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 o ) 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 o 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 o if the [Cl−] c was reduced to 20 mm; the P o was 0.58 ± 0.10 at 100 nm [Ca2+] c 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 .
The Journal of Membrane Biology – Springer Journals
Published: Mar 1, 2001
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