Transepithelial Na and Urea Transport Pathways in Rat Inner Medullary Thin Limbs of Henle's Loop

Abstract

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 (Pf) is high in DTLupper and low in DTLlower, ATLupper and ATLlower. In vivo microperfusion studies confirmed the low Pf in papillary DTLs and ATLs. Mean transepithelial Na and urea permeabilities (PNa, Purea) with in vitro lumen perfusion rates of ~30 nl/min in DTLupper were ~60 E‐5 cm/sec and in DTLlower and ATL were ~350 to 450 E‐5 cm/sec; Na and urea fluxes in DTLlower are directly proportional to lumen perfusion rates between 5 to 30 nl/min. In vivo microperfusion studies confirmed the high PNa and Purea in papillary DTLs and ATLs. With in vitro microperfusion, Purea in DTLupper and DTLlower 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, Purea in DTLlower 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, Purea in DTLupper and DTLlower were almost completely inhibited by peritubular lanthanum (5 mM), with Purea recovering in both segments following 20 min lanthanum washout. Lanthanum (5 mM) also completely inhibited PNa in DTLlower; however, PNa 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 DTLupper (~1–2 kJ/mol) compared to DTLlower and ATL (~13 kJ/mol). Urea transport in DTLupper and Na and urea transport in DTLlower 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 PNa and Purea in upper and lower DTLs, respectively.Support or Funding InformationNIDDK 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.