Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Because tumor necrosis factor (TNF)-alpha can upregulate alveolar fluid cle arance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-alpha -mediated increase in epithelial fluid transport were st udied. In rats, 5 mug of TNF-alpha in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. W e also tested a triple-mutant TNF-alpha that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effec t; the mutant also has decreased binding affinity to both TNF-alpha recepto rs. The triple-mutant TNF-alpha did not increase AFC. Perfusion of human A5 49 cells, patched in the whole cell mode, with TNF-alpha (120 ng/ml) result ed in a sustained increase in Na+ currents from 82 +/- 9 to 549 +/- 146 pA (P < 0.005; n = 6). The TNF-<alpha>-elicited Na+ current was inhibited by a miloride, and there was no change when A549 cells were perfused with the tr iple-mutant TNF-alpha or after preincubation with blocking antibodies to th e two TNF-alpha receptors before perfusion with TNF-alpha. In conclusion, a lthough TNF-alpha can initiate acute inflammation and edema formation in th e lung, TNF-alpha can also increase AFC by an amiloride-sensitive, cAMP-ind ependent mechanism that enhances the resolution of alveolar edema in pathol ogical conditions by either binding to its receptors or activating Na+ chan nels by means of its lectinlike domain.