Detergent solubilization, functional reconstitution, and partial purification of epithelial amiloride-binding protein

Identification of small molecule inhibitors that distinguish between non-transferrin bound iron uptake and transferrin-mediated iron transport

Chemical genetics is an emerging field that takes advantage of combinatorial chemical and small molecule libraries to dissect complex biological processes. Here we establish a fluorescence-based assay to screen for inhibitors of iron uptake by mammalian cells. Using this approach, we screened the National Cancer Institute's Diversity Set library for inhibitors of non-transferrin bound iron uptake. This screen identified 10 novel small molecule inhibitors of iron transport with IC(50) values that ranged from 5 to 30 microM. Of these ten compounds, only two blocked uptake of iron mediated by transferrin. Thus, this study characterizes the first small molecule inhibitors that distinguish between different pathways of iron transport.

Epithelial sodium channel activity in detergent-resistant membrane microdomains

The activity of epithelial Na(+) selective channels is modulated by various factors, with growing evidence that membrane lipids also participate in the regulation. In the present study, Triton X-100 extracts of whole cells and of apical membrane-enriched preparations from cultured A6 renal epithelial cells were floated on continuous-sucrose-density gradients. Na(+) channel protein, probed by immunostaining of Western blots, was detected in the high-density fractions of the gradients (between 18 and 30% sucrose), which contain the detergent-soluble material but also in the lighter, detergent-resistant 16% sucrose fraction. Single amiloride-sensitive Na(+) channel activity, recorded after incorporation of reconstituted proteoliposomes into lipid bilayers, was exclusively localized in the 16% sucrose fraction. In accordance with other studies, high- and low-density fractions of sucrose gradients likely represent membrane domains with different lipid contents. However, exposure of the cells to cholesterol-depleting or sphingomyelin-depleting agents did not affect transepithelial Na(+) current, single-Na(+) channel activity, or the expression of Na(+) channel protein. This is the first reconstitution study of native epithelial Na(+) channels, which suggests that functional channels are compartmentalized in discrete domains within the plane of the apical cell membrane.

Oxidative stress induced by L-buthionine-(S,R)-sulfoximine, a selective inhibitor of glutathione metabolism, abrogates mouse kidney mineralocorticoid receptor function

In vitro studies have demonstrated that cysteine groups present in most of the steroid receptors play an essential role in the steroid binding process as well as in the ability of this superfamily of signaling proteins to function as transcription factors. However, there is poor experimental evidence, if any, which demonstrates that under conditions of oxidative stress the steroid receptors in general, and the mineralocorticoid receptor in particular, are affected in vivo in a similar fashion as has been described for cell-free systems or cells in culture. In the present work we report that when mice are exposed to oxidative stress by treatment with L-buthionine-(S,R)-sulfoximine (L-(S,R)-BSO), a glutathione depleting agent, the aldosterone-dependent mineralocorticoid biological response (measured as sodium retention and potassium elimination) was diminished in a directly proportional manner with respect to the depletion of renal glutathione. Accordingly, the steroid binding capacity of the mineralocorticoid receptor was also abrogated, whereas the receptor protein level remained unchanged. The harmful effects observed in mice after glutathione depletion were efficiently prevented by co-treatment with glutathione monoethyl ester. Similar inhibition in the steroid binding capacity was also generated in vitro by receptor alkylation and receptor oxidation, an effect which was prevented in the presence of reducing agents. Since the glutathione deficit generated in vivo by treatment with L-(S,R)-BSO did not significantly affect other renal proteins which are known to be required for the mineralocorticoid mechanism of action, we suggest that in renal cells a low redox potential exerts drastic and uncompensated inhibition of the receptor-mediated mineralocorticoid biological response. This effect was ascribed to the loss of steroid binding capacity of oxidized receptor, most likely by modification of essential cysteines as supported by experiments where a decreased number of reactive thiols and reduced covalent binding of thiol-reactive ligand were evidenced on immunopurified receptor after in vivo treatment with L-(S,R)-BSO.

