Ion channels, exchangers, and transporters (ion transport systems, ITSs) represent important factors in the mechanism of cell volume regulation. Among these ITSs include the Na–K–Cl cotransporter (NKCC), volumesensitive osmolyte anion channels, Na+/Ca2+ exchanger, the Na+/H+ exchanger, the SUR1-regulated nonselective cation channel (NCCa-ATP), acid-sensing ion channels(ASIC), and the transient receptor potential (TRP) ion channels. The Na–K–Cl cotransporter-1(NKCC1), in particular, has been shown to play an important role in the mediation of astrocyte swelling/brain edema. The Na–K–Cl cotransport systems has been identified in a wide variety of animal cells and tissues. Two isoforms of NKCC have been identified (NKCC1 and NKCC2). NKCC1 is present in many cell types, including astrocytes, neurons and oligodendrocytes. 
The NKCC2 isoform is localized exclusively to the kidney. NKCC is involved in the regulation of ion transport across secretory and absorptive epithelia, NH4+ transport, and in the maintenance and regulation of cell volume and ion gradients. The electroneutral NKCC transports Na+, K+, and Cl− into cells under both physiological and pathophysiological conditions with a stoichiometry of 1Na+:1 K+:2Cl− and it can be inhibited by either bumetanide or furosemide. The transport of these ions and obligated water restores cell volume in states of dehydration, whereas inappropriate activation of NKCC will lead to cell swelling and tissue edema. Exposure of cultured rabbit ciliary epithelial cells to hydrogen peroxide (H2O2) results in increased NKCC activity, along with increased intracellular Na+ levels. Several signaling kinases have been proposed to mediate NKCC1 phosphorylation. Among these include, oxidative stress response kinase (OSR1) and STE20/SPS-1 related, proline-alanine-rich kinase (SPAK), which are serine/ threonine kinases. In addition to OSR1 and SPAK, activation of the mitogenactivated protein kinases (MAPKs), protein kinases A and C (PKA and PKC) and calcium/calmodulin-dependent protein kinases II (CaMKII) also phosphorylates NKCC1. 
Activation of Rho, Janus kinase 2, cAMP-PK and myosin light chain kinase has also been shown to be involved in NKCC1 phosphorylation. Activation of NKCC1 is an important factor in the mechanism of astrocyte swelling/brain edema in various neurological conditions, including ALF. While several factors have been reported to be involved in its regulation, increased NKCC1 protein abundance, phosphorylation, oxidation, and nitration play major roles in such activation. Understanding the mechanisms of NKCC activation, and its contribution to cell swelling may significantly assist in the development of new therapeutic strategies for the treatment of brain edema after ischemia, trauma and ALF, all conditions associated with astrocyte swelling.

References

1.Jayakumar AR, Norenberg MD. Metab Brain Dis. 2010;25(1):31–38.