Properties of an anion-selective channel from rat colonic enterocyte plasma membranes reconstituted into planar phospholipid bilayers

Presence of the nucleic acid channel in renal brush-border membranes: allosteric modulation by extracellular calcium

We have previously described a cell surface channel complex that is highly selective for nucleic acid ( 6 , 7 ). The channel complex was purified to homogeneity by solubilizing renal brush-border membranes (BBM) with CHAPS and separation by liquid chromatography. It was characterized by reconstitution in planar lipid bilayers. The channel consists of a pore-forming subunit that is blocked by heparan sulfate and a regulatory subunit that is blocked by l-malate ( 7 ). The current studies were performed to compare the characteristics of the nucleic acid-conducting channel in native BBM with the characteristics that have been determined for the complex reconstituted from purified proteins. BBM were purified by differential centrifugation and reconstituted in lipid bilayers. Current was not observed until oligodeoxynucleotide (ODN) was added. Conductance was 9.1 ± 0.9 pS; rectification and voltage dependence were not observed. Reversal potential ( Erev) shifted to +14 ± 0.1 mV by a 10-fold gradient for ODN but was not altered when gradients were created for any other ion. Open probability increased significantly with an increase in Ca2+on the trans chamber of the bilayer apparatus. Changes in cis Ca2+were without effect. Addition of l-malate to the cis chamber or heparan sulfate to the trans chamber significantly reduced the open probability of the channel. These data demonstrate that the nucleic acid channel in BBM is electrophysiologically and pharmacologically identical to that previously reported for purified protein and demonstrate that a nucleic acid-conducting channel is a component of renal BBM.

Basolateral outward rectifier chloride channel in isolated crypts of mouse colon

Single channel patch-clamp techniques were used to demonstrate the presence of outwardly rectifying chloride channels in the basolateral membrane of crypt cells from mouse distal colon. These channels were rarely observed in the cell-attached mode and, in the inside-out configuration, only became active after a delay and depolarizing voltage steps. Single channel conductance was 23.4 pS between -100 and -40 mV and increased to 90.2 pS between 40 and 100 mV. The channel permeability sequence for anions was: I(-) > SCN(-) > Br(-) > Cl(-) > NO(3)(-) > F(-)>> SO(4)(2-) approximately gluconate. In inside-out patches, the channel open probability was voltage dependent but insensitive to intracellular Ca(2+) concentration. In cell-attached mode, forskolin, histamine, carbachol, A-23187, and activators of protein kinase C all failed to activate the channel, and activity could not be evoked in inside-out patches by exposure to the purified catalytic subunit of cAMP-dependent protein kinase A. The channel was inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate, 9-anthracenecarboxylic acid, and DIDS. Stimulation of G proteins with guanosine 5'-O-(3-thiotriphosphate) decreased the channel open probability and conductance, whereas subsequent addition of guanosine 5'-O-(2-thiodiphosphate) reactivated the channel.

Permeability of wild-type and mutant cystic fibrosis transmembrane conductance regulator chloride channels to polyatomic anions

Permeability of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel to polyatomic anions of known dimensions was studied in stably transfected Chinese hamster ovary cells by using the patch clamp technique. Biionic reversal potentials measured with external polyatomic anions gave the permeability ratio (P/P) sequence NO > Cl > HCO > formate > acetate. The same selectivity sequence but somewhat higher permeability ratios were obtained when anions were tested from the cytoplasmic side. Pyruvate, propanoate, methane sulfonate, ethane sulfonate, and gluconate were not measurably permeant (P/P < 0.06) from either side of the membrane. The relationship between permeability ratios from the outside and ionic diameters suggests a minimum functional pore diameter of approximately 5.3 A. Permeability ratios also followed a lyotropic sequence, suggesting that permeability is dependent on ionic hydration energies. Site-directed mutagenesis of two adjacent threonines in TM6 to smaller, less polar alanines led to a significant (24%) increase in single channel conductance and elevated permeability to several large anions, suggesting that these residues do not strongly bind permeating anions, but may contribute to the narrowest part of the pore.

HCO3-dependent pHi regulation in tracheal epithelial cells

Regulation of intracellular pH (pHi) was studied in cultured bovine tracheal epithelial cells using microspectrofluorimetry of the fluorescent indicator 2',7'-biscarboxyethyl- 5(6)-carboxyfluorescein (BCECF). The cells, which were grown on coverslips and superfused in a chamber on the stage of a microscope, were acidified by NH4Cl-prepulses, and pHi recovery was measured (in DeltapH/min) at approximately pHi 6.7. In HCO3-free solutions the recovery rate was 0.14 pH/min, and addition of amiloride or Na-free solution reduced this rate to 0.02-0.03 pH/min. In HCO3/CO2-buffered Ringer's, the rate of recovery was 0.32 pH/min, and amiloride or Na-free reduced the rate to 0.08-0.10 pH/min. This residual Na-independent and HCO3-dependent pHi recovery was studied by using inhibitors of HCO3 and H transporters. Bafilomycin (inhibits H-ATPases) at 100 nM did not significantly affect pHi recovery, while 100 microM SCH28080 (inhibits H,K-ATPase) had a variable inhibitory effect (25-75%), indicating that a gastric-like H, K-ATPase, but not electrogenic H pump, may contribute in a minor way to the recovery from acidification. Cl-free solution and 500 microM H2DIDS (dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, blocks anion exchange and the outwardly rectifying Cl channel, ORCC), both blocked apparent anion exchange activity, but had no effect on the recovery; 100 microM DNDS (4-4''-dinitro-2-2'-stilbenedisulfonate blocks the ORCC but not the cystic fibrosis transmembrane conductance regulator, CFTR) had no effect on pHi recovery; DPC (diphenylamine carboxylate, blocks the CFTR and the ORCC) caused a complete and reversible inhibition of the recovery. When [K] was increased ten fold to depolarize the cell's membrane potential, the magnitude of the pHi recovery (though not the rate) was enhanced. Thus, the HCO3-dependent, Na- and Cl-independent, DPC-blockable pHi recovery may be largely due to an influx of HCO3 via CFTR Cl channels. Under physiological conditions, when the electrochemical gradient for HCO3 is likely to be outwardly rather than inwardly directed, the CFTR (or another HCO3-permeable channel) may mediate HCO3 secretion and contribute to regulation of pH of the periciliary fluid.

