AMILORIDE-SENSITIVE NA-II PNEUMOCYTES ARE REGULATED BY G-PROTEINS( CHANNELS IN FETAL TYPE)

We have used the patch-clamp technique to record single channels in ex cised membrane patches from type II pneumocytes freshly isolated from fetal guinea pig lung by elastase digestion and differential filtratio n. The 10/56 patches exhibited spontaneous channel activity with a mea n open-state probability (NP0) of 0.5 +/- 0.1. In symmetrical Na+-rich solutions, the channels had a unitary conductance of 11.1 +/- 0.5 pS and showed current reversal at similar to 0 mV. Superfusing the inner membrane leaflet of the patch with a K+-rich solution resulted in sing le-channel current activity with a conductance of 5.6 +/- 0.2 pS being resolved. Current reversed at +22.1 +/- 1.9 mV, which is compatible w ith a P-Na+/P-K+ of 1.8 +/- 0.1. The addition of 0.1 mM guanosine 5'-O -(3-thiotriphosphate) to the cytoplasmic face of the patch elicited ch annel activity in 12/31 previously quiescent patches, whereas, in spon taneously active patches, channel NP0, was increased. Amiloride, in th e concentration range 0.4-4 mu M, reduced the frequency of observed sp ontaneous (or activatable) channel activity, reduced NP0, and induced flickery channel behavior. No activity was seen in the presence of 10 mu M amiloride in the pipette. This is the first direct observation of a G protein regulated Na+-conductive pathway in alveolar epithelium, and it may represent one route by which the alveolar epithelium of the fetus can regulate the Na+-driven fluid reabsorption necessary for th e adaptation of the newborn lung to air breathing at birth.