Amiloride-sensitive Na+ channels in fetal type II pneumocytes are regulated by G proteins

We have used the patch-clamp technique to record single channels in excised membrane patches from type II pneumocytes freshly isolated from fetal guinea pig lung by elastase digestion and differential filtration. The 10/56 patches exhibited spontaneous channel activity with a mean open-state probability (NPo) 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 approximately 0 mV. Superfusing the inner membrane leaflet of the patch with a K(+)-rich solution resulted in single-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 with a PNa+/PK+ of 1.8 +/- 0.1. The addition of 0.1 mM guanosine 5'-O-(3-thiotriphosphate) to the cytoplasmic face of the patch elicited channel activity in 12/31 previously quiescent patches, whereas, in spontaneously active patches, channel NPo was increased. Amiloride, in the concentration range 0.4-4 microM, ...