Osmotic water permeabilities in specific segments of rat inner medullary thin limbs of Henle's loops

Abstract

Three‐dimensional reconstructions of thin limbs of Henle's loops in rat renal inner medulla (IM) involving antibody to constitutive water channel aquaporin 1 (AQP1) have revealed that descending thin limbs (DTLs) of loops extending deeper than 1.0 mm express AQP1 for first 40% of their length, but fail to express it for remaining 60%. Ascending thin limbs (ATLs) and prebend segments do not express AQP1. Therefore, we examined water permeabilities in isolated perfused segments of DTLs from specific regions of IM and in ATLs. We perfused DTL segments from two IM regions: 1) final 2.0‐2.5 mm, where all DTLs lack AQP1, and 2) initial 2.5 mm. In initial 2.5 mm, we chose DTLs extending beyond first 1 mm because all of these should contain AQP1. DTLs from AQP1‐null region showed no osmotic water permeability (Pf = ‐292 ± 166 μm/s; mean ± SE; n=4). ATLs also showed no osmotic water permeability (Pf = 69 ± 69 μm/s; mean ± SE; n = 10). In contrast, DTL segments from initial 2.5 mm had mean osmotic water permeability of 1670 ± 169 μm/s (mean ± SE; n = 6). However, values for individual tubule segments varied from about 1200 to 2100 μm/s. Since there is no way to determine if segments perfused expressed AQP1 for their entire length, it seems likely that segments with lower values had regions that did not express AQP1. These data indicate that presence or absence of AQP1 assessed by immunocytochemistry accurately reflects differences in water permeability. DK16294