Fig. 1.

Four structurally distinct segments of rat medullary thin limbs of Henle's loops (designated as types 1–4). Descending thin limbs (DTLs) that descend less than about 1 mm below the outer medulla (OM) express detectable aquaporin 1 (AQP1) in their OM segments and little or no AQP1 in their inner medullary (IM) segments. DTLs that descend more than about 1 mm below the OM express detectable AQP1 protein only along the initial 40% of their IM length; the terminal 60% expresses no detectable AQP1 protein (32, 34). The ascending thin limb (ATL) and prebend segment express the Cl⁻ channel ClCK1 along their entire lengths (18, 32, 40). Urea transporter UT-A2 protein has previously been shown to be weakly expressed in some IM DTLs, but only in short portions close to the OM-IM boundary (not shown) (19, 25). For the experiments conducted in this study, the upper DTL (DTL_upper) and upper ATL (ATL_upper) [red and green segments, respectively, from the upper IM (IM_upper)] were dissected from 0.5 to 2.5 mm below the OM; the lower DTL (DTL_lower) and lower ATL (ATL_lower) [yellow and green segments, respectively, from the lower IM (IM_lower)] were dissected from the terminal 2.5 mm of the IM (see methods for additional details).