Progressive increase in collecting duct basal/apical AQP2 expression ratio with increasing depth along the corticopapillary axis positively correlates with maximum urine concentrating capacity

Abstract

Our goal is to determine if AQP2 expression patterns in the renal medullary collecting duct (CD) contribute to the ability of the kangaroo rat to produce a more highly concentrated urine than that of the Munich‐Wistar rat. Inner medullas from kidneys of moderately concentrating animals were embedded in Spurrs and 1 μm thick transverse sections cut at 500 μm intervals throughout the corticopapillary axis and labeled for AQP2. Apical and basal plasma membrane AQP2 expression levels were quantified on a pixel‐by‐pixel basis in each CD in 0.11 mm2 sections. The basal/apical AQP2 expression ratio at 1000 μm below the outer medulla is nearly equal in both species (~0.50), whereas at 4500 μm below the outer medulla, the basal/apical AQP2 expression ratio is greater in the CD of kangaroo rat (~1.2) relative to Munich‐Wistar rat (~0.70). Prior studies have shown that transepithelial water permeability of the inner two‐thirds of Sprague‐Dawley rat isolated perfused inner medullary CDs markedly exceeds that of the outer third without vasopressin. The high water permeable segment correlates with high basal/apical AQP2 expression ratios. A heightened basal/apical AQP2 expression ratio in kangaroo rat CD is consistent with prior studies indicating peritubular hypertonicity is a regulatory factor of CD water permeability and basal/apical AQP2 expression ratios. NIDDK DK083338, NSF IOS095285, APS/NIDDK STEP‐UP, WAESO.