Aqueous cigarette smoke extract induces a voltage-dependent inhibition of CFTR expressed in Xenopus oocytes.

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel inhabits the apical membrane of airway epithelia, where its function is essential for mucus hydration, mucociliary clearance, and airway defense. Chronic obstructive pulmonary disease (COPD), most often a consequence of cigarette smoke (CS) exposure, affects 15 million persons in the US. Clinically, COPD is characterized by many of the salient features of cystic fibrosis lung disease, where CFTR is either absent or reduced in function. CS is an acidic aerosol (pH 5.3 to 6.3) reported to contain over 4,000 constituents. Acute CS exposure has been reported to decrease airway transepithelial voltage in vivo and short-circuit current in vitro; however, the mechanistic basis of these effects is uncertain. The goal of the studies described here was to develop a bioassay to characterize the effects of aqueous CS preparations on the channel function of CFTR. We studied aqueous CS extract (CSE) prepared in our laboratory, as well as commercial cigarette smoke condensate (CSC) in Xenopus oocytes expressing human CFTR. Application of CSE at pH 5.3 produced a reversible, voltage-dependent inhibition of CFTR conductance. CSE neutralized to pH 7.3 produced less inhibition of CFTR conductance. Serial dilution of CSE revealed a dose-dependent effect at acidic and neutral pH. In contrast, CSC did not inhibit CFTR conductance in oocytes. We conclude that one or more components of CSE inhibits CFTR in a manner similar to diphenylamine-2-carboxylate, a negatively charged, open-channel blocker.

CFTR: covalent and noncovalent modification suggests a role for fixed charges in anion conduction.

The goal of the experiments described here was to explore the possible role of fixed charges in determining the conduction properties of CFTR. We focused on transmembrane segment 6 (TM6) which contains four basic residues (R334, K335, R347, and R352) that would be predicted, on the basis of their positions in the primary structure, to span TM6 from near the extracellular (R334, K335) to near the intracellular (R347, R352) end. Cysteines substituted at positions 334 and 335 were readily accessible to thiol reagents, whereas those at positions 347 and 352 were either not accessible or lacked significant functional consequences when modified. The charge at positions 334 and 335 was an important determinant of CFTR channel function. Charge changes at position 334--brought about by covalent modification of engineered cysteine residues, pH titration of cysteine and histidine residues, and amino acid substitution--produced similar effects on macroscopic conductance and the shape of the I-V plot. The effect of charge changes at position 334 on conduction properties could be described by electrodiffusion or rate-theory models in which the charge on this residue lies in an external vestibule of the pore where it functions to increase the concentration of Cl adjacent to the rate-limiting portion of the conduction path. Covalent modification of R334C CFTR increased single-channel conductance determined in detached patches, but did not alter open probability. The results are consistent with the hypothesis that in wild-type CFTR, R334 occupies a position where its charge can influence the distribution of anions near the mouth of the pore.

CFTR: covalent modification of cysteine-substituted channels expressed in Xenopus oocytes shows that activation is due to the opening of channels resident in the plasma membrane.

Some studies of CFTR imply that channel activation can be explained by an increase in open probability (P(o)), whereas others suggest that activation involves an increase in the number of CFTR channels (N) in the plasma membrane. Using two-electrode voltage clamp, we tested for changes in N associated with activation of CFTR in Xenopus oocytes using a cysteine-substituted construct (R334C CFTR) that can be modified by externally applied, impermeant thiol reagents like [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET+). Covalent modification of R334C CFTR with MTSET+ doubled the conductance and changed the I-V relation from inward rectifying to linear and was completely reversed by 2-mercaptoethanol (2-ME). Thus, labeled and unlabeled channels could be differentiated by noting the percent decrease in conductance brought about by exposure to 2-ME. When oocytes were briefly (20 s) exposed to MTSET+ before CFTR activation, the subsequently activated conductance was characteristic of labeled R334C CFTR, indicating that the entire pool of CFTR channels activated by cAMP was accessible to MTSET+. The addition of unlabeled, newly synthesized channels to the plasma membrane could be monitored on-line during the time when the rate of addition was most rapid after cRNA injection. The addition of new channels could be detected as early as 5 h after cRNA injection, occurred with a half time of approximately 24-48 h, and was disrupted by exposing oocytes to Brefeldin A, whereas activation of R334C CFTR by cAMP occurred with a half time of tens of minutes, and did not appear to involve the addition of new channels to the plasma membrane. These findings demonstrate that in Xenopus oocytes, the major mechanism of CFTR activation by cAMP is by means of an increase in the open probability of CFTR channels.

