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

Department of Pharmacology and Toxicology, University of Lausanne, Lausanne, Switzerland Submitted 8 August 2008 ; accepted in final form 10 March 2009 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 cl1 ) 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 cl1 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. cultured mCCD cells; urinary potassium excretion; aldosterone Address for reprint requests and other correspondence: H. Fodstad, Dept. of Pharmacology and Toxicology, Univ. of Lausanne, Rue du Bugnon 27, CH-1005 Lausanne, Switzerland (e-mail: heidi.fodstad{at}unil.ch )