The effect of neuropeptide Y on sodium, chloride and potassium transport across the rat distal colon

1. Neuropeptide Y (NPY; 10(-10)-10(-7) mol l-1) reduced basal short-circuit current (Isc) in a concentration-dependent manner in the rat distal colon but was ineffective in the proximal colon. 2. The action of NPY was dependent upon the presence of Cl- and HCO3- anions and was blocked by prior treatment of the tissue with a Cl- channel blocker. The decrease in Isc was associated with an increase in mucosa-to-serosa fluxes of Na+, Rb+ (K+) and Cl-, whereas the serosa-to-mucosa flux of Cl- was decreased. 3. The size of the inhibitory NPY effect was linearly correlated with the height of the basal Isc, i.e. it inhibited 55% of basal secretory Isc. 4. The action of NPY was unaffected by indomethacin and tetrodotoxin, when given alone, but was abolished, when the basal Isc was decreased to values near zero by a combination of both inhibitors. This inhibition could be overcome by restoring basal Isc with prostaglandin E2, indicating that the effect of NPY is not mediated by nerves or prostaglandins, but that NPY is only effective, when anion secretion is stimulated by the spontaneous release of neurotransmitters and prostaglandins. 5. NPY inhibited the increase in Isc induced by veratridine and prostaglandin E2, but it had no effect on the Isc induced by direct stimulation of the adenylate cyclase with forskolin, or on Isc induced by stimulation of the Ca(2+)-pathway with carbachol. Inhibition of the response to veratridine or prostaglandin E2 by NPY showed the same dependence on the height of the ISC just prior to addition of NPY as seen in control conditions, i.e. NPY inhibited 55% of cyclic AMP-mediated secretion.6. These results suggest that the effect of NPY is mediated by an inhibition of cyclic AMP-stimulated secretion, which is stimulated in the rat distal colon by a continuous release of prostaglandins and neurotransmitters.

Stable and functional expression of the CIC-3 chloride channel in somatic cell lines

The CIC family is the superfamily of voltage-gated Cl- channels. Although the CIC channels expressed in Xenopus oocytes have been characterized, their channel properties are still poorly understood. We recently cloned a unique member of the CIC family, CIC-3, that is expressed abundantly in neurons. Its channel activity was regulated by phorbol esters. Now, we have established a stably transfected somatic cell line expressing functional CIC-3 channels and examined the CIC-3 single-channel current by patch-clamp techniques. In inside-out patches from the stably transfected cells, a rise of bath Ca2+ concentration in the physiological range of intracellular Ca2+ concentrations inhibited the CIC-3 single-channel currents. This inhibition by Ca2+ was independent of phosphorylation and ATP. Thus, the CIC-3 channel is a Ca(2+)-sensitive Cl- channel localized in neuronal cells, and its Ca2+ sensitivity implies a physiological role in neuronal functions.

Characterization of carbohydrate binding proteins in Trypanosoma cruzi

Trypanosoma cruzi is an obligatory intracellular protozoan parasite that causes Chagas' disease in humans and invades a great variety of mammalian cells. The nature of the ligand(s) and receptor components in both T. cruzi and target cells remains controversial, although it seems to involve an interaction with oligosaccharides. In an attempt to identify possible ligands on the parasite, we have searched for the presence of carbohydrate binding proteins (CBPs) in T. cruzi. By fluorescence-activated cell sorter analysis using a panel of fluoresceinated glyco- and neoglycopeptides with well characterized glycans, the presence of at least two different CBPs was identified on the surface of T. cruzi epimastigotes and trypomastigotes. The specificity of binding of the two CBPs seems to be mediated by galactose and mannose residues. The mannose- and galactose-mediated CBPs from epimastigotes and trypomastigotes were purified to homogeneity by affinity chromatography on immobilized thyroglobulin and identified as 60-70-kDa glycoproteins. Purified CBPs were able to specifically bind with high affinity to murine and human macrophages as well as other cell types susceptible to infection by T. cruzi but not to fat or neuronal cells. This binding was inhibited by the corresponding ligands. Moreover, the mannose-mediated CBP binding was completely abolished by alpha-mannosidase treatment of the cells. These results suggest a possible role for the CBPs in the recognition events between the parasite and target cells and/or in the interaction of the epimastigotes with the insect gut cells.

Parathyroid hormone inhibits plasma membrane Pi transport without changing endocytic activity in opossum kidney cells

Parathyroid hormone (PTH) inhibits Na(+)-dependent Pi uptake in renal epithelial cells from opossum kidney (OK). This requires an intact endocytic pathway, suggesting that one action of PTH may be to promote endocytic removal of Na+/Pi cotransporters from the cell membrane. The present study tested if PTH, at a dose that inhibited membrane Pi transport, also produced an increase in endocytic activity. Pi transport was measured in isolated plasma membrane vesicles. Endocytosis was measured by allowing cells to take up horseradish peroxidase (HRP) followed by assay of triton-sensitive (latent) HRP activity in subcellular fractions isolated by density gradient centrifugation. Incubation of OK cells with 10(-7) M PTH for 3 h decreased Na+/Pi cotransport by membrane vesicles to 328 +/- 54 pmol/mg/min compared to 448 +/- 67 pmol/mg/min (mean +/- S.E., P < 0.03) in controls. Latent HRP content of endosomal fractions was dependent on the time and temperature used to load cells with HRP and on the concentration of HRP. However, incubation of OK cells with 10(-7) M PTH for either 1 or 3 h produced no change in latent HRP activity. Thus the action of PTH on the Na+/Pi cotransporter in the plasma membrane of OK cells does not require a change in the rate of endocytosis.

Comparison of -nitro versus -amino 4,4'-substituents of disulfonic stilbenes as chloride channel blockers

We showed previously that the disulfonic stilbene DNDS (4,4'-dinitrostilben-2,2'-disulfonic acid) was a potent blocker of outwardly rectifying chloride channels (ORCC). The studies reported here were designed to quantify the relationship between electron withdrawal by the 4,4'-substituents and blocker potency. Specifically we compared the blocking effects and molecular properties of the symmetrically substituted 4,4'-diaminostilben-2, 2'-disulfonic acid (DADS) and the hemi-substituted 4-amino, 4'-nitrostilben-2,2'-disulfonic acid (ANDS) with those of DNDS. Blockade was studied using outwardly rectifying colonic chloride channels incorporated into planar lipid bilayers. DADS was 430-fold and ANDS 44-fold less potent than DNDS as blockers of ORCC. Amplitude distribution analysis revealed that all three disulfonic stilbenes act as open channel blockers. Furthermore, this kinetic analysis indicated that the lower potency of DADS and ANDS was due to an increase in off rate. These results support the conclusion that the 4,4'-substituents make an important contribution to blockade by stabilizing the channel-blocker complex. Isopotential electron contour maps illustrated the dramatic shift in charge at the 4,4'-poles of the disulfonic stilbene molecule from electronegative in DNDS to electropositive in DADS as well as the bipolar contour of ANDS. Thus, the greater potency of DNDS results from the symmetric electronegative regions at the 4,4'-poles of the molecule. We hypothesize that the channel protein has two corresponding electropositive areas at the blocker binding site.

