Chloride channels of high conductance in the microvillous membrane of term human placenta

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Knowledge needed about the exchange physiology of the placenta

There is now a basic understanding of the driving forces and mechanisms underlying rates of solute exchange across the placenta but there are still major gaps in knowledge. Here we summarise this basic understanding, whilst highlighting gaps in knowledge. We then focus on two particular areas where more knowledge is needed: (1) the electrical potential difference (PD) across the placenta and (2) the paracellular permeability of the placenta to hydrophilic solutes. In many species a PD has been recorded between a catheter in a maternal blood vessel and one in a fetal vessel. However, the key question is whether this PD is the same as that across the placental exchange barrier. We addressed this in the human placenta using microelectrodes to measure the PD in isolated villi in vitro; the transtrophoblast PD so measured had a median value of -3 mV (range 0-15 mV). There have been no subsequent studies to validate this measurement. The syncytiotrophoblast of haemochorial placentas lacks any obvious extracellular water filled paracellular space between the syncytial nuclei. However, in mouse, rat, guinea pig and human there is an inverse relationship between the rate of diffusion of inert hydrophilic solutes across the placenta and their molecular size. The simplest explanation is that a paracellular route exists but its morphological identity is still uncertain. Areas of syncytial denudation could provide a paracellular route but this has not been proven. Answers to these and similar questions are required to fully understand the exchange physiology of the normal placenta and how this is affected in pathology.

Oxygen-Sensitive K+ Channels Modulate Human Chorionic Gonadotropin Secretion from Human Placental Trophoblast

Human chorionic gonadotropin (hCG) is a key autocrine/paracrine regulator of placental syncytiotrophoblast, the transport epithelium of the human placenta. Syncytiotrophoblast hCG secretion is modulated by the partial pressure of oxygen (pO₂), reactive oxygen species (ROS) and potassium (K⁺) channels. Here we test the hypothesis that K⁺ channels mediate the effects of pO₂ and ROS on hCG secretion. Placental villous explants from normal term pregnancies were cultured for 6 days at 6% (normoxia), 21% (hyperoxia) or 1% (hypoxia) pO₂. On days 3–5, explants were treated with 5mM 4-aminopyridine (4-AP) or tetraethylammonium (TEA), blockers of pO₂-sensitive voltage-gated K⁺ (K_V) channels, or ROS (10–1000μM H₂O₂). hCG secretion and lactate dehydrogenase (LDH) release, a marker of necrosis, were determined daily. At day 6, hCG and LDH were measured in tissue lysate and ⁸⁶Rb (K⁺) efflux assessed to estimate syncytiotrophoblast K⁺ permeability. hCG secretion and ⁸⁶Rb efflux were significantly greater in explants maintained in 21% pO₂ than normoxia. 4-AP/TEA inhibited hCG secretion to a greater extent at 21% than 6% and 1% pO₂, and reduced ⁸⁶Rb efflux at 21% but not 6% pO₂. LDH release and tissue LDH/hCG were similar in 6%, 21% and 1% pO₂ and unaffected by 4-AP/TEA. H₂O₂ stimulated ⁸⁶Rb efflux and hCG secretion at normoxia but decreased ⁸⁶Rb efflux, without affecting hCG secretion, at 21% pO₂. 4-AP/TEA-sensitive K⁺ channels participate in pO₂-sensitive hCG secretion from syncytiotrophoblast. ROS effects on both hCG secretion and ⁸⁶Rb efflux are pO₂-dependent but causal links between the two remain to be established.

