Choroid plexus epithelial cells (CPECs) secrete cerebrospinal fluid (CSF) and regulate its ionic composition through mechanisms that are not fully understood. These cells express the Na + /K + ATPase and the Na + -K + -2Cl - cotransporter 1 (NKCC1) on their apical membrane, which faces the CSF, contrary to their typical basolateral localization in chloride-secreting epithelia. Because of this unusual polarity, the direction of basal net ion fluxes mediated by apical NKCC1 and associated water fluxes in CPECs under normal conditions remains controversial, and the functional role of the apical cotransporter has been debated (Alvarez-Leefmans, 2020. J Physiol 598.21: 47332-4736; MacAulay and Rose, 2020. J Physiol 598.21: 4737-4739). A key factor determining the direction and magnitude of NKCC1 fluxes is the external K + concentration (K + o), which in CSF is typically 2.9-3.0 mM. We hypothesize that at this K + o, the apical NKCC1 transports ions and water inwardly, and that the flux reversal point (FRP) must occur at K + o < < 3.0 mM. Since NKCC1 is an electroneutral transporter, ion fluxes cannot be measured with electrophysiological methods. To estimate the FRP of NKCC1 in single CPECs as a function of K + o, we used calcein fluorescence to monitor relative changes in cell water volume caused by NKCC1 activity, as a surrogate for net ion and water flux direction and magnitude (Gregoriades et al., 2019, Am J Physiol Cell Physiol 316: C525-C544). At constant osmolality (290 ± 1 mOsm), in HEPES-buffered saline, CPECs responded to slight variations in K + o of ± 2 mM relative to the baseline of 3 mM. At K + o = 1 mM, the cells shrank by 11.8 ± 1.7 % (n=11 animals), indicating the fluxes were outwardly directed, whereas at 5mM K + o, the cells swelled by 10.4 ± 1.6% (n=8), indicating that fluxes were inward. A similar flux reversal was observed in CO 2 /HCO 3 - solutions; at 1mM K + the cells shrank by 13.1 ± 2.3 %, n=4, and in 5mM K + solutions, the cells swelled by 16.6± 2.9%, n=4. Cells from NKCC1 knockout mice lose this sensitivity to external K + , strongly suggesting that NKCC1 mediates these responses. By modeling the net free energy driving NKCC1 as a function of K + o, using measured intra- and extracellular Na + and Cl - concentrations, and an intracellular K + of 110 mM, the flux reversal point occurred at K + o = 1.7-2.0 mM. Based on these and previous observations, we propose that apical NKCC1 acts as a “sensor” and “regulator” of CSF K + levels. NKCC1 is suitable for efficient extracellular K + buffering because it is reversible, and the direction of net ion and water transport is sensitive to small changes in K + o. Further, its Km for K + (2.7 ± 0.7 mM, hNKCC1) aligns with the typical CSF K + range (2.9–3 mM). Because NKCC1 functions inwardly at normal K + o, it is unlikely to directly contribute to CSF secretion. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Gregoriades et al. (Fri,) studied this question.