Single-channel characterization of a nonselective cation channel from human placental microvillous membranes. Large conductance, multiplicity of conductance states, and inhibition by lanthanides

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) ≃ 0.024), is reversibly inhibited (presumably blocked) by submillimolar La³⁺, has very unstable kinetics, and displays a large number (>10) of current sub-levels 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.