Effect of polycations on barrier and transport properties of alveolar epithelium in situ.

We examined the effect of polycations, classes of which are released by activated leukocytes, on the transport properties of the alveolar epithelium in isolated-perfused rat lungs. Protamine, polylysines, and ruthenium red produced rapid, dose-dependent increases in mannitol permeability (PAmann) when instilled into airspaces. The coupling between active transepithelial Na+ transport and alveolar fluid absorption was not altered, despite > 10-fold increases in PAmann. The increase in albumin permeability compared with that in mannitol suggested preservation of alveolar barrier-size selectivity. Tracheal instillation of protamine produced no cellular abnormality, whereas its addition to the perfusate resulted in damage to endothelial and type I cells. Protamine produced an even larger (P < 0.05) increase in PAmann in the presence of isoproterenol or dibutyryl adenosine 3',5'-cyclic monophosphate + 3-isobutyl-1-methyl-xanthine. The stimulation of Na+ and fluid transport by these agents was unaffected by protamine. Mastoparan, a peptide that activates G proteins, produced effects comparable to those of the polycations. The protamine- and mastoparan-induced increase in PAmann was abolished by barium, a K+ channel blocker, but not by zinc, a membrane-protective cation. Other K+ channel blockers, tetraethylammonium and quinine, had no effect. Thus short-term apical application of polycations and mastoparan alter alveolar epithelium paracellular permeability by a noncytotoxic mechanism that is inhibited by barium. The resulting increase in paracellular permeability does not alter fluid absorption driven by active Na+ transport. Polycations have very different effects, depending on whether they are present on one side or the other side of the alveolar capillary barrier.