Amiloride-sensitive Na channels from the apical membrane of the rat cortical collecting tubule

Potassium permeable channels in primary cultures of rabbit cortical collecting tubule

Rabbit cortical collecting tubule (RCCT) primary cultures, were grown on permeable, collagen supports with 1.5 microM aldosterone. Single K+ permeable channels in principal cell apical membranes were examined. At applied patch pipette potential (Vapp) from -60 to +60 mV (cell interior with respect to pipette interior), outward currents (cell to pipette) with a unitary conductance of 8 to 10 pS were seen in cell-attached (N = 31) and excised inside-out (N = 15) patches. At resting membrane potential (Vapp = 0 mV), mean open probability (Po = 0.85 +/- 0.16) decreased by 50% with 0.75 mM luminal BaCl2 exposure. In cell-attached patches, a second type of outward current was seen only at extreme depolarization, Vapp greater than +80 mV (N = 9). Usually in the closed state (Po less than 0.0005) at no applied potential, Po for this 150 pS channel increased dramatically with depolarization and/or raising cytoplasmic Ca2+. With a calculated K+ equilibrium potential of -84 mV, excised patch reversal potentials were less than -50 mV for both the above channel types, indicating high selectivity for K+ over Na+. In cultures grown without aldosterone low conductance K+ channels were rarely observed, while mineralocorticoid status did not appear to affect high conductance K+ channel frequency. Finally, a 30 pS cation channel was found to be nonselective for K+ over Na+, and insensitive to voltage, intracellular Ca2+ or luminal Ba2+. We conclude that: 1) Principal cell apical membranes from aldosterone-stimulated, RCCT primary cultures contain (a) low conductance, Ba(2+)-inhibitable and (b) high conductance, Ca2+/voltage-dependent K+ channels; and c) nonselective cation channels. 2) The low conductance K+ channel may play an important physiologic role in native RCCT mineralocorticoid-controlled K+ secretion, while the latter two channels' functions are unknown, although similar channels have been suggested to play a role in cell volume regulation.

Sodium dependence of the epithelial sodium conductance expressed in Xenopus laevis oocytes

The epithelial Na+ conductance was expressed in Xenopus laevis oocytes by injection of size-fractionated mRNA of bovine tracheal epithelium. Fractionation was achieved by sucrose density gradient centrifugation. Successful expression was analysed by recording current/voltage (I/V) curves in the presence and absence of amiloride (10 mumol/l). The newly expressed conductance was half-maximally inhibited by 44 nmol/l amiloride and exhibited a selectivity for Na+ over K+ of 140:1. I/V curves obtained at different extracellular Na+ concentrations ([Na+]o) were subjected to a Goldman-fit analysis to obtain the relation between Na+ permeability (PNa) and [Na+]o. The data show that decreasing [Na+]o from 85 mmol/l to 0.85 mmol/l increased PNa by more than threefold, which is thought to reflect Na+ channel inhibition by increasing [Na+]o. This effect clearly exceeded what can be attributed to concentration saturation of single Na+ channel conductance (Palmer and Frindt (1986) Proc Natl Acad Sci USA 83:2767). No correlation of inhibition with intracellular Na+ concentration was observed. Preservation of the [Na+]o-dependent self-inhibition by the newly expressed Na+ conductance suggests that it is an intrinsic property of the Na+ channel protein, probably mediated by an extracellular Na+ binding site.

