The Shrinkage-activated Na+ Conductance of Rat Hepatocytes and its Possible Correlation to rENaC
At moderate cell shrinkage, activation of Na+ channels is the most prominent mechanism of regulatory cell volume increase in rat hepatocytes. The amiloride sensitivity of these channels suggests a relation to the family of epithelial Na+ channels (ENaCs). The present study was performed to determine the pharmacological profile of shrinkage-activated Na+ channels and to test for ENaC expression in primary cultures of rat hepatocytes; in addition, the influence of the cell volume regulated serine/threonine kinase hSGK on activity and pharmacological profile of rENaC was examined in Xenopus oocytes. Conventional electrophysiology in hepatocytes reveals that the shrinkage-activated Na+ channel is inhibited by amiloride and EIPA with IC50 values of 6.0 and 0.12 μmol/l, respectively. Western blots and RT-PCR demonstrate that rat hepatocytes do express all three subunits (α, β, γ) of ENaC. Coexpression of hSGK with rENaC in Xenopus oocytes reveals that the kinase stimulates ENaC by a factor of 4. Moreover, hSGK decreases the affinity to amiloride (increase of IC50 from 0.12 to 0.26 μmol/l) and increases the affinity to EIPA (decrease of IC50 from 250 to 50 μmol/l). In conclusion, rat hepatocytes express ENaC, which is activated by the cell volume-sensitive kinase hSGK. ENaC may contribute to the Na+ channels activated by osmotic cell shrinkage in hepatocytes, whereby the relatively low amiloride and high EIPA sensitivity of the channel could at least be partially due to modification by SGK, which decreases the amiloride and increases the EIPA sensitivity of ENaC.