Biophysical properties and molecular characterization of amiloride-sensitive sodium channels in A549 cells

Amiloride-sensitive Na+ channels, present in fetal and adult alveolar epithelial type II (ATII) cells, play a critical role in the reabsorption of fetal fluid shortly after birth and in limiting the extent of alveolar edema across the adult lung. Because of the difficulty in isolating and culturing ATII cells, there is considerable interest in characterizing the properties of ion channels and their response to injury of ATII cell-like cell lines such as A549 that derive from a human alveolar cell carcinoma. A549 cells were shown to contain alpha-, beta-, and gamma-epithelial Na+ channel mRNAs. In the whole cell mode of the patch-clamp technique (bath, 145 mM Na+; pipette, 145 mM K+), A549 cells exhibited inward Na+ currents reversibly inhibited by amiloride, with an inhibition constant of 0.83 mu M. Ion substitution studies showed that these channels were moderately selective for Na+ (Na+-to-K+ permeability ratio = 6:1). Inward Na+ currents were activated by forskolin (10 mu M) and inhibited by nitric oxide (300 nM) and cGMP. Recordings in cell-attached mode revealed the presence of an amiloride-sensitive Na+ channel with a unitary conductance of 8.6 +/- 0.04 (SE) pS. Channel activity was increased by forskolin and decreased by nitric oxide and the cGMP analog 8-bromo-cGMP. These data demonstrate that A549 cells contain amiloride-sensitive Nat channels with biophysical properties similar to those of ATII cells.