Reduced water permeability and altered ultrastructure in thin descending limb of Henle in aquaporin-1 null mice

It has been controversial whether high water permeability in the thin desce nding limb of Henle (TDLH) is required for formation of a concentrated urin e by the kidney. Freeze-fracture electron microscopy (FFEM) of rat TDLH has shown an exceptionally high density of intramembrane particles (IMPs), whi ch were proposed to consist of tetramers of aquaporin-1 (AQP1) water channe ls. In this study, transepithelial osmotic water permeability(P-f) was meas ured in isolated perfused segments (0.5-1 mm) of TDLH in wild-type (+/+), A QP1 heterozygous (+/-), and AQP1 null (-/-) mice. P-f was measured at 37 de grees C using a 100 mM bath-to-lumen osmotic gradient of raffinose, and flu orescein isothiocyanate (FITC)-dextran as the luminal volume marker. P-f wa s (in cm/s): 0.26 +/- 0.02 ([+/+]; SE, n = 9 tubules), 0.21 +/- 0.01 ([+/-] ; n = 12), and 0.031 +/- 0.007 ([-/-]; n = 6) (P < 0.02, [+/+] vs. [+/-]; P < 0.0001, [+/+] vs. [-/-]). FFEM of kidney medulla showed remarkably fewer IMPs in TDLH from (-/-) vs. (+/+) and (+/-) mice. IMP densities were (in m u m(-2), SD, 5-12 micrographs): 5,880 +/- 238 (+/+); 5,780 +/- 450 (+/-); a nd 877 +/- 420 (-/-). IMP size distribution analysis revealed mean LMP diam eters of 8.4 nm ([+/+] and [+/-]) and 5.2 nm([-/-]). These results demonstr ate that AQP1 is the principal water channel in TDLH and support the view t hat osmotic equilibration along TDLH by water transport plays a key role in the renal countercurrent concentrating mechanism. The similar P-f and AQP1 expression in TDLH of(+/+) and (+/-) mice was an unexpected finding that p robably accounts for the unimpaired urinary concentrating ability in (+/-) mice.