An outwardly rectifying chloride channel in BeWo choriocarcinoma cell line

In this study, an outwardly rectifying chloride channel was characterized in the trophoblastic cell line BeWo, a human hormone-synthesizing cell which displays many biochemical and morphological properties similar to those reported for the human cytotrophoblast. Ion channel activity was recorded in the cell attached and inside-out configurations with standard patch-clamp technology. In most of the BeWo cells studied, the channel under symmetrical N-methyl-d-glucamine (NMDG-Cl) concentration (Na(+) free solution) in both sides of the membrane exhibited spontaneous activity, an outwardly rectifying current/voltage relationship and single-channel conductances of 15 pS and 48 pS for inwards and outwards currents, respectively. The channel has a low permeability for gluconate with a relative permeability P(gluconate)/P(Cl) of 0.23, and a higher permeability to I(-). The open probability (Po) of the channel exhibited dependence with the applied membrane potential with greater activity at positive pulses. The channel activity was inhibited by the sulphonylurea hypoglycemic agent glibenclamide (50 μM) or by diphenylamine-2-carboxylate (DPC, 500 μM) added to the cytoplasmic side of the patch whereas conductances remained unchanged. The blockade with glibenclamide and DPC was independent of the applied membrane potential. All these results are characteristic of the outwardly rectifying Cl channel (ORCC) found in other types of cells. Neither Po, conductances nor reversal potential (Er) values were affected by the absence of intracellular Ca(2+), suggesting that the channel is not sensitive to Ca(2+).

Placental chloride channels: a review

The human placental syncytiotrophoblast (hSTB) is a polarized epithelial structure, that forms the main barrier to materno-fetal exchange. The chloride (Cl(-)) channels in other epithelial tissues contribute to several functions, such as maintenance of the membrane potential, volume regulation, absorption and secretion. Additionally, the contributions of Cl(-) channels to these functions are demonstrated by certain diseases and knock-out animal models. There are multiple lines of evidence for the presence of Cl(-) channels in the hSTB, which could contribute to different placental functions. However, both the mechanism by which these channels are involved in the physiology of the placenta, and their molecular identities are still unclear. Furthermore, a correlation between altered Cl(-) channels functions and pathological pregnancies is beginning to emerge. This review summarizes recent developments on conductive placental chloride transport, and discusses its potential implications for placental physiology.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

The maxi-chloride channel in human syncytiotrophoblast: a pathway for taurine efflux in placental volume regulation?

Taurine (Tau), the most abundant amino acid in fetal blood, is highly concentrated in human placenta. During pregnancy, Tau is involved in the neurological development of the fetus, and in volume regulation of the placenta. The placenta may release taurine in parallel with K(+) and Cl(-) in response to an increase in cell volume. However, the pathway for the volume-activated taurine efflux is unknown. One candidate is a voltage-dependent Maxi-chloride channel from apical syncytiotrophoblast membrane (MVM), with a conductance over 200pS and multiple subconductance states. Our aim was to study whether this channel could be a Tau conductive pathway in the MVM. Purified human placental MVM were reconstituted into giant liposomes suitable for patch clamp recordings. Typical Maxi-chloride channel activity was detected in symmetrical chloride (Cl(-)) solutions, and then taurine (Tau), Aspartate (Asp), and glutamate (Glu) solutions were used in the bath of excised patches to detect single channel currents carried by these anions. The relative permeabilities (P), estimated from the shift in reversal potential of current-voltage curves after anion replacement, were as follows: Chloride>Taurine=Glutamate=Aspartate. In Tau symmetric conditions using equivalent Cl(-) concentrations, the slope conductance was 62.4+/-7.3pS. The data shows that Tau and other amino acids diffuse through the Maxi-chloride channel, which could be of great importance as part of the mechanism involved in the volume regulation process in human placenta.

Purinergic receptors in human placenta: evidence for functionally active P2X4, P2X7, P2Y2, and P2Y6

