41 ± 0.04 nmol/cm2×s), while it was increased in the transition from youth to adulthood in the WKY rat (0.59 ± 0.05 vs. 1.26 ± 0.11 nmol/cm2×s). Higher NHE3 activity in the Y-SHR compared with A-SHR was associated with a predominant microvilli confinement and a lower ratio of phosphorylated NHE3 at serine-552 to total NHE3 (P-NHE3/total). After development of hypertension, P-NHE3/total increased and NHE3 was retracted out of the microvillar microdomain along with the regulator dipeptidyl peptidase IV (DPPIV). Collectively, our data suggest that the PT is playing a role in adapting to the hypertension in the SHR. The molecular mechanisms of this adaptation possibly include an increase of P-NHE3/total and a redistribution of the NHE3-DPPIV complex from the body to the base of the PT microvilli, both predicted to decrease sodium reabsorption.

The effect of dietary protein on intestinal calcium absorption in rats

Increasing dietary protein intake in humans acutely increases urinary calcium. Isotopic absorption studies have indicated that, at least in the short term, this is primarily due to increased intestinal Ca absorption. To explore the mechanisms underlying dietary protein's effect on intestinal Ca absorption, female Sprague Dawley rats were fed a control (20%), low (5%), or high (40%) protein diet for 7 d, and Ca balance was measured during d 4-7. On d 7, duodenal mucosa was harvested and brush border membrane vesicles (BBMVs) were prepared to evaluate Ca uptake. By d 7, urinary calcium was more than 2-fold higher in the 40% protein group compared with control (4.2 mg/d vs. 1.7 mg/d; P < 0.05). Rats consuming the 40% protein diet both absorbed and retained more Ca compared with the 5% protein group (absorption: 48.5% vs. 34.1% and retention: 45.8% vs. 33.7%, respectively; P < 0.01). Ca uptake was increased in BBMVs prepared from rats consuming the high-protein diet. Maximum velocity (V(max)) was higher in the BBMVs prepared from the high-protein group compared with those from the low-protein group (90 vs. 36 nmol Ca/mg protein x min, P < 0.001; 95% CI: 46-2486 and 14-55, respectively). The Michaelis Menten constant (K(m)) was unchanged (2.2 mm vs. 1.8 mm, respectively; P = 0.19). We conclude that in rats, as in humans, acute increases in protein intake result in hypercalciuria due to augmented intestinal Ca absorption. BBMV Ca uptake studies suggest that higher protein intake improves Ca absorption, at least in part, by increasing transcellular Ca uptake.

Anionic leak currents through the Na+/monocarboxylate cotransporter SMCT1

SMCT1 is a Na-coupled cotransporter of short chain monocarboxylates, which is expressed in the apical membrane of diverse epithelia such as colon, renal cortex, and thyroid. We previously reported that SMCT1 cotransport was reduced by extracellular Cl− replacement with cyclamate− and that the protein exhibited an ostensible anionic leak current. In this paper, we have revisited the interaction between small monovalent anions and SMCT cotransport and leak currents. We found that the apparent Cl− dependence of cotransport was due to inhibition of this protein by the replacement anion cyclamate, whereas several other replacement anions function as substrates for SMCT1; a suitable replacement anion (MES−) was identified. The observed outward leak currents represented anionic influx and favored larger anions (NO3−>I−>Br−>Cl−); currents in excess of 1 μA (at +50 mV) could be observed and exhibited a quasilinear relationship with anion concentrations up to 100 mM. Application of 25 mM bicarbonate did not produce measurable leak currents. The leak current displayed outward rectification, which disappeared when external Na+ was replaced by N-methyl-d-glucamine+. More precisely, external Na+ blocked the leak current in both directions, but its Ki value rose rapidly when membrane potential became positive. Thus SMCT1 possesses a anionic leak current that becomes significant whenever external Na+ concentration is reduced. The presence of this leak current may represent a second function for SMCT1 in addition to cotransporting short chain fatty acids, and future experiments will determine whether this function serves a physiological role in tissues where SMCT1 is expressed.

NHE3 function and phosphorylation are regulated by a calyculin A-sensitive phosphatase

Na+/H+ exchanger 3 (NHE3) is phosphorylated and regulated by multiple kinases, including PKA, SGK1, and CK2; however, the role of phosphatases in the dephosphorylation and regulation of NHE3 remains unknown. The purpose of this study was to determine whether serine/threonine phosphatases alter NHE3 activity and phosphorylation and, if so, at which sites. To this end, we first examined the effects of calyculin A [a combined protein phosphatase 1 (PP1) and PP2A inhibitor] and okadaic acid (a PP2A inhibitor) on general and site-specific NHE3 phosphorylation. Calyculin A induced a phosphorylation-dependent NHE3 gel mobility shift and increased NHE3 phosphorylation at serines 552 and 605. No change in NHE3 phosphorylation was detected after okadaic acid treatment. An NHE3 gel mobility shift was also evident in calyculin A-treated COS-7 cells transfected with either wild-type or mutant (S552A, S605G, S661A, S716A) rat NHE3. Since the NHE3 gel mobility shift occurred despite mutation of known phosphorylation sites, novel sites of phosphorylation must also exist. Next, we assayed NHE3 activity in response to calyculin A and okadaic acid and found that calyculin A induced a 24% inhibition of NHE3 activity, whereas okadaic acid had no effect. When all known NHE3 phosphorylation sites were mutated, calyculin A induced a stimulation of NHE3 activity, demonstrating a functional significance for the novel phosphorylation sites. Finally, we established that the PP1 catalytic subunit can directly dephosphorylate immunopurified NHE3 in vitro. In conclusion, our data demonstrate that a calyculin A-sensitive phosphatase, most likely PP1, is involved in the regulation and dephosphorylation of NHE3 at known and novel sites.

Prestin's anion transport and voltage-sensing capabilities are independent

The integral membrane protein prestin, a member of the SLC26 anion transporter family, is responsible for the voltage-driven electromotility of mammalian outer hair cells. It was argued that the evolution of prestin's motor function required a loss of the protein's transport capabilities. Instead, it was proposed that prestin manages only an abortive hemicycle that results in the trapped anion acting as a voltage sensor, to generate the motor's signature gating charge movement or nonlinear capacitance. We demonstrate, using classical radioactive anion ([(14)C]formate and [(14)C]oxalate) uptake studies, that in contrast to previous observations, prestin is able to transport anions. The prestin-dependent uptake of both these anions was twofold that of cells transfected with vector alone, and comparable to SLC26a6, prestin's closest phylogenetic relative. Furthermore, we identify a potential chloride-binding site in which the mutations of two residues (P328A and L326A) preserve nonlinear capacitance, yet negate anion transport. Finally, we distinguish 12 charged residues out of 22, residing within prestin's transmembrane regions, that contribute to unitary charge movement, i.e., voltage sensing. These data redefine our mechanistic concept of prestin.

