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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 (mCCDc11) 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 intracelIular 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, mCCDc11 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. [ABSTRACT FROM AUTHOR]

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.