UREA TRANSPORT IN INITIAL IMCD OF RATS FED A LOW-PROTEIN DIET - FUNCTIONAL-PROPERTIES AND MESSENGER-RNA ABUNDANCE

Feeding rats a low-protein (8%) diet (LPD) for 2 wk induces a facilita ted urea transporter in rat initial inner medullary collecting ducts ( IMCDs). To determine whether this is preceded by an increase in mRNA a bundance, we designed degenerate polymerase chain reaction primers to the rabbit facilitated urea transporter (UT2; G. You, C. P. Smith, Y. Kanai, W.-S. Lee, M. Stelzner, and M. A. Hediger. Nature Lend. 365: 84 4-847, 1993) and amplified a 716-bp cDNA fragment to perform Northern analysis of the base or tip of rat inner medulla. In the base, the pre dominant transcript was a 2.9-kb band, which increased 55% after 1 wk on an LPD; there was no change in a 4-kb band. In the tip, the 4-kb ba nd predominated, but neither band varied with an LPD. Next, we functio nally characterized the induced urea transporter using microperfused i nitial IMCDs from rats fed an LPD for 2 wk. First, 100 pM arginine vas opressin (AVP) stimulated urea permeability (P-urea); 10 nM AVP increa sed P-urea; further. Second, raising perfusate and bath osmolality to 690 mosmol/kgH(2)O (NaCl added) stimulated P-urea; adding AVP (10 nM) increased P-urea further. Third, thiourea reversibly inhibited AVP-sti mulated P-urea. In conclusion, 1) 1 wk. on an LPD increases mRNA abund ance of rat UT2 in the inner medullary base; 2) an LPD does not change mRNA abundance of rat UT2 in the inner medullary tip; and 3) although the predominant-rat UT2 transcript differs betw een base and tip regi ons, the facilitated urea transporter induced in initial IMCDs by an L PD has the same functional properties as the urea transporter normally expressed in terminal LMCDs, suggesting that both transcripts encode the same urea transporter.