A chloride channel reconstituted from fetal rat brain growth cones

Chloride channels were reconstituted into planar lipid bilayers isolated from a preparation of growth cone particles (GCPs) isolated from fetal rat brain. One type of channel was predominantly seen and some of its biophysical and pharmacological properties were studied. The single channel i-V relationship was curvilinear with a chord conductance of 75 pS at +30 mV in symmetric 200 mM NaCl solutions buffered with phosphate. The channel was inactivated by depolarization, and this inactivation was reversed rapidly upon returning to -25 mV. The Cl- channel was significantly permeant to Na+ ions (PNa/PCl = 0.26), and the relative halide permeabilities were determined to be: I(1.92) > Br(1.73) > Cl(1.0) > F(0.34). The channel was inhibited by the common stilbene compounds (DIDS, SITS, DNDS), as well as by Zn2+ ions and an indanyloxyacetic acid derivative. A developmental role for the GCP Cl- channel is suggested by the observation that adult rat brain synaptosomal membranes were nearly devoid of this type of Cl- channel.

Expression and polarized targeting of a rab3 isoform in epithelial cells

Pathways of polarized membrane traffic in epithelial tissues serve a variety of functions, including the generation of epithelial polarity and the regulation of vectorial transport. We have identified a candidate regulator of polarized membrane traffic in epithelial cells (i.e., rab3B), which is a member of the rab family of membrane traffic regulators. Rab3B is highly homologous to a brain-specific rab3 isoform (rab3A) that targets in a polarized fashion to the presynaptic nerve terminal, where it probably regulates exocytosis. The coding region for human rab3B was cloned from epithelial mRNA using a reverse-transcription polymerase chain reaction strategy. This cDNA clone hybridized to a single mRNA species in Northern blots of poly(A)+ RNA isolated from epithelial cell lines. A rab3B-specific antibody that was raised against recombinant fusion protein recognized a 25-kD band in immunoblots of cell lysates prepared from cultured epithelial cells (e.g., T84 and HT29-CL19A), but not from a variety of nonepithelial cells (e.g., PC12 neuroendocrine cells). Immunofluorescence analysis confirmed that rab3B protein is preferentially expressed in cultured epithelial cells as well as in a number of native epithelial tissues, including liver, small intestine, colon, and distal nephron. Rab3B localized to the apical pole very near the tight junctions between adjacent epithelial cells within all of these cell lines and native epithelial tissues, as determined by immunofluorescence and immunoelectron microscopic analysis. Moreover, this pattern of intracellular targeting was regulated by cell contact; namely, rab3B was reversibly retrieved from the cell periphery as epithelial cell contact was inhibited by reducing the extracellular Ca2+ concentration. Our results indicate that neurons and epithelial cells express homologous rab3 isoforms that target in a polarized fashion within their respective tissues. The pattern and regulation of rab3B targeting in epithelial cells implicates this monomeric GTPase as a candidate regulator of apical and/or junctional protein traffic in epithelial tissues.

Calcium-dependent chloride channels in endosomes from rabbit kidney cortex

Ion channels in endosomal membranes from rabbit kidney cortex were studied after reconstitution into planar lipid bilayers. The most frequently observed ion channel was anion selective (PCl/PK = 13) and had a single-channel conductance of 116 pS when the cis and trans solutions contained 410 and 150 mM KCl, respectively, and a conductance of 90 pS in symmetrical 150 mM KCl solutions. The anion selectivity sequence of the channel was NO3- > F- > Br- > Cl- >> I-. The activity of the channel was voltage dependent such that hyperpolarization of the cis, or cytoplasmic, surface of the channel increased the open probability (Po). The activity of the channel was also highly dependent on the calcium activity of the cis but not the trans solution. Channels were fully active (Po > 0.7) at Ca2+ concentration > 1 microM, but channel activity was completely absent (Po < 0.001) at Ca2+ concentration < 250 nM. The effects of calcium on Po were not voltage dependent. The Cl(-)-channel blocker 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H-inden -5- yl)oxy]-acetic acid (IAA-94/95) produced a concentration-dependent reversible flickering block of the endosomal channel with a Ki of 15 microM. 4,4'-Dinitrostilbene-2,2'-disulfonic acid, a disulfonic stilbene, also produced a flickering block of the channel with a Ki of approximately 5 microM. Because endosomal Cl- channels are believed to facilitate endosomal acidification, we tested the effects of IAA-94/95 and deletion of Ca2+ on the rate of acidification of intact endosomes. Because neither maneuver affected acidification, we conclude that the 116-pS channel does not participate in endosomal acidification. This channel may be involved in other endosomal processes, e.g., cell volume regulation and control of membrane trafficking.

Anion channels in the sea urchin sperm plasma membrane

Ionic fluxes in sea urchin sperm plasma membrane regulate cell motility and the acrosome reaction (AR). Although cationic channels mediate some of the ionic movements, little is known about anion channels in these cells. The fusion of sperm plasma membranes into lipid bilayers allowed identification of a 150 pS anion channel. This anion channel was enriched from detergent-solubilized sperm plasma membranes using a wheat germ agglutinin Sepharose column. Vesicles formed from this preparation were fused into black lipid membranes (BLM), yielding single channel anion-selective activity with the properties of those found in the sperm membranes. The following anion selectivity sequence was found: NO3- > CNS- > Br- > Cl-. This anion channel has a high open probability at the holding potentials tested, it is partially blocked by 4,4'-diisothiocyano-2,2'-stilbendisulfonic acid (DIDS), and it often displays substates. The sperm AR was also inhibited by DIDS.

Trinitrophenyl-ATP blocks colonic Cl- channels in planar phospholipid bilayers. Evidence for two nucleotide binding sites

