PARATHYROID HORMONE-DEPENDENT DEGRADATION OF TYPE-II NA+ P-I COTRANSPORTERS/

Parathyroid hormone (PTH) inhibits proximal tubular brush border membr ane Na+/P-i cotransport activity; this decrease in the transport activ ity was found to be associated with a decrease in type II Na+/P-i cotr ansporter protein content in rat brush border membranes, In the presen t study we investigated the PTH-dependent regulation of the type II Na +/P-i cotransporter in opossum kidney cells, a previously established model to study cellular mechanisms involved in the regulation of proxi mal tubular Na+/P-i cotransport. We transfected opossum kidney cells w ith a cDNA coding for NaPi-2 (rat renal type II Na+/P-i cotransporter) , This allowed the study of PTH-dependent regulation of the transfecte d NaPi-2 and of the corresponding intrinsic cotransporter (NaPi-4). Th e results show (i) that the intrinsic and the transfected cotransporte rs are functionally (transport) and morphologically (immunofluorescenc e) localized at the apical membrane, (ii) that the intrinsic as well a s the transfected Na+/P-i cotransport activities are inhibited by PTH, (iii) that PTH leads to a retrieval of both cotransporters from the a pical membrane, (iv) that both cotransporters are rapidly degraded in response to PTH, and (v) that the reappearance/recovery of type II Na/P-i cotransporter protein and function from PTH inhibition requires d e novo protein synthesis. These results document that PTH leads to a r emoval of type II Na+/P-i cotransporters from the apical membrane and to their subsequent degradation.