Effects of vasopressin and cAMP on single amiloride-blockable Na channels

To determine the mechanism by which vasopressin increases sodium transport in sodium-transporting, tight epithelia, we examined single amiloride-blockable Na channels in membrane patches from cultured distal nephron cells (A6) either before or after treatment with arginine vasopressin. Pretreatment of cells with vasopressin (40 mU/ml) for 40-50 min increases NPo (N, the number of Na channels; Po, the open probability of an individual Na channel). The increase in NPo is due to an increase in the number of conductive Na channels with little or no change in the open probability of individual Na channels. Pretreatment of cells for 1 h with 1 mM N6,2'-O-dibutyryladenosine 3', 5'-cyclic monophosphate (DBcAMP) also increased NPo. The increase in NPo caused by DBcAMP pretreatment is also due to the increase in the number of conductive Na channels with no change in the open probability of individual Na channels. Cells pretreated with cholera toxin (CTX; 250 ng/ml) for 4 h appeared similar to cells that had been treated with vasopressin or DBcAMP; that is, the number of Na channels per patch increased with little or no effect on the open probability of individual Na channels. For patches from many untreated cells, when the frequency of occurrence is plotted against the number of channels in an individual patch, the histogram consists of a single peak with a number of channels per patch of 2.0 +/- 1.5 (+/- SD, 126 patches). After pretreatment of cells with vasopressin, DBcAMP, or CTX, the same histogram contains two peaks after vasopressin of 1.8 +/- 1.2 and 9.2 +/- 1.5 (+/- SD, 38 and 53 patches, respectively). These observations suggest that pretreatment of cells with vasopressin, DBcAMP, or CTX may act by promoting insertion of clusters of new sodium channels.

Stimulation by cGMP of apical Na channels and cation channels in toad urinary bladder

The effects of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) on apical membrane cation conductances in the toad urinary bladder were investigated. 8-BrcGMP (1 mM) added to the serosal solution increased the amiloride-sensitive short-circuit current (INa) after a delay of 5 min to a steady-state value 1.8 times that of controls achieved after 30 min. Similar effects were seen when the bladders were bathed on the serosal side with a normal NaCl Ringer solution and with a high-K sucrose solution to depolarize the basolateral membrane. Under the latter conditions, the amiloride-sensitive transepithelial conductance increased in parallel with the short-circuit current, indicating stimulation of apical membrane Na channels. The threshold concentration for observing the stimulation of INa was 100 microM, 10-100 times larger than the concentration of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) required to elicit an increase in INa. Currents through an outwardly rectifying Ca-sensitive cation conductance (Iout) were also increased by 1.8-fold relative to controls. This stimulatory effect occurred after a delay of 15 min and reached maximal levels 90-120 min after addition of the nucleotide. The effects of cGMP on INa were not additive with those of 8-BrcAMP or with antidiuretic hormone, an agent known to act by increasing cAMP within the cell. Addition of 1 mM 3-isobutyl-1-methylxanthine to the serosal side of the bladders stimulated INa by 1.3-fold and Iout by 2.4-fold. In both cases, subsequent addition of cGMP produced no further activation of either conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

How do loop diuretics act?

In the thick ascending limb of the loop of Henle, NaCl reabsorption is mediated by a Na+/2Cl-/K+ cotransport system, present in the luminal membrane of this nephron segment. Loop diuretics such as furosemide (frusemide), piretanide, bumetanide and torasemide bind reversibly to this carrier protein, thus reducing or abolishing NaCl reabsorption. This leads to a decrease in interstitial hypertonicity and thus to a reduced water reabsorption. In nephron segments other than the thick ascending limb, loop diuretics have no quantitative importance with respect to their saluretic and diuretic activities. Loop diuretics also reduce Ca++ and Mg++ reabsorption in the thick ascending limb in a way which is still not clear. Furthermore, these drugs increase the urinary K+ excretion by enhancing distal tubular K+ secretion and reducing K+ reabsorption in the loop of Henle. Finally, by reduction of active NaCl transport, loop diuretics drastically reduce the substrate requirement and oxygen dependence of the thick ascending limb cells. This renders these cells, which are characterised by high transport rates and only limited substrate reserves, less vulnerable in acute renal failure.

Tonoplast ion channels from sugar beet cell suspensions : inhibition by amiloride and its analogs

The properties of the vacuolar membrane (tonoplast) ion channels of sugar beet (Beta vulgaries) cell cultures were studied using the patch-clamp technique. Tonoplast currents displayed inward rectification in the whole vacuole and isolated outside-out patch configurations and permeability ratios P(K+)/P(Na+) = 1 and P(K+)/P(Cl-) = 5. Amiloride and two of its analogs, 5-(N-methyl-N-isobutyl)-amiloride and benzamil, inhibitors of Na(+) channels in animal systems, blocked inward currents by reducing single-channel openings. Concentrations for 50% inhibition of vacuolar currents of 730 nanomolar, 130 nanomolar, and 1.5 micromolar for amiloride, benzamil, and 5-(N-methyl-N-isobutyl)-amiloride, respectively, were obtained from whole-vacuole recordings. The high inhibitory action (affinity) of amiloride and its analogs for the tonoplast cation channel suggests that these compounds could be used for the isolation and biochemical characterization of this protein.