Absence of Na+/sugar cotransport activity in Barrett’s metaplasia

AIM: To evaluate the presence of Na⁺-dependent, active, sugar transport in Barrett's epithelia as an intestinal biomarker, based on the well-documented, morphological intestinal phenotype of Barrett's esophagus (BE).

METHODS: We examined uptake of the nonmeta-bolizable glucose analogue, alpha-methyl-D-glucoside (AMG), a substrate for the entire sodium glucose cotransporter (SGLT) family of transport proteins. During upper endoscopy, patients with BE or with uncomplicated gastroesophageal reflux disease (GERD) allowed for duodenal, gastric fundic, and esophageal mucosal biopsies to be taken. Biopsies were incubated in bicarbonate-buffered saline (KRB) containing 0.1 mmol/L ¹⁴C-AMG for 60 min at 20°C. Characterized by abundant SGLT, duodenum served as a positive control while gastric fundus and normal esophagus, known to lack SGLT, served as negative controls.

RESULTS: Duodenal biopsies accumulated 249.84 ± 35.49 (SEM) picomoles AMG/&mgr;g DNA (n = 12), gastric fundus biopsies 36.20 ± 6.62 (n = 12), normal esophagus 12.10 ± 0.59 (n = 3) and Barrett’s metaplasia 29.79 ± 5.77 (n = 8). There was a statistical difference (P < 0.01) between biopsies from duodenum and each other biopsy site but there was no statistically significant difference between normal esophagus and BE biopsies. 0.5 mmol/L phlorizin (PZ) inhibited AMG uptake into duodenal mucosa by over 89%, but had no significant effect on AMG uptake into gastric fundus, normal esophagus, or Barrett’s tissue. In the absence of Na⁺ (all Na⁺ salts replaced by Li⁺ salts), AMG uptake in duodenum was decreased by over 90%, while uptake into gastric, esophageal or Barrett’s tissue was statistically unaffected.

CONCLUSION: Despite the intestinal enterocyte phenotype of BE, Na⁺-dependent, sugar transport activity is not present in these cells.

NHE3 phosphorylation at serines 552 and 605 does not directly affect NHE3 activity

Direct phosphorylation of sodium hydrogen exchanger type 3 (NHE3) is a well-established physiological phenomenon; however, the exact role of NHE3 phosphorylation in its regulation remains unclear. The objective of this study was to evaluate whether NHE3 phosphorylation at serines 552 and 605 is physiologically regulated in vivo and, if so, whether changes in phosphorylation at these sites are tightly coupled to changes in transport activity. To this end, we directly compared PKA-induced NHE3 inhibition with site-specific changes in NHE3 phosphorylation in vivo and in vitro. In vivo, PKA was activated using an intravenous infusion of parathyroid hormone in Sprague-Dawley rats. In vitro, PKA was activated directly in opossum kidney (OKP) cells using forskolin and IBMX. NHE3 activity was assayed in microvillar membrane vesicles in the rat model and by 22Na uptake in the OKP cell model. In both cases, NHE3 phosphorylation at serines 552 and 605 was determined using previously characterized monoclonal phosphospecific antibodies directed to these sites. In vivo, we found dramatic changes in NHE3 phosphorylation at serines 552 and 605 with PKA activation but no corresponding alteration in NHE3 activity. This dissociation between NHE3 phosphorylation and activity was further verified in OKP cells in which phosphorylation clearly preceded transport inhibition. We conclude that although phosphorylation of NHE3 at serines 552 and 605 is regulated by PKA both in vivo and in vitro, phosphorylation of these sites does not directly alter Na+/H+ exchange activity.

Immunolocalization of anion transporter Slc26a7 in mouse kidney

Previous studies have indicated that a major fraction of the filtered Cl−is reabsorbed via apical membrane Cl−/base exchange in the proximal tubule. Recent studies in Slc26a6 null mice have suggested that this transporter mediates only a portion of proximal tubule Cl−/base exchange, raising the possibility that one or more unidentified apical membrane transporters may additionally contribute. Recent studies have identified Slc26a7 as another Cl−/base exchanger expressed in the kidney. We therefore generated Slc26a7-specific polyclonal and monoclonal antibodies to examine cellular and subcellular sites of expression in mouse kidney. The specificity of each antibody was verified by immunoblotting and immunofluorescence of COS-7 cells transiently transfected with mouse Slc26a7. Immunofluorescence microscopy of mouse kidney detected the expression of Slc26a7 subapically in proximal tubule cells, and on the basolateral surface of thick ascending limb cells. Similar staining patterns were demonstrated with two antibodies shown to react with different epitopes on Slc26a7. Immunolocalization of Slc26a7 to proximal tubule and thick ascending limb was also observed in rat kidney. We conclude that Slc26a7 is expressed in the proximal tubule and thick ascending limb of the loop of Henle, and it may therefore contribute to anion transport in these nephron segments.

Role of PDZK1 in membrane expression of renal brush border ion exchangers

Na-H exchanger NHE3 and Cl-anion exchanger CFEX (SLC26A6, PAT1) play principal roles in the reabsorption of Na and Cl in the proximal tubule of the mammalian kidney. The mechanisms by which NHE3 and CFEX are localized to and maintained in the brush border of the proximal tubule are largely unknown. To investigate the possible interaction of NHE3 and CFEX with the PDZ-domain-containing scaffolding protein PDZK1, we performed a series of in vitro interaction assays with GST-fusion proteins and native brush border membrane proteins. These studies demonstrated that, not only were NHE3 and CFEX capable of directly interacting with PDZK1, but that this interaction was mediated through their C-terminal PDZ-interaction sites. To determine whether PDZK1 interaction is essential for brush border localization of NHE3 and CFEX in vivo, we examined the expression of NHE3 and CFEX in kidneys of wild-type and PDZK1-null mutant mice by both Western analysis and immunocytochemistry. These studies indicated that, although brush border expression of NHE3 was unaffected by the loss of PDZK1, the expression of CFEX was markedly reduced. Finally, we assayed CFEX functional activity as Cl-oxalate exchange in brush border membrane vesicles and oxalate-stimulated volume absorption in microperfused proximal tubules. Consistent with the observed decrease in CFEX protein expression, both measures of CFEX functional activity were dramatically reduced in PDZK1-null animals. In conclusion, the scaffolding protein PDZK1 is essential for the normal expression and function of Cl-anion exchanger CFEX in the proximal tubule of the mammalian kidney.