Molecular cloning and expression of an inwardly rectifying K(+) channel from bovine corneal endothelial cells.

PURPOSE: To determine the presence of a putative inwardly rectifying K(+) channel in bovine corneal endothelial (BCE) cells and to characterize its molecular and electrophysiological properties. METHODS: An RT-PCR strategy was used to clone an IRK1 channel sequence from BCE mRNA. Northern blot analysis was used to confirm expression of this sequence in cultured BCE cells. Two-electrode voltage-clamp and whole-cell patch-clamp recordings were used to characterize the cloned channel expressed in Xenopus oocytes and the native channels in cultured BCE cells, respectively. RESULTS: A full-length (1284 bp) coding sequence that shares 99.7% nucleotide sequence and 100% amino acid sequence identity to bovine lens IRK1 (Kir2.1) was cloned. The authors designate this sequence BCE IRK1 or BCIRK1. Northern blot analysis indicated that BCIRK1 mRNA is expressed in cultured BCE cells with two major transcripts of 7.5 and 5.5 kb. BCIRK1 cDNA was subcloned into the vector, pcDNA3.1(-), and cRNA transcribed from the BCIRK1 cDNA clone was injected into Xenopus oocytes. Two-electrode voltage-clamp recordings from injected oocytes revealed inwardly rectifying K(+) currents that were blocked by external Ba(2+) and Cs(+) in a concentration- and voltage-dependent manner. Whole-cell patch-clamp recordings from dissociated cultured BCE cells revealed strongly inwardly rectifying K(+) currents with similar properties. CONCLUSIONS: Corneal endothelial cells express IRK1 (Kir2.1) inwardly rectifying K(+) channels. Consistent with the properties of IRK1 channels, BCIRK1 is likely involved in regulating membrane potential and possibly other cellular functions in corneal endothelial cells.

Cystic fibrosis transmembrane conductance regulator. Physical basis for lyotropic anion selectivity patterns.

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl channel exhibits lyotropic anion selectivity. Anions that are more readily dehydrated than Cl exhibit permeability ratios (P(S)/P(Cl)) greater than unity and also bind more tightly in the channel. We compared the selectivity of CFTR to that of a synthetic anion-selective membrane [poly(vinyl chloride)-tridodecylmethylammonium chloride; PVC-TDMAC] for which the nature of the physical process that governs the anion-selective response is more readily apparent. The permeability and binding selectivity patterns of CFTR differed only by a multiplicative constant from that of the PVC-TDMAC membrane; and a continuum electrostatic model suggested that both patterns could be understood in terms of the differences in the relative stabilization of anions by water and the polarizable interior of the channel or synthetic membrane. The calculated energies of anion-channel interaction, derived from measurements of either permeability or binding, varied as a linear function of inverse ionic radius (1/r), as expected from a Born-type model of ion charging in a medium characterized by an effective dielectric constant of 19. The model predicts that large anions, like SCN, although they experience weaker interactions (relative to Cl) with water and also with the channel, are more permeant than Cl because anion-water energy is a steeper function of 1/r than is the anion-channel energy. These large anions also bind more tightly for the same reason: the reduced energy of hydration allows the net transfer energy (the well depth) to be more negative. This simple selectivity mechanism that governs permeability and binding acts to optimize the function of CFTR as a Cl filter. Anions that are smaller (more difficult to dehydrate) than Cl are energetically retarded from entering the channel, while the larger (more readily dehydrated) anions are retarded in their passage by "sticking" within the channel.

A mechanism for branchial acid excretion in marine fish: identification of multiple Na+/H+ antiporter (NHE) isoforms in gills of two seawater teleosts.

