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

The cortical collecting duct (CCD) plays a key role in regulated [K.sup.+] secretion, which is mediated mainly through renal outer medullary [K.sup.+] (ROMK) channels located in the apical membrane. However, the mechanisms of the regulation of urinary [K.sup.+] excretion with regard to [K.sup.+] balance are not well known. We took advantage of a recently established mouse CCD cell line (mCC[D.sub.c11]) to investigate the regulation of [K.sup.+] secretion by mineralocorticoid and [K.sup.+] concentration. We show that this cell line expresses ROMK mRNA and a barium-sensitive [K.sup.+] 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.sup.+] conductance, while it increased the amiloride-sensitive [Na.sup.+] transport. Overnight exposure to a high [K.sup.+] (7 mM) concentration produced a small but significant increase in the apical membrane barium-sensitive [K.sup.+] conductance. The mRNA levels of all ROMK isoforms measured by qRT-PCR were not changed by altering the basolateral [K.sup.+] 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.sup.+] loss which would otherwise result from the increased electrogenic [Na.sup.+] transport and associated depolarization of the apical membrane in the CCD. In conclusion, mCC[D.sub.c11] cells demonstrate a significant [K.sup.+] secretion, probably mediated by ROMK, which is not stimulated by aldosterone but increased by overnight exposure to a high [K.sup.+] concentration. cultured mCCD cells; urinary potassium excretion; aldosterone