Use of phospho-specific antibodies to determine the phosphorylation of endogenous Na+/H+ exchanger NHE3 at PKA consensus sites

Transfection studies using mutant constructs have implicated one or both protein kinase A (PKA) consensus phosphorylation sites [serines 552 and 605 in rat Na+/H+ exchanger type 3 (NHE3)] as critical for mediating inhibition of NHE3 in response to several stimuli including dopamine. However, whether one or both of these sites is actually phosphorylated in endogenous NHE3 in proximal tubule cells is unknown. The purpose of this study was to generate phosphospecific antibodies so that the state of phosphorylation of these serine residues in endogenous NHE3 could be assessed in vitro and in vivo. To this end, polyclonal and monoclonal phosphospecific peptide antibodies were generated against each PKA consensus site. Phosphospecificity was established by ELISA and Western blot assays. We then used these antibodies in vitro to evaluate the effect of dopamine on phosphorylation of the corresponding PKA sites (serines 560 and 613) in NHE3 endogenously expressed in opossum kidney cells. Baseline phosphorylation of both sites was detected that was significantly increased by dopamine. Next, we determined the baseline phosphorylation state of each serine in rat kidney NHE3 in vivo. We found that serine 552 of NHE3 is phosphorylated to a much greater extent than serine 605 at baseline in vivo. Moreover, we detected a distinct subcellular localization for NHE3 phosphorylated at serine 552 compared with total NHE3. Specifically, NHE3 phosphorylated at serine 552 localized to the coated pit region of the brush-border membrane, where NHE3 is inactive, while total NHE3 was found throughout the brush-border membrane. These findings strongly suggest that phosphorylation of NHE3 plays a role in its subcellular trafficking in vivo. In conclusion, we successfully generated phosphospecific antibodies that should be useful to assess the phosphorylation of endogenous NHE3 at its two PKA consensus sites under a variety of physiological conditions in vitro and in vivo.

Linking Receptor-mediated Endocytosis and Cell Signaling

Megalin, a member of the low density lipoprotein receptor gene family, is required for efficient protein absorption in the proximal tubule. Recent studies have shown that the low density lipoprotein receptor-related protein, another member of this gene family, is proteolytically processed by gamma-secretase implying a role for low density lipoprotein receptor-related protein in a Notchlike signaling pathway. This pathway has been shown to involve: 1) metalloprotease-mediated ectodomain shedding and gamma-secretase-mediated intramembrane proteolysis of some receptors. Experiments were performed to determine whether megalin undergoes similar processing. By immunocytochemistry, immunoblotting, and a fluorogenic enzyme assay presenilin-1 (required for gamma-secretase activity) and gamma-secretase activity were found in the brush border of proximal kidney tubules where megalin is localized. Using a fluorogenic peptide containing an amyloid precursor protein gamma-secretase cleavage site and Compound E, a specific gamma-secretase inhibitor, we found high levels of gamma-secretase activity in renal brush border membrane vesicles. Immunoblotting analysis of renal microsomes and opossum kidney proximal tubule (OKP) cells using antibodies directed to the cytosolic domain of megalin showed a 35-40-kDa, membrane-associated, carboxyl-terminal fragment of megalin (MCTF). When cells were incubated with 200 nm phorbol 12-myristate 13-acetate, the appearance of the MCTF increased 2.5-fold and was blocked by metalloprotease inhibitors. When the cells were incubated with gamma-secretase inhibitor Compound E, it caused a 2-fold increase in MCTF. Finally, incubating the cells with 1 microm vitamin D-binding protein resulted in a 25% increase in the appearance of the MCTF. In summary, the MCTF is produced by protein kinase C regulated, metalloprotease-mediated ectodomain shedding and is the substrate for gamma-secretase. We postulate that the enzymatic processing of megalin represents part of a novel ligand-dependent signaling pathway in the proximal tubule that links receptor-mediated endocytosis with cell signaling.

Impairing actin filament or syndapin functions promotes accumulation of clathrin-coated vesicles at the apical plasma membrane of acinar epithelial cells

In this article, we investigate the contributions of actin filaments and accessory proteins to apical clathrin-mediated endocytosis in primary rabbit lacrimal acini. Confocal fluorescence and electron microscopy revealed that cytochalasin D promoted apical accumulation of clathrin, alpha-adaptin, dynamin, and F-actin and increased the amounts of coated pits and vesicles at the apical plasma membrane. Sorbitol density gradient analysis of membrane compartments showed that cytochalasin D increased [14C]dextran association with apical membranes from stimulated acini, consistent with functional inhibition of apical endocytosis. Recombinant syndapin SH3 domains interacted with lacrimal acinar dynamin, neuronal Wiskott-Aldrich Syndrome protein (N-WASP), and synaptojanin; their introduction by electroporation elicited remarkable accumulation of clathrin, accessory proteins, and coated pits at the apical plasma membrane. These SH3 domains also significantly (p </= 0.05) increased F-actin, with substantial colocalization of dynamin and N-WASP with the additional filaments. Coelectroporation with the VCA domain of N-WASP blocked the increase in F-actin and reversed the morphological changes indicative of impaired apical endocytosis. We suggest that transient modulation of actin polymerization by syndapins through activation of the Arp2/3 complex via N-WASP coordinates dynamin-mediated vesicle fission at the apical plasma membrane of acinar epithelia. Trapping of assembled F-actin intermediates during this process by cytochalasin D or syndapin SH3 domains impairs endocytosis.

Renal expression of novel Na+/H+ exchanger isoform NHE8

Although Na(+)/H(+) exchanger isoform 3 (NHE3) mediates most Na(+)/H(+) exchange in the proximal tubule, studies of NHE3/NHE2 null mice suggest residual Na(+)-dependent proton secretion (Choi JY, Shah M, Lee MG, Schultheis PJ, Shull GE, Muallem S, and Baum M. J Clin Invest 105: 1141-1146, 2000). To characterize additional NHE isoforms that might be expressed in the kidney, we identified the partial sequence of a novel NHE. PCR was used to define the 5'- and 3'-ends, and a cDNA encoding the complete open reading frame was amplified from mouse kidney. The predicted protein of 576 amino acids, which we have named NHE8, has 30-35% amino acid identity to known mammalian isoforms (NHE1-7) but has >50% identity to Drosophila melanogaster "NHE1," suggesting it is the mammalian ortholog of this ancient invertebrate isoform. Northern blot of mouse tissues revealed ubiquitous expression. Western blot using anti-NHE8 antibodies demonstrated protein expression in apical membranes purified from rat renal cortex by divalent cation precipitation. In situ hybridization revealed that NHE8 message was present in both cortex and medulla. In the cortex, NHE8 was present in the majority of cortical tubules, consistent with proximal tubule (S1 and S2) localization. In the medulla, NHE8 message was most highly expressed in the proximal tubules (S3) of the outer stripe of the outer medulla. Thus NHE8 is expressed in the proximal tubule, where it may contribute to apical membrane ion transport.

Formate-stimulated NaCl absorption in the proximal tubule is independent of the pendrin protein

A significant fraction of active chloride reabsorption across the apical membrane of the proximal tubule is mediated by a chloride/formate exchange process, whereby intracellular formate drives the transport of chloride into the cell. When chloride/formate exchange operates in parallel with Na(+)/H(+) exchange and H(+)-coupled recycling of formate, the net result is electroneutral NaCl reabsorption. Pendrin is the protein product of the PDS gene (SLC26A4) and functions in several different anion exchange modes, including chloride/formate exchange. Pendrin is expressed in the kidney and may serve as the transporter responsible for formate-dependent NaCl reabsorption. In the present study, Pds-knockout mice were used to determine the role of pendrin in proximal tubule chloride reabsorption. We show that formate-dependent NaCl absorption in microperfused proximal tubules is similar between wild-type and pendrin-deficient mice. In addition, there is no difference in the rate of formate-mediated chloride transport in brush-border membrane vesicles isolated from wild-type and pendrin-deficient mice. These studies demonstrate that pendrin is not responsible for formate-dependent NaCl reabsorption in the proximal tubule.