Appropriate regulation of ion transport by the human placental syncytiotrophoblast is important for fetal growth throughout pregnancy. In nonplacental tissues, ion transport can be modulated by extracellular nucleotides that raise intracellular calcium ([Ca2+]i) via activation of purinergic receptors. We tested the hypothesis that purinergic receptors are expressed by human placental cytotrophoblast cells and that their activation by extracellular nucleotides modulates ion (K+) efflux and [Ca2+]i. P2X/P2Y receptor agonists 5-bromouridine 5'-triphosphate (5-BrUTP), ADP, ATP, 2',3'-O-(4-benzoyl-benzoyl)adenosine 5'-triphosphate (BzATP), and UTP stimulated 86Rb (K+ tracer) efflux from cultured cytotrophoblast cells at early (mononuclear) or later (multinucleate syncytiotrophoblast-like) stages of differentiation, with ATP and UTP particularly potent. 2-Methylthioadenosine 5'-triphosphate (2-MeS-ATP), and UDP elevated 86Rb efflux only from multinucleated cells. All agonists caused a significant peak and plateau increase in [Ca2+]i, although the magnitude of responses was variable. The effect of BzATP, UTP, and UDP in multinucleated cells was unaffected, and that of ATP partially inhibited, by removal of extracellular Ca2+, implicating P2Y receptor activation. mRNA encoding P2X1, P2X2, P2X4, and P2X7 and P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 were identified in mono- and multinucleated cells, whereas P2X3 and P2X5 mRNA were absent from all samples. Western blot analysis revealed P2X4, P2X7, P2Y2, and P2Y6 protein in cytotrophoblast cells, but P2Y4 was not detected. On the basis of published agonist selectivity, the data indicate the presence of functionally active P2X4, P2X7, P2Y2, and P2Y6 receptors in cytotrophoblast cells. We propose that activation of these receptors, and subsequent elevation of [Ca2+]i, modulates syncytiotrophoblast homeostasis and/or maternofetal ion exchange in response to extracellular nucleotides.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Annexin 6 modulates the maxi-chloride channel of the apical membrane of syncytiotrophoblast isolated from human placenta

The syncytiotrophoblast separates the maternal and fetal blood and constitutes the primary barrier for maternal-fetal transport. The Maxi-chloride channel from the apical membrane of the syncytiotrophoblast plays a role in the chloride conductance. Annexins can play an important role in the regulation of membrane events. In this study we evaluate the role of annexin 6 in the Maxichloride channel properties. The results showed that annexin 6 is bound in the apical placenta membranes in a calcium-dependent phospholipid-binding manner but also in a calcium-independent fashion. The neutralization of annexin 6 decreased the total current by 39 +/- 1.9% in the range of +/-80 mV, and the currents decrease with the time. The single-channel slope conductance was decreased from 253 +/- 7.4 pS (control) to 105 +/- 13 pS, and the amplitude decreased by 50%. The open probability was also affected when higher voltage steps were used, changes in either the positive or negative direction induced the channel to close, and the open probability (P(o)) did not decrease. In channels with neutralized annexin 6, it was maintained at 1 at +/-40 mV and at +/-80 mV. These results suggest that endogenous annexin 6 could regulate the Maxi-chloride channel. The results obtained with normal placentae, in which annexin 6 was neutralized, are similar to those described for the Maxi-chloride channel isolated from pre-eclamptic placenta. Together these data suggest that annexin 6 could play an important role in ion transport of the placenta.

Channels, pumps, and exchangers in the gill and kidney of freshwater fishes: Their role in ionic and acid-base regulation

In freshwater fishes, the gill and kidney are intricately involved in ionic and acid-base regulation owing to the presence of numerous ion channels, pumps, or exchangers. This review summarizes recent developments in branchial and renal ion transport physiology and presents several models that integrate epithelial ion and acid-base movements in freshwater fishes. At the gill, three cell types are potentially involved in ionic uptake: pavement cells, mitochondria-rich (MR) PNA(+) cells, and MR PNA(-) cells. The transfer of acidic or basic equivalents between the fish and its environment is accomplished largely by the gill and is appropriately regulated to correct acid-base imbalances. The kidney, while less important than the gill in overall acid or base excretion, has an essential role in regulating systemic acid-base balance by controlling HCO(3) (-) reabsorption from the filtrate.

