NITRIC-OXIDE INHIBITS LUNG SODIUM-TRANSPORT THROUGH A CGMP-MEDIATED INHIBITION OF EPITHELIAL CATION CHANNELS

We used the patch-clamp technique to study the effect of nitric oxide (NO) on a cation channel in rat type II pneumocytes [alveolar type II (AT II) cells]. Single-channel recordings from the apical surface of A T II cells in primary culture showed a predominant cation channel with a conductance of 20.6 +/- 1.1 (SE) p(S) (n = 9 cell-attached patches) and Na+-to-K+ selectivity of 0.97 +/- 0.07 (n = 7 cell-attached patch es). An NO donor, S-nitrosoglutathione (GSNO; 100 mu M), inhibited the basal cation-channel activity by 43% [open probability (P-o), control 0.28 +/- 0.05 vs. GSNO 0.16 +/- 0.03; P < 0.001; n = 16 cell-attached patches], with no significant change in the conductance. GSNO reduced the Po by reducing channel mean open and increasing mean closed times . GSNO inhibition was reversed by washout. The inhibitory effect of NO was confirmed by using a second donor of NO, S-nitroso-N-acetylpenici llamine (100 mu M; P-o, control 0.53 +/- 0.05 vs. S-nitroso-N-acetylpe nicillamine 0.31 +/- 0.04; -42%; P < 0.05; n = 5 cell-attached patches ). The GSNO effect was blocked by methylene blue (a blocker of guanyly l cyclase; 100 mu M), suggesting a role for cGMP. The permeable analog of cGMP, 8-bromo-cGMP (8-BrcGMP; 1 mM), inhibited the cation channel in a manner similar to GSNO CP,, control 0.38 +/- 0.06 vs. 8-BrcGMP 0. 09 +/- 0.02; P < 0.05; n = 7 cell-attached patches). Pretreatment of c ells with 1 mu M KT-5823 (a blocker of protein kinase G) abolished the inhibitory effect of GSNO. The NO inhibition of channels was not due to changes in cell viability. Intracellular cGMP was found to be eleva ted in AT II cells treated with NO (control 13.4 +/- 3.6 vs. GSNO 25.4 +/- 4.1 fmol/ml; P < 0.05; n = 6 cell-attached patches). We conclude that NO suppresses the activity of an Na+-permeant cation channel on t he apical surface of AT II cells. This action appears to be mediated b y a cGMP-dependent protein kinase.