Cho M J, Adson A, Kezdy F J
Drug Delivery Systems Research, Upjohn Company, Kalamazoo, Michigan 49001.
Pharm Res. 1990 Apr;7(4):325-31. doi: 10.1023/a:1015802918845.
Transport of 14C-labeled acetic, propionic (PA), butyric, valeric, heptanoic (HA), and octanoic (OA) acids across the Madin Darby canine kidney (MDCK) epithelial cell monolayer grown on a porous polycarbonate membrane was studied in Hanks' balanced salt solution (HBSS) at 37 degrees C in both apical-to-basolateral and basolateral-to-apical directions. At micromolar concentrations of solutes, metabolic decomposition was significant as evidenced by [14C]CO2 production during the OA transport. The apparent permeability (Pe) indicates that as lipophilicity increases, diffusion across the "unstirred" boundary layer becomes rate limiting. In support of this notion, transport of OA and HA was enhanced by agitation, showed an activation energy of 3.7 kcal/mol for OA, and resulted in identical Pe values for both transport directions. Analysis of Pe changes with varying alkyl chain length resulted in a delta G of -0.68 +/- 0.09 kcal/mol for -CH2-group transfer from an aqueous phase to the MDCK cells. When the intercellular tight junctions were opened by the divalent chelator EGTA in Ca2+/Mg2(+)-free HBSS, transport of the fluid-phase marker Lucifer yellow greatly increased because of paracellular leakage. PA transport also showed a significant increase, but OA transport was independent of EGTA. Although albumin also undergoes paracellular transport in the presence of EGTA and OA binds strongly to albumin, OA transport in EGTA solution was unchanged by albumin. These observations indicate that transmembrane transport is the major mechanism for lipophilic substances. The present study, together with earlier work on the transport of polar substances, shows that the MDCK cell monolayer is an excellent model of the transepithelial transport barrier.
在37摄氏度的汉克斯平衡盐溶液（HBSS）中，研究了14C标记的乙酸、丙酸（PA）、丁酸、戊酸、庚酸（HA）和辛酸（OA）在生长于多孔聚碳酸酯膜上的麦氏犬肾（MDCK）上皮细胞单层中的跨膜运输，运输方向包括从顶端到基底外侧以及从基底外侧到顶端。在微摩尔浓度的溶质条件下，代谢分解较为显著，这在OA运输过程中[14C]CO2的产生中得到了证明。表观渗透率（Pe）表明，随着亲脂性增加，跨“未搅动”边界层的扩散成为限速步骤。支持这一观点的是，OA和HA的运输通过搅拌得到增强，OA的活化能为3.7千卡/摩尔，并且两个运输方向的Pe值相同。分析Pe随烷基链长度变化的情况，得出从水相到MDCK细胞转移-CH2-基团的ΔG为-0.68±0.09千卡/摩尔。当在无Ca2+/Mg2+的HBSS中用二价螯合剂EGTA打开细胞间紧密连接时，由于细胞旁渗漏，液相标记物路西法黄的运输大大增加。PA的运输也显著增加，但OA的运输与EGTA无关。尽管白蛋白在EGTA存在时也会进行细胞旁运输，且OA与白蛋白强烈结合，但在EGTA溶液中OA的运输不受白蛋白影响。这些观察结果表明，跨膜运输是亲脂性物质的主要运输机制。本研究与早期关于极性物质运输的工作一起表明，MDCK细胞单层是跨上皮运输屏障的一个优秀模型。