Integrative physiology and functional genomics of epithelial function in a genetic model organism

Classically, biologists try to understand their complex systems by simplifying them to a level where the problem is tractable, typically moving from whole animal and organ-level biology to the immensely powerful "cellular" and "molecular" approaches. However, the limitations of this reductionist approach are becoming apparent, leading to calls for a new, "integrative" physiology. Rather than use the term as a rallying cry for classical organismal physiology, we have defined it as the study of how gene products integrate into the function of whole tissues and intact organisms. From this viewpoint, the convergence between integrative physiology and functional genomics becomes clear; both seek to understand gene function in an organismal context, and both draw heavily on transgenics and genetics in genetic models to achieve their goal. This convergence between historically divergent fields provides powerful leverage to those physiologists who can phrase their research questions in a particular way. In particular, the use of appropriate genetic model organisms provides a wealth of technologies (of which microarrays and knock-outs are but two) that allow a new precision in physiological analysis. We illustrate this approach with an epithelial model system, the Malpighian (renal) tubule of Drosophila melanogaster. With the use of the beautiful genetic tools and extensive genomic resources characteristic of this genetic model, it has been possible to gain unique insights into the structure, function, and control of epithelia.

A low conductance, non-selective cation channel from human placenta

Non-selective cation channels have been identified in the plasma membranes of many different cells. Previous research using fluorescent techniques has demonstrated the presence of cation conductances in membranes from human trophoblast. The purpose of this work was to explore, by electrophysiological methods, a non-selective cation channel in apical membranes from human placenta. Human placental apical membranes were purified by differential centrifugation and reconstituted in giant liposomes. These giant liposomes were then used for electrophysiological studies and were probed for the presence of cation channels by the patch-clamp method. The channel identified had a linear current-potential relationship with a conductance of around 16 pS in symmetrical Na(+) solution. Under asymmetrical conditions the reversal potential was close to the reversal potential for Na(+). The channel was equally permeable to sodium and potassium and the permeability sequence was NH+4>Cs(+) approximately Rb(+)>Na(+) approximately K(+)>Li(+). The channel also showed permeability to calcium and barium. The channel was insensitive to calcium but was blocked by millimolar concentration of Mg(2+). We have demonstrated the presence of a low conductance, non-selective cation channel in placental apical membranes. These channels share some properties with non-selective cation channels previously described in other different cells. The precise role of these channels in placental physiology has yet to be determined.

Okadaic acid-sensitive activation of Maxi Cl(-) channels by triphenylethylene antioestrogens in C1300 mouse neuroblastoma cells

1. The regulation of Maxi Cl(-) channels by 17beta-oestradiol and non-steroidal triphenylethylene antioestrogens represents a rapid, non-classical effect of these compounds. In the present study we have investigated the signalling pathways used for the regulation of Maxi Cl(-) channel activity by oestrogens and antioestrogens in C1300 neuroblastoma cells. 2. Whole-cell Maxi Cl(-) currents were readily and reversibly activated by tamoxifen, toremifene and the membrane-impermeant ethyl-bromide tamoxifen, only when applied to the extracellular medium. 3. Pre-treatment of C1300 cells with oestrogen or cAMP prevented the antioestrogen-induced activation of Maxi Cl(-) channels. The inhibitory effect of 17beta-oestradiol and cAMP was abolished by the kinase inhibitor staurosporine. 4. Current activation was unaffected by the removal of intracellular Ca(2+) and Mg(2+), but was completely abolished in the presence of okadaic acid. These results are consistent with the participation of an okadaic acid-sensitive serine/threonine protein phosphatase in the activation of Maxi Cl(-) channels. However, neither oestrogen or antioestrogen treatment modified the total activity of the two major serine/threonine phosphatases, PP1 and PP2A, in C1300 cells. 5. Although the role of these Maxi Cl(-) channels remains unknown, our findings suggest strongly that their modulation by oestrogens and antioestrogens is linked to intracellular signalling pathways.

Extramitochondrial porin: facts and hypotheses

Mitochondrial porin, or VDAC, is a pore-forming protein abundant in the outer mitochondrial membrane. Several publications have reported extramitochondrial localizations as well, but the evidence was considered insufficient by many, and the presence of porin in nonmitochondrial cellular compartments has remained in doubt for a long time. We have now obtained new data indicating that the plasma membrane of hematopoietic cells contains porin, probably located mostly in caveolae or caveolae-like domains. Porin was purified from the plasma membrane of intact cells by a procedure utilizing the membrane-impermeable labeling reagent NH-SS-biotin and streptavidin affinity chromatography, and shown to have the same properties as mitochondrial porin. A channel with properties similar to that of isolated VDAC was observed by patch-clamping intact cells. This review discusses the evidence supporting extramitochondrial localization, the putative identification of the plasma membrane porin with the "maxi" chloride channel, the hypothetical mechanisms of sorting porin to various cellular membrane structures, and its possible functions.

