Grassl S M
Department of Pharmacology, State University of New York Health Science Center, Syracuse 13210, USA.
Biochim Biophys Acta. 1996 Jun 13;1282(1):115-23. doi: 10.1016/0005-2736(96)00048-x.
Membrane transport pathways for transplacental transfer of sulfate were investigated by assessing the possible presence of a bicarbonate-coupled anion exchange mechanism for sulfate in the maternal facing membrane of human placental epithelial cells. The presence of a SO42-/HCO3- exchange mechanism was determined from 35SO42-tracer flux measurements in preparations of purified brush-border membrane vesicles. Under 10% CO2/90% N2 the imposition of an outwardly directed bicarbonate gradient (pH0 6/pHi 7.5) stimulated sulfate uptake to levels approximately 4-fold greater than observed at equilibrium. Maneuvers designed to offset the development of ion gradient-induced diffusion potentials (valinomycin, [K+]0 = [K+]i) significantly reduced bicarbonate gradient-induced sulfate uptake but concentrative accumulation of sulfate persisted. Early time point determinations performed in the presumed absence of membrane potential suggest the reduced level of bicarbonate gradient-induced sulfate uptake resulted from a more rapid dissipation of the imposed bicarbonate gradient. Concentrative accumulation of sulfate was not observed in the presence of a pH gradient alone under 100% N2. suggesting a preference of bicarbonate over hydroxyl ions as substrates for exchange. Static head determinations of opposing sulfate and bicarbonate gradients resulting in zero net flux of sulfate suggests the anion exchange mechanism mediates the electroneutral exchange of 2 bicarbonate or 1 carbonate for each sulfate. Sulfate uptake was increased with increasing intravesicular concentrations of carbonate at constant bicarbonate but was constant with increasing intravesicular concentrations of bicarbonate at constant carbonate suggesting carbonate as a substrate for anion exchange. The mechanism mediating bicarbonate gradient-induced sulfate uptake was sensitive to inhibition by stilbene derivatives, furosemide, bumetanide and probenecid. Substrate specificity studies suggest possible interactions of the anion exchange mechanism with salicylate, butyrate, thiosulfate, sulfite, selenate, chromate and oxalate. The results of this study provide evidence for the presence of a bicarbonate-coupled anion exchange mechanism as an electroneutral pathway for sulfate transport across the maternal-facing membrane of human placental epithelial cells.
通过评估人胎盘上皮细胞母体侧膜中是否存在硫酸盐的碳酸氢盐偶联阴离子交换机制，对硫酸盐经胎盘转运的膜运输途径进行了研究。从纯化的刷状缘膜囊泡制剂中的³⁵SO₄²⁻示踪通量测量结果确定了SO₄²⁻/HCO₃⁻交换机制的存在。在10%CO₂/90%N₂条件下，施加外向性碳酸氢盐梯度（pH₀6/pHi7.5）刺激硫酸盐摄取至比平衡时观察到的水平高约4倍。旨在抵消离子梯度诱导扩散电位发展的操作（缬氨霉素，[K⁺]₀ = [K⁺]i）显著降低了碳酸氢盐梯度诱导的硫酸盐摄取，但硫酸盐的浓缩积累仍然存在。在假定不存在膜电位的情况下进行的早期时间点测定表明，碳酸氢盐梯度诱导的硫酸盐摄取水平降低是由于施加的碳酸氢盐梯度更快消散。在100%N₂下仅存在pH梯度时未观察到硫酸盐的浓缩积累，这表明碳酸氢盐比氢氧根离子更适合作为交换底物。对相反的硫酸盐和碳酸氢盐梯度进行静态头部测定，导致硫酸盐净通量为零，这表明阴离子交换机制介导了每一个硫酸盐与两个碳酸氢盐或一个碳酸盐的电中性交换。在碳酸氢盐浓度恒定的情况下，随着囊泡内碳酸盐浓度的增加，硫酸盐摄取增加，但在碳酸盐浓度恒定的情况下，随着囊泡内碳酸氢盐浓度的增加，硫酸盐摄取保持恒定，这表明碳酸盐是阴离子交换的底物。介导碳酸氢盐梯度诱导的硫酸盐摄取的机制对芪衍生物、速尿、布美他尼和丙磺舒的抑制敏感。底物特异性研究表明阴离子交换机制可能与水杨酸盐、丁酸盐、硫代硫酸盐、亚硫酸盐、硒酸盐、铬酸盐和草酸盐相互作用。本研究结果为存在一种碳酸氢盐偶联阴离子交换机制提供了证据，该机制是硫酸盐跨人胎盘上皮细胞母体侧膜运输的电中性途径。