Human kidney amiloride-binding protein: cDNA structure and functional expression

Phenamil, an analog of amiloride, is a potent blocker of the epithelial Na+ channel. It has been used to purify the porcine kidney amiloride-binding protein. Synthetic oligonucleotides derived from partial sequences have been used to screen a human kidney cDNA library and to isolate the cDNA encoding the human amiloride-binding protein. The primary structure was deduced from the DNA sequence analysis. The protein is 713 residues long, with a 19-amino acid signal peptide. The mRNA was expressed in 293-S and NIH 3T3 cells, yielding a glycoprotein (i) that binds amiloride and amiloride analogs with affinities similar to the amiloride receptor associated with the apical Na+ channel in pig kidney membranes and (ii) that is immunoprecipitated with monoclonal antibodies raised against pig kidney amiloride-binding protein.

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.

Effect of diuretics on cell potassium transport: an electron microprobe study

To study the short-term uptake of potassium across the basolateral membrane into individual tubule cells, rubidium was used and measured by electron microprobe analysis. Changes of rubidium uptake were interpreted to reflect altered sodium entry and basolateral Na-K-ATPase activity. The effects of hydrochlorothiazide, amiloride and furosemide were determined in saline-loaded animals. Hydrochlorothiazide inhibited rubidium uptake in proximal convoluted and distal convoluted tubule cells. The effect was largest in distal convoluted tubule cells. Amiloride reduced rubidium uptake in principal cells as well as in proximal convoluted, distal convoluted and connecting tubule cells. Furosemide depressed rubidium uptake in distal convoluted tubule cells, but increased uptake in principal cells. Rubidium uptake into intercalated cells was not affected by any of the diuretics used. Hydrochlorothiazide and amiloride altered rubidium uptake also in cells not associated with the main diuretic action. These effects of hydrochlorothiazide and amiloride may be due to interference with cell transport mechanisms of Na-H and anion exchange.

Blocker-related changes of channel density. Analysis of a three-state model for apical Na channels of frog skin

Blocker-induced noise analysis of apical membrane Na channels of epithelia of frog skin was carried out with the electroneutral blocker (CDPC, 6-chloro-3,5-diamino-pyrazine-2-carboxamide) that permitted determination of the changes of single-channel Na currents and channel densities with minimal inhibition of the macroscopic rates of Na transport (Baxendale, L. M., and S. I. Helman. 1986. Biophys. J. 49:160a). Experiments were designed to resolve changes of channel densities due to mass law action (and hence the kinetic scheme of blocker interaction with the Na channel) and to autoregulation of Na channel densities that occur as a consequence of inhibition of Na transport. Mass law action changes of channel densities conformed to a kinetic scheme of closed, open, and blocked states where blocker interacts predominantly if not solely with open channels. Such behavior was best observed in "pulse" protocol experiments that minimized the time of exposure to blocker and thus minimized the contribution of much longer time constant autoregulatory influences on channel densities. Analysis of data derived from pulse, staircase, and other experimental protocols using both CDPC and amiloride as noise-inducing blockers and interpreted within the context of a three-state model revealed that Na channel open probability in the absence of blocker averaged near 0.5 with a wide range among tissues between 0.1 and 0.9.

