PARATHYROID-HORMONE INHIBITS PLASMA-MEMBRANE PI-TRANSPORT WITHOUT CHANGING ENDOCYTIC ACTIVITY IN OPOSSUM KIDNEY-CELLS

Parathyroid hormone (PTH) inhibits Na+-dependent Pi uptake in renal ep ithelial cells from opossum kidney (OK). This requires an intact endoc ytic pathway, suggesting that one action of PTH may be to promote endo cytic removal of Na+/Pi cotransporters from the cell membrane. The pre sent study tested if PTH, at a dose that inhibited membrane Pi transpo rt, also produced an increase in endocytic activity. Pi transport was measured in isolated plasma membrane vesicles. Endocytosis was measure d by allowing cells to take up horseradish peroxidase (HRP) followed b y assay of triton-sensitive (latent) HRP activity in subcellular fract ions isolated by density gradient centrifugation. Incubation of OK cel ls 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 cell s 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 late nt 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.