Inwardly rectifying currents in human term placental cells

Inwardly rectifying K(+) current and differentiation of human placental cytotrophoblast cells in culture

Ion transport is important for driving nutrient transport across the syncytiotrophoblast and yet is poorly understood. We have examined K(+)currents under basal conditions in cultured cytotrophoblast cells, at various stages of differentiation, using the whole cell patch clamp technique. Cytotrophoblast cells were isolated from human term placenta and maintained in culture for up to 3 days. Cells were studied at four stages of progressive morphological differentiation: (i) mononuclear cells, (ii) mononuclear cells in aggregates, (iii) small multinucleate cells and (iv) large multinucleate syncytiotrophoblast-like cells. In the conditions of whole cell recording the only K(+) selective current identified in all cell types was a strong inwardly rectifying current which was sensitive to Ba(2+) and Cs(+). This current was unaffected by intracellular ATP whereas intracellular GTPgammas caused either run down of the current or activated a linear current. The characteristics of the current described are consistent with those of the inwardly rectifying K(+) channel Kir2.1. The inwardly rectifying K(+) current was observed in three out of 19 (16 per cent ) mononuclear cells, seven out of 21 (33 per cent ) mononuclear aggregates, eight out of 21 (38 per cent ) small multinucleate cells and 16 out of 19 (84 per cent ) large multinucleate cells. This inwardly rectifying K(+) current is likely to have an important role in determining net K(+) diffusion across the syncytiotrophoblast cell membrane, perhaps increasing in importance as the cells terminally differentiate.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

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.

Whole-cell outward currents in freshly dissociated rat placental cells

1. We applied whole cell voltage clamp techniques to freshly dissociated rat placental cells (20-21 days gestation). Tetraethylammonium (TEA)-sensitive outward currents were recorded in about 50% of cells. 2. The outward current had a reversal potential of -50 mV which is more positive than the potassium equilibrium potential (-82 mV). 3. The bath solution without NaCl decreased the outward current amplitude, while the elimination of only Na ions from the bath solution did not modify the outward current. 4. The results suggest a possible contribution of chloride ions to the outward currents in rat placental cells.

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.