Reconstitution of immunopurified alveolar type II cell Na+ channel protein into planar lipid bilayers

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Abstract
Low-amiloride-affinity (L-type) Na+ channels have been functionally and immunologically localized to alveolar type II (ATII) cells. Purified rabbit ATII epithelial cells were isolated by elastase digestion and solubilized with 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate. The solubilized proteins were purified by ion-exchange chromatography, followed by immunoaffinity purification over a column to which rabbit polyclonal antibodies raised against purified bovine renal Na+ channel protein were bound. The proteins eluted from the immunoaffinity column were assayed for specific binding of [3H]Br-benzamil and reconstituted into planar lipid bilayers. Sequential purification steps gave a final enrichment in specific [3H]Br-benzamil binding of > 2,000 compared with the homogenate. Single-channel currents of 25 pS were recorded from the immunopurified rabbit ATII cell protein. Addition of the catalytic subunit of protein kinase A (PKA) plus ATP to the presumed cytoplasmic side of the bilayer resulted in a significant increase in the single-channel open probability (Po), from 0.40 +/- 0.14 to 0.8 +/- 0.12, without altering single-channel conductance. The addition of amiloride or ethylisopropyl amiloride (EIPA) to the side opposite that in which PKA acts reduced Po with no change in single-channel conductance. Rabbit ATII Na+ channels in bilayers had an inhibitory constant for amiloride of 8 microM and 1 microM for EIPA. These data confirm the presence of L-type Na+ channels in adult mammalian ATII cells.