Regulation of human placental chloride channel by arachidonic acid and other cis unsaturated fatty acids

OBJECTIVE

Arachidonic acid has been implicated in the modulation of various transport processes, including conductive chloride transport in brush border membranes in the human placenta. The purpose of this work was to explore the effects of some cis unsaturated fatty acids on the electrophysiologic properties of the maxi chloride channels present in apical membranes from human placenta.

STUDY DESIGN

Apical membrane chloride channels from human term placentas were reconstituted in giant liposomes. These cell-sized liposomes, generated by a cycle of dehydration and rehydration, are suitable for electrophysiologic studies by the patch-clamp method.

RESULTS

Low micromolar concentrations of arachidonic acid reversibly inhibit maxi chloride channels in excised patches. Other cis unsaturated fatty acids, such as oleic and linoleic acids, show similar blockade. The inhibition was dose dependent. The maxi chloride channel can also be inhibited by 4,4 -diisothiocyanatostilbene-2,2 -disulfonic acid, a known chloride channel inhibitor.

CONCLUSIONS

Our results identify the apical membrane maxi chloride channel as a possible electrophysi ologic counterpart of 4,4 -diisothiocyanatostilbene-2, 2 -disulfonic acid and cis unsaturated fatty acid-inhibited conductance previously described in brush border membranes of the human placenta. From a functional point of view the control of these channels by arachidonic acid may be of great importance in placental physiologic characteristics. Regulation of chloride channels could be important in the control of electrolyte and fluid transfer across the placenta. In addition, if these channels contribute to setting the membrane potential their regulation could have consequences for nutrient transport and delivery to the fetus. The electrophysiologic identification of these channels and their regulation might help to unravel their possible role in transplacental transport in normal and pathologic placental tissue.

Identification of endogenous outward currents in the human embryonic kidney (HEK 293) cell line

Human embryonic kidney cells (HEK 293) are widely used as an expression system in studies of ion channels. However, their endogenous ionic currents remain largely unidentified. To characterize these currents, we performed patch clamp experiments on this expression system. In whole-cell voltage clamp mode, the HEK 293 cells showed mainly outward currents using physiological concentrations of Na+ and K+ and symmetric concentrations of Cl- (150 mM) across the plasma membranes. K+ currents contributed to a small portion of these outward currents, since a shift of the reversal potentials of only approximately 20 mV was seen with a change of extracellular K+ concentration from 3 to 150 mM. In contrast, the reversal potential shifted approximately 25 mV when extracellular Cl- was reduced to 50 mM, indicating that most of the outward currents are carried by Cl-. In inside-out patches, several distinct Cl- currents were identified. They were: (1) 350 pS Cl- current, which was voltage-activated and had a moderate outward rectification; (2) 240 pS Cl- current with a weak outward rectification; and (3) 55 pS Cl- current, which was voltage-activated, sensitive to DIDS, and showed a strong outward rectification. Activation of these Cl- currents did not require an elevation of free Ca2+ level in the cytosol. Besides these three currents, we observed two other Cl- currents with much smaller conductances (25 and 16 pS, respectively). Two different K+ currents were seen in the HEK 293 cells, with one of them (125 pS) showing inward rectification and the other (70 pS) outward rectification. Moreover, a 50 pS cation channel was recorded in these cells. The presence of a variety of ion channels in the HEK 293 cells suggests that a great precaution needs to be taken when this expression system is used in studies of several similar ion channels.