Association of the epithelial sodium channel with Apx and alpha-spectrin in A6 renal epithelial cells

Recent molecular cloning of the epithelial sodium channel (ENaC) provides the opportunity to identify ENaC-associated proteins that function in regulating its cell surface expression and activity. We have examined whether ENaC is associated with Apx (apical protein Xenopus) and the spectrin-based membrane cytoskeleton in Xenopus A6 renal epithelial cells. We have also addressed whether Apx is required for the expression of amiloride-sensitive Na(+) currents by cloned ENaC. Sucrose density gradient centrifugation of A6 cell detergent extracts showed co-sedimentation of xENaC, alpha-spectrin, and Apx. Immunoblot analysis of proteins co-immunoprecipitating under high stringency conditions from peak Xenopus ENaC/Apx-containing gradient fractions indicate that ENaC, Apx, and alpha-spectrin are associated in a macromolecular complex. To examine whether Apx is required for the functional expression of ENaC, alphabetagamma mENaC cRNAs were coinjected into Xenopus oocytes with Apx sense or antisense oligodeoxynucleotides. The two-electrode voltage clamp technique showed there was a marked reduction in amiloride-sensitive current in oocytes coinjected with antisense oligonucleotides when to compared with oocytes coinjected with sense oligonucleotides. These studies indicate that ENaC is associated in a macromolecular complex with Apx and alpha-spectrin in A6 cells and suggest that Apx is required for the functional expression of ENaC in Xenopus epithelia.

Guanine nucleotide binding proteins in cultured renal epithelia: studies with pertussis toxin and aldosterone

The GTP-binding proteins from cultured A6 epithelia were examined in isolated membrane preparations. Binding of [35S]GTPgammaS revealed a class of binding sites with an apparent Kd value of 100 nM and a Bmax of 220 pmol/mg protein. Short-term aldosterone treatment of the cells did not modify the binding kinetics, whereas pertussis toxin (PTX) decreased Bmax by 50%. The mRNA levels for Galphai-3, Galpha0, Galphas, and Galphaq were not increased after aldosterone. The patterns of small Mr G proteins and of PTX-ribosylated proteins were identical in membranes of both control and aldosterone-treated cells. Cross-linking of [alpha-32P]GTP, in control membranes, showed either no labeling or a faint band of Mr 59.5 kDa. This protein became prominent after aldosterone, and its labeling decreased with spironolactone. Thus short-term aldosterone does not promote increased expression of known heterotrimeric G proteins in epithelial membranes but activates resident PTX-sensitive Gi proteins and stimulates the expression of a specific GTP-binding protein of Mr 59.5 kDa.

Immunopurification and functional reconstitution of a Na+ channel complex from rat lymphocytes

Patch-clamp experiments have demonstrated an amiloride-sensitive Na+ conductance in human B lymphoid cells. We measured whole cell currents in rat lymphocytes and observed a similar Na(+)-specific inward conductance. The presence of 400 microM 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate in the bath significantly increased the inward current, and this adenosine 3',5'-cyclic monophosphate activation was abolished by 2 microM amiloride. We immunopurified a protein complex from rat lymphocyte membranes using an anti-bovine kidney Na+ channel antibody. The complex consisted of five distinct polypeptides with apparent M(r) values of 110,000, 92,000, 59,000, 48,000, and 42,000. This putative channel complex was incorporated into planar lipid bilayers, where we observed single Na+ channel activity that was blocked by amiloride in a concentration-dependent manner. The addition of protein kinase A and ATP to the "intracellular" solution elicited a twofold increase in channel activity. Reverse transcription-polymerase chain reaction analysis was used to determine if the rat lymphocytes express the message for the recently cloned Na+ channel of the rat colon (rENaC). Primers for the alpha-subunit of rENaC identified no message in the lymphocyte RNA, while primers for the beta-subunit of the clone produced low levels of the expected product. Thus it appears that a rENaC-like beta-subunit may be an essential component of the lymphocyte Na+ channel that was isolated. At the same time, this channel is different from those recently cloned in that it does not include an alpha-subunit homologous to that of rENaC.

