SPAK sensitive regulation of the epithelial Na⁺ channel ENaC

Background/Aims: The WNK-dependent STE20/SPS1-related proline/alanine-rich kinase SPAK participates in the regulation of NaCl and Na⁺,K+,2Cl- cotransport and thus renal salt excretion. The present study explored whether SPAK has similarly the potential to regulate the epithelial Na⁺ channel (ENaC). Methods: ENaC was expressed in Xenopus oocytes with or without additional expression of wild type SPAK, constitutively active T233ESPAK, WNK insensitive T233ASPAK or catalytically inactive D212ASPAK, and ENaC activity estimated from amiloride (50 μM) sensitive current (I_amil) in dual electrode voltage clamp experiments. Moreover, Ussing chamber was employed to determine I_amil in colonic tissue from wild type mice (spakwt/wt) and from gene targeted mice carrying WNK insensitive SPAK (spaktg/tg). Results: I_amil was observed in ENaC-expressing oocytes, but not in water-injected oocytes. In ENaC expressing oocytes I_amil was significantly increased following coexpression of wild-type SPAK and T233ESPAK, but not following coexpression of T233ASPAK or D212ASPAK. Colonic I_amil was significantly higher in spakwt/ wt than in spaktg/tg mice. Conclusion: SPAK has the potential to up-regulate ENaC.