Renal Na–K–Cl cotransporter NKCC2 in Dahl salt-sensitive rats

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Abstract
Dahl salt-sensitive (DS) rats are characterized by enhanced NaCl reabsorption in the loop of Henle, but the responsible ion transport protein is unknown. To investigate renal Na-K-Cl cotransporter NKCC2 function and expression in DS rats under a low-salt diet. NKCC2 functioning was assessed in vitro by measuring bumetanide-sensitive rubidium uptake and cytosolic chloride concentration in isolated medullary thick ascending limb (mTAL) tubules, and in vivo by measuring the salidiuretic action of orally given bumetanide. NKCC2 expression was assessed by Western blot analysis of outer medullary proteins using T4 monoclonal antibody. mTAL tubules from DS rats exhibited significantly higher bumetanide-sensitive rubidium uptake (85.1 +/- 4.8 versus 66.2 +/- 4.4 nmol/min per mg protein in DS and DR, (Dahl salt-resistant) rats, respectively; P = 0.011) and significantly higher cytosolic chloride (32.8 +/- 1.7 versus 25.0 +/- 1.5 mmol/l in DS and DR rats, respectively). Moreover, DS rats showed a significantly higher (P < 0.001) natriuretic response to bumetanide (1.13 +/- 0.05 versus 0.64 +/- 0.09 mmole/3 h in DS and DR rats, respectively). Finally, Western blot analysis revealed less NKCC2 expression in DS rats. We conclude that DS rats have increased renal NKCC2 activity, thus explaining, at least in part, their genetic renal inability to excrete sodium. Moreover, DS rats have a decreased renal NKCC2 expression, which can be a compensatory phenomenon against NKCC2 hyperactivity.