Malmström K, Murer H
Am J Physiol. 1986 Jul;251(1 Pt 1):C23-31. doi: 10.1152/ajpcell.1986.251.1.C23.
Na+-dependent phosphate transport and its response to parathyroid hormone (PTH) has been investigated in three continuous cell lines of renal epithelial origin (LLC-PK1, JTC-12.P3, and OK). The apparent Km for phosphate was similar, but the maximal transport rate (Vmax) was markedly different in the three cell lines. PTH and forskolin produced an increase of cellular adenosine 3',5'-cyclic monophosphate (cAMP) in all cell lines, but Na+-dependent phosphate transport was inhibited exclusively in the OK cells (a threefold reduction of influx after 4 h of exposure to 10(-10) M PTH). The change in phosphate transport is accounted for by a lowered Vmax (30.8 +/- 5.3 vs. 10.2 +/- 1.1 pmol X mg-1 X 3 min-1). The reduction in phosphate transport was reversible, such that 5 h after removal of PTH the Vmax had increased threefold over the inhibited state. Addition of PTH did not alter Na+-dependent L-alanine influx in the OK cells. Experiments with apical membrane vesicles showed that the change in Vmax occurred at the membrane level. It is concluded that the regulatory event responsible for PTH-reduced phosphate transport is beyond cAMP. Of the cell lines studied, only OK cells have a complete regulatory cascade.
在三种源自肾上皮的连续细胞系（LLC-PK1、JTC-12.P3和OK）中，研究了钠依赖性磷酸盐转运及其对甲状旁腺激素（PTH）的反应。三种细胞系中磷酸盐的表观米氏常数（Km）相似，但最大转运速率（Vmax）明显不同。PTH和福斯可林在所有细胞系中均使细胞内环磷酸腺苷（cAMP）增加，但仅OK细胞中的钠依赖性磷酸盐转运受到抑制（暴露于10^(-10) M PTH 4小时后，流入量降低了三倍）。磷酸盐转运的变化是由于Vmax降低所致（30.8±5.3对10.2±1.1 pmol·mg^(-1)·3 min^(-1)）。磷酸盐转运的降低是可逆的，即去除PTH 5小时后，Vmax比抑制状态增加了三倍。添加PTH并未改变OK细胞中钠依赖性L-丙氨酸的流入。顶膜囊泡实验表明，Vmax的变化发生在膜水平。得出的结论是，导致PTH降低磷酸盐转运的调节事件超出了cAMP的范围。在所研究的细胞系中，只有OK细胞具有完整的调节级联反应。