Both Na+/H+ exchange and the electrogenic extrusion of H+ via an H+-ATPase have been postulated to drive acid excretion across the branchial epithelium of fishes. While the H+-ATPase/Na+ channel system appears to be the predominant mechanism in some freshwater species, it may play a reduced role in seawater and brackish-water animals, where high external Na+ concentrations may thermodynamically favor Na+/H+ exchange driven by a Na+/H+ antiporter (NHE). In this study, we used molecular and immunological methods to assess the role of NHE isoforms in the branchial epithelium of the marine long-horned sculpin (Myoxocephalus octodecimspinosus) and the euryhaline killifish (Fundulus heteroclitus). Northern blot analysis of RNA probed with the human NHE-1 BamHI fragment suggested the presence of homologous gill NHE mRNA in sculpin. RT-PCR on gill RNA isolated from sculpin recovering from metabolic acidosis provided evidence for two distinct NHE isoforms; one with 76 % amino acid homology to mammalian NHE-2, and another 92 % homologous to trout erythrocytic beta-NHE. Killifish also have transcripts with 91 % homology to beta-NHE. Immunological detection using monoclonal antibodies for mammalian NHE-1 revealed a protein antigenically similar to this isoform in the gills of both species. Metabolic acidosis caused an approximately 30-fold decrease in expression of the NHE-1-like protein in sculpin. We speculate that beta-NHE in the gills plays the intracellular 'housekeeping' roles described for mammalian NHE-1. During systemic acidosis, apical gill NHE-2 (which is sensitive to external amiloride and low [Na+]) in parallel with a dramatic suppression of basolateral NHE-1 activity enhances net capdelta H+ transfers to the water.

CFTR: mechanism of anion conduction.

CFTR: Mechanism of Anion Conduction. Physiol. Rev. 79, Suppl.: S47-S75, 1999. - The purpose of this review is to collect together the results of recent investigations of anion conductance by the cystic fibrosis transmembrane conductance regulator along with some of the basic background that is a prerequisite for developing some physical picture of the conduction process. The review begins with an introduction to the concepts of permeability and conductance and the Nernst-Planck and rate theory models that are used to interpret these parameters. Some of the physical forces that impinge on anion conductance are considered in the context of permeability selectivity and anion binding to proteins. Probes of the conduction process are considered, particularly permeant anions that bind tightly within the pore and block anion flow. Finally, structure-function studies are reviewed in the context of some predictions for the origin of pore properties.

2 barrier-1 site pore: an interactive spreadsheet model for two permeating ions.

An interactive ion channel permeation tutorial was developed using a Microsoft Excel v5.0 spreadsheet to describe a two barrier, one site channel model by means of Eyring rate theory (ERT). The spreadsheet is inherently interactive so that the user receives immediate feedback about how changes in the energy barrier profile or ion concentrations affect current-voltage relations and single channel conductance. The spreadsheet model is easy to use, allows direct access to intermediate calculated values and all equations, contains graphical displays of parameters and lends itself to customization by a user having only a basic knowledge of spreadsheet operations.

Cystic fibrosis transmembrane conductance regulator (CFTR) anion binding as a probe of the pore.

We compared the effects of mutations in transmembrane segments (TMs) TM1, TM5, and TM6 on the conduction and activation properties of the cystic fibrosis transmembrane conductance regulator (CFTR) to determine which functional property was most sensitive to mutations and, thereby, to develop a criterion for measuring the importance of a particular residue or TM for anion conduction or activation. Anion substitution studies provided strong evidence for the binding of permeant anions in the pore. Anion binding was highly sensitive to point mutations in TM5 and TM6. Permeability ratios, in contrast, were relatively unaffected by the same mutations, so that anion binding emerged as the conduction property most sensitive to structural changes in CFTR. The relative insensitivity of permeability ratios to CFTR mutations was in accord with the notion that anion-water interactions are important determinants of permeability selectivity. By the criterion of anion binding, TM5 and TM6 were judged to be likely to contribute to the structure of the anion-selective pore, whereas TM1 was judged to be less important. Mutations in TM5 and TM6 also dramatically reduced the sensitivity of CFTR to activation by 3-isobutyl 1-methyl xanthine (IBMX), as expected if these TMs are intimately involved in the physical process that opens and closes the channel.

Muscarinic modulation of voltage-dependent Ca2+ channels in insulin-secreting HIT-T15 cells.