Patch-clamp profile of ion channels in resting murine B lymphocytes

Patch-clamp studies of single ion channel currents in freshly isolated murine B lymphocytes are characterized here according to their respective unitary conductances, ion selectivities, regulatory factors, distributions and kinetic behavior. The most prevalent ion channel in murine B lymphocytes is a large conductance (348 pS) nonselective anion channel. This report characterizes additional conductances including: two chloride channels (40 and 128 pS), a calcium-activated potassium channel (93 pS), and an outwardly rectifying potassium channel which displays two distinct conductances (18 and 30 pS). Like the anion channel, both chloride channels exhibit little activity in the cell-attached patch configuration. The kinetic behavior of all of these channels is complex, with variable periods of bursting and flickering activity interspersed between prolonged closed/open intervals (dwell times). It is likely that some of these channels play an important role in the signal transduction of B cell activation.

Chloride channels in the apical membrane of a distal nephron A6 cell line

In this report, single-channel recording methods were used to determine whether there are Cl- conductive pathways in the apical membrane of cultured renal distal nephron cells (A6). Two different types of single Cl- channels were observed. In cell-attached patches, one had a unit conductance of 3 pS, whereas the unit conductance of the other was 8 pS. In cell-attached patches, the currents associated with the 3-pS Cl- channel outwardly rectified, whereas the current voltage relationship for the 8-pS Cl-channel was linear. The 3-pS Cl- channel has one open and one closed state; the 8-pS Cl- channel has one open and two closed states. The open probability of the 3-pS Cl- channel was voltage dependent (increasing with depolarization of the membrane) but even at very depolarized potentials (+140 mV) remained small (always less than 0.1). On the other hand, the open probability of the 8-pS Cl- channel was large (approximately 0.8) and voltage independent. The closing rate of the 3-pS Cl- channel was decreased when the patch membrane was depolarized, whereas the opening rate was increased. In contrast, the closing rate of the 8-pS Cl- channel decreased with depolarization, but the opening rates were voltage independent. The outward rectification of the 3-pS channel was markedly reduced in inside-out patches when high calcium concentrations (10-800 microM) were present on the intracellular surface. The open probability of the 3-pS Cl- channel is increased by membrane permeable analogues of adenosine 3',5'-cyclic monophosphate primarily by decreasing the mean closed time.

[3H]phenamil binding protein of the renal epithelium Na+ channel. Purification, affinity labeling, and functional reconstitution

This paper describes a large-scale purification procedure of the amiloride binding component of the epithelium Na+ channel. [3H]Phenamil was used as a labeled ligand to follow the purification. The first two steps are identical with those previously described [Barbry, P., Chassande, O., Vigne, P., Frelin, C., Ellory, C., Cragoe, E. J., Jr., & Lazdunski, M. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 4836-4840]. A third step was a hydroxyapatite column. The purified material consisted of a homodimer of two 88-kDa proteins that migrated anomalously in SDS-PAGE to give an apparent Mr of 105,000. Deglycosylation by treatment with neuraminidase and endoglycosidase F or with neuraminidase and glycopeptidase F indicated that less than 5% of the mass of the native receptor was carbohydrate. Sedimentation analysis of the purified Na+ channel in H2O and D2O sucrose gradients and gel filtration experiments led to an estimated molecular weight of the [3H]phenamil receptor protein-detergent-phospholipid complex of 288,000 and of the native [3H]phenamil receptor protein of 158,000. [3H]Br-benzamil is another labeled derivative of amiloride that recognized binding sites that had the same pharmacological properties as [3H]phenamil binding sites and that copurified with them. Upon irradiation of kidney membranes, [3H]Br-benzamil incorporated specifically into a 185-kDa polypeptide chain under nonreducing electrophoretic conditions and a 105-kDa protein under reducing conditions. The same labeling pattern was observed at the different steps of the purification. Reconstitution of the purified phenamil receptor into large unilamellar vesicles was carried out. A low but significant phenamil- and amiloride-sensitive electrogenic Na+ transport was observed.