Outwardly rectifying 30-50-pS Cl- channels mediate cell volume regulation and transepithelial transport. Several recent reports indicate that rectifying Cl- channels are blocked after addition of ATP to the extracellular bath (Alton, E. W. F. W., S. D. Manning, P. J. Schlatter, D. M. Geddes, and A. J. Williams. 1991. Journal of Physiology. 443:137-159; Paulmichl, M., Y. Li, K. Wickman, M. Ackerman, E. Peralta, and D. Clapham. 1992. Nature. 356:238-241). Therefore, we decided to conduct a more detailed study of the ATP binding site using a higher affinity probe. We tested the ATP derivative, 2',3',O-(2,4,6-trinitrocyclohexadienylidene) adenosine 5'-triphosphate (TNP-ATP), which has a high affinity for certain nucleotide binding sites. Here we report that TNP-ATP blocked colonic Cl- channels when added to either bath and that blockade was consistent with the closed-open-blocked kinetic model. The TNP-ATP concentration required for a 50% decrease in open probability was 0.27 microM from the extracellular (cis) side and 20 microM from the cytoplasmic (trans) side. Comparison of the off rate constants revealed that TNP-ATP remained bound 28 times longer when added to the extracellular side compared with the cytoplasmic side. We performed competition studies to determine if TNP-ATP binds to the same sites as ATP. Addition of ATP to the same bath containing TNP-ATP reduced channel amplitude and increased the time the channel spent in the open and fast-blocked states (i.e., burst duration). This is the result expected if TNP-ATP and ATP compete for block, presumably by binding to common sites. In contrast, addition of ATP to the bath opposite to the side containing TNP-ATP reduced amplitude but did not alter burst duration. This is the result expected if opposite-sided TNP-ATP and ATP bind to different sites. In summary, we have identified an ATP derivative that has a nearly 10-fold higher affinity for reconstituted rectifying colonic Cl- channels than any previously reported blocker (Singh, A. K., G. B. Afink, C. J. Venglarik, R. Wang, and R. J. Bridges. 1991. American Journal of Physiology. 260 [Cell Physiology. 30]:C51-C63). Thus, TNP-ATP should be useful in future studies of ion channel nucleotide binding sites and possibly in preliminary steps of ion channel protein purification. In addition, we have obtained good evidence that there are at least two nucleotide binding sites located on opposite sides of the colonic Cl- channel and that occupancy of either site produces a blocked state.

Purification and characterization of chlorotoxin, a chloride channel ligand from the venom of the scorpion

We have previously demonstrated that the venom of the scorpion Leiurus quinquestriatus blocks small-conductance Cl- channels, derived from epithelial cells, when applied to the cytoplasmic surface. We have now purified to near homogeneity, and characterized, the component responsible for this blocking activity. It is a small basic peptide of 4,070 Da. The primary amino acid structure shows considerable homology to a class of previously described putative short insectotoxins. A brief characterization of the kinetics of Cl- channel block as well as a demonstration of toxicity to arthropods is also presented.

Prostaglandin- and theophylline-induced C1 secretion in rat distal colon is inhibited by microtubule inhibitors

The aim of the present study was to examine the possible role of microtubules in chloride secretion by distal rat colon stimulated by prostaglandin (PGE2) and theophylline. Distal colonic tissue from male rats was mounted in Ussing chambers, and short-circuit current (Isc) was measured to assess chloride secretion. Three microtubule inhibitors, colchicine, nocodazole, and taxol, all inhibited the stimulated Isc and reduced the 60-min integrated secretory response to PGE2 and theophylline (integral of Iscdt) by 39-52%, whereas the inactive colchicine analog lumicolchicine did not. Atropine and tetrodotoxin had no effect on stimulated chloride secretion. To confirm the source of Isc, unidirectional 22Na+ and 36Cl- fluxes were measured in tissues exposed to lumicolchicine (control) or colchicine. Control tissues absorbed both chloride [5.0 (1.1-8.6) (median and 95% confidence interval) mueq/cm2/hr] and sodium [2.8 (0.9-7.2) mueq/cm2/hr], and this net absorption was reduced by 96% and 79%, respectively, by treatment with PGE2 and theophylline due to an increase in serosal-to-mucosal chloride and sodium movement. Colchicine-treated tissues exhibited similar net basal chloride and sodium absorption that was reduced by 71% and 75%, respectively, by treatment with PGE2 and theophylline. Thus the PGE2- and theophylline-induced increase in chloride secretion was significantly reduced by colchicine (P < 0.05 by Wilcoxon rank-sum test), whereas colchicine had no effect on PGE2- and theophylline-induced changes in sodium fluxes. Furthermore, the colchicine-related changes in stimulated chloride secretion were numerically similar to colchicine-related changes in stimulated Isc.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and functional reconstitution of the cystic fibrosis transmembrane conductance regulator (CFTR)

Circumstantial evidence has accumulated suggesting that CFTR is a regulated low-conductance Cl- channel. To test this postulate directly, we have purified to homogeneity a recombinant CFTR protein from a high-level baculovirus-infected insect cell line. Evidence of purity included one- and two-dimensional gel electrophoresis, N-terminal peptide sequence, and quantitative amino acid analysis. Reconstitution into proteoliposomes at less than one molecule per vesicle was accomplished by established procedures. Nystatin and ergosterol were included in these vesicles, so that nystatin conductance could serve as a quantitative marker of vesicle fusion with a planar lipid bilayer. Upon incorporation, purified CFTR exhibited regulated chloride channel activity, providing evidence that the protein itself is the channel. This activity exhibited the basic biophysical and regulatory properties of the type of Cl- channel found exclusively in CFTR-expressing cell types and believed to underlie cAMP-evoked secretion in epithelial cells.

Regulated endocytosis in a chloride secretory epithelial cell line

The colonic epithelial cell line T84 has been shown to be a good model to investigate the regulation of Cl- secretion by the adenosine 3',5'-cyclic monophosphate (cAMP)-mediated second messenger cascade. Regulated exocytic insertion and endocytic retrieval of transport proteins, or proteins that regulate transport proteins, is one mechanism proposed to regulate plasma membrane solute permeabilities. The aims of our studies were to characterize endocytic processes in T84 cells and to investigate their regulation by known activators of Cl- secretion that are mediated by the cAMP second messenger cascade. Forskolin, an activator of adenylate cyclase, caused a marked inhibition of endocytic uptake of the fluid-phase marker horseradish peroxidase (HRP) and the adsorptive marker wheat germ agglutinin conjugated to HRP. Similar inhibition was obtained with vasoactive intestinal peptide, a secretagogue whose receptor is coupled to adenylate cyclase, and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, a membrane-permeable cAMP analogue. 1,9-Dideoxy-forskolin, a forskolin analogue that fails to activate adenylate cyclase, was without effect on endocytosis. Our data show that the net rate of endocytosis, as measured by fluid-phase uptake, is decreased by a cAMP-mediated mechanism. Because the number of Cl- channels or associated regulatory proteins in the plasma membrane reflects a balance between their exocytic insertion and endocytic retrieval, we propose that the cAMP-mediated decrease in endocytosis could contribute to the concomitant increase in plasma membrane Cl- permeability.

