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/kgH₂O; 116 mM NaCl) and then to either a 138-hyposmotic (138 mosmol/kgH₂O; 37 mM NaCl) or 170-hyposmotic (170 mosmol/kgH₂O; 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/kgH₂O) 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 of 5 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.

potassium; chloride; conductance; placenta; volume regulation

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