Sikka P K, McMartin K E
Department of Pharmacology and Therapeutics, Louisiana State University Medical Center, Shreveport 71130, USA.
Chem Biol Interact. 1998 Jul 3;114(1-2):15-31. doi: 10.1016/s0009-2797(98)00038-6.
Deficiency of the vitamin folic acid has recently been linked with increased incidence of neural tube defects and of cardiovascular disease, through elevated plasma homocysteine levels. The kidney has an important role in conserving folate to counteract development of deficiency. Urinary folate excretion is regulated by the degree of reabsorption of folate by the proximal tubule cell. To evaluate an in vitro model for studies of the regulation of urinary folate excretion, the present studies examined the transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu), the primary form of folate in the glomerular filtrate, by normal rat proximal tubule (RPT) cells in confluent monolayer cultures. Specific binding of 5-CH3-H4PteGlu to the apical membrane was saturable (K(D) = 27 nM), but intracellular transport was not saturated up to 100 nM concentrations. 5-CH3-H4PteGlu transport was decreased 50% by concentrations of folic acid that completely blocked 5-CH3-H4PteGlu binding by the apical folate receptor. Probenecid (10 mM), an anion exchange (reduced folate carrier) inhibitor, reduced 5CH3-H4PteGlu transport by 50% without significantly affecting binding. Aspirin (3 mM) did not alter 5-CH3-H4PteGlu transport, but significantly enhanced the inhibition due to probenecid. Similarly, indomethacin (5 microM) potentiated the inhibition of 5-CH3-H4PteGlu transport by probenecid. These data suggest that RPT cells take up 5-CH3-H4PteGlu by both the folate receptor and the reduced folate carrier, implying a role for both pathways in regulating urinary folate excretion.
最近，维生素叶酸缺乏与神经管缺陷和心血管疾病发病率增加有关，这是通过血浆同型半胱氨酸水平升高实现的。肾脏在保存叶酸以对抗缺乏症的发展方面起着重要作用。尿叶酸排泄受近端小管细胞对叶酸重吸收程度的调节。为了评估用于研究尿叶酸排泄调节的体外模型，本研究检测了正常大鼠近端小管（RPT）细胞在汇合单层培养物中对5-甲基四氢叶酸（5-CH3-H4PteGlu）的转运，5-甲基四氢叶酸是肾小球滤液中叶酸的主要形式。5-CH3-H4PteGlu与顶膜的特异性结合是可饱和的（K(D)=27 nM），但在高达100 nM的浓度下细胞内转运不饱和。完全阻断顶叶叶酸受体对5-CH3-H4PteGlu结合的叶酸浓度使5-CH3-H4PteGlu转运降低了50%。丙磺舒（10 mM），一种阴离子交换（还原叶酸载体）抑制剂，使5CH3-H4PteGlu转运降低50%，而对结合无明显影响。阿司匹林（3 mM）不改变5-CH3-H4PteGlu转运，但显著增强了丙磺舒的抑制作用。同样，吲哚美辛（5 microM）增强了丙磺舒对5-CH3-H4PteGlu转运的抑制作用。这些数据表明，RPT细胞通过叶酸受体和还原叶酸载体摄取5-CH3-H4PteGlu，这意味着这两条途径在调节尿叶酸排泄中均起作用。