Potentiation of insulin secretion from pancreatic beta-cells by acetylcholine requires ongoing cyclic electrical activity initiated by other depolarizing secretagogues. Patch-clamp recordings in glucose-free solutions were made from the clonal beta-cell line HIT-T15 to determine whether the muscarinic agonist bethanechol (BCh) modulated voltage-dependent Ca2+ channels independent of effects on membrane potential. Only high-threshold, dihydropyridine-sensitive (L-type) Ca2+ channels with a mean conductance of 26 pS were observed in cell-attached patches. BCh (100 microM) caused a two- to threefold increase in both fractional open time and mean current of single Ca2+ channels. These changes resulted from a 44% decrease in the longer of two apparent mean closed times and a 25% increase in the mean open time. Similar BCh-stimulated increases in macroscopic Ca2+ currents were recorded in whole cell, perforated-patch recordings. The role of protein kinase C (PKC) in the muscarinic activation of Ca2+ channels was tested using a variety of PKC activators and inhibitors. Acute application of either the active phorbol ester phorbol 12-myristate 13-acetate (PMA) or the membrane-permeable diacylglycerol analog 1,2-didecanoyl-rac-glycerol mimicked the effects of BCh, whereas an inactive phorbol (4 alpha) had no effect. Depletion of PKC activity by chronic exposure to PMA or acute application of the PKC inhibitor staurosporine greatly reduced or abolished muscarinic activation of Ca2+ channels. These results are consistent with muscarinic activation of L-type, voltage-dependent Ca2+ channels mediated in large part by PKC.

CFTR activation: additive effects of stimulatory and inhibitory phosphorylation sites in the R domain.

To investigate the functional significance of individual consensus phosphorylation sites within the R domain of cystic fibrosis transmembrane conductance regulator (CFTR), serines were eliminated by substituting them with alanine. Included in this analysis were serine-660, -670, -686, -700, -712, -737, -768, -795, and -813, which lie within protein kinase A consensus sequences, and serine-641, which does not. Elimination of single potential phosphorylation sites altered the sensitivity of CFTR (expressed in Xenopus oocytes) to activating conditions in a manner that was highly site dependent. Substitution at serine-660, -670, -700, -795, or -813 significantly increased the half-maximal activation constant (KA) for activation by 3-isobutyl-1-methylxanthine, which is consistent with the hypothesis that phosphorylation at any of these sites promotes CFTR activation. The effect of substitution at serine-813 was significantly greater than at the other sites. In contrast, alanine substitution at serine-737 or -768 actually decreased the KA for activation, suggesting that phosphorylation at either of these sites is inhibitory. Substitution at serine-641, -686, and -712 had no significant effect on activation sensitivity. The effects of multiple serine to alanine substitutions were consistent with the notion that phosphorylation at individual sites produced roughly additive effects, suggesting that the effect produced by phosphorylation of any one serine was not dependent on the phosphorylation state of other serines. These results are consistent with the notion that, although none of the phosphorylation sites studied here are absolutely necessary for activation of CFTR, individual sites contribute differently to the gating of the channel.

Epithelial localization of a reptilian Na+/H+ exchanger homologous to NHE-1.

Basolateral membranes of turtle (Pseudemys scripta) colon epithelial cells exhibit robust Na+/H+ exchange activity that can be activated by cell shrinkage and is blocked by amiloride [M. A. Post and D. C. Dawson. Am. J. Physiol. 262 Cell Physiol. 31):C1089-C1094, 1992]. The colonic epithelium actively absorbs Na+ and secretes K+ and HCO3-, but the role of basolateral Na+/H+ exchange, if any, in transepithelial transport is unknown. The current studies were undertaken to identify the gene product(s) responsible for the observed basolateral Na+/H+ exchange activity and to determine the cellular localization of the reptilian Na+/H+ exchange protein. We cloned and sequenced partial-length cDNAs that are likely to encode a reptilian homologue of the mammalian NHE-1 Na+/H+ exchanger isoform. The partial-length cDNAs were > 80% identical to mammalian NHE-1 homologues at the nucleotide level and recognized a transcript (approximately 5.8-6.0 kb) in RNA isolated from turtle colon, small intestine, stomach, kidney, urinary bladder, heart, and liver. In situ hybridization showed that mRNA encoding the reptile homologue of NHE-1 was expressed predominantly in the epithelial cells of these tissues. Immunofluorescent localization of the reptilian Na+/H+ exchanger protein using an antibody raised against a human NHE-1 fusion protein confirmed that protein expression paralleled abundant mRNA expression in epithelial cells of turtle stomach and colon, as well as in some nephron segments, and showed that the reptile NHE-1 homologue was localized exclusively to the basolateral membranes of these cells. The relatively high level of NHE-1 expression in epithelial cells, particularly those of the colon and stomach, suggests that NHE-1 function is important for the maintenance or regulation of ion transport processes that occur in these cell types.

