Ultrastructural localization of UT-A and UT-B in rat kidneys with different hydration status

Urea transport in the kidney is mediated by a family of transporter proteins, including renal urea transporters (UT-A) and erythrocyte urea transporters (UT-B). We aimed to determine whether hydration status affects the subcellular distribution of urea transporters. Male Sprague-Dawley rats were divided into three groups: dehydrated rats (WD) given minimum water, hydrated rats (WL) given 3% sucrose in water for 3 days before death, and control rats given free access to water. We labeled kidney sections with antibodies against UT-A₁ and UT-A₂ (L194), UT-A₃ (Q2), and UT-B using preembedding immunoperoxidase and immunogold methods. In control animals, UT-A₁ and UT-A₃ immunoreactivities were observed throughout the cytoplasm in inner medullary collecting duct (IMCD) cells, and weak labeling was observed on the basolateral plasma membrane. UT-A₂ immunoreactivity in the descending thin limbs (DTL) was observed mainly on the apical and basolateral membranes of type I epithelium, and very faint labeling was observed in the long-loop DTL at the border between the outer and inner medulla. UT-A₁ immunoreactivity intensity was markedly lower, and UT-A₃ immunoreactivity was higher in IMCD of WD vs. controls. UT-A₂ immunoreactivity intensities in the plasma membrane and cytoplasm of type I, II, and III epithelia of DTL were greater in WD vs. controls. In contrast, UT-A₁ expression was greater and UT-A ₂ and UT-A₃ expressions were lower in WL vs. controls. The subcellular distribution of UT-A in DTL or IMCD did not differ between control and experimental animals. UT-B was expressed in the plasma membrane of the descending vasa recta of both control and experimental animals. UT-B intensity was higher in WD and lower in WL vs. controls. These data indicate that changes in hydration status over 3 days affected urea transporter protein expression without changing its subcellular distribution.