Mechanisms of chloride transport across the syncytiotrophoblast basal membrane in the human placenta

Chloride transport mechanisms in isolated plasma membrane vesicles were studied to characterize pathways for transcellular transport of chloride. Microvillous membrane (MVM) and basal membranes (BM) vesicles were isolated from term placentae. Western blot analysis of the anion exchanger isoform 1 (AE1) demonstrated that the density of AE1 was 12-fold higher on the MVM compared to the BM. At 30 sec, the Cl- uptake in the absence of a potential difference (p.d.) was 457.3 +/- 69.7 and 111.0 +/- 29.1 pmol/mg protein in MVM and BM, respectively (mean +/- SEM, n=6). Chloride transport pathways were characterized using diisothiocyano-2'2-disulphonic stilbene. (DIDS, 0.1 mM) and diphenylamine-2-carboxylate (DPC, 0.5 mM) in the absence or presence of inside positive membrane potentials. Anion exchange (DIDS-sensitive uptake at zero mV) was found in the MVM only. Both MVM and BM showed increased chloride uptake in the presence of inside positive potentials, suggesting the presence of chloride conductance pathways. The chloride uptake with a 25-mV inside positive p.d. could be inhibited by both DIDS and DPC in MVM and BM. However greater potentials (50 mV) showed no significant inhibition by DIDS or DPC in BM. In conclusion, the anion exchanger is unlikely to contribute significantly to chloride fluxes across BM. The data also suggest the presence of Cl- conductance pathways in both the MVM and BM which are sensitive to both DIDS and DPC.

Hormonally controlled chloride movement across Drosophila tubules is via ion channels in stellate cells

Anion conductance across the Drosophila melanogaster Malpighian (renal) tubule was investigated by a combination of physiological and transgenic techniques. Patch-clamp recordings identified clusters of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive "maxi-chloride" channels in a small domain of the apical membrane. Fluid secretion assays demonstrated sensitivity to the chloride channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, anthracene-9-carboxylic acid, and niflumic acid. Electrophysiological analysis showed that the calcium-mediated increase in anion conductance was blocked by the same agents. Vibrating probe analysis revealed a small number of current density hot spots, coincident with "stellate" cells, that were abolished by low-chloride saline or the same chloride channel blockers. GAL-4-targeted expression of an aequorin transgene revealed that the neurohormone leucokinin elicits a rapid increase in intracellular calcium levels in stellate cells that precedes the fastest demonstrable physiological effect. Taken together, these data show that leucokinins act on stellate cells through intracellular calcium to increase transcellular chloride conductance through channels. As electrogenic cation conductance is confined to principal cells, the two pathways are spatially segregated in this tissue.

Properties of two multisubstate Cl- channels from human syncytiotrophoblast reconstituted on planar lipid bilayers

We describe the first successful reconstitution of placental ionic channels on planar lipid bilayers. An apical plasma membrane-enriched vesicle fraction from human syncytiotrophoblast at term was prepared by following isotonic agitation, differential centrifugation, and Mg2+-induced selective precipitation of nonapical membranes, and its purity was assessed by biochemical and morphological marker analysis. We have already reported that, unlike previous patch-clamp studies, nonselective cation channels were incorporated in most cases, a result consistent with the higher permeability for cations as compared with Cl- and with the low apical membrane potential difference at term revealed by fluorescent probe partition studies, and microelectrode techniques. In this paper, we report that Cl--selective channels were incorporated in 4% of successful reconstitutions (14 out of 353) and that their analysis revealed two types of activity. One of them was consistent with a voltage-dependent, 100-pS channel while the other was consistent with the lateral association of 47-pS conductive units, giving rise to multibarrelled, DIDS-sensitive channels of variable conductance (300 to 650 pS). The latter displayed a very complex behavior which included cooperative gating of conductive units, long-lived substates, voltage-dependent entry into an apparent inactivated state, and flickering activity. The role of the reported Cl- channels in transplacental ion transport and/or syncytium homeostasis remains to be determined.

