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 ( spak ^wt/wt ) and from gene targeted mice carrying WNK insensitive SPAK ( spak ^tg/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 spak ^wt/wt than in spak ^tg/tg mice. Conclusion: SPAK has the potential to up-regulate ENaC.