Effects of vasopressin and aldosterone on the lateral mobility of epithelial Na+ channels in A6 renal epithelial cells

We have previously demonstrated that apical Na+ channels in A6 renal epithelial cells are associated with spectrin-based membrane cytoskeleton proteins and that the lateral mobility of these channels, as determined by fluorescence photobleach recovery (FPR) analysis, is severely restricted by this association (Smith et al., 1991. Proc. Natl. Acad. Sci. USA 88:6971-6975). Recent data indicate that the actin component of the cytoskeleton may play a role in modulating Na+ channel activity (Cantiello et al., 1991. Am. J. Physiol. 261:C882-C888); however, it is unknown if the Na+ channel's linkage to the spectrin-based membrane cytoskeleton is also involved in regulating channel activity. In this study, we have used FPR to examine if the linkage of the Na+ channels to the membrane cytoskeleton is a site for modulation of Na+ channel activity in filter grown A6 cells by vasopressin and aldosterone. We hypothesized that if the linkage of the Na+ channels to the membrane cytoskeleton is a site for regulation of Na+ channel activity by vasopressin and aldosterone, then hormone-mediated changes in either the membrane cytoskeleton or the affinity of the Na+ channel for the membrane cytoskeleton, should be reflected in changes in the lateral mobility and/or mobile fraction of Na+ channels on the cell surface. FPR revealed that although the rates of lateral mobility were not affected, there was a twofold increase in mobility fraction (f) of apical Na+ channels in aldosterone-treated (16 hr) monolayers (f = 32.31 +/- 5.42%) when compared to control (unstimulated) (f = 14.2 +/- 0.77%) and vasopressin-treated (20 min) (f = 12.7 +/- 2.4%) monolayers. The twofold increase in mobile fraction of Na+ channels corresponds to the average increase in Na+ transport in response to aldosterone in A6 cells. The aldosterone-induced increase in Na+ transport and mobile fraction can be inhibited by the methylation inhibitor, 3-deazaadenosine, consistent with the hypothesis that a methylation event is involved in aldosterone induced upregulation of Na+ transport. We propose that the membrane cytoskeleton is involved in the aldosterone-mediated activation of epithelial Na+ channels.

The highly selective low-conductance epithelial Na channel of Xenopus laevis A6 kidney cells

In Na-reabsorbing tight epithelia, the rate-limiting step for Na transport is the highly selective low-conductance amiloride-sensitive epithelial Na channel (type 1 ENaC). In rat distal colon, type 1 ENaC is made of three homologous subunits. The aim of this study was to identify the corresponding genes of the renal channel from the kidney-derived A6 cell line of Xenopus laevis. Three homologous subunits were identified and coexpressed in the Xenopus oocyte system. The reconstituted channel had all the characteristics of the native type 1 ENaC described in A6 cells: 1) high selectivity, 2) low single-channel conductance, 3) slow gating kinetics, and 4) high affinity for amiloride. Transcripts for alpha-, beta-, and gamma-subunits of the Xenopus epithelial Na channel (xENaC) were detected in A6 kidney cells, Xenopus kidney, lung, and to a lesser extent in stomach and skin. Each subunit of the xENaC shares approximately 60% overall identity with the corresponding rat homologue (alpha, beta, and gamma rENaC). Our data suggest that the triplication of the ENaC subunits occurred before the divergence between mammalian and amphibian lineages.

Amiloride-sensitive apical membrane sodium channels of everted Ambystoma collecting tubule

Patch clamp methods were used to characterize sodium channels on the apical membrane of Ambystoma distal nephron. The apical membranes were exposed by everting and perfusing initial collecting tubules in vitro. In cell-attached patches, we observed channels whose mean inward unitary current averaged 0.39 +/- 0.05 pA (9 patches). The conductance of these channels was 4.3 +/- 0.2 pS. The unitary current approached zero at a pipette voltage of -92 mV. When clamped at the membrane potential the channel expressed a relatively high open probability (0.46). These characteristics, together with observation that doses of 0.5 to 2 microM amiloride reversibly inhibited the channel activity, are consistent with the presence of the high amiloride affinity, high sodium selectivity channel reported for rat cortical collecting tubule and cultured epithelial cell lines. We used antisodium channel antibodies to identify biochemically the epithelial sodium channels in the distal nephron of Ambystoma. Polyclonal antisodium channel antibodies generated against purified bovine renal, high amiloride affinity epithelial sodium channel specifically recognized 110, 57, and 55 kDa polypeptides in Ambystoma and localized the channels to the apical membrane of the distal nephron. A polyclonal antibody generated against a synthetic peptide corresponding to the C-terminus of Apx, a protein associated with the high amiloride affinity epithelial sodium channel expressed in A6 cells, specifically recognized a 170 kDa polypeptide. These data corroborate that the apically restricted sodium channels in Ambystoma are similar to the high amiloride affinity, sodium selective channels expressed in both A6 cells and the mammalian kidney.

Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

We have used polyclonal antibodies generated against purified bovine renal amiloride-sensitive Na+ channels to localize amiloride-sensitive Na+ channels within the lower intestine (colon and coprodeum) of the hen. These antibodies cross-reacted with two polypeptides exhibiting M(r)'s of 235 and 150 kDa on immunoblots of detergent-solubilized apical membrane fractions from both the colon and coprodeum. The apparent molecular masses of theses polypeptides are in agreement with the M(r)'s of 2 of the subunits of the renal high amiloride-affinity Na+ channel, namely the alpha and the beta (= amiloride binding) subunits. The cellular distribution of Na+ channels was determined by immunoperoxidase and indirect immunofluorescence cytochemical techniques. The apical (luminal) membrane and cytoplasm of villar principal cells in both colon and coprodeum exhibited immunoreactivity, whereas goblet cells were negative. Both principal and goblet cells of the crypts were also negative. We conclude that the amiloride-sensitive Na+ channels are localized to the principal cells of the intestinal villi and that these cells are responsible for intestinal Na+ absorption.

Primary structure of an apical protein from Xenopus laevis that participates in amiloride-sensitive sodium channel activity

High resistance epithelia express on their apical side an amiloride-sensitive sodium channel that controls sodium reabsorption. A cDNA was found to encode a 1,420-amino acid long polypeptide with no signal sequence, a putative transmembrane segment, and three predicted amphipathic alpha helices. A corresponding 5.2-kb mRNA was detected in Xenopus laevis kidney, intestine, and oocytes, with weak expression in stomach and eyes. An antibody directed against a fusion protein containing a COOH-terminus segment of the protein and an antiidiotypic antibody known to recognize the amiloride binding site of the epithelial sodium channel (Kleyman, T. R., J.-P. Kraehenbuhl, and S. A. Ernst. 1991. J. Biol. Chem. 266:3907-3915) immunoprecipitated a similar protein complex from [35S]methionine-labeled and from apically radioiodinated Xenopus laevis kidney-derived A6 cells. A single integral of 130-kD protein was recovered from samples reduced with DTT. The antibody also cross-reacted by ELISA with the putative amiloride-sensitive sodium channel isolated from A6 cells (Benos, D. J., G. Saccomani, and S. Sariban-Sohraby. 1987. J. Biol. Chem. 262:10613-10618). Although the protein is translated, cRNA injected into oocytes did not reconstitute amiloride-sensitive sodium transport, while antisense RNA or antisense oligodeoxynucleotides specific for two distinct sequences of the cloned cDNA inhibited amiloride-sensitive sodium current induced by injection of A6 cell mRNA. We propose that the cDNA encodes an apical plasma membrane protein that plays a role in the functional expression of the amiloride-sensitive epithelial sodium channel. It may represent a subunit of the Xenopus laevis sodium channel or a regulatory protein essential for sodium channel function.

Single-channel behavior of a purified epithelial Na+ channel subunit that binds amiloride

The apical membrane of high electrical resistance epithelia, which is selectively permeable to Na+, plays an essential role in the maintenance of salt balance. Na+ entry from the apical fluid into the cells is mediated by amiloride-blockable Na(+)-specific channels. The channel protein, purified from both amphibian and mammalian sources, is composed of several subunits, only one of which the 150-kDa polypeptide, specifically binds the Na+ transport inhibitor amiloride. The goal of the present study was to investigate whether the isolated amiloride-binding subunit of the channel could conduct Na+. The patch-clamp technique was used to study the 150-kDa polypeptide incorporated into a lipid bilayer formed on the tip of a glass pipette. Unitary conductance jumps averaged 4.8 pS at 100 mM Na2HPO4. Open times ranged from 24 ms to several seconds. The channel spent most of the time in the closed state. Channel conductance and gating were independent of voltage between -60 and +100 mV. Amiloride (0.1 microM) decreased the mean open time of the channel by 98%. We conclude that the 150-kDa subunit of the amiloride-blockable Na+ channel conducts current and may be sufficient for the Na+ transport function of the whole channel.

