Paraiso M S, McAteer J A, Kempson S A
Department of Physiology and Biophysics, Indiana University School of Medicine, Indianapolis 46202-5120, USA.
Biochim Biophys Acta. 1995 Apr 28;1266(2):143-7. doi: 10.1016/0167-4889(95)00008-g.
Parathyroid hormone (PTH) inhibits Na(+)-dependent Pi uptake in renal epithelial cells from opossum kidney (OK). This requires an intact endocytic pathway, suggesting that one action of PTH may be to promote endocytic removal of Na+/Pi cotransporters from the cell membrane. The present study tested if PTH, at a dose that inhibited membrane Pi transport, also produced an increase in endocytic activity. Pi transport was measured in isolated plasma membrane vesicles. Endocytosis was measured by allowing cells to take up horseradish peroxidase (HRP) followed by assay of triton-sensitive (latent) HRP activity in subcellular fractions isolated by density gradient centrifugation. Incubation of OK cells with 10(-7) M PTH for 3 h decreased Na+/Pi cotransport by membrane vesicles to 328 +/- 54 pmol/mg/min compared to 448 +/- 67 pmol/mg/min (mean +/- S.E., P < 0.03) in controls. Latent HRP content of endosomal fractions was dependent on the time and temperature used to load cells with HRP and on the concentration of HRP. However, incubation of OK cells with 10(-7) M PTH for either 1 or 3 h produced no change in latent HRP activity. Thus the action of PTH on the Na+/Pi cotransporter in the plasma membrane of OK cells does not require a change in the rate of endocytosis.
甲状旁腺激素（PTH）可抑制负鼠肾（OK）肾上皮细胞中依赖钠离子的磷酸盐摄取。这需要完整的内吞途径，提示PTH的一种作用可能是促进细胞膜上钠离子/磷酸盐共转运体的内吞性清除。本研究检测了在抑制膜磷酸盐转运的剂量下，PTH是否也会使内吞活性增加。在分离的质膜囊泡中测量磷酸盐转运。通过让细胞摄取辣根过氧化物酶（HRP），然后在通过密度梯度离心分离的亚细胞组分中检测对 Triton 敏感的（潜伏的）HRP 活性来测量内吞作用。与对照组中 448±67 pmol/mg/min（平均值±标准误，P<0.03）相比，用 10⁻⁷ M PTH 孵育 OK 细胞 3 小时后，膜囊泡的钠离子/磷酸盐共转运降低至 328±54 pmol/mg/min。内体组分的潜伏 HRP 含量取决于用 HRP 加载细胞所用的时间和温度以及 HRP 的浓度。然而，用 10⁻⁷ M PTH 孵育 OK 细胞 1 小时或 3 小时，潜伏 HRP 活性均无变化。因此，PTH 对 OK 细胞质膜中钠离子/磷酸盐共转运体的作用并不需要内吞速率的改变。
