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1 Medicine, Renal Division, Emory University, Atlanta, Georgia, United States
2 Medicine, Renal Division, Emory University, Atlanta, Georgia, United States; Physiology, Emory University, Atlanta, Georgia, United States
* To whom correspondence should be addressed. E-mail: mabloun{at}emory.edu.
UT-A1 is regulated by vasopressin and localized to the apical membrane and intracellular compartment of inner medullary collecting duct (IMCD) cells. UT-A3 is also expressed in the IMCD and regulated by forskolin in heterologous systems. The goal of the present study is to investigate mechanisms by which vasopressin regulates UT-A3 in rat IMCD. In fresh rat IMCD suspensions, forskolin increases the phosphorylation of UT-A3. Biotinylation studies indicate that UT-A3 is located in the plasma membrane. Forskolin treatment increases the abundance of UT-A3 in the plasma membrane similar UT-A1. However, these two transporters do not form a complex through a protein-protein interaction, suggesting that transporter function is unique to each protein. While immunohistochemistry localized UT-A3 to the basal and lateral membranes, a majority of the staining was cytosolic. Immunohistochemistry of vasopressin-treated rat kidney sections also localized UT-A3 primarily to the cytosol with basal and lateral membrane staining but also showed apical membrane staining in some IMCD cells. This suggests that under normal conditions, UT-A3 functions as the basolateral transporter but in a high cAMP-environment, the transporter may move from the cytosol to all plasma membranes to increase urea flux in the IMCD. In summary, this study confirms that UT-A3 is located in the inner medullary tip where it is expressed in the basolateral membrane, shows that IMCD UT-A3 is a phosphoprotein in rat that can be trafficked to the plasma membrane independent of UT-A1, and suggests that vasopressin may induce UT-A3 expression in the apical plasma membrane of IMCD.
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