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 prominen t mechanism of regulatory cell volume increase in rat hepatocytes, The amil oride sensitivity of these channels suggests a relation to the family of ep ithelial Na+ channels (ENaCs), The present study was performed to determine the pharmacological profile of shrinkage-activated Na+ channels and to tes t 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 oocyt es, Conventional electrophysiology in hepatocytes reveals that the shrinkag e-activated Na+ channel is inhibited by amiloride and EIPA with IC50 values of 6.0 and 0.12 mu mol/l, respectively. Western blots and RT-PCR demonstra te that rat hepatocytes do express all three subunits (alpha, beta, gamma) of ENaC. Coexpression of hSGK with rENaC in Xenopus oocytes reveals that th e kinase stimulates ENaC by a factor of 4. Moreover, hSGK decreases the aff inity to amiloride (increase of IC50 from 0.12 to 0.26 mu mol/l) and increa ses the affinity to EIPA (decrease of IC50 from 250 to 50 mu mol/l). In con clusion, rat hepatocytes express ENaC, which is activated by the cell volum e-sensitive kinase hSGK, ENaC may contribute to the Na+ channels activated by osmotic cell shrinkage in hepatocytes, whereby the relatively low amilor ide and high El PA sensitivity of the channel could at least be partially d ue to modification by SGK, which decreases the amiloride and increases the EIPA sensitivity of ENaC. Copyright (C) 2000 S. Karger AG, Basel.