Inhibition of chloride secretion by furosemide