RESUMEN
The rate-limiting step for the maternofetal exchange of low molecular-weight solutes in humans is constituted by transport across a single epithelial layer (syncytiotrophoblast) of the placenta. Other than the well-established presence of a large-conductance, multisubstate Cl- channel, the ionic channels occurring in this syncytial tissue are, for the most part, unknown. We have found that fusion of apical plasma membrane-enriched vesicle fractions with planar lipid bilayers leads, mainly (96% of 353 reconstitutions), to the reconstitution of nonselective cation channels. Here we describe the properties of this novel placental conductance at the single-channel level. The channel has a large (>200 pS) and variable conductance, is cation selective (P(Cl)/P(K) approximately or approximately equal 0.024), is reversibly inhibited (presumably blocked) by submillimolar La3+, has very unstable kinetics, and displays a large number (>10) of current sublevels with a "promiscuous" connectivity pattern. The occurrence of both "staircaselike" and "all-or-nothing" transitions between the minimum and maximum current levels was intriguing, particularly considering the large number of conductance levels spanned at a time during the concerted current steps. Single-channel data simulated according to a multistate linear reaction scheme, with rate constants that can vary spontaneously in time, reproduce many aspects of the recorded subconductance behavior. The channel's sensitivity to lanthanides is reminiscent of stretch-sensitive channels which, in turn, suggests a physiological role for this ion channel as a mechanotransducer during syncytiotrophoblast-volume regulation.
Asunto(s)
Vellosidades Coriónicas/fisiología , Canales Iónicos/fisiología , Lantano/farmacología , Metales de Tierras Raras/farmacología , Placenta/fisiología , Vellosidades Coriónicas/efectos de los fármacos , Conductividad Eléctrica , Electrofisiología , Células Epiteliales/efectos de los fármacos , Células Epiteliales/fisiología , Femenino , Humanos , Canales Iónicos/efectos de los fármacos , Membrana Dobles de Lípidos , Potenciales de la Membrana/efectos de los fármacos , Potenciales de la Membrana/fisiología , EmbarazoRESUMEN
The capacity of Al3+ to promote oxidative damage to brain membranes was investigated both in vitro and in vivo. In vitro, Al3+ and related metals (Sc3+, Ga3+, In3+, Be2+, Y3+, and La3+) stimulated Fe2+-initiated lipid and protein oxidation in brain myelin and synaptic membranes. Al3+, Sc3+, Y3+, and La3+ significantly promoted protein-associated carbonyl production in myelin, while in synaptic membranes, the stimulatory effect was observed in the presence of Ga3+, In3+, Y3+, Sc3+, and La3+. In myelin the magnitude of the stimulation of lipid oxidation followed the order Sc3+, Y3+, La3+ > Al3+, Ga3+, In3+ > Be2+. When compared to mitochondria and microsomal and synaptic membranes, myelin showed a marked susceptibility to Al3+-mediated lipid peroxidation. The differential susceptibility of myelin compared to synaptic membranes could not be explained by differences in membrane composition, since the relative content of negatively charged phospholipids (binding sites) was similar for both membranes, and myelin had a lower content of poly-unsaturated fatty acids (substrates of lipid oxidation) and a higher concentration of alpha-tocopherol compared to synaptic membranes. In a model of Al3+ intoxication imposed to mice during pregnancy and early development, a 72% higher content of lipid peroxidation products was found in brain myelin. The fluidity of myelin evaluated by the polarization fluorescence of 1,3-diphenylhexatriene was significantly higher in the Al3+-intoxicated mice than in controls. Since myelin has a high relative content of lipid:protein compared to other membranes, these results support our hypothesis that ions without redox capacity can stimulate in vitro and in vivo lipid oxidation by promoting phase separation and membrane rigidification, thus accelerating lipid oxidation.