CFTR: the nucleotide binding folds regulate the accessibility and stability of the activated state.

The functional roles of the two nucleotide binding folds, NBF1 and NBF2, in the activation of the cystic fibrosis transmembrane conductance regulator (CFTR) were investigated by measuring the rates of activation and deactivation of CFTR Cl- conductance in Xenopus oocytes. Activation of wild-type CFTR in response to application of forskolin and 3-isobutyl-1-methylxanthine (IBMX) was described by a single exponential. Deactivation after washout of the cocktail consisted of two phases: an initial slow phase, described by a latency, and an exponential decline. Rate analysis of CFTR variants bearing analogous mutations in NBF1 and NBF2 permitted us to characterize amino acid substitutions according to their effects on the accessibility and stability of the active state. Access to the active state was very sensitive to substitutions for the invariant glycine (G551) in NBF1, where mutations to alanine (A), serine (S), or aspartic acid (D) reduced the apparent on rate by more than tenfold. The analogous substitutions in NBF2 (G1349) also reduced the on rate, by twofold to 10-fold, but substantially destabilized the active state as well, as judged by increased deactivation rates. In the putative ATP-binding pocket of either NBF, substitution of alanine, glutamine (Q), or arginine (R) for the invariant lysine (K464 or K1250) reduced the on rate similarly, by two- to fourfold. In contrast, these analogous substitutions produced opposite effects on the deactivation rate. NBF1 mutations destabilized the active state, whereas the analogous substitutions in NBF2 stabilized the active state such that activation was prolonged compared with that seen with wild-type CFTR. Substitution of asparagine (N) for a highly conserved aspartic acid (D572) in the ATP-binding pocket of NBF1 dramatically slowed the on rate and destabilized the active state. In contrast, the analogous substitution in NBF2 (D1370N) did not appreciably affect the on rate and markedly stabilized the active state. These results are consistent with a hypothesis for CFTR activation that invokes the binding and hydrolysis of ATP at NBF1 as a crucial step in activation, while at NBF2, ATP binding enhances access to the active state, but the rate of ATP hydrolysis controls the duration of the active state. The relatively slow time courses for activation and deactivation suggest that slow processes modulate ATP-dependent gating.

Missense mutation (G480C) in the CFTR gene associated with protein mislocalization but normal chloride channel activity.

We have identified a novel CFTR missense mutation associated with a protein trafficking defect in mammalian cells but normal chloride channel properties in a Xenopus oocyte assay. The mutation, a cysteine for glycine substitution at residue 480 (G480C), was detected in a pancreatic insufficient, African-American, cystic fibrosis (CF) patient. G480C was found on one additional CF chromosome and on none of 220 normal chromosomes, including 160 chromosomes from normal African-American individuals. Western blot analysis and immunofluorescence studies revealed that, in 293T cells, the encoded mutant protein was not fully glycosylated and failed to reach the plasma membrane, suggesting that the G480C protein was subject to defective intracellular processing. However, in Xenopus oocytes, a system in which mutant CFTR proteins are less likely to experience an intracellular processing/trafficking deficit, expression of G480C CFTR was associated with a chloride conductance that exhibited a sensitivity to activation by forskolin and 3-isobutyl-1-methylxanthine (IBMX) that was similar to that of wild-type CFTR. This appears to be the first identification of a CFTR mutant with a single amino acid substitution in which the sole basis for disease is mislocalization of the protein.

Coupled secretion of chloride and mucus in skin of Xenopus laevis: possible role for CFTR.

We used the isolated skin of Xenopus laevis to investigate the relationship between the secretion of salt, water, and mucus by submucosal glands expressing the cystic fibrosis transmembrane conductance regulator (CFTR). In situ hybridization and immunofluorescence provided evidence for specific expression of CFTR in the mucus-secreting cells of the subepidermal glands. Stimulation of isolated sheets of skin with 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate produced active Cl secretion and a marked increase in tissue conductance that was correlated with mucous cell degranulation and the distention of the glandular ducts. This coordinated increase in active secretion of salt and mucus was abolished by pretreatment of skins with bumetanide or by removing Cl from the bathing solutions. These results provide evidence for an intimate coupling between electrolyte transport and mucus secretion that may illuminate the pathophysiology of mucus-producing glands in cystic fibrosis lung disease.

Basolateral Na(+)-H+ antiporter. Mechanisms of electroneutral and conductive ion transport.