Anion permeation in an apical membrane chloride channel of a secretory epithelial cell

Single channel currents though apical membrane Cl channels of the secretory epithelial cell line T84 were measured to determine the anionic selectivity and concentration dependence of permeation. The current-voltage relation was rectified with single channel conductance increasing at positive potentials. At 0 mV the single channel conductance was 41 +/- 2 pS. Permeability, determined from reversal potentials, was optimal for anions with diameters between 0.4 and 0.5 nm. Anions of larger diameter had low permeability, consistent with a minimum pore diameter of 0.55 nm. Permeability for anions of similar size was largest for those ions with a more symmetrical charge distribution. Both HCO3 and H2PO4 had lower permeability than the similar-sized symmetrical anions, NO3 and ClO4. The permeability sequence was SCN greater than I approximately NO3 approximately ClO4 greater than Br greater than Cl greater than PF6 greater than HCO3 approximately F much greater than H2PO4. Highly permeant anions had lower relative single channel conductance, consistent with longer times of residence in the channel for these ions. The conductance sequence for anion efflux was NO3 greater than SCN approximately ClO4 greater than Cl approximately I approximately Br greater than PF6 greater than F approximately HCO3 much greater than H2PO4. At high internal concentrations, anions with low permeability and conductance reduced Cl influx consistent with block of the pore. The dependence of current on Cl concentration indicated that Cl can also occupy the channel long enough to limit current flow. Interaction of Cl and SCN within the conduction pathway is supported by the presence of a minimum in the conductance vs. mole fraction relation. These results indicate that this 40-pS Cl channel behaves as a multi-ion pathway in which other permeant anions could alter Cl flow across the apical membrane.

Properties of single- and double-barreled Cl channels of shark rectal gland in planar bilayers

Chloride channels from the apical plasma membrane fraction of rectal gland of Squalus acanthias were characterized by incorporation into planar bilayers in the presence of cAMP-PK/ATP. In a total of 80 bilayer preparations, 21 Cl-selective channels were observed as single channels and 13 as pairs. This was a significantly greater number of double Cl channels than expected from a binomial distribution. The double Cl channels were divided into two groups based on kinetic and voltage-dependent behavior. One group had properties identical to the single channels (gb1) while the other was consistent with a double-barreled channel (gb2) with coordinated activity between proto-channels. The single-channel slope conductances of gb1 and gb2 from -60 to +20 mV with a 250/70 mM KCl gradient were 41 and 75 pS, respectively. With symmetrical 250 mM KCl, the I-V relation of gb1 showed outward rectification with 47.8 +/- 6.6 pS at cis negative potentials and 68.9 +/- 6.1 pS at cis positive potentials. gb1 was open from 70 to 95% at all electrochemical potentials from -80 to +40 mV. gb2 was steeply voltage dependent between -80 and -20 mV. Both gb1 and gb2 were insensitive to Ca (from 100 nm to 1 microM), blocked by 0.1 mM DIDS and highly selective for chloride. These data suggest that double-barreled Cl channels are related to the family of small, outwardly rectifying Cl channels of epithelial membranes.

Anion channels from rat brain synaptosomal membranes incorporated into planar bilayers

Synaptic membranes from rat brain were incorporated into planar lipid bilayers, and the characteristics of two types of anion-selective channels (type I and type II) were investigated. In asymmetric BaCl2 buffers (cis, 100 mM/trans, 25 mM), single channel conductances at -40 mV were 70 pS (type I) and 120 pS (type II). Permeability ratios (PNa:PBa:PCl) calculated from the Goldman-Hodgkin-Katz current equation for type I and type II channels were 0.23:0.04:1 and 0.05:0.03:1, respectively. Both channels exhibited characteristic voltage-dependent bursting activities. Open probability for type I channels had a maximum of approximately 0.7 at about 0 mV and decreased to zero at greater transmembrane potentials of either polarity. Type II channels were relatively voltage independent at negative voltages and were inactivated at highly positive voltages. Type I channels showed spontaneous irreversible inactivation often preceded by sudden transition to subconducting states. DIDS blocked type I channels only from the cis side, while it blocked type II channels from either side.

Chloride channel inhibition by the venom of the scorpion Leiurus quinquestriatus

The venom of the scorpion Leiurus quinquestriatus produced a significant, reversible inhibition of reconstituted Cl- channels of the small conductance type found in rat colonic epithelial cells. The kinetics of single-channel block by this venom were consistent with a first-order binding reaction in which the binding of one ligand molecule is sufficient to induce channel block. Single-channel mean block times were c.6 sec at -20 mV, and a KI in the submicromolar range is predicted. The active component has a mol. wt of roughly 5000 as judged by molecular sieve chromatography.

Quinine inhibits chloride and nonselective cation channels in isolated rat distal colon cells

Isolated cells from rat distal colon were investigated with the patch-clamp technique. In cell-attached and cell-excised patches (inside-out) single chloride channels with outward-rectifying properties were observed. In excised patches the single-channel conductance g was 47 +/- 5 pS at positive and 22 +/- 2 pS at negative clamp potentials (n = 6). The Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 10 microM) induced fast closing events, whereas 10 microM of 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) had no effect when applied to the cytosolic side. Quinine in the bath inhibited the Cl- channel by reducing its single-channel amplitude and increased open channel noise. With 0.1 mM the current amplitude decreased by 54% and with 1 mM quinine by 67%. Ca2(+)-dependent nonselective cation channels where observed after excision of the membrane patch. This channel was completely and reversibly inhibited by 100 microM DCDPC. Application of 1 mM quinine to the bath induced flickering and reduced the open-state probability from 0.94 to 0.44. In summary, besides its well established effects on K+ channels, quinine also inhibits nonselective cation channels and chloride channels by inducing fast closing events.

Outward-rectifying chloride channels in cultured adult and fetal human nasal epithelial cells

The patch-clamp technique was used to characterize ion channels in the apical membranes of cultured human nasal epithelial cells, dissociated from fetal nasal mucosa and from adult nasal polyps. Outward-rectifying chloride channels were found in 4.3% of the cell-attached patches from fetal cells (n = 258) and in 3.1% of the patches from adult cells (n = 320). After excision the number of patches containing active chloride channels increased threefold to 13% of the patches from the fetal cells and 10% from adult cells. The single-channel conductance at 0 mV in symmetrical 150 mM NaCl solutions was 24.3 +/- 0.9 pS (n = 28) and 26.0 +/- 1.2 pS (n = 30), respectively, in adult and fetal cells and showed outward rectification in the potential range from -80 to +80 mV. In fetal cells as well as in adult cells the channels were anion selective, and were almost impermeable for larger anions and monovalent cations. In cell-free patches the channels were Ca2+ independent. In most of the channels the open probability was voltage independent and high (+/- 0.86); in 20% of the channels, however, the open probability increased with depolarization. In conclusion, fetal nasal epithelial cells contain chloride channels in their apical membranes with single-channel properties and regulatory mechanisms similar to those found in cells from adults.

