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 lt; 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 (ISC) measurements, in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted vs. normal rats, as measured by 86Rb+ fluxes under voltage-clamped conditions (net K+ secretion = -2.1 ± 0.6 vs -0.4 ± 0.2 mEq/cm2.hr; p lt; 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 ISC (146.7 ± 16.8 vs. 93.7 ± 8.7 mA/cm2; p lt; 0.05), as well as bumetanide-sensitive, Ca2+- and cAMP-stimulated ISC (i.e., Cl- secretion) were significantly enhanced by dietary Na+ depletion (ΔISC = 239.6 ± 5.6 vs. 105.7 ± 33.5 and 154.3 ± 8.7 vs. 115.3 ± 8.7 mA/cm2 for Ca2+ and cAMP, respectively; p lt; 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 lt; 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.
Support or Funding Information
This study was supported by the National Institute of Health NIDDK R01DK104791 and DK112085 grants.
