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

Abstract

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 (mCCDcl1) 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, mCCDcl1cells 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.