Apical membrane Cl- channels in airway epithelia: anion selectivity and effect of an inhibitor

Previous work has investigated the anion selectivity of transepithelial Cl- secretion by airway epithelia and its inhibition by the Cl(-)-channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB). Here we report the anion selectivity of the apical membrane and of the outwardly rectifying Cl- channel and the effect of NPPB on the Cl- channel. The anion selectivity sequence of the apical membrane determined with conventional microelectrodes in the native epithelium was SCN- greater than I- greater than Br- greater than NO3- approximately Cl- much greater than SO(4)2- approximately gluconate-. This contrasts with the observation that Cl- and Br- support transepithelial secretion but that I- does not. Thus the anion selectivity of transepithelial transport is determined by the basolateral membrane Cl- entry step. The anion selectivity of the outwardly rectifying Cl- channel studied in excised patches was the same as that of the apical membrane. We also found that NPPB reversibly blocked the outwardly rectifying Cl- channel from both the internal and external surfaces of the patch. NPPB, 10 microM, completely blocked the channel; lower concentrations caused a decrease in the probability of finding the channel in the open state. NPPB also caused the appearance of a subconductance state of the channel, an occurrence which is rarely observed in the absence of NPPB. These data provide further support for the conclusion that the outwardly rectifying Cl- channel is responsible for Cl- exit from the cell across the apical membrane.

cAMP-activated chloride channel in the basolateral membrane of the thick ascending limb of the mouse kidney

The properties of an anion-selective channel observed in basolateral membranes of microdissected, collagenase-treated, cortical thick ascending limbs of Henle's loop from mouse kidney were investigated using patch-clamp single-channel recording techniques. In basal conditions, single Cl- currents were detected in 8% of cell-attached and excised, inside-out, membrane patches whereas they were observed in 24% of cell-attached and 67% of inside-out membrane patches when tubular fragments were preincubated with Forskolin (10(-5) M) or 8-bromo-cAMP (10(-4) M) and isobutylmethylxanthine (10(-5) M). The channel exhibited a linear current-voltage relationship with conductances of about 40 pS in both cell-attached and cell-free membrane configurations. A PNa+/PCl- ratio of 0.05 was estimated in the presence of a 142/42 mM NaCl concentration gradient applied to inside-out membrane patches. Anionic selectivity of the channel followed the sequence Cl- greater than Br- greater than NO3- much greater than F-; gluconate was not a permeant species. The open-state probability of the channel increased with membrane depolarization in cell-attached, i.e., in situ membrane patches. In excised, inside-out, membrane patches, the channel was predominantly open with the open-state probability close to 0.8 over the whole range of potentials tested (-60 to +60 mV). The channel activity was not a function of internal calcium concentration between 10(-9) and 10(-3) M. We suggest that this Cl- channel, whose properties are distinct from those in other epithelia, could account for the well-documented conductance which mediates Cl- exit in the basolateral step of NaCl absorption in thick ascending limb of Henle's loop.

Cl- transport in basolateral renal medullary vesicles: II. Cl- channels in planar lipid bilayers

The present studies examined some of the properties of Cl- channels in renal outer medullary membrane vesicles incorporated into planar lipid bilayers. The predominant channel was anion selective having a PCl/PK ratio of 10 and a unit conductance of 93 pS in symmetric 320 mM KCl. In asymmetric KCl solutions, the I-V relations conformed to the Goldman-Hodgkin-Katz equation. Channel activity was voltage-dependent with a gating charge of unity. This voltage dependence of channel activity may account, at least in part, for the striking voltage dependence of the basolateral membrane Cl- conductance of isolated medullary thick ascending limb segments. The Cl- channels incorporated into the planar bilayers were asymmetrical: the trans surface was sensitive to changes in ionized Ca2+ concentrations and insensitive to reducing KCl concentrations to 10 mM, while the cis side was insensitive to changes in ionized Ca2+ concentrations, but was inactivated by reducing KCl concentrations to 50 mM.

Bicarbonate permeability of the outwardly rectifying anion channel

Single anion-selective channels have been studied in cultured human epithelial cells using the patch-clamp technique. Three cell types were used as models for different anion transport systems: (i) PANC-1, a cell line derived from the pancreatic duct, (ii) T84, a Cl-secreting colonic cell line, and (iii) primary cultures of sweat duct epithelium. Outwardly rectifying anion-selective channels were observed in all three preparations and were indistinguishable with respect to conductance, selectivity and gating. Striking similarities between HCO3- and Cl-secreting epithelia, and the high density of outward rectifiers in pancreatic cells prompted us to study HCO3 permeation through this channel. HCO3 permeability was significant when channels were bathed in symmetrical 150 mM HCO3 solutions, Cl-HCO3 mixtures, and under bi-ionic conditions with outwardly and inwardly directed HCO3 gradients. Permeability ratios (PHCO3/PCl) estimated from bi-ionic reversal potentials ranged from 0.50 to 0.64, although conductance ratios greater than 1.2 were observed with high extracellular pH. Chloride did not inhibit HCO3 permeation noticeably but rather had a small stimulatory effect when present on the opposite side of the membrane. The prevalence of outward rectifiers in PANC-1 and their permeability to bicarbonate suggests the channel may have a dual role in HCO3 secretion; to allow Cl recycling at the apical membrane and to mediate some of the HCO3 flux. Defective modulation of this channel in cystic fibrosis might provide a common basis for dysfunction in epithelia having very different anion transport properties (e.g., HCO3 secretion, Cl secretion and Cl absorption.

Single chloride channels in endosomal vesicle preparations from rat kidney cortex

Endocytotic vesicles from rat kidney cortex, isolated by differential centrifugation and enriched on a Percoll gradient, contain both an electrogenic H+ translocation system and a conductive chloride pathway. Using the dehydration/rehydration method, we fused vesicles of enriched endosomal vesicle preparations and thereby made them accessible to the patch-clamp technique. In the fused vesicles, we observed Cl- channels with a single-channel conductance of 73 +/- 2 pS in symmetrical 140 mM KCl solution (n = 25). The current-voltage relationship was linear in the range of -60 to +80 mV, but channel kinetic properties depended on the clamp potential. At positive potentials, two sublevels of conductance were discernible and the mean open time of the channel was 10-15 msec. At negative voltages, only one substate could be resolved and the mean open time decreased to 2-6 msec. Clamp voltages more negative than -50 mV caused reversible channel inactivation. The channel was selective for anions over cations. Ion substitution experiments revealed an anion permeability sequence of Cl- = Br- = I- greater than SO4(2-) approximately F-. Gluconate, methanesulfonate and cyclamate were impermeable. The anion channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 1.0 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 0.1 mM) totally inhibited channel activity. Comparisons with data obtained from radiolabeled Cl(-)-flux measurements and studies on the H+ pump activity in endocytotic vesicle suspensions suggest that the channel described here is involved in maintenance of electroneutrality during ATP-driven H+ uptake into the endosomes.

