The essential role of cytosolic Cl- in Ca2+ regulation of an amiloride-sensitive channel in fetal rat pneumocyte

An amiloride-sensitive, Ca2+-activated nonselective cation (NSC) channel in the apical membrane of fetal rat alveolar epithelium plays an important role in stimulation of Na+ transport by a beta adrenergic agonist (beta agonist). We studied whether Ca2+ has an essential role in the stimulation of the NSC channel by beta agonists. In cell-attached patches formed on the epithelium, terbutaline, a beta agonist, increased the open probability (P-0) of the NSC channel to 0.62 +/- 0.07 from 0.03 +/- 0.01 (mean +/- SE; n = 8) 30 min after application of terbutaline in a solution containing 1 mM Ca2+. The P-0 of the terbutaline-stimulated NSC channel was diminished in the absence of extracellular Ca2+ to 0.26 +/- 0.05 (n = 8). The cytosolic Ca2+ concentration ([Ca2+](c)) in the presence and absence of extracellular Ca2+ was, respectively, 100 +/- 6 and 20 +/- 2 nM (n = 7) 30 min after application of terbutaline. The cytosolic Cl- concentration ([Cl-](c)) in the presence and absence of extracellular Ca2+ was, respectively, 20 +/- 1 and 40 +/- 2 mM (n = 7) 30 min after application of terbutaline. The diminution of [Ca2+](c) from 100 to 20 nM itself had no significant effects on the P-0 if the [Cl-](c) was reduced to 20 mM; the P-o was 0.58 +/- 0.10 at 100 nM [Ca2+], and 0.55 +/- 0.09 at 20 nM [Ca2+](c) (n = 8) with 20 mM [Cl-](c) in inside-out patches. On the other hand, the P-o (0.28 +/- 0.10) at 20 nM [Ca2+](c) with 40 mM [Cl-](c) was significantly lower than that (0.58 +/- 0.10; P < 0.01; n = 8) at 100 nM [Ca2+](c) with 20 mM [Cl-](c) suggesting that reduction of [Cl-](c) is an important factor stimulating the NSC channel. These observations indicate that the extracellular Ca2+ plays an important role in the stimulatory action of beta agonist on the NSC channel via reduction of [Cl-](c).