The 2.9 kb mRNA of the urea transporter ut2 expressed in the upper part of the rat inner medulla is selectively increased by acute adh administration

Using a cDNA probe specific for rat UT2, we investigated by Northen blot (NB) analysis in normal rat kidneys, (1) the expression of UT2 mRNA in different subregions of the outer (OM) and inner medulla (IM), and (2) the shortterm influence of ADH. RNA was extracted from superficial and deep halves of inner stripe (IS) of OM, and from upper third and lower two thirds of IM. (1) Confirming and extending previous findings (Smith et al, JCI 96:1556, 1995), NB revealed the short transcript (2.9 kb) of UT2 in deep IS and upper IM, whereas the long transcript (4.0 kb) was mostly confined to deep IM. Only a weak signal was found in superficial IS. As suggested by others, the IS signal probably corresponds to the late thin descending limbs of short loops and the 4.0 kb signal in deep IM to terminal collecting ducts (CD). The structure expressing 2.9 kb message in upper tM is not yet identified. (2) In rats given 1 ug dDAVP (a pure V2 agonist of ADH) in oil s.c. 18 hrs prior to sacrifice, NB revealed a marked increase in 2.9 kb UT2 mRNA in upper IM (UT2/GAPDH, 2.29 ±0.3 vs 1.02 ±0.05 in rats given vehicle only) and no significant changes in deep IS and deep IM. These results suggest that a renal structure expresses the short UT2 transcript in the early IM and that this expression is ADH-sensitive. Considering that long loops should possess a facilitated urea transport in their thin limb (as do short loops), and because their thin ascending limb (ATL) is known to possess an ADHsensitive adenylate cyclase, it is conceivable that the ADH-responsive 2.9 kb message in upper IM originates from ATL. This would mean that most of the urea recycled from the CD reenters both short and long loops just before entry in thick ascending limbs. The possible localization of UT2 in ATL and the possible influence of ADH on urea permeability in this segment need to be confirmed.