Renal proximal tubule cells express in their apical brush border membrane (BBM) a Na/P i cotransporter type IIa that is rapidly downregulated in response to parathyroid hormone (PTH). We used the rat renal Na/P i cotransporter type IIa (NaPi-2) as an in vivo model to assess early cellular events in the rapid downregulation of this transporter. When rats were treated with PTH for 15 minutes, NaPi-2 abundance in the BBM was decreased. In parallel, transporter accumulated in intracellular vesicles. Concomitantly, microtubules (MTs) were found to form dense bundles of apical-to-basal orientation. After 60 minutes of PTH action, the cells were vastly depleted of NaPi-2, whereas their microtubular cytoskeleton had returned to its normal appearance. Prevention of MT rearrangement by taxol resulted in accumulation of NaPi-2 in the subapical cell portion after 15 minutes and a strong delay in depletion of intracellular transporter after 60 minutes of PTH action. Furthermore, the subapical accumulation of NaPi-2 was associated with the expansion of dense apical tubules of the subapical endocytic apparatus (SEA). Depolymerization of MTs by colchicine likewise caused a retardation of intracellular NaPi-2 depletion.These results suggest that NaPi-2 is downregulated in response to PTH through a rapid endocytic process in 2 separate steps: (a) internalization of the transporter into the SEA, and (b) its delivery to degradative organelles by a trafficking mechanism whose efficiency depends on a taxol-sensitive rearrangement of MTs.J. Clin. Invest. 104:483-494 (1999). the plasma membrane: the insulin-stimulated glucose transporter in cells of fat and muscle tissue, GLUT-4, and the vasopressin-responsive water channel in principal cells of the renal collecting duct, AQP2, respectively (10, 11). After these transport systems have been inserted into the plasma membrane upon the specific hormone stimulus, removal of the respective hormone results in rapid internalization of GLUT-4 and AQP2, probably by constitutive endocytosis via clathrin-coated pits (12)(13)(14). Most likely, GLUT-4 and AQP2 enter the recycling pathway to be reinserted, eventually, into the plasma membrane. This transporter recycling strongly contrasts with the fate of the Na/P i cotransporter, which is apparently degraded within a few hours in the process of downregulation. The downregulation of the Na/P i cotransporter is, therefore, mechanistically more reminiscent of the downregulation of liganded receptors for growth factors through receptor-mediated endocytosis (RME) than any known transporter (15). In this respect, the further investigation of Na/P i cotransporter downregulation is expected to provide insight into novel cell biological principles of transport regulation.The purpose of this study was to reveal early cellular events in Na/P i cotransporter downregulation that fill the mechanistic gap between transporter removal from the BBM and transporter degradation in lysosomes. More specifically, by studying the PTH-induced downregulation of the rat renal...