Immunocytochemical and functional characterization of Na+ conductance in adult alveolar pneumocytes

The purpose of this study was to document the existence, assess the spatial localization, and characterize some of the transport properties of proteins antigenically related to epithelial Na+ channels in freshly isolated rabbit and rat alveolar type II (ATII) cells. ATII cells, isolated by elastase digestion of lung tissue and purified by density-gradient centrifugation, were incubated with polyclonal antibodies raised against Na+ channel protein purified from beef kidney papilla (NaAb), followed by a secondary antibody (goat antirabbit immunoglobulin G conjugated to fluorescein isothiocyanate). Rat ATII cells exhibited specific staining with NaAb at the level of the plasma membrane, which, in most cells, colocalized with that of the lectin Maclura pomiferra agglutinin, an apical surface marker. In Western blots, NaAb specifically recognized a 135 +/- 10-kDa protein in rat ATII membrane vesicles. When patch clamped in the whole cell mode using symmetrical solutions (150 mM Na+ glutamate), ATII cells exhibited outwardly rectified Na+ currents that were diminished by amiloride (10-100 microM) instilled into the bath solution. Ion substitution studies showed that the conductive pathways were three times more permeable to Na+ than K+. Amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride were equally effective in diminishing 22Na+ flux into rabbit and rat ATII cells (45% inhibition at 100 microM, with IC50 of approximately 1 microM for all inhibitors). Tetraethylammonium chloride (10 mM) or BaCl2 (2 mM), well-known K+ channel blockers, had no effect on 22Na+ uptake. These results indicate that ATII cells express an amiloride-sensitive Na+ conductance, probably a channel, with a lower affinity for amiloride and its structural analogues than the well-established amiloride-sensitive Na+ channels found in bovine renal papila and cultured amphibian A6 kidney cells.

Regulation of Na+ channels in the cortical collecting duct by AVP and mineralocorticoids

A variety of experimental approaches have shown that AVP and mineralocorticoids stimulate Na+ transport through their effects on the number and kinetic properties of amiloride-sensitive Na+ channels in the apical membrane. The different mechanisms by which AVP and mineralocorticoid act on the Na+ channel provide a basis for synergism in their actions, perhaps by a scheme such as that proposed in Figure 5. However, the details of this interaction will require a better understanding of the molecular details involved in activating quiescent channels, increasing their open probability, and reorientating or inserting channels to an operational position in the apical membrane. Electrophysiological and biochemical approaches have gone a long way toward elucidating some of these molecular details. But the latter approach in particular has indicated that the Na+ channel may have multiple regulatory subunits and thus be a target for several intracellular second messengers and autacoids other than those involved in the actions of AVP and aldosterone. The challenges for future research in this area are multiple. It seems likely that the primary amino acid sequence of the channel subunits will soon become available from cloning and sequencing approaches, but the application of this knowledge to understanding how the subunits are integrated into the complete protein and mediate regulatory signals will be a formidable task. It will be important to determine the normal extracellular signals (other than aldosterone and AVP) and the associated intracellular second messengers that alter channel activity. It will also be important to understand how some species such as the rabbit may "turn off" the stimulatory effect of AVP on Na+ reabsorption in the CCD, and how this regulatory process is altered when these cells are cultured. At the whole animal level, it will also be important to investigate whether changes in one or more of the normal regulatory pathways that impinge on the Na+ channel might be involved in a diminished ability to excrete a salt load, as is observed in some models of hypertension. All of these issues need to be understood at the molecular level, and it seems likely they will provide exciting physiological insights at all levels.