Effect of human chorionic gonadotrophin on chloride current in human syncytiotrophoblasts in culture

Human trophoblast differentiates in vivo and in vitro by the fusion of cytotrophoblastic cells to form syncytiotrophoblasts. A large amount of human chorionic gonadotrophin (hCG) is produced by the syncytiotrophoblasts, which express hCG luteinizing hormone (LH) receptors. Since recent investigations with electrophysiological techniques support the conclusion that hormonal effects can be mediated by modulations of the membrane ionic conductances of the cells, a perforated patch-clamp technique was used to investigate the possible presence of a chloride current evoked by hCG. The perifusion of hCG (500 mIU/ml) activated a time-independent current, which presents a linear current-voltage (I/V) relationship in symmetrical chloride concentrations. The reversal potential was -1.8 mV with 142 mM Cl- external solution and 134 mM cl- internal solution. This reversal potential shifted with changes in the transmembrane Cl- gradient. Moreover, this hCG-induced current was sensitive to 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (50 microM), to diphenylalamine-2-carboxylic acid (DPC) (0.5 mM) and to 9-AC (1 mM), three known chloride channel blockers. These results confirm the autocrine action of hCG in the physiology of the trophoblast.

A maxi Cl- channel coupled to endothelin B receptors in the basolateral membrane of guinea-pig parietal cells

1. To study endothelin (ET) receptors in guinea-pig stomach, ET-binding assays and in vitro autoradiography were performed on fundic cell suspensions and on sections of the fundus, respectively. ETA and ETB receptor subtypes were found to coexist in the parietal cells. 2. Endothelin 1 (ET-1) added to the (basolateral) bathing solution was found to activate noisy whole-cell Cl- currents within about 1 min in both single, isolated parietal cells and those within gastric glands obtained from the fundus. 3. ET-1-induced Cl- currents were rapidly blocked by a Cl- channel blocker (NPPB) added to the (basolateral) bathing solution in a concentration-dependent manner with a half-maximum inhibition concentration of 33 microM. 4. The anion selectivity sequence of the ET-1-induced conductance was I- > Br- > Cl- > F-, corresponding to Eisenman's sequence I. 5. Changes in extracellular pH between 5 and 8 did not affect the ET-1-induced activation of Cl- currents. 6. Similar activating effects were also observed with ET-3 and a specific ETB receptor agonist (IRL1620). An ETB receptor antagonist (IRL1720) prevented the ET-1 effect, whereas an ETA-selective antagonist (FR139317 or BQ123) failed to antagonize the ET-1 effect. 7. In the whole-cell mode, unitary Cl- channel events could be observed in association with ET-1-activated macroscopic currents. The single-channel conductances were around 200 and 350 pS at negative and positive membrane potentials, respectively. 8. It is concluded that gastric parietal cells of guinea-pig possess pH-insensitive 'maxi' Cl- channels coupled to ETB receptors in the basolateral membrane.

A chloride channel from human placenta reconstituted into giant liposomes

OBJECTIVE

Ion channels play important roles in epithelial transport, but they are difficult to access for conventional electrophysiologic studies in intact placenta. The purpose of this work was to explore the suitability of purified trophoblast plasma membrane as a source of ion channels for reconstitution in artificial lipid membranes.

STUDY DESIGN

Human placental brush border membranes were purified by differential and gradient centrifugation and fused with small liposomes. Giant liposomes were then generated by a cycle of dehydration and rehydration. These giant liposomes are suitable for electrophysiologic studies and were probed for the presence of active ion channels by the patch-clamp method.

RESULTS

The results reported here indicate the presence of a high conductance chloride channel showing some similarities with "maxi" chloride channels described in secreting and absorbing epithelia. The channel had a slight outward rectification with conductances of 232 and 300 pS at negative and positive potentials, respectively.

CONCLUSIONS

For the first time successful reconstitution of a human placental ion channel is achieved in a system suited for electrophysiologic studies. The chloride channel described might play a role in transplacental transport.