The gamma-Na+,K+-ATPase subunit assembles selectively with alpha1/beta1-Na+,K+-ATPase but not with the colonic H+,K+-ATPase

BACKGROUND

The ubiquitous Na+-pump (Na+,K+-ATPase) assembles as a heterodimer of composition alpha/beta in some nephron segments, while in other segments it may exist as a heterotrimer of composition alpha/beta/gamma. The gamma-subunit has been reported to increase the affinity of the Na+-pump for adenosine 5'-triphosphate (ATP), and decrease affinity for both Na+ and K+. The alpha-subunit of the colonic H+,K+-ATPase (cHK) shares 75% sequence similarity with alpha1-Na+,K+-ATPase (alpha1) and assembles with beta1-Na+,K+-ATPase (beta1) in distal colon and renal medulla. Differences in pharmacological properties have been ascribed to when heterologously expressed function has been compared to function in vitro. The purpose of this study was to determine if cHK might associate with the gamma-subunit of the Na+,K+-ATPase (gamma) as a possible explanation for these variations in function.

METHODS

An antibody specific for the gamma was used in coimmunoprecipitation experiments to determine if the gamma assembles stably in vitro with cHK and beta1 in rat renal medulla or distal colon.

RESULTS

Our results demonstrate that the gamma-subunit assembles specifically with the Na+-pump, but not with cHK. Furthermore, the gamma-subunit assembly was specific for rat kidney and was not observed in distal colon.

CONCLUSION

Since the gamma-subunit did not assemble with the cHK/beta1 complex, gamma-subunit assembly cannot explain those variations in ex vivo and in vitro pharmacologic properties ascribed to cHK.

Association of Na(+)-H(+) exchanger isoform NHE3 and dipeptidyl peptidase IV in the renal proximal tubule

In an attempt to identify proteins that assemble with the apical membrane Na(+)-H(+) exchanger isoform NHE3, we generated monoclonal antibodies (mAbs) against affinity-purified NHE3 protein complexes isolated from solubilized renal microvillus membrane vesicles. Hybridomas were selected based on their ability to immunoprecipitate NHE3. We have characterized in detail one of the mAbs (1D11) that specifically co-precipitated NHE3 but not villin or NaPi-2. Western blot analyses of microvillus membranes and immunoelectron microscopy of kidney sections showed that mAb 1D11 recognizes a 110-kDa protein highly expressed on the apical membrane of proximal tubule cells. Immunoaffinity chromatography was used to isolate the antigen against which mAb 1D11 is directed. N-terminal sequencing of the purified protein identified it as dipeptidyl peptidase IV (DPPIV) (EC ), which was confirmed by assays of DPPIV enzyme activity. We also evaluated the distribution of the NHE3-DPPIV complex in microdomains of rabbit renal brush border. In contrast to the previously described NHE3-megalin complex, which principally resides in a dense membrane population (coated pits) in which NHE3 is inactive, the NHE3-DPPIV complex was predominantly in the microvillar fraction in which NHE3 is active. Serial precipitation experiments confirmed that anti-megalin and anti-DPPIV antibodies co-precipitate different pools of NHE3. Taken together, these studies revealed an unexpected association of the brush border Na(+)-H(+) exchanger NHE3 with dipeptidyl peptidase IV in the proximal tubule. These findings raise the possibility that association with DPPIV may affect NHE3 surface expression and/or activity.

Active (9.6 s) and inactive (21 s) oligomers of NHE3 in microdomains of the renal brush border

We have previously shown that Na(+)-H(+) exchanger isoform NHE3 exists as both 9.6 and 21 S (megalin-associated) oligomers in the renal brush border. To characterize the oligomeric forms of the renal brush border Na(+)-H(+) exchanger in more detail, we performed membrane fractionation studies. We found that similar amounts of NHE3 were present in microvilli and a nonmicrovillar membrane domain of high density (dense vesicles). Horseradish peroxidase-labeled endosomes were not prevalent in the dense membrane fraction. However, megalin, which localizes primarily to the intermicrovillar microdomain of the brush border, was enriched in the dense vesicles, implicating this microdomain as the likely source of these membranes. Immunolocalization of NHE3 confirmed that a major fraction of the transporter colocalized with megalin in the intermicrovillar region of the brush border. Immunoprecipitation studies demonstrated that in microvilli the majority of NHE3 was not bound to megalin, while in the dense vesicles most of the NHE3 coprecipitated with megalin. Moreover, sucrose velocity gradient centrifugation experiments revealed that most NHE3 in microvilli sedimented with an S value of 9.6, while the S value of NHE3 in dense vesicles was 21. Finally, we examined the functional state of NHE3 in both membrane fractions. As assayed by changes in acridine orange fluorescence, imposing an outwardly directed Na(+) gradient caused generation of an inside acid pH gradient in the microvilli, indicating Na(+)-H(+) exchange activity, but not in the dense vesicles. Taken together, these data demonstrate that renal brush border NHE3 exists in two oligomeric states: a 9.6 S active form present in microvilli and a 21 S, megalin-associated, inactive form in the intermicrovillar microdomain of the apical plasma membrane. Thus, regulation of renal brush border Na(+)-H(+) exchange activity may be mediated by shifting the distribution between these forms of NHE3.

Role of sodium/hydrogen exchanger isoform NHE3 in fluid secretion and absorption in mouse and rat cholangiocytes

Na+/H+ exchanger (NHE) isoforms play important roles in intracellular pH regulation and in fluid absorption. The isoform NHE3 has been localized to apical surfaces of epithelia and in some tissues may facilitate the absorption of NaCl. To determine whether the apical isoform NHE3 is present in cholangiocytes and to examine whether it has a functional role in cholangiocyte fluid secretion and absorption, immunocytochemical studies were performed in rat liver with NHE3 antibodies and functional studies were obtained in isolated bile duct units from wild-type and NHE3-/- mice after stimulation with forskolin, using videomicroscopic techniques. Our results indicate that NHE3 protein is present on the apical membranes of rat cholangiocytes and on the canalicular membrane of hepatocytes. Western blots also detect NHE3 protein in rat cholangiocytes and isolated canalicular membranes. After stimulation with forskolin, duct units from NHE3-/- mice fail to absorb the secreted fluid from the cholangiocyte lumen compared with control animals. Similar findings were observed in isolated bile duct units from wild-type mice and rats in the presence of the Na+/H+ exchanger inhibitor 5-(N-ethyl-N-isopropyl)-amiloride. In contrast, we could not demonstrate absorption of fluid from the canalicular lumen of mouse or rat hepatocyte couplets after stimulation of secretion with forskolin. These findings indicate that NHE3 is located on the apical membrane of rat cholangiocytes and that this NHE isoform can function to absorb fluid from the lumens of isolated rat and mouse cholangiocyte preparations.