Amiloride-sensitive sodium channel is linked to the cytoskeleton in renal epithelial cells

Amiloride-sensitive sodium channels are localized to the microvillar domain of apical membranes in sodium-transporting renal epithelial cells. To elucidate the elements that maintain sodium channel distribution at the apical membrane, we searched for specific proteins associating with the channel. Triton X-100 extraction of A6 epithelial cells reveals that sodium channels are associated with detergent-insoluble and assembled cytoskeleton. Indirect immunofluorescence and confocal microscopy show that sodium channels are segregated to the apical microvillar membrane and colocalize with ankyrin, fodrin, and actin. We document by immunoblot analysis that ankyrin and fodrin remain associated with sodium channels after isolation and purification from bovine renal papillae. 125I-labeled ankyrine can be precipitated by anti-sodium-channel antibodies only in the presence of purified bovine sodium-channel complex. Direct binding of 125I-labeled ankyrin shows ankyrin binds to the 150-kDa subunit of the channel. Fluorescence photobleach lateral-diffusion measurements indicate sodium channels are severely restricted in their lateral mobility. We conclude that ankyrin links the amiloride-sensitive sodium channel to the underlying cytoskeleton and this association may sequester sodium channels at apical microvilli and maintain their polarized distribution in renal epithelial cells.

Cellular and molecular aspects of the renal effects of diuretic agents

In the past few years, increased knowledge of the nature of transport proteins and their molecular regulation in the translocation of ions across kidney membranes has emerged. We are beginning to better understand the characteristics of the interaction of diuretics with these transport proteins. It is likely that this knowledge will permit further insight into nephron function regulation.

Cation transport probes: the amiloride series

The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Inhibition of colonic Na+ transport by amiloride analogues

The potency of several amiloride analogues to inhibit electrogenic Na+ transport in colon from dexamethasone-treated rats was compared. Short-circuit current (Isc) across the colonic mucosa and 22Na+ uptake into membrane vesicles derived from colonic enterocytes was determined in dexamethasone-treated rats. Kinetic analysis of inhibition of Isc and 22Na+ uptake revealed the presence of a high- and low-affinity amiloride pathway. One pathway had a high affinity [(Ki-Isc; Ki uptake] to benzamil (15.5 nM; 5.4 nM), phenamil (19.4 nM; 7.0 nM), 3',4'-dichlorobenzamil (29.0 nM; 25.2 nM), and amiloride (115 nM; 12.4 nM) but a much lower affinity to 5-(N-ethyl-N-isopropyl)amiloride (EIPA) (greater than 100 microM; greater than 9.9 microM) and 5-(N-propyl-N-butyl)-2'-4'-dichlorobenzamil (PBDCB) (greater than microM; greater than 32.8 microM). The high-affinity pathway accounted for 75-83% of the transport of Na+. The second pathway had nearly the same low affinity for each of the analogues (e.g., amiloride Ki-Isc 1 microM; Ki uptake 4 microM) and accounted for only 15-25% of the transport of Na+. The results demonstrate that the structure-inhibitory pattern of these amiloride analogues for the high-affinity pathway is the pattern observed in other electrogenic Na+-transporting epithelia and that this pharmacological profile is preserved in membrane vesicles derived from colonic enterocytes. In addition, the potency of EIPA and benzamil to inhibit electroneutral Na+ transport across the colon from normal rats (i.e., not treated with dexamethasone) was also investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of chloride channels in membrane vesicles from the kidney outer medulla

The basolateral membrane of the thick ascending loop of Henle (TALH) of the mammalian kidney is highly enriched in Na+/K+ ATPase and has been shown by electrophysiological methods to be highly conductive to Cl-. In order to study the Cl- conductive pathways, membrane vesicles were isolated from the TALH-containing region of the porcine kidney, the red outer medulla, and Cl- channel activity was determined by a 36Cl uptake assay where the uptake of the radioactive tracer is driven by the membrane potential (positive inside) generated by an outward Cl- gradient. The accumulation of 36Cl- inside the vesicles was found to be dependent on the intravesicular Cl- concentration and was abolished by clamping the membrane potential with valinomycin. The latter finding indicated the involvement of conductive pathways. Cl- channel activity was also observed using a fluorescent potential-sensitive carbocyanine dye, which detected a diffusion potential induced by an imposed inward Cl- gradient. The anion selectivity of the channels was Cl- greater than NO3- = I- much greater than gluconate. Among the Cl- transport inhibitors tested, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPAB), 4,4'-diisothiocyano-stilbene-2,2'-disulfonate (DIDS), and diphenylamine-2-carboxylate (DPC) showed IC50 of 110, 200 and 550 microM, respectively. Inhibition of 36Cl uptake by NPPAB and two other structural analogues was fully reversible, whereas that by DIDS was not. The nonreactive analogue of DIDS, 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS), was considerably less inhibitory than DIDS (25% inhibition at 200 microM). The irreversible inhibition by DIDS was prevented by NPPAB, whereas DPC was ineffective, consistent with its low inhibitory potency.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibodies against purified epithelial sodium channel protein from bovine renal papilla