Volume-activated amino acid efflux from term human placental tissue: stimulation of efflux via a pathway sensitive to anion transport inhibitors

The effect of a hyposmotic challenge and hence cell-swelling upon the efflux of a variety of solutes from isolated human placental tissue has been examined. A hyposmotic shock increased the fractional release of taurine, the most abundant free amino acid in placental tissue, via a pathway sensitive to niflumic acid, DIDS (4,4'-Diisothiocyanatostilbene-2',2'-disulphonic acid,) NPPB (5-Nitro-2(3-phenylpropylamino)benzoic acid) and DIOA (R(+)[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden -5-y) oxy] acetic acid). In contrast, tamoxifen was without effect. The cell-swelling induced efflux of taurine was attenuated (40 per cent) by replacing external Cl- with NO3-. The efflux of glutamic acid was also markedly increased by a hyposmotic challenge. Niflumic acid inhibited both basal and volume-activated glutamic acid efflux. A hyposmotic shock also increased alpha-aminoisobutyric acid efflux but not that of 3-O-methylglucose and SO4(2)-. The results suggest that the human placenta can respond to cell-swelling by releasing organic osmolytes such as amino acids via a pathway which is sensitive to anion transport inhibitors. However, it appears that the volume-activated amino acid transport system is independent from the placental anion-exchange pathways. The efflux of these compounds may act with K+ and Cl- efflux to effect a regulatory volume decrease in placental tissue. In addition, volume-activated transport may play a role in transplacental amino acid transfer.

Chloride transport by human placental microvillous membrane vesicles

Unidirectional uptake of chloride by microvillous membrane vesicles prepared from human term placentas was studied over a range of membrane potentials in the presence and absence of chloride transport inhibitors alone and in combination at maximally effective concentrations. At 0 mV, inhibition of chloride uptake by 0.1 mM DIDS, 0.5 mM DPC, and 0.5 mM DPC plus 0.1 mM DIDS was similar, suggesting a common action upon an anion exchanger; neither 0.1 mM furosemide nor 0.1 mM bumetanide alone had any effect. An inside-positive membrane potential was created by imposing an inwardly directed potassium ion gradient in the presence of valinomycin. Total chloride uptake increased with increasing membrane potential (0, 4.6, 17.3, 25.8 and 32.0 mV). The inhibition of uptake by DPC and DPC/DIDS increased with the membrane potential. The effect of DPC compared to DPC/DIDS was significantly different at 4.6, 17.3 and 25.8 mV, suggesting a degree of additivity of inhibition. Neither furosemide nor bumetanide had any effect at any potential. There was a significant increase in inhibition due to DIDS alone until the membrane potential reached 25.8 mV. But there was no significant difference between the level of inhibition at 32 mV as compared to 0 mV, providing evidence of a DIDS-sensitive conductance similar to that previously seen in patch clamp studies. We suggest that uptake of chloride across the microvillous membrane of the human placenta may be by at least three different pathways; an electroneutral, DIDS-sensitive anion exchanger, a DPC-sensitive chloride conductance and a DIDS-sensitive chloride conductance.

Transtrophoblast and microvillus membrane potential difference in mature intermediate human placental villi

Single mature intermediate villi from term placentas after normal gestation and vaginal or cesarian delivery were identified microscopically and mounted in a bath for conventional micro-electrode studies. With the application of strict selection criteria, the following observations on electrical potential difference (PD) were made. 1) With the tissue bathed in Earle's medium (37 degrees C) the PD across the syncytiotrophoblast microvillus membrane with respect to the bath was not normally distributed. The median PD was -22 mV (range -12 to -60 mV, n = 200). This fell to -6 mV after prior incubation for 4 h with cyanide (3 mM) or iodoacetate (2 mM) but was not altered by short-term application of these agents or by 0.1 mM ouabain. Substituting Na+ in Earle's medium with choline had no effect on PD, but replacing Cl- with gluconate caused a depolarization of 6 mV (P < 0.002). Increasing KCl in the bath fluid revealed an apparently low microvillus membrane K+ conductance. The low microvillus membrane PD may reflect a low Na(+)-K(+)-ATPase activity and/or a low membrane permeability to K+. 2) The transtrophoblast PD measured by insertion and withdrawal of the electrode was significantly different from zero [P < 0.003, median -3 mV (range 0 to -15 mV), n = 11]; and PD measured by insertion of the electrode into the villus core and beyond was -6 mV (significantly different from zero P < 0.003, range -2.5 to -10 mV, n = 6). If a similar PD were to exist in vivo, it could have a significant influence on ion transport across the placenta at term.