The colonic H+,K+-ATPase functions as a Na+-dependent K+(NH4+)-ATPase in apical membranes from rat distal colon

Recent studies have suggested that the colonic H+,K+-ATPase (HKalpha2) can secrete either Na+ or H+ in exchange for K+. If correct, this view would indicate that the transporter could function as either a Na+ or a H+ pump. To investigate this possibility a series of experiments was performed using apical membranes from rat colon which were enriched in colonic H+,K+-ATPase protein. An antibody specific for HKalpha2 was employed to determine whether HKalpha2 functions under physiological conditions as a Na+-dependent or Na+-independent K+-ATPase in this same membrane fraction. K+-ATPase activity was measured as [gamma-32P]ATP hydrolysis. The Na+-dependent K+-ATPase accounted for approximately 80% of overall K+-ATPase activity and was characterized by insensitivity to Sch-28080 but partial sensitivity to ouabain. The Na+-independent K+-ATPase activity was insensitive to both Sch-28080 and ouabain. Both types of K+-ATPase activity substituted NH4+ for K+ in a similar manner. Furthermore, our results demonstrate that when incubated with native distal colon membranes, the blocking antibody inhibited dramatically Na+-dependent K+-ATPase activity. Therefore, these data demonstrate that HKalpha2 can function in native distal colon apical membranes as a Na+-dependent K+-ATPase. Elucidation of the role of the pump as a transporter of Na+ versus H+ or NH4+ versus K+ in vivo will require additional studies.

Kinetic mechanisms of inhibitor binding: relevance to the fast-acting slow-binding paradigm

Although phlorizin inhibition of Na+-glucose cotransport occurs within a few seconds, 3H-phlorizin binding to the sodium-coupled glucose transport protein(s) requires several minutes to reach equilibrium (the fast-acting slow-binding paradigm). Using kinetic models of arbitrary dimension that can be reduced to a two-state diagram according to Cha's formalism, we show that three basic mechanisms of inhibitor binding can be identified whereby the inhibitor binding step either (A) represents, (B) precedes, or (C) follows the rate-limiting step in a binding reaction. We demonstrate that each of mechanisms A-C is associated with a set of unique kinetic properties, and that the time scale over which one may expect to observe mechanism C is conditioned by the turnover number of the catalytic cycle. In contrast, mechanisms A and B may be relevant to either fast-acting or slow-binding inhibitors. However, slow-binding inhibition according to mechanism A may not be compatible with a fast-acting behavior on the steady-state time scale of a few seconds. We conclude that the recruitment hypothesis (mechanism C) cannot account for slow phlorizin binding to the sodium-coupled glucose transport protein(s), and that mechanism B is the only alternative that may explain the fast-acting slow-binding paradigm.

Expression of HKalpha2 protein is increased selectively in renal medulla by chronic hypokalemia

Our laboratory has demonstrated by Northern analysis that chronic hypokalemia increases HKalpha2 (i.e., alpha-subunit of the colonic H+-K+-ATPase) mRNA abundance in the rat. To determine whether the increase in mRNA correlated with an increase in HKalpha2 protein, an antibody was raised against a synthetic peptide derived from amino acids 686-698 of the HKalpha2 sequence. The anti-HKalpha2 antibody hybridized to rat distal colon membranes which migrated at approximately 100 kDa (expected mobility of HKalpha2). HKalpha2 protein was not detected in plasma membranes from rat whole kidney or stomach (100 microg) derived from control animals. The antibody was then used to investigate changes in expression of HKalpha2 in renal cortex, renal medulla, and distal colon in two pathophysiological conditions: 1) chronic hypokalemia (LK) and 2) chronic metabolic acidosis (CMA). In LK rats there was a marked, but selective, increase in the abundance of HKalpha2 protein in membranes prepared from renal medulla. Nevertheless, a corresponding increase in HKalpha2 protein abundance was not observed in membranes prepared from the distal colon of LK rats. HKalpha2 protein abundance in CMA was indistinguishable from controls. Moreover, chronic hypokalemia had no effect on expression of alpha1-Na+-K+-ATPase or HKalpha1 in kidney or distal colon under any experimental condition. Therefore, HKalpha2 protein is tissue- and site-specifically upregulated in response to chronic hypokalemia but not by CMA. Furthermore, this regulatory response is localized to the renal medulla.

Membrane topology of NHE3. Epitopes within the carboxyl-terminal hydrophilic domain are exoplasmic

Experimental data indicate that the relatively hydrophilic carboxyl-terminal domains of Na+-H+ exchangers mediate the regulation of transporter activity through interactions with cytoskeletal effectors. It has therefore been assumed that this entire domain lies on the cytoplasmic surface of the plasma membrane. The purpose of the present study was to determine the membrane orientation of the COOH-terminal 131 amino acids of Na+-H+ exchanger isoform NHE3 by use of three monoclonal antibodies that recognize at least two distinct epitopes within this region. Enzyme-linked immunosorbent assay studies demonstrated binding of these monoclonal antibodies (mAbs) to intact right-side-out renal brush border membrane vesicles in the absence of detergent. Moreover, when coupled to an affinity matrix to isolate membrane vesicles, the anti-NHE3 mAbs bound structures that were morphologically identical to intact microvilli. To confirm the identity of the exoplasmic antigen bound by the antibodies, immunoprecipitation studies were performed. Intact right-side-out brush border membrane vesicles were incubated with the mAbs in the absence of detergent. The membranes were pelleted, supernatant with unbound antibody was removed, the pellet was solubilized, and then immunoprecipitation with secondary antibody was performed. Immunoblot analysis indicated that NHE3 was precipitated after binding of the mAbs to intact membranes. Finally, the localization of the mAb epitopes was determined using high resolution immunocytochemistry. Ultrathin cryosections of rat kidney were labeled with the mAbs and bound antibody detected with the colloidal gold technique. Labeling was restricted to the exoplasmic surface of microvilli of the proximal tubule. Taken together, these findings indicate that epitopes within the carboxyl terminus of the Na+-H+ exchanger isoform NHE3 are exposed to the outside of the plasma membrane.