Two types of chloride channel on duct cells cultured from human fetal pancreas

Using the patch-clamp technique, we have identified two types of chloride channel on duct cells cultured from human fetal pancreas. The channel we observed most frequently exhibited slight outward rectification, had a conductance of 4-7 pS in cell-attached patches, and was present on the apical plasma membrane where it usually occurred in clusters. Its open-state probability was not markedly voltage dependent but was increased up to threefold by exposing duct cells to secretin (10 nM), dibutyryl cyclic AMP (1 mM), or forskolin (1 microM). The other type of chloride channel was only rarely observed. Its current-voltage relationship exhibited marked outward rectification, giving chord conductances of 19 pS for inward currents and 53 pS for outward currents. Although this channel could be activated by sustained depolarization of excised patches, once activated the open state probability was not voltage dependent. While the physiological role of these channels is not firmly established, the small-conductance channel might function in parallel with a Cl- -HCO-3 exchanger to provide a mechanism for electrogenic bicarbonate secretion from the duct cell.

Indirect evidence for the presence of non-specific anion channels in rabbit mandibular salivary gland acinar cells

1. Intracellular pH (pHi) was measured using the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein in acini isolated from the rabbit mandibular salivary gland. 2. Stimulation of the acinar cells with acetylcholine (ACh) evoked an intracellular acidosis, the size of which was dependent on the HCO3-concentration in the bathing medium. A half-maximal acidosis was observed at approximately 10 mM-HCO3-. ACh also evoked an acidosis in HCO3(-)-free solutions containing acetate; a half-maximal acidosis was observed at about 10 mM-acetate. 3. Propionate, lactate and butyrate were also able to support the ACh-evoked acidosis to varying extents. In contrast, formate, pyruvate and salicylate did not support the ACh-induced acidosis to any great extent. 4. Acetazolamide greatly reduced the size of the acidosis in HCO3(-)-buffered medium, but had no effect in acetate-buffered medium, suggesting that the inhibitory effect of acetazolamide was due to a specific inhibition of carbonic anhydrase activity. 5. The Cl- channel blockers diphenylamine-2-carboxylic acid (DPC, 1 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (0.5 mM) abolished the ACh-evoked acidosis in both HCO3(-) -and acetate-buffered media. 6. The data are consistent with the presence in the acinar cell of relatively non-specific anion channels sensitive to DPC and its derivatives. Such channels, activated on stimulation with ACh, would allow HCO3- and other weak acid ions to leave the cell, leading to the observed acidosis. The existence of such channels, located in the apical membrane, could explain why HCO3- or acetate can sustain fluid secretion in the intact perfused rabbit mandibular gland in the absence of Cl-.

Closed-time distribution of ionic channels. Analytical solution to a one-dimensional defect-diffusion model

A one-dimensional version of the model recently proposed by Läuger (1988) to explain the closed-time distribution of ionic channels in cell membranes is solved analytically. While the probability density f(t) for closed-time lengths may show a well-defined exponential behavior at short times, a power-law decay is predicted at long times. The influence of an additional random distribution of defects in the current-conducting protein is investigated and found to be dominating at long times. Explicit expressions that may be used for fitting experimental data are given for the closed-time distribution. Some of the available data are discussed and shown to be in good agreement with the predictions of the model.

Voltage-gated chloride currents in cultured canine tracheal epithelial cells

Chloride ions (Cl-) are concentrated in airway epithelial cells and subsequently secreted into the tracheal lumen by downhill flux through apical Cl- channels. We have studied Cl- currents in cultured canine tracheal cells using the whole-cell voltage-clamp technique. Ultrastructural techniques demonstrated that the cells used in the electrophysiological experiments possessed apical membrane specializations known to be present in the intact, transporting cell type. Cultured cells 2-6 days old were characterized by an input resistance of 3.4 +/- 0.8 G omega (n = 11) and a capacitance of 63.8 +/- 10.8 pF (n = 26). A comparison of 3 and 4 day-old cells with 5 and 6 day-old cells showed that the input resistance decreased almost 50%, and the cell capacitance and the inward and outward currents increased concomitantly approximately 200%. Cultured cells 3-4 days old held at -40 mV produced currents of 196 +/- 22 pA at 50 mV and -246 +/- 27 pA at -90 mV (n = 212) with pipette and bath solutions containing primarily 140 KCl and 140 NaCl, respectively. The chloride channel blocker diphenylamine-2-carboxylate (DPC, 100 microM) suppressed whole-cell currents by 76.8% at 60 mV; however, currents were unaffected by the stilbenes SITS (1 mM) and DNDS (1-30 microM). Replacement of K+ with Cs+ in the pipette solution did not affect the outward current, the current reversal potential, or the input resistance of the cells, indicating that the current was not significantly K+ dependent when the intrapipette solution was buffered to a Ca2+ concentration of 20 nM. The Cl-/Na+ permeability ratio was estimated to be greater than 11 as calculated from reversal potential measurements in the presence of an internal to external NaCl concentration ratio of 1:2. Current equilibrium permeabilities, relative to Cl- were: I- (2.9) much greater than NO3- (1.1) greater than or equal to Br- (1.1) greater than or equal to Cl- (1.0) greater than or equal to F- (0.93) much greater than MeSO4- (0.19) greater than or equal to gluconate (0.18) greater than or equal to aspartate (0.14). Depolarizations to potentials greater than 20 mV elicited a time-dependent component in the outward current in 71% of the cells studied. Currents inactivated with a double exponential time course at the most depolarized voltages. Recovery from inactivation was fast, holding potential-dependent, and followed a double exponential time course.(ABSTRACT TRUNCATED AT 250 WORDS)

Single chloride channels in colon mucosa and isolated colonic enterocytes of the rat

Chloride channels from rat colonic enterocytes were studied using the patch-clamp technique. After removal of mucus, inside-out patches were excised from the apical membrane of intact epithelium located at the luminal surface. They contained spontaneously switching Cl- channels with a conductance of 35-40 pS. The channels were blocked reversibly by anthracene-9-carboxylic acid (1mM). In excised patches from single enterocytes, isolated by calcium removal, the Cl- channels were studied in more detail. The I-V relation was linear between +/- 80 mV. The selectivity was I- greater than Br- greater than Cl- = NO3- greater than F- = HCO3-. Thirty pS Cl- channels were also found on the basolateral membrane of crypts isolated by brief calcium removal. The I-V curve of these Cl- channels was also linear. The results provide direct evidence for the existence of Cl- channels in the apical membrane of surface cells in colonic mucosa. The properties of these channels are similar to those previously observed when incorporating membrane vesicles into planar lipid bilayers. Both results support the validity of the theoretical models describing intestinal secretion.