Monoclonal and polyclonal antibodies against amiloride-sensitive sodium channel protein purified from bovine renal papilla have been produced. These antibodies show considerable specificity in enzyme-linked immunosorbent assay (ELISA) and dot-blot assays and can immunoprecipitate radiolabeled channel protein. The polyclonal antibodies bind two sodium channel subunits on Western blots, namely, the 300- and 110-kDa polypeptides, and cross react with channel protein isolated from the amphibian A6 renal epithelial cell line. They can be used to immunoaffinity purify the channel in relatively high yield from a crude, detergent-solubilized bovine kidney homogenate. These antibodies should be useful in isolating the sodium channel gene, in studying the channel protein structure, in studying immunocytochemical localization, and in allowing purification of the channel from other tissue sources.

Amiloride and its analogs as tools in the study of ion transport

Amiloride inhibits most plasma membrane Na+ transport systems. We have reviewed the pharmacology of inhibition of these transporters by amiloride and its analogs. Thorough studies of the Na+ channel, the Na+/H+ exchanger, and the Na+/Ca2+ exchanger, clearly show that appropriate modification of the structure of amiloride will generate analogs with increased affinity and specificity for a particular transport system. Introduction of hydrophobic substituents on the terminal nitrogen of the guanidino moiety enhances activity against the Na+ channel; whereas addition of hydrophobic (or hydrophilic) groups on the 5-amino moiety enhances activity against the Na+/H+ exchanger. Activity against the Na+/Ca2+ exchanger and Ca2+ channel is increased with hydrophobic substituents at either of these sites. Appropriate modification of amiloride has produced analogs that are several hundred-fold more active than amiloride against specific transporters. The availability of radioactive and photoactive amiloride analogs, anti-amiloride antibodies, and analogs coupled to support matrices should prove useful in future studies of amiloride-sensitive transport systems. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, as well as effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. One must consider whether the effects seen on various cellular processes are direct or due to a cascade of events triggered by an effect on an ion transport system.

Sodium transport in rat renal papillary collecting tubule cells in culture

Sodium transport in the papillary collecting duct (PCD) is poorly understood because of the inaccessibility of the distal nephron to micropuncture. Cultured rat renal papillary collecting tubule (RPCT) cells were investigated as a model for the PCD. RPCT cells have the morphologic appearance and hormonal responsiveness of the papillary collecting tubule. Sodium transport was studied using 22Na+ uptake measurements. Sodium uptake, measured at 23 degrees C in the absence of K+ and in the presence of 0.5 mM ouabain, was saturable at 100 mM extracellular NaCl, and half-maximal uptake occurred at 40 mM NaCl. The accumulation of 22Na+ appeared to be intracellular and was regulated by (Na+,K+)-ATPase activity, since activation of the Na+/K+ pump with K+ reduced 22Na+ accumulation by 90%. The time course for uptake was linear, showed only a single component, and followed first order kinetics with a t1/2 of 16 min. Amiloride and lithium inhibited 22Na+ influx, and a Dixon plot was linear, with a Ki of 16 microM amiloride. Chloride replacement of 1 mM furosemide, with or without K+, reduced uptake by only 20%. Sodium efflux from RPCT cells in the presence of ouabain showed a similar time course (t1/2, 15 min) and was also inhibited by amiloride (IC50 = 20 microM). Increased extracellular pH stimulated 22Na+ uptake and inhibited 22Na+ efflux. Addition of permeable organic acids, acetate, and bicarbonate, enhanced 22Na+ uptake. These results are consistent with Na+/H+ and Na+/Na+ exchange as mechanisms of 22Na+ uptake in the RPCT cell. This exchanger may be important in regulation of transepithelial sodium flux, maintenance of intracellular pH and cell volume, and hormonal stimulation of the papillary collecting duct.

