Regulation of an amiloride-sensitive Na+-permeable channel by a β2-adrenergic agonist cytosolic Ca2+ and Cl− in fetal rat alveolar epithelium

摘要

class="enumerated" style="list-style-type:decimal">In cell-attached patches formed on the apical membrane of fetal alveolar epithelium, terbutaline (a specific β2-adrenergic agonist) increased the open probability (Po) of an amiloride-sensitive Na⁺-permeable non-selective cation (NSC) channel (control, 0.03 ± 0.04; terbutaline, 0.62 ± 0.18; n = 8, P < 0.00001) by increasing the mean open time 100-fold without any significant change in the mean closed time and without any change in the single channel conductance (control, 27.8 ± 2.3 pS; terbutaline, 28.2 ± 2.1 pS; n = 8).The Po of the unstimulated channel increased when the apical membrane was depolarized due to a decrease in the closing rate and an increase in the opening rate, while the Po of the terbutaline-stimulated channel did not depend on the membrane potential.Increased cytosolic [Ca²⁺] also increased the Po of the channel in a manner consistent with one Ca²⁺-binding site on the cytosolic surface of the channel. Terbutaline increased the sensitivity of the channel to cytosolic Ca²⁺ by shifting the concentration of cytosolic Ca²⁺ ([Ca²⁺]c) required for half-maximal activation to a lower [Ca²⁺]c value, leading to an increase in Po.An increase in the cytosolic Cl⁻ concentration ([Cl⁻]c) decreased the Po of the channel consistent with two Cl⁻-binding sites by increasing the closing rate without any significant change in the opening rate. Terbutaline increased Po by reducing the effect of cytosolic Cl⁻ to promote channel closing.Taken together, these observations indicate that terbutaline activates a Ca²⁺-activated, Cl⁻-inhibitable, amiloride-sensitive, Na⁺-permeable NSC channel in fetal rat alveolar epithelium in two ways: first, through an increase in Ca²⁺ sensitivity, and second, through a reduction in the effect of cytosolic Cl⁻ to promote channel closing.