G-PROTEIN-REGULATED LARGE-CONDUCTANCE CHLORIDE CHANNELS IN FRESHLY ISOLATED FETAL TYPE-II ALVEOLAR EPITHELIAL-CELLS

Using the patch-clamp technique, we have recorded single channels in c ell-attached and inside-out excised patches from the plasma membrane o f type II alveolar epithelial cells freshly isolated from fetal guinea pig lung by elastase digestion and differential filtration. In cell-f ree patches the channels were highly selective for Cl- (P(Cl):P(cat) = 9:1), had a large unitary conductance (375 pS +/- 23 pS), and current reversal of 0 mV in either symmetrical Na-rich solutions or when the inner membrane leaflet was bathed in a K+-rich solution. The large-con ductance Cl- channel exhibited little or no voltage inactivation at po sitive potentials, remained open for a significant amount of time at p otentials negative to -40 mV, and was blocked at, all potentials by 0. 1 mM -acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Channel activity was independent of intracellular calcium concentration. Bath addition of the nonmetabolizable analogue of GTP, GTPgammaS (0.1 mM), caused a voltage-dependent inhibition of channel activity [open probab ility (P(O)) plot was shifted by at least +25 mV]. Smaller channels (2 5 +/- 3 pS) were recorded in the cell-attached configuration with a cu rent-voltage (I-V) relationship which was compatible with a Cl- conduc tance. On excision, the patches previously containing small-conductanc e channels exhibited only large-conductance Cl- channel behavior. Thes e large-conductance, G1 protein-regulatable Cl- channels may provide a route for alveolar cell Cl- exit and as such may be an integral part of the mechanism responsible for secretion of fetal lung fluid.