Segmental variability in the regulation of NKCC1 by aldosterone in rat proximal and distal colon.

The corticosteroid hormone, aldosterone, markedly enhances K secretion throughout the colon - a mechanism critical to its role in maintaining overall K balance. Previous studies from our lab have demonstrated that basolateral Na -K -2Cl cotransporter 1 (NKCC1) is transcriptionally up-regulated by aldosterone in the distal colon specifically to support large conductance K (BK) channel-mediated K secretion. This function is distinct from the more well-established role of NKCC1 in supporting luminal Cl secretion throughout the gastrointestinal tract. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by corticosteroids. Although active K secretion has also been described in the proximal colon, the molecular identity of the K channel(s) that mediates aldosterone-induced K secretion in the proximal colon is not known, nor is the involvement of NKCC1 in the process. Experiments were therefore designed to test the hypothesis that aldosterone up-regulates NKCC1 in the proximal colon to support BK channel-mediated K secretion, congruent with its effects in the distal colon. Using dietary Na -depletion as a model of secondary hyperaldosteronism in rats, we found that NKCC1 protein expression in the proximal colonic mucosa was enhanced 2-fold (p < 0.05) by Na depletion (i.e., aldosterone), whereas mRNA abundance was unaffected (p = 0.32). Surprisingly, electrogenic K secretion was not detectable by short-circuit current (I ) measurements, in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba (non-selective K channel blocker), despite enhanced K secretion in Na -depleted vs. normal rats, as measured by Rb fluxes under voltage-clamped conditions (net K secretion = -2.1 ± 0.6 vs -0.4 ± 0.2 mEq/cm .hr; p < 0.05). Expression of both BK and intermediate conductance K (IK) channels was also found to be unaltered by dietary Na depletion, at both the protein and mRNA level. However, basal I (146.7 ± 16.8 vs. 93.7 ± 8.7 mA/cm ; p < 0.05), as well as bumetanide-sensitive, Ca - and cAMP-stimulated I (i.e., Cl secretion) were significantly enhanced by dietary Na depletion (ΔI = 239.6 ± 5.6 vs. 105.7 ± 33.5 and 154.3 ± 8.7 vs. 115.3 ± 8.7 mA/cm for Ca and cAMP, respectively; p < 0.05 for both). In parallel to this was an increase in cystic fibrosis transmembrane conductance regulator (CFTR) Cl channel protein expression (~1.7-fold; p < 0.05). We therefore reject our original hypothesis and conclude that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon, where they are biased toward Cl and K secretion, respectively. Development of therapeutic strategies in treating pathologies related to aberrant or altered colonic K /Cl transport - such as pseudo-obstruction, ulcerative colitis, or end-stage renal disease (ESRD) - may benefit from these findings.