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Transepithelial Na and Urea Transport Pathways in Rat Inner Medullary Thin Limbs of Henle's Loop
Author(s) -
Evans Kristen K.,
Yu Alan S. L.,
Pannabecker Thomas
Publication year - 2018
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.2018.32.1_supplement.624.31
Subject(s) - urea , chemistry , phloretin , in vivo , lumen (anatomy) , perfusion , ouabain , biophysics , medicine , biochemistry , sodium , biology , microbiology and biotechnology , organic chemistry
Identification of key transport pathways for Na and urea in thin limbs of Henle's loops will further explain how solute recycling and countercurrent mechanisms are integrated to produce the corticomedullary osmotic gradient. We studied transepithelial Na and urea fluxes by in vivo microperfusion of papillary descending and ascending thin limbs (DTLs and ATLs, respectively) and by in vitro microperfusion of isolated DTLs and ATLs from the upper and lower 50% of the inner medulla (IM). In vitro studies have shown that transepithelial water permeability (P f ) is high in DTL upper and low in DTL lower , ATL upper and ATL lower . In vivo microperfusion studies confirmed the low P f in papillary DTLs and ATLs. Mean transepithelial Na and urea permeabilities (P Na , P urea ) with in vitro lumen perfusion rates of ~30 nl/min in DTL upper were ~60 E‐5 cm/sec and in DTL lower and ATL were ~350 to 450 E‐5 cm/sec; Na and urea fluxes in DTL lower are directly proportional to lumen perfusion rates between 5 to 30 nl/min. In vivo microperfusion studies confirmed the high P Na and P urea in papillary DTLs and ATLs. With in vitro microperfusion, P urea in DTL upper and DTL lower was not inhibited by peritubular phloretin (0.25 mM) indicating that urea fluxes occur independently of known facilitated urea transporters such as UT‐A1, UT‐A2 or UT‐A3. Furthermore, P urea in DTL lower was not inhibited by ouabain (10 mM) or by a lumen‐to‐bath Na concentration gradient (143 mM/lumen and 0 mM/bath) indicating the absence of Na‐dependent urea transport. On the other hand, P urea in DTL upper and DTL lower were almost completely inhibited by peritubular lanthanum (5 mM), with P urea recovering in both segments following 20 min lanthanum washout. Lanthanum (5 mM) also completely inhibited P Na in DTL lower ; however, P Na remained completely inhibited following 20 min washout. Activation energy for transepithelial Na and urea transport determined by in vitro microperfusion during 37°‐ to −16° C temperature transitions was found to be low in DTL upper (~1–2 kJ/mol) compared to DTL lower and ATL (~13 kJ/mol). Urea transport in DTL upper and Na and urea transport in DTL lower and ATL likely occur in large part, by way of a plasma membrane or paracellular channel‐like pathway. The distinctive transport characteristics suggest that there are different mechanisms involved with P Na and P urea in upper and lower DTLs, respectively. Support or Funding Information NIDDK DK083338 (TP) This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .