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

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Abstract
Because tumor necrosis factor (TNF)-α can upregulate alveolar fluid clearance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-α-mediated increase in epithelial fluid transport were studied. In rats, 5 μg of TNF-α in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. We also tested a triple-mutant TNF-α that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effect; the mutant also has decreased binding affinity to both TNF-α receptors. The triple-mutant TNF-α did not increase AFC. Perfusion of human A549 cells, patched in the whole cell mode, with TNF-α (120 ng/ml) resulted in a sustained increase in Na+currents from 82 ± 9 to 549 ± 146 pA (P < 0.005; n = 6). The TNF-α-elicited Na+current was inhibited by amiloride, and there was no change when A549 cells were perfused with the triple-mutant TNF-α or after preincubation with blocking antibodies to the two TNF-α receptors before perfusion with TNF-α. In conclusion, although TNF- α can initiate acute inflammation and edema formation in the lung, TNF-α can also increase AFC by an amiloride-sensitive, cAMP-independent mechanism that enhances the resolution of alveolar edema in pathological conditions by either binding to its receptors or activating Na+ channels by means of its lectinlike domain.