Stilbene disulfonate blockade of colonic secretory Cl- channels in planar lipid bilayers

We studied blockade by 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) of a secretory Cl- channel from colonic enterocyte plasma membrane vesicles incorporated into planar lipid bilayer membranes. Except for intermittent long-lived closed periods (100 ms to several min), the control channel open probability (Po) was greater than 90%. DNDS, added to the cis or vesicle-containing side, which corresponds to the outer membrane side of the channel, caused a dramatic increase in the number of current transitions from the open-to-closed state. DNDS caused a concentration-dependent decrease in Po with a maximum inhibition of 95 +/- 2.0% and a half-maximal inhibitory concentration of 3.3 +/- 1.4 microM. DNDS added to the trans side of the channel had no effect on either the single-channel conductance or kinetic behavior of the channel. Kinetic analysis revealed that DNDS blockade from the cis side could be explained by a linear, closed-open-blocked, kinetic scheme. The estimated DNDS block rate constants were kon = 3.2 X 10(7) M-1.s-1 and koff = 52 s-1, yielding an equilibrium dissociation constant (KD) of 2.1 +/- 0.38 microM, similar to the Ki for inhibition of Po. The effects of DNDS were fully reversible after perfusion of the cis compartment with DNDS-free solution. In contrast, the covalently reactive 4,4'-diisothiocyano-substituted stilbene disulfonate caused an irreversible blockade of the Cl- channel.

Secretin-regulated chloride channel on the apical plasma membrane of pancreatic duct cells

Using the patch clamp technique we have identified a small conductance ion channel that typically occurs in clusters on the apical plasma membrane of pancreatic duct cells. The cell-attached current/voltage (I/V) relationship was linear and gave a single channel conductance of about 4 pS. Since the reversal potential was close to the resting membrane potential of the cell, and unaffected by changing from Na+-rich to K+-rich pipette solutions, the channel selects for anions over cations in cell-attached patches. The open state probability was not voltage-dependent. Adding 25 mM-bicarbonate to the bath solution caused a slight outward rectification of the I/V relationship, but otherwise, the characteristics of the channel were unaffected. In excised, inside-out, patches the I/V relationship was linear and gave a single channel conductance of about 4 pS. A threefold chloride concentration gradient across the patch (sulphate replacement) shifted the single channel current reversal potential by -26 mV, indicating that the channel is chloride selective. Stimulation of duct cells with secretin (10 nM), dibutyryl cyclic AMP (1 mM) and forskolin (1 microM) increased channel open state probability and also increased the number of channels, and/or caused disaggregation of channel clusters, in the apical plasma membrane. Coupling of this channel to a chloride/bicarbonate exchanger would provide a mechanism for electrogenic bicarbonate secretion by pancreatic duct cells.

Apical membrane chloride channels in a colonic cell line activated by secretory agonists

We characterized the anion channel responsible for the increase in apical membrane Cl secretion using a model salt-secreting epithelium, the T84 colonic cell line. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated secretagogues, prostaglandin E2, forskolin, and 8-bromo-cAMP, evoked activity of an outwardly rectifying Cl channel in previously quiet cell-attached membrane patches. The channel remained active in excised, inside-out membranes, where its single-channel conductance was 40-45 pS at 0 mV with 160 mM NaCl in pipette and bath. Selectivities were PCl/PNa = 50 and for halides I(1.8)/Br(1.4)/Cl(1.0)/F(0.4). This halide sequence illustrates that the ability of various anions to undergo transepithelial secretion is determined by the selectivity of the basolateral membrane Cl entry step rather than by the apical Cl channel. Open-channel probability increased with depolarization, an effect that would adjust the rate of Cl exit across secretory cell apical membranes with agonist-induced changes in apical membrane potential. Comparison with the properties of Cl channels detected in other cell types suggests that this cAMP-stimulated Cl channel is uniquely present in the apical membranes of salt-secreting epithelial cells.

Single anion-selective channel and its ion selectivity in the vascular smooth muscle cell

The properties of voltage-gated Cl channels of cultured smooth muscle cell prepared from embryonic rat aorta were studied. In the excised patch (inside-out configuration), we observed the activity of channels, opening and closing spontaneously, when the membrane potential was held at around 0 mV. The channels were active at a potential range between +10 and -10 mV. A step change of the membrane potential from the active potential range in either a positive or a negative direction closed the channel to an apparently inactivated state. The time course of this inactivation process became faster as the amplitude of the step change was increased. Returning the membrane potential to 0 mV allowed the channel to recover from the inactivated state. The channel had at least two open conductance states. Ca ions at the cytoplasmic face were not required for the activation of the channel. Adenosine nucleotides at the same side of the membrane had no effect on channel activity. The channels were selective to anions rather than cations, and they had a large single channel conductance of 340.5 +/- 20.4 pS in symmetrical 150 mM TEA-Cl. The reversal potential of the channel was shifted by -15.2 +/- 2.6 and 17.0 +/- 1.7 mV, when the Cl concentration at the intracellular side was changed to 75 mM or 300 mM, respectively. The permeability sequence of halides was I- greater than Br- greater than Cl- greater than F- (1.4:1.3:1.0:0.7), whereas the conductance sequence was Cl- greater than Br- greater than F- greater than I- (1.00:0.89:0.86:0.83).(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical properties of a chloride channel in the apical membrane of a secretory epithelial cell

1. Patch clamp studies on colonic tumor cell line T84 show the presence of chloride channels. 2. The channels are activated by forskolin, PGE2, or 8-Br-cAMP. 3. Single channel conductance was ca 40 pS at the reversal potential, increasing to 70 pS at +80 mV and decreasing to 25 pS at -80 mV. 4. Relative permeabilities were I greater than Br greater than Cl greater than F.

Properties of an anion-selective channel from rat colonic enterocyte plasma membranes reconstituted into planar phospholipid bilayers

Vesicles derived from epithelial cells of the colonic mucosa of the rat were fused to planar phospholipid bilayer membranes, revealing spontaneously switching anion-conducting channels of 50 pS conductance (at -30 mV with 200 mM Cl- each side). The equilibrium selectivity series was I- (1.7)/Br- (1.3)/Cl- (1.0)/F- (0.4)/HCO3- (0.4)/Na (less than 0.11). Only one dominant open-state conductance could be resolved, which responded linearly to Cl- concentrations up to 600 mM. The single-channel current-voltage curve was weakly rectifying with symmetrical solutions. When 50 mV were exceeded at the high-conductance branch of the curve, switching was arrested in the closed state. At more moderate voltages (+/- 40 mV) kinetics were dominated by one open state of about 35-msec lifetime and two closed states of about 2 and 9-msec lifetime. Of these, the more stable closed state occurred less often. At these voltages one additional closed state of significantly longer lifetime (greater than 0.5 sec) was observed.