Na+ uptake into colonic enterocyte membrane vesicles

Na+ uptake was studied in colonic enterocyte membrane vesicles prepared from normal and dexamethasone-treated rats. Vesicles from rats treated with dexamethasone demonstrated a fivefold greater 22Na+ uptake compared with vesicles from normal rats. Most of the tracer uptake in membranes derived from treated rats occurred through a conductive, amiloride-blockable pathway located in vesicles with low native K+ permeability and high Cl- permeability. Kinetic analysis of the amiloride inhibition curve revealed the presence of two amiloride-blockable pathways, one with a high affinity (Ki = 9 +/- 1.8 nM), accounting for 85% of the uptake, and one with a low affinity (Ki = 2.2 +/- 0.71 microM), accounting for only 12% of the uptake. Only the low-affinity pathway was detected with vesicles from normal rats. The high sensitivity to amiloride, the dependence on dexamethasone pretreatment, and the relative permeabilities to K+ and Cl- indicate that most of the 22Na+ uptake in membranes derived from treated rats is through a Na+-specific channel located in apical membrane vesicles. Preincubation of the isolated cells from dexamethasone-treated rats at 37 degrees C in Ca2+-free solutions before homogenization and membrane vesicle purification caused a 5- to 10-fold increase in amiloride-blockable 22Na+ uptake compared with vesicles derived from cells maintained at 0 degrees C. The addition of Ca2+, but not of Mg2+, to the incubation solution markedly reduced this temperature-dependent enhancement in 22Na+ uptake. The uptake of 22Na+ into vesicles from normal rats was unaffected by preincubation at 37 degrees C or the addition of Ca+ to the incubation solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and subunit structure of the [3H]phenamil receptor associated with the renal apical Na+ channel

Sodium crosses the apical membrane of tight epithelia through a sodium channel, which is inhibited by the diuretic amiloride and by analogs such as phenamil. Target size analysis indicated that the functional size of the [3H]phenamil binding sites associated with the epithelial Na+ channel from pig kidney is 92 +/- 10 kDa. The [3H]phenamil receptor was solubilized by using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. The solubilized material displayed the same properties of interaction with amiloride and its derivatives as the membrane-bound receptor. A two-step purification of the epithelial Na+ channel was achieved by using QAE Sephadex chromatography and affinity chromatography on a Bandeiraea simplicifolia lectin column. It results in an 1100-fold purification of the Na+ channel as compared to pig kidney microsomes with a yield of 15% +/- 5%. The maximal specific activity was 3.7 nmol/mg of protein. NaDodSO4/poly-acrylamide gel electrophoresis of the purified Na+ channel under nonreducing conditions showed the presence of a single major polypeptide chain of apparent molecular mass 185 kDa. Under disulfide-reducing conditions, the purified epithelial Na+ channel migrated as a single band of apparent molecular mass 105 kDa. It is suggested that the epithelial Na+ channel from pig kidney has a total molecular mass of 185 kDa and consists of two nearly identical 90- to 105-kDa polypeptide chains crosslinked by disulfide bridges.

Purification and characterization of the amiloride-sensitive sodium channel from A6 cultured cells and bovine renal papilla

The amiloride-binding Na+ channel protein of high electrical resistance epithelia was solubilized and purified from cultured A6 toad kidney cells and bovine renal papilla. Purification was assessed by enrichment in [3H]methylbromoamiloride specific binding. Chromatography of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS)-solubilized plasma membrane vesicles on agarose-immobilized wheat-germ agglutinin provided a 130-fold enrichment of the amiloride-binding component compared to the cell homogenate. Further purification was achieved by either amiloride-affinity chromatography or size-exclusion HPLC. When the HPLC and amiloride affinity-purified material was injected into a second higher molecular weight exclusion HPLC column, only a single peak with Mr 800,000 was found. Further HPLC separation of the Mr 800,000 material at low ionic strength resolved two peaks with apparent Mrs 800,000 and 700,000. Only the 700-kDa component displayed specific [3H]methylbromoamiloride binding activity. The final binding specific activity achieved was 1300 pmol/mg of protein, corresponding to 91% homogeneity of the protein.