Cl- regulation of a Ca(2+)-activated nonselective cation channel in beta-agonist-treated fetal distal lung epithelium

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Abstract
Nonselective cation (NSC) channels have been identified in the apical membrane of fetal distal lung epithelium (FDLE). However, their physiological role in Na+ transport is uncertain. Because terbutaline, a beta 2-agonist, increases Na+ transport by FDLE, we studied its effect and selected signal transduction mechanisms on NSC channel activity. Using patch-clamp and single-cell imaging techniques, we found that terbutaline activated the NSC channel by 1) increasing its sensitivity to cytosolic Ca2+ concentration ([Ca2+]c) by 100- to 1,000-fold, 2) increasing [Ca2+]c from 35 nM to 3.3 microM, 3) producing a dependency of the NSC channel activity on the cytosolic Cl- concentration ([Cl-]c) at a physiological [Ca2+]c, and 4) inducing a reduction in the [Cl-]c from 45 to 25 mM, which directly activates the beta 2-treated NSC channel. These observations indicate that a beta 2-agonist physiologically activates an amiloride-blockable NSC channel in FDLE through an increase in its sensitivity to [Ca2+]c, resulting in the development of a [Cl-]c dependency at a physiological [Ca2+]c associated with both an increase in [Ca2+]c and a reduction in [Cl-]c. A development of the [Cl-]c dependency and a reduction in [Cl-]c act as a second messenger of the beta-agonist signal transduction pathway in this Na(+)-transporting epithelium.