Effect of hyposmotic challenge on microvillous membrane potential in isolated human placental villi

This study examined the effect of hyposmotic solutions on the syncytiotrophoblast microvillous membrane potential (E-m) in mature intermediate villi isolated from term human placentas. When villi were exposed to a control solution (280 mosmol/kgH(2)O; 116 mM NaCl) and then to either a 138-hyposmotic (138 mosmol/kgH(2)O; 37 mM NaCl) or 170-hyposmotic (170 mosmol/kgH(2)O; 55 mM NaCl) solution, there was a significant hyperpolarization of E-m (-5.1 +/- 1.5 mV, P < 0.01 and -5.0 +/- 0.5 mV, P < 0.001, respectively; n = 10), which was reversible on removal of the hyposmotic stimulus. Low-NaCl (37 and 55 mM) solutions made isosmotic with control (i.e., 280 mosmol/kgH(2)O) by addition of raffinose did not significantly alter E-m, suggesting that reducing NaCl concentration per se had no effect on E-m. Exposure to 170-hyposmotic solution in the presence of5 mM BaCl2 depolarized E-m by +4.1 +/- 0.7 mV (P < 0.001, n = 6); BaCl2 similarly depolarized E-m when added in control solution (+5.6 +/- 1.1 mV, n = 5). Exposure to 170-hyposmotic solution containing 1 mM DIDS hyperpolarized E-m by -9.0 +/- 1.7 mV (P < 0.001, n = 5). This degree of hyperpolarization was significantly greater than that observed in hyposmotic solution alone (P < 0.01) but was not different from the hyperpolarization when DIDS was added to control solution (-7.4 +/- 0.2 mV, n = 6). We conclude 1) that Ba2+-sensitive K+ conductances and DIDS-sensitive anion conductances contribute to the resting potential of the syncytiotrophoblast microvillous membrane and 2) that the syncytiotrophoblast microvillous membrane responds to a hyposmotic stimulus by activating both Ba2+-sensitive K+ and DIDS-sensitive anion conductances.