Sodium leak pathway and substrate binding order in the Na+-glucose cotransporter

The Na+-glucose cotransporter (SGLT1) expressed in Xenopus laevis oocytes was shown to generate a phlorizin-sensitive sodium leak in the absence of sugars. Using the current model for SGLT1, where the sodium leak was presumed to occur after two sodium ions are bound to the free carrier before glucose binding, a characteristic concentration constant (Kc) was introduced to describe the relative importance of the sodium leak versus Na+-glucose cotransport currents. Kc represents the glucose concentration at which the Na+-glucose cotransport current is equal to the sodium leak. As both the sodium leak and the Na+-glucose cotransport current are predicted to occur after the binding of two sodium ions, the model predicted that Kc should be sodium-independent. However, by using a two-microelectrode voltage-clamp technique, the observed Kc was shown to depend strongly on the external sodium concentration ([Na+]o): it was four times higher at 5 mM [Na+]o than at 20 mM [Na+]o. In addition, the magnitude of the sodium leak varied as a function of [Na+]o in a Michaelian fashion, and the sodium affinity constant for the sodium leak was 2-4 times lower than that for cotransport in the presence of low external glucose concentrations (50 or 100 microM), whereas the current model predicted a sigmoidal sodium dependence of the sodium leak and identical sodium affinities for the sodium leak and the Na+-glucose cotransport. These observations indicate that the sodium leak occurs after one sodium ion is associated with the carrier and agree with predictions from a model with the binding order sodium-glucose-sodium. This conclusion was also supported by experiments performed where protons replaced Na+ as a "driving cation."

Effect of acute increases in filtered HCO3- on renal hydrogen transporters: II. H(+)-ATPase

Adaptive increases in renal bicarbonate reabsorption occur in response to acute increases in filtered bicarbonate (FLHCO3). In a previous study, we showed that an increase in FLHCO3 induced by plasma volume expansion increased the Vmax for Na+/H+ exchange activity in renal cortical brush border membrane vesicles (BBMV), providing a potential mechanism for the adaptive increase in HCO3- reabsorption. The present studies were undertaken to determine whether the increase in FLHCO3 induced by plasma expansion also stimulates the other major H+ transporter in cortical BBMV, the H(+)-ATPase. H(+)-ATPase activity was assessed in BBMV obtained from hydropenic and plasma expanded Munich-Wistar rats, using a NADH-linked ATPase assay. H(+)-ATPase activity was measured as the ouabain and oligomycin-insensitive, bafilomycin A1-sensitive component of total ATPase activity. Acute plasma expansion doubled single nephron FLHCO3, and this change was associated with a 64% increase in the Vmax for H(+)-ATPase activity, with no change in apparent Km. The Vmax for H(+)-ATPase activity correlated directly with whole kidney GFR and FLHCO3 (r = 0.68 and 0.72, respectively), and with single nephron GFR and FLHCO3 (r = 0.76 and 0.80, respectively). Thus, the mechanism for the adaptive increase in proximal tubular HCO3- reabsorption that occurs in response to acute increases in FLHCO3 appears to be related to increased activity of both H(+)-ATPase and Na+/H+ exchange in the apical membrane of the proximal tubule epithelium.

Role of NHE3 in mediating renal brush border Na+-H+ exchange. Adaptation to metabolic acidosis

The aims of the present study were to estimate the fraction of renal brush border membrane Na+-H+ exchange activity mediated by the isoform NHE3 and to evaluate whether the increased brush border Na+-H+ exchange observed in metabolic acidosis is due to increased expression of NHE3 protein. Compared with other isoforms, NHE3 is known to have a unique profile of sensitivity to pharmacologic inhibitors, including relative resistance to amiloride analogs and HOE694. We therefore assessed the inhibitor sensitivity of pH gradient-stimulated 22Na uptake in renal brush border vesicles isolated from normal rats. The I50 values for amiloride (30 microM), dimethylamiloride (10 microM), ethylisopropylamiloride (6 microM), and HOE694 (>100 microM) were markedly dissimilar from those reported for NHE1 and NHE2 but were nearly identical to reported values for NHE3. Na+-H+ exchange activity in renal brush border vesicles isolated from rats with 5 days of NH4Cl-induced metabolic acidosis was increased 1.5-fold compared with control rats, with no change in inhibitor sensitivity. Western blot analysis indicated that NHE3 protein expression was greater in brush border membranes from acidotic compared with control rats. We conclude that virtually all measured Na+-H+ exchange activity in brush border membranes from control and acidotic rats is mediated by NHE3 and that metabolic acidosis causes increased expression of renal brush border NHE3 protein.

Pathways for oxalate transport in rabbit renal microvillus membrane vesicles

Recent evidence suggests that apical membrane Cl--oxalate exchange plays a major role in mediating Cl- absorption in the renal proximal tubule. To sustain steady-state Cl- absorption by a mechanism of exchange for intracellular oxalate requires the presence of one or more pathways for recycling oxalate from lumen to cell. Accordingly, we evaluated the mechanisms of oxalate transport in luminal membrane vesicles isolated from the rabbit renal cortex. We found that transport of oxalate by Na+ cotransport is negligible compared to the transport of sulfate. In contrast, we demonstrated that oxalate shares the electroneutral pathway mediating Na+-independent sulfate-carbonate exchange. We also demonstrated the presence of OH--oxalate exchange (indistinguishable from H+-oxalate cotransport). The process of OH--oxalate exchange was electrogenic and partially inhibited by Cl-, indicating that it occurs, at least in part, as a mode of the Cl--oxalate exchanger described previously. An additional component of OH--oxalate exchange was insensitive to inhibition by either Cl- or sulfate, suggesting that it takes place by neither the Cl--oxalate exchanger nor the sulfate-carbonate exchanger. We conclude that multiple anion exchange mechanisms exist by which oxalate can recycle from lumen to cell to sustain Cl- absorption occurring via apical membrane Cl--oxalate exchange in the renal proximal tubule.

The molecular mechanism and potential dependence of the Na+/glucose cotransporter

Activity of the Na+/glucose cotransporter endogenously expressed in LLC-PK1 cells was measured using whole cell recording techniques under three different sodium concentration conditions: 1) externally saturating, zero trans; 2) 40 mM external, zero trans; and 3) externally saturating, 50 mM trans. Activity of the transporter with increasing concentrations of sugar was measured for each set of conditions, from which the maximal current for saturating sugar, Im, was determined. The Im measured shows substantial potential dependence for each set of conditions. The absolute Im and the relative potential dependence of Im compared among the various solute conditions were used to identify which loci in the transport cycle are responsible for potential-dependent changes in function. The experimental data were compared with the predicted Im values calculated from an eight-state, sequential, reversible model of a transport reaction kinetic scheme. Predictions derived from assignment of rate limitation and/or potential dependence to each of the 16 transitions in the transport pathway were derived and compared with the measured data. Most putative models were dismissed because of lack of agreement with the measured data, indicating that several steps along the transport pathway are not rate limiting and/or not potential dependent. Only two models were found that can completely account for the measured data. In one case, translocation of the free carrier must be rate limiting, and both extracellular sodium-binding events as well as translocation of both free and fully loaded carrier forms must be potential-dependent transitions. In the second case, translocation of the free carrier and dissociation of the first sodium to be released intracellularly must be equivalently rate limiting. In this case only the two translocation events are required to be potential dependent. The two external sodium-binding events might still be potential dependent, but this is not required to fit the data. Previous reports suggest that the first model is correct; however, no direct experimental data compel us to dismiss the second option as a feasible model.

