Loghman-Adham M, Dousa T P
Department of Pediatrics, University of Utah School of Medicine, Salt Lake City 84132.
Am J Physiol. 1992 Aug;263(2 Pt 2):F301-10. doi: 10.1152/ajprenal.1992.263.2.F301.
Phosphonoformic acid (PFA, foscarnet) was found to exert both an inhibitory and a stimulatory effect on Na(+)-dependent Pi transport in opossum kidney (OK) cells. When added in the uptake media, PFA produced a dose-dependent inhibition of Na(+)-Pi cotransport. PFA had no effect on the Na(+)-dependent transports of methyl-alpha-D-glucopyranoside (AMG) or L-alanine or on amiloride-sensitive Na(+)-H+ antiport. The inhibition of Na(+)-Pi cotransport was competitive [inhibitory constant (Ki) = 6.0 mM], reversible by dilution, and solute specific. When OK cells were incubated with PFA for longer time periods (1-15 h), the Na(+)-Pi uptake measured after removal of PFA was significantly increased, i.e., "upregulated." The extent of Na(+)-Pi cotransport upregulation was dependent on time (greater than or equal to 30 min) and dose of PFA (2-10 mM). The increase in Na(+)-Pi cotransport by upregulation with PFA was due to higher apparent Vmax with no change in apparent Michaelis constant (Km) for Pi and was solute specific: uptakes of AMG or L-proline were not changed. Removal of PFA from culture medium resulted in a fast reversal of upregulation. Upregulation was not inhibited by cycloheximide, actinomycin D, or cordycepin. Solute-specific increase of Na(+)-Pi cotransport was also found when measured in apical membrane vesicles isolated from OK cells. Thus PFA exerts a dual action on the Na(+)-Pi cotransporter of OK cells: 1) acute, competitive inhibition and 2) after prolonged exposure it increases Na(+)-Pi uptake, probably by insertion of Na(+)-Pi cotransporters into apical membrane.
膦甲酸（PFA，膦甲酸钠）对负鼠肾（OK）细胞中依赖钠离子的无机磷转运既有抑制作用，又有刺激作用。当添加到摄取培养基中时，PFA对钠离子-无机磷共转运产生剂量依赖性抑制。PFA对甲基-α-D-吡喃葡萄糖苷（AMG）或L-丙氨酸的钠离子依赖性转运或对氨氯地平敏感的钠离子-氢离子反向转运均无影响。钠离子-无机磷共转运的抑制是竞争性的[抑制常数（Ki）=6.0 mM]，可通过稀释逆转，且具有溶质特异性。当OK细胞与PFA孵育较长时间（1-15小时）时，去除PFA后测得的钠离子-无机磷摄取量显著增加，即“上调”。钠离子-无机磷共转运上调的程度取决于时间（大于或等于30分钟）和PFA的剂量（2-10 mM）。PFA上调导致的钠离子-无机磷共转运增加是由于表观最大转运速率（Vmax）升高，而无机磷的表观米氏常数（Km）没有变化，且具有溶质特异性：AMG或L-脯氨酸的摄取量没有改变。从培养基中去除PFA会导致上调的快速逆转。上调不受环己酰亚胺、放线菌素D或虫草素的抑制。在从OK细胞分离的顶端膜囊泡中进行测量时，也发现了钠离子-无机磷共转运的溶质特异性增加。因此，PFA对OK细胞的钠离子-无机磷共转运体有双重作用：1）急性竞争性抑制和2）长时间暴露后增加钠离子-无机磷摄取，可能是通过将钠离子-无机磷共转运体插入顶端膜实现的。
