Parathyroid hormone-induced translocation of Na-H antiporters in rat proximal tubules

C. B. Hensley, M. E. Bradley and A. K. Mircheff Department of Physiology, University of Southern California School of Medicine, Los Angeles 90033. Parathyroid hormone (PTH) is believed to inhibit bicarbonate reabsorption by inhibiting Na-H antiport activity in proximal tubular brush-border membranes. The sequence of events triggered by PTH was investigated in a crude preparation of proximal tubules obtained by mechanical disruption and filtration through nylon mesh filters. Tubule samples were subjected to analytical subcellular fractionation after 2-, 5-, and 30-min treatments with 1 IU/ml PTH. These PTH-treatment intervals caused 54, 63, and 68% decreases in the Na-H antiport activity of a population of brush-border membrane vesicles that was resolved from a PTH-unresponsive brush-border population by density-gradient centrifugation. The rapid loss of Na-H antiport activity from the responsive population was accompanied by a transient increase in the Na-H antiport activity of a region of the density gradient, designated density window III, which was shown to contain two distinct membrane populations; these populations were both enriched in acid phosphatase activity, and one of them was also an important locus of galactosyltransferase activity. The increase in the Na-H antiport activity of window III accounted for 52% of the activity lost from the PTH-responsive population after 2 min, and for 43% of the activity lost after 5 min, but it was completely abolished after 25 more minutes in the presence of PTH. These observations suggest that PTH triggers a rapid translocation of Na-H antiporters from the microvillus membrane to a distinct membrane domain, where they are subsequently inactivated.