Ligand binding to the serotonin transporter: equilibria, kinetics, and ion dependence

The effects of Na+ and Cl- on the binding of [3H]imipramine and the cocaine analog [125I]-beta-carbomethoxy-3 beta-(4-iodophenyl)tropane([125I]-beta-CIT) to the human platelet serotonin transporter have been measured. The ion dependence of beta-CIT binding is consistent with binding beta-CIT together with one Na+ ion, but not in an ordered sequence. Imipramine affinity, like beta-CIT affinity, is increased by Na+, but imipramine binding involves at least two Na+ ions. This conclusion is based on the observation that both imipramine association rate constants and equilibrium affinity constants show a sigmoidal Na+ dependence. As with beta-CIT, the imipramine and Na+ binding sequence is not strictly ordered. Cl- increases imipramine affinity, apparently by slowing dissociation. beta-CIT binding occurs even in the absence of Na+ and Cl-. This provided a means to measure substrate and inhibitor affinity in both the presence and absence of cotransported ions. Nontransported inhibitors, such as imipramine and citalopram, as well as the transport substrates serotonin and 3,4-(methylenedioxy)methamphetamine all displaced beta-CIT binding in the absence of NaCl. In the absence of Cl-, Na+ increased the affinity of nontransported inhibitors but not of substrates. The results suggest that Na+ and Cl- induce independent changes in the transporter binding site and that binding of substrates and inhibitors is affected differently by these changes.

Classical and channel-like urate transporters in rabbit renal brush border membranes

The precise mechanism by which urate is transported across rabbit renal proximal tubule luminal membranes has not been defined. To determine whether urate flux across this membrane represents simple diffusion or transport on specific carriers, urate uptake was examined in brush border membrane vesicles that were prepared by a Mg+(+)-aggregation technique and then exposed to CuCl2. Na(+)-independent, voltage sensitive urate transport was demonstrated in these Cu+(+)-exposed vesicles. Transport was trans-stimulated by urate and cis inhibited by pyrazinoic acid and oxonate. A small fraction of transported urate and urate in the extravesicular fluid was oxidized to allantoin. Kinetic analysis revealed the presence of two kinetically distinct transporters; a channel-like carrier that was inhibited by pyrazinoic acid and oxonate, and a high-affinity, classical, saturable carrier that was inhibited by higher concentrations of oxonate. These studies provide the first direct evidence for carrier-mediated urate transport in rabbit renal brush-border membranes and demonstrate that the rabbit transporter(s) share a number of properties with the urate uniporter in rat proximal tubule cell membranes.

Function and presumed molecular structure of Na(+)-D-glucose cotransport systems

Functional characterization of Na(+)-D-glucose cotransport in intestine and kidney indicates the existence of heterogeneous Na(+)-D-glucose cotransport systems. Target size analysis of the transporting unit and model analysis of substrate binding have been performed and proteins have been cloned which mediate (SGLT1) and modulate (RS1) the expression of Na(+)-D-glucose cotransport. The experiments support the hypothesis that functional Na(+)-D-glucose cotransport systems in mammals are composed of two SGLT1-type subunits and may contain one or two RS1-type proteins. SGLT1 contains up to twelve membrane-spanning alpha-helices, whereas RS1 is a hydrophilic extracellular protein which is anchored in the brush-border membrane by a hydrophobic alpha-helix at the C-terminus. SGLT1 alone is able to translocate glucose together with sodium; however, RS1 increases the Vmax of transport expressed by SGLT1. In addition, the biphasic glucose dependence of transport, which is typical for kidney and has been often observed in intestine, was only obtained after coexpression of SGLT1 and RS1.

Na+ alters the affinity for glucose and phosphate in rat renal brush-border membranes: a study of NMR relaxation rates

Transport of D-glucose (Glc) and Pi across renal microvilli brush-border membrane vesicles (BBMV) is coupled to Na+.Na+ may alter substrate binding and dissociation or the mobility of the translocator and its complexes. To evaluate substrate binding in NaCl or KCl solutions containing BBMV, we measured nuclear magnetic resonance transverse relaxation rates (1/T2) of [1-13C]Glc and 31P(i), as well as their temperature, magnetic field, and pH dependencies. The data indicate the following: 1) the alpha-anomer, but not the beta-anomer, of Glc binds to BBMV, more in the presence of Na+ than of K+; 2) interactions of P(i) with BBMV that increase its 1/T2 are significant in the presence of K+ but not of Na+; 3) temperature and magnetic field dependencies of the 1/T2 of P(i) bound to BBMV are consistent with more rapid exchange with free P(i) in the presence of Na+ than of K+; and 4) in the presence of Na+, [H+] promoted interactions of P(i) with BBMV that increase its 1/T2, probably by interfering with formation of rapidly dissociating Na(+)-P(i)-carrier complexes. In conclusion, Na+ promotes association of alpha-Glc to BBMV but promotes rapid exchange and dissociation of P(i) from BBMV. In both cases, Na+ effects coupling by altering the affinity of the cotransporters for the substrate.

Effect of acute changes in glomerular filtration rate on Na+/H+ exchange in rat renal cortex

Studies were undertaken in Munich-Wistar rats to assess the influence of changes in filtered bicarbonate (FLHCO3), induced by changes in GFR, on Na+/H+ exchange activity in renal brush border membrane vesicles (BBMV). Whole-kidney and micropuncture measurements of GFR, FLHCO3, and whole-kidney and proximal tubule HCO3 reabsorption (APRHCO3) were coupled with BBMV measurements of H+ gradient-driven 22Na+ uptake in each animal studied. 22Na+ uptake was measured at three Na+ concentration gradients to allow calculation of Vmax and Km for Na+/H+ exchange. GFR was varied by studying animals under conditions of hydropenia, plasma repletion, and acute plasma expansion. The increase in GFR, FLHCO3, and APRHCO3 induced by plasma administration correlated directly with an increase in the Vmax for Na+/H+ exchange in BBMV. The Km for sodium was unaffected. In the plasma-expanded rats, the Vmax for Na+/H+ exchange was 22% greater than in the hydropenic rats (P less than 0.025) whereas APRHCO3 was 86% greater (P less than 0.001). These results indicate that increases in FLHCO3, induced by acute increases in GFR, stimulate Na+/H+ exchange activity in proximal tubular epithelium. This stimulation is a mechanism which can, in part, account for the delivery dependence of proximal bicarbonate reabsorption.

