Urea permeabilities in defined segments of rat renal inner medullary thin limbs of Henle's loops

Abstract

Urea permeabilities and resulting urea fluxes in specific segments of thin limbs of Henle's loops are critical to concentrating urine in rat renal inner medulla (IM) but have not yet been well‐defined. Immunohistochemistry has shown that urea transporter UT‐A2 is expressed in rat descending thin limbs (DTLs) near outer medullary (OM)‐IM border but not more than 0.5 mm below that point. No other urea transporters have been identified in rat IM. To understand the role that urea fluxes may play in the urine concentrating mechanism, we determined urea permeabilities (Purea) from 14C‐urea fluxes in perfused rat DTLs and ascending thin limbs (ATLs) isolated from 1) initial 2.5 mm below OM‐IM border (upper IM) and 2) terminal 2.5 mm of the IM (lower IM). Regardless of flux direction, Purea (x 10−5 cm/s) in upper IM was 49.6 ± 7.7 (mean ± SE) in DTLs and 192.9 ± 23.6 in ATLs. In lower IM, it was 208 ± 31 in DTLs and 225 ± 38 in ATLs. Phloretin (0.25 mM) had no effect on Purea in upper DTLs, suggesting that Purea was not due to the UT‐A2 transporter. Purea is similar (~200 × 10−5 cm/s) in lower DTLs and upper and lower ATLs, all segments that lack water channel AQP1 and have no osmotic water permeability, but is significantly lower (~50 × 10−5 cm/s) in upper DTLs, segments that express AQP1 and have high osmotic water permeabilities. Pathways or transporters accounting for urea permeabilities are unknown.