The basolateral Na-H antiporter of the turtle colon exhibits both conductive and electroneutral Na+ transport (Post and Dawson. 1992. American Journal of Physiology. 262:C1089-C1094). To explore the mechanism of antiporter-mediated current flow, we compared the conditions necessary to evoke conduction and exchange, and determined the kinetics of activation for both processes. Outward (cell to extracellular fluid) but not inward (extracellular fluid to cell) Na+ or Li+ gradients promoted antiporter-mediated Na+ or Li+ currents, whereas an outwardly directed proton gradient drove inward Na+ or Li+ currents. Proton gradient-driven, "counterflow" current is strong evidence for an exchange stoichiometry of > 1 Na+ or Li+ per proton. Consistent with this notion, outward Na+ and Li+ currents generated by outward Na+ or Li+ gradients displayed sigmoidal activation kinetics. Antiporter-mediated proton currents were never observed, suggesting that only a single proton was transported per turnover of the antiporter. In contrast to Na+ conduction, Na+ exchange was driven by either outwardly or inwardly directed Na+, Li+, or H+ gradients, and the activation of Na+/Na+ exchange was consistent with Michaelis-Menten kinetics (K1/2 = 5 mM). Raising the extracellular fluid Na+ or Li+ concentration, but not extracellular fluid proton concentration, inhibited antiporter-mediated conduction and activated Na+ exchange. These results are consistent with a model for the Na-H antiporter in which the binding of Na+ or Li+ to a high-affinity site gives rise to one-for-one cation exchange, but the binding of Na+ or Li+ ions to other, lower-affinity sites can give rise to a nonunity, cation exchange stoichiometry and, hence, the net translocation of charge. The relative proportion of conductive and nonconductive events is determined by the magnitude and orientation of the substrate gradient and by the serosal concentration of Na+ or Li+.

Functional roles of the nucleotide-binding folds in the activation of the cystic fibrosis transmembrane conductance regulator.

The cystic fibrosis transmembrane conductance regulator (CFTR), a member of the traffic ATPase superfamily, possesses two putative nucleotide-binding folds (NBFs). The NBFs are sufficiently similar that sequence alignment of highly conserved regions can be used to identify analogous residues in the two domains. To determine whether this structural homology is paralleled in function, we compared the activation of chloride conductance by forskolin and 3-isobutyl-1-methylxanthine in Xenopus oocytes expressing CFTRs bearing mutations in NBF1 or NBF2. Mutation of a conserved glycine in the putative linker domain in either NBF produced virtually identical changes in the sensitivity of chloride conductance to activating conditions, and mutation of this site in both NBFs produced additive effects, suggesting that in the two NBFs this region plays a similar and critical role in the activation process. In contrast, amino acid substitutions in the Walker A and B motifs, thought to form an integral part of the nucleotide-binding pockets, produced strikingly different effects in NBF1 and NBF2. Substitutions for the conserved lysine (Walker A) or aspartate (Walker B) in NBF1 resulted in a marked decrease in sensitivity to activation, whereas the same changes in NBF2 produced an increase in sensitivity. These results are consistent with a model for the activation of CFTR in which both NBF1 and NBF2 are required for normal function but in which either the nature or the exact consequences of nucleotide binding differ for the two domains.

Mercury blockade of thiazide-sensitive NaCl cotransport in flounder urinary bladder.

The effects of HgCl2 on ion transport were investigated using isolated sheets of flounder urinary bladder, a model epithelium that is capable of electrically silent NaCl absorption and electrogenic K secretion. Exposure of the mucosal surface of the bladder to submicromolar doses of HgCl2 reduced K secretion, but the effect was not due to blockade of apical K channels. Rather, the effects of HgCl2 were virtually identical to those seen with experimental maneuvers that blocked the thiazide-sensitive NaCl cotransporter in the apical membrane, e.g., hydrochlorothiazide, Cl-free solutions, and Na-free solutions. Mucosal HgCl2 also blocked 22Na absorption, suggesting that the effect of the metal was mediated by blockade of NaCl entry. The effects of HgCl2 had a rapid onset and were readily reversed by washing, suggesting a noncovalent binding reaction. The abundance of polyanionic Hg complexes in salt solutions prompts the speculation that one of these may bind to the Cl-binding site on the cotransporter, thereby blocking it. The results provide the first evidence that the thiazide-sensitive NaCl cotransporter is a specific site of action for inorganic mercury.