Na+ binding to the Na(+)-glucose cotransporter is potential dependent

Activity of the Na(+)-glucose cotransporter in LLC-PK1 epithelial cells was assayed by measuring sugar-induced currents (IAMG) using whole cell recording techniques. IAMG was compared among cells by standardizing the measured currents to cell size using cell capacitance measurements. IAMG at a given membrane potential was measured as a function of alpha-methylglucoside (AMG) concentration and can be fit to Michaelis-Menten kinetics. IAMG at varying Na+ concentrations can be described by the Hill equation with a Hill coefficient of 1.6 at all tested potentials. At high external Na+ levels (155 mM), Na+ is at least 90% saturating at all tested potentials. Maximal currents at a given membrane potential (Im) are calculated from the Michaelis-Menten equation fit to data measuring IAMG vs. AMG concentration at a constant Na+ concentration. Im showed potential dependence under all conditions. Potential-dependent Na+ binding rate(s) cannot alone explain the observed potential dependence of Im under saturating Na+ conditions. Therefore, because Im is potential dependent, at least one step of the transport cycle other than external Na+ binding must be potential dependent. Im was also calculated from data taken at 40 mM external Na+. At all potentials studied, Im at 155 mM Na+ is greater than Im calculated at 40 mM Na+. This implies that the rate of external Na+ binding to the transporter at 40 mM also affects the maximal transport rate. Furthermore, Im at 40 mM external Na+ increases with hyperpolarization faster than Im at 155 mM Na+. Together, these facts indicate that the rate at which Na+ binds to the transporter is also potential dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity phlorizin binding in Mytilus gill

The gill of the marine mussel, Mytilus, contains a high affinity, Na-dependent D-glucose transporter capable of accumulating glucose directly from sea water. We examined the ability of the beta-glucoside, phlorizin, to act as a high-affinity ligand of this process in intact gills and isolated brush border membrane vesicles (BBMV). The time course of association of nanomolar [3H]phlorizin to gills and BBMV was slow, with t50 values between 10 and 30 min, and a half-time for dissociation of approx. 30 min. 1 mM D-glucose reduced equilibrium binding of 1 nM phlorizin by 90-95%, indicating that there was little non-specific binding of this ligand to the gill. In addition, there was little, if any, hydrolysis by the gill of phlorizin to its constituents, glucose and phloretin. Phlorizin binding to gills and BBMV was significantly inhibited by the addition of 50 microM concentrations of D-glucose and alpha-methyl-D-glucose, and unaffected by the addition of L-glucose and fructose. Binding to gills and BBMV was reduced by greater than 90% when Na+ was replaced by K+. Replacement of Na+ by Li+ effectively blocked binding to the intact gill, although Li+ did support a limited amount of glucose-specific phlorizin binding in BBMV. The Kd values for glucose-specific phlorizin binding in intact gills and BBMV were 0.5 nM and 6 nM, respectively. We conclude that phlorizin binds with extremely high affinity to the Na-dependent glucose transporter of Mytilus gill, which may be useful in future efforts to isolate and purify the protein(s) involved in integumental glucose transport.

Solubilization and partial purification of the thiazide diuretic receptor from rabbit renal cortex

This study was designed to solubilize, characterize and begin to purify the thiazide-sensitive Na/Cl transporter from mammalian kidney. Metolazone, a thiazide-like diuretic drug, binds to receptors in rat renal cortex closely related to the thiazide-sensitive Na/Cl transport pathway of the renal distal tubule. In the current study, [3H]metolazone bound to receptors in rabbit renal cortical microsomes. The portion of [3H]metolazone binding that was inhibited by hydrochlorothiazide reflected binding to a high-affinity class of receptor. The affinity (Kd 2.0 +/- 0.1 nM) and number (Bmax = 0.9 +/- 0.4 pmol/mg protein) of high-affinity receptors in rabbit renal cortical membranes were similar to values reported previously for rat. When proximal and distal tubule fragments were separated by Percoll gradient centrifugation, receptors were restricted to the fraction that contained distal tubules. When compared with cortical homogenates, receptor density was enriched 12-fold by magnesium precipitation and differential centrifugation. The zwitterionic detergent CHAPS solubilized 25-35% of the receptors (at 6 mM). Chloride inhibited and Na stimulated binding of [3H]metolazone to solubilized high-affinity receptors. The receptors could be purified significantly by hydroxyapatite chromatography and size exclusion high performance liquid chromatography (HPLC). The combination of magnesium precipitation and differential centrifugation, hydroxyapatite chromatography, and size exclusion HPLC resulted in a 213-fold enrichment of receptors, compared to renal cortical homogenate. The current results indicate that thiazide receptors from rabbit kidney share characteristics with receptors from rat, and that rabbit receptors can be solubilized in CHAPS and purified significantly by hydroxyapatite chromatography and size exclusion HPLC.

Heterogeneity in the effects of membrane potentials on pantothenate and glucose uptakes by rabbit renal apical membranes

1. Previous studies using renal brush-border membrane vesicles have established that both the pantothenate and the low Km (Michaelis-Menten constant), low Vmax (maximal rate) D-glucose systems have a stoichiometry of 2 Na+: 1 organic molecule. In this study, we compared the mechanisms by which the membrane potential energizes pantothenate and D-glucose uptakes by brush-border membrane vesicles isolated from the whole cortex of rabbit kidney. 2. In the absence of Na+, varying the membrane potential from +60 to -60 mV decreased pantothenate uptake, whereas D-glucose uptake was increased in a linear manner. These results suggested the existence of a conductive pathway for pantothenate in these membranes. They also suggested that the pantothenate free carrier is electroneutral, while the glucose free carrier is negatively charged. 3. In the presence of an inwardly directed Na+ gradient, varying the membrane potential from +60 to -60 mV increased Na(+)-dependent pantothenate influx linearly. In contrast, a shift from +60 to +40 mV in the membrane potential had no influence on Na(+)-dependent D-glucose influx, whereas influx was a linear function of the membrane potential from +40 to -60 mV, indicating that there is a threshold membrane potential required for membrane potential-dependent D-glucose movement to occur. 4. Kinetic studies revealed that the effect of membrane potential on pantothenate uptake is through changes in the Km, while Vmax was unchanged. On the other hand, the membrane potential exerted its effect on D-glucose transport solely on the Vmax. 5. Finally, binding studies revealed that membrane potential, both in the presence and absence of a Na+ gradient, elicited effects on phlorizin binding qualitatively similar to those observed for D-glucose transport. 6. Implications of these findings for tubular regulation of these electrogenic secondary active transport systems are discussed.

Reconstitution and partial purification of an amiloride-sensitive, cation channel from rabbit kidney

The aim of the present study was to reconstitute and purify an epithelial potassium channel from rabbit kidney. Renal brush border membrane vesicles (BBMV) were found to contain a potassium conductance which was inhibited by amiloride, 5-(N-methyl-N-isobutyl)amiloride (MIA) and by barium. Membrane vesicle proteins were solubilized and reconstituted in proteoliposomes. Channel activity was assayed using Acridine orange and the voltage sensitive dye, 3,3'-diethylthiadicarbocyanine iodide (DiSC2(5)). Both methods yielded similar results which indicated the presence of an amiloride-sensitive, cation channel in the proteoliposomes. This channel was more permeable to K than to Na and its activity was increased in reconstituted proteoliposomes as compared to native brush border membranes. We conclude that rabbit BBMV possess an amiloride sensitive cation channel. Channel activity was successfully reconstituted in proteoliposomes and the protein was partially purified during reconstitution.