Mechanisms of chemosensory transduction in taste cells

The application of new techniques to the study of taste cells has revealed much about both the basic physiology of these cells and also about the mechanisms of taste transduction. The taste cells are electrically excitable cells with a variety of voltage-dependent ion currents. These ionic currents have an important role in the transduction of salt taste in mammals and frogs. In mudpuppies different ion channels are involved in the transduction of acidic-sour stimuli. The role of ion currents in the transduction of sweet taste is less clear. Some proposed mechanisms suggest an important role for ion currents and others suggest that the transduction process may be a biochemical event involving cell surface receptors and intracellular second messengers, possibly cAMP. The transduction of bitter taste seems to be a biochemical event involving cell surface receptors and intracellular second messengers in the inositol trisphosphate pathway. Thus, one cannot talk about "the mechanism" of taste transduction. Different taste modalities are transduced by different mechanisms. A corollary to this is that taste cells are not a homogeneous population of cells. In order to provide animals with the ability to discriminate between different taste modalities the taste cells consist of distinct subpopulations of cells based on their primary taste modality. The primary taste modality in a given cell is determined by the receptors and transduction mechanism(s) expressed in that cell. Evidence suggests that modality-specific receptors are expressed in a segregated manner in distinct subpopulations of taste cells. Secondary responses observed in gustatory axons may arise due to a lack of absolute specificity in the transduction processes and nonspecific effects of low pH and high ionic strength and osmolarity on the taste cells. An interesting area for future work will be to elucidate the mechanism(s) by which basal cells become committed to a given taste modality and how the gustatory neurons influence this process of differentiation. The involvement of the gustatory neurons is critical as they must synapse with taste cells of the correct taste modality to preserve the integrity of the information transferred to the CNS. This process of synaptogenesis is presumably mediated by the expression of taste-modality-specific, cell surface antigens on the basolateral domain of a taste cell and receptors on the appropriate neurons, but much work will be necessary to elucidate this process.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of aldosterone-induced increase of K+ conductance in early distal renal tubule cells of the frog

Isolated early distal tubule cells (EDC) of frog kidney were incubated for 20-28 hr in the presence of aldosterone and then whole-cell K+ currents were measured at constant intracellular pH by the whole-cell voltage-clamp technique. Aldosterone increased barium-inhibitable whole-cell K+ conductance (gK+) threefold. This effect was reduced by amiloride and totally abolished by ouabain. However, aldosterone could still raise gK- in ouabain-treated cells in the presence of furosemide. We tested whether changes in intracellular pH (pHi) could be a signal for cells to regulate gK+. After removal of aldosterone, the increase in gK+ was preserved by subsequent incubation for 8 hr at pH 7.6 but abolished at pH 6.6. In the complete absence of aldosterone, incubation of cells at pH 8.0 for 20-28 hr raised pHi and doubled gK+. Using the patch-clamp technique, three types of K+-selective channels were identified, which had conductances of 24, 45 and 59 pS. Aldosterone had no effect on the conductance or open probability (Po) of any of the three types of channels. However, the incidence of observing type II channels was increased from 4 to 22%. Type II channels were also found to be pH sensitive, Po was increased by raising pH. These results indicate that prolonged aldosterone treatment raises pHi and increases gK+ by promoting insertion of K+ channels into the cell membrane. Channel insertion is itself triggered by raising both pHi and increasing the activity of the Na+/K+ pump in early distal cells of frog kidney.

Expression of amiloride-blockable sodium channels in Xenopus oocytes

This report describes the expression of a sodium-selective, amiloride-blockable conductance in Xenopus oocytes that have been injected with RNA prepared from a distal nephron cell line (A6). After injecting the RNA into mature oocytes (stage V or VI) and incubating the oocytes for 2-4 days, the oocytes were examined for amiloride-blockable current. The RNA induced a substantial amiloride-blockable current. Uninjected or water-injected oocytes had no measurable amiloride-blockable current. RNA prepared from aldosterone-treated A6 cells was much more effective in inducing amiloride-blockable sodium current than RNA prepared from aldosterone-depleted A6 cells. Oocytes injected with RNA prepared from mineralocorticoid-depleted cells appeared very similar to water-injected oocytes. The amiloride-blockable current in oocytes has a reversal potential of approximately +50 - +60 mV, which varies 61 mV/decade change in external sodium concentration, suggesting that the current is highly selective for sodium over other ions. In addition, the concentration of amiloride that produces half block of the current is 48 +/- 8 nM. Thus the current expressed in oocytes appears very similar to sodium-selective currents observed from the apical membranes of various tight epithelial tissues.

Identification and properties of a novel type of Na+-permeable amiloride-sensitive channel in thyroid cells

Amiloride-sensitive cationic channels are present in the apical membrane of porcine thyroid cells in primary culture. An amiloride-sensitive (K0.5 = 150 +/- 28 nM where K0.5 is the concentration of unlabelled ligand which reduces the specific binding of the same labelled ligand by 50%) 22Na+-flux component (Km for Na+ at 18 mM) has been identified which was also blocked by the potent amiloride derivative phenamil (K0.5 = 47 +/- 21 nM). The most potent inhibitor of Na+/H+ exchange, ethylisopropyl-amiloride, hardly inhibited this 22Na+-influx component at a concentration of 21 microM. Amiloride binding sites were characterized using [3H]phenamil. The tritiated ligand binds to a single family of binding sites in thyroid membranes with a Kd value of 50 +/- 10 nM and a maximal binding capacity of 5 +/- 1 pmol/mg protein. Patch-clamp experiments have directly demonstrated the existence of a phenamil- and amiloride-sensitive cationic channel, with a conductance of 2.6 pS, which is permeable to sodium, but not very selective (PNa+/PK+ = 1.2). This channel is an important element in the regulation of the resting membrane potential of thyroid cells.

The biology of amiloride-sensitive sodium channels

Studies of the molecular nature of epithelial sodium channels are in their infancy and have largely involved experiments in which the interaction between amiloride and this transport process has been examined. Because of the inherent geometric complexity of epithelial tissues, these studies have in large measure been macroscopic in nature, with the molecular details of transport being deduced. In the past five years, however, the molecular biology of these critical ion channels has been studied directly. The development of radioactive high-affinity probes, the application of patch-clamp and reconstitution techniques, the generation of specific antibodies, and the formulation of epithelial cDNA expression libraries have propelled the field of epithelial ion channels into a new era. Now, for the first time, we can rigorously address questions concerning the molecular nature of the amiloride block, the channel's selectivity to alkali metal cations, and the modulation of ion transport through this channel by other ions (such as calcium), hormones (such as vasopressin, aldosterone, and atrial natriuretic factor), or intracellular second messengers (such as cAMP or cGMP). The complexity of the epithelial sodium channel's structure may reflect the constitutive and regulatory role this protein plays in sodium homeostasis. The epithelial sodium channel is continually operating, constantly changing its activity on a second-to-second basis. Hence, its tonic functions are probably modulated by a myriad of factors, most of which are unknown. With the application of molecular techniques, a much clearer understanding of the nature and regulation of epithelial sodium channel processes in health and disease will emerge in the years to come.

Characterization of a partially degraded Na+ channel from urinary tract epithelium

The mammalian urinary bladder contains in its apical membrane and cytoplasmic vesicles, a cation-selective channel or activating fragment which seems to partition between the apical membrane and the luminal (or vesicular space). To determine whether it is an activating fragment or whole channel, we first demonstrate that solution known to contain this moiety can be concentrated and when added back to the bladder causes a conductance increase, with a percent recovery of 139 +/- 25%. Next, we show that using tip-dip bilayer techniques (at 21 degrees C) and a patch-clamp recorder, the addition of concentrated solution resulted in the appearance of discrete current shots, consistent with the incorporation of a channel (as opposed to an activating fragment) into the bilayer. The residency time of the channel in the bilayer was best described by the sum of two exponentials, suggesting that the appearance of the channel involves an association of the channel with the membrane before insertion. The channel is cation selective and more conductive to K+ than Na+ (by a factor of 1.6). It has a linear I-V relationship, but a single-channel conductance that saturates as KCl concentration is raised. This saturation is best described by the Michaelis-Menten equation with a Km of 160 mM KCl and a Gmax of 20 pS. The kinetics of the channel are complex, showing at least two open and two closed states.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-channel recordings from the apical membrane of the toad urinary bladder epithelial cell

The patch-clamp technique for the recording of single-channel currents was used to investigate the activity of ion channels in the intact epithelium of the toad urinary bladder. High resistance seals were obtained from the apical membrane of tightly stretched tissue. Single-channel recordings revealed the activity of a variety of ion channels that could be classified in 4 groups according to their mean ion conductances, ranging from 5 to 59 pS. In particular, we observed highly selective, amiloride-sensitive Na channels with a mean conductance of 4.8 pS, channels with a similar conductance that were not Na-selective and channels with mean conductance values of 17-58 pS that were mostly seen after stimulation of the tissue with vasopressin or cAMP. When inside-out patches from the apical membrane were exposed to 110 mM fluoride, large conductances (86-490 pS) appeared.

Amiloride-blockable sodium currents in isolated taste receptor cells

Isolated taste receptor cells from the frog tongue were investigated under whole-cell patch-clamp conditions. With the cytosolic potential held at -80 mV, more than 50% of the cells had a stationary inward Na current of 10 to 700 pA in Ringer's solution. This current was in some cells partially, in others completely, blockable by low concentrations of amiloride. With 110 mM Na in the external and 10 mM Na in the internal solution, the inhibition constant of amiloride was (at -80 mV) near 0.3 microM. In some cells the amiloride-sensitive conductance was Na specific; in others it passed both Na and K. The Na/K selectivity (estimated from reversal potentials) varied between 1 and 100. The blockability by small concentrations of amiloride resembled that of channels found in some Na-absorbing epithelia, but the channels of taste cells showed a surprisingly large range of ionic specificities. Receptor cells, which in situ express these channels in their apical membrane, may be competent to detect the taste quality "salty." The same cells also express TTX-blockable voltage-gated Na channels.

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-independent in vitro induction of Na+,K+-ATPase by aldosterone in renal target cells: permissive effect of triiodothyronine

The aim of this study was to develop an in vitro system in which we could study the causal relationship between short-term stimulation of Na+,K+-ATPase in the collecting tubule by aldosterone on the one hand and protein synthesis and changes in intracellular Na+ concentration on the other hand. Previous in vivo studies suggested that triiodothyronine might facilitate aldosterone-induced stimulation of Na+,K+-ATPase. Results show that when segments of cortical collecting tubules microdissected from collagenase-treated kidneys of adrenalectomized rats were incubated for 3 hr in the presence of either 10(-8) M aldosterone or 10(-8) M triiodothyronine alone Na+,K+-ATPase activity was not altered, whereas the addition of both hormones markedly stimulated the activity and the number of catalytic sites of Na+,K+-ATPase. This stimulation was abolished by actinomycin D and cycloheximide, whereas it was not altered in the absence of extracellular sodium or in the presence of the luminal Na+-channel blocker amiloride. Thus, triiodothyronine facilitates the in vitro induction of Na+,K+-ATPase synthesis by aldosterone. Aldosterone action on Na+,K+-ATPase is independent of Na+ availability.

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.

Ion selectivity of epithelial Na channels

Epithelial Na channels are apparently pore-forming membrane proteins which conduct Na much better than any other biologically abundant ion. The conductance to Na can be 100 to 1000 times higher than that to K. The only other ions that can readily get through this channel are protons and Li. Small organic cations cannot pass through the channel, and water may also be impermeant. The selectivity properties of epithelial Na channels appear to be determined by at least three factors: A high field-strength anionic site, most likely a carboxyl residue of glutamic or aspartic acid residues on the channel protein, probably accounts for the high conductance through these channels of Na and Li and to the low conductance of K, Rb and Cs. A restriction in the size of the pore at its narrowest point probably accounts for the low conductance of organic cations as well as the possible exclusion of water molecules. The outer mouth of the channel appears to be negatively charged and may control access to the region of highest selectivity and may serve as a preliminary selectivity filter, attracting cations over anions. These conclusions are illustrated by the cartoon of the channel in Fig. 3. This picture is obviously both fanciful and simplified, but its general points will hopefully be testable. It leaves open a number of important questions, including: does amiloride block the channel by binding within the outer mouth? what does the inner mouth of the channel look like, and does this part of the channel contribute to selectivity? and what, if any, are the interactions between the features of the channel that impart selectivity and those that control the regulation of the channel by hormonal and other factors?