Rapid downregulation of rat renal Na/P-i cotransporter in response to parathyroid hormone involves microtubule rearrangement

Renal proximal tubule cells express in their apical brush border membrane ( BBM) a Na/P-i cotransporter type IIa that is rapidly downregulated in respo nse to parathyroid hormone (PTH). We used the rat renal Na/P-i cotransporte r 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 P TH for 15 minutes, NaPi-2 abundance in the BBM was decreased. In parallel, transporter accumulated in intracellular vesicles. Concomitantly, microtubu les (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, w hereas their microtubular cytoskeleton had returned to its normal appearanc e. 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 dep letion of intracellular transporter after 60 minutes of PTH action. Further more, the subapical accumulation of NaPi-2 was associated with the expansio n of dense apical tubules of the subapical endocytic apparatus (SEA). Depol ymerization of MTs by colchicine likewise caused a retardation of intracell ular NaPi-2 depletion. These results suggest that NaPi-2 is downregulated i n 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.