Abstract
It has long been stated that the K^+^‐Cl^−^ cotransporters (KCCs) are activated during cell swelling through dephosphorylation of their cytoplasmic domains by a protein phosphatase (PP) but that other enzymes are involved by targeting this PP or the KCCs directly. To date, however, the role of signaling intermediates in KCC regulation has been deduced from indirect evidence rather than in vitro phosphorylation studies, and examined after simulation of ion transport through cell swelling or N‐ethylmaleimide treatment. In this study, the oocyte expression system was used to examine the effects of changes in cell volume (C~VOL~) and intracellular [Cl^−^] ([Cl^−^]~i~) on the activity and phosphorylation levels (P~LEV~) of KCC4, and determine whether these effects are mediated by PP1 or phorbol myristate acetate (PMA)‐sensitive effectors. We found that (1) low [Cl^−^]~i~ or low C~VOL~ leads to decreased activity but increased P~LEV~, (2) high C~VOL~ leads to increased activity but no decrease in P~LEV~ and (3) calyculin A (Cal A) or PMA treatment leads to decreased activity but no increase in P~LEV~. Thus, we have shown for the first time that one of the KCCs can be regulated through direct phosphorylation, that changes in [Cl^−^]~i~ or C~VOL~ modify the activity of signaling enzymes at carrier sites, and that the effectors directly involved do not include a Cal A‐sensitive PP in contrast to the widely held view. J. Cell. Physiol. 219: 787–796, 2009. © 2009 Wiley‐Liss, Inc.