Effects of mineralocorticoid and K+ concentration on K+ secretion and ROMK channel expression in a mouse cortical collecting duct cell line

Fodstad H, Gonzalez-Rodriguez E, Bron S, Gaeggeler HP, Guisan B, Rossier BC, Horisberger JD. Effects of mineralocorticoid and K+ concentration on K+ secretion and ROMK channel expression in a mouse cortical collecting duct cell line. Am J Physiol Renal Physiol 296: F966-F975, 2009. First published March 18, 2009; doi:10.1152/ajprenal.90475.2008.-The cortical collecting duct (CCD) plays a key role in regulated K+ secretion, which is mediated mainly through renal outer medullary K+ ( ROMK) channels located in the apical membrane. However, the mechanisms of the regulation of urinary K+ excretion with regard to K+ balance are not well known. We took advantage of a recently established mouse CCD cell line (mCCD(c11)) to investigate the regulation of K+ secretion by mineralocorticoid and K+ concentration. We show that this cell line expresses ROMK mRNA and a barium-sensitive K+ conductance in its apical membrane. As this conductance is sensitive to tertiapin-Q, with an apparent affinity of 6 nM, and to intracellular acidification, it is probably mediated by ROMK. Overnight exposure to 100 nM aldosterone did not significantly change the K+ conductance, while it increased the amiloride-sensitive Na+ transport. Overnight exposure to a high K+ ( 7 mM) concentration produced a small but significant increase in the apical membrane barium-sensitive K+ conductance. The mRNA levels of all ROMK isoforms measured by qRT-PCR were not changed by altering the basolateral K+ concentration but were decreased by 15-45% upon treatment with aldosterone ( 0.3 or 300 nM for 1 and 3 h). The paradoxical response of ROMK expression to aldosterone could possibly work as a preventative mechanism to avoid excessive K+ loss which would otherwise result from the increased electrogenic Na+ transport and associated depolarization of the apical membrane in the CCD. In conclusion, mCCD(c11) cells demonstrate a significant K+ secretion, probably mediated by ROMK, which is not stimulated by aldosterone but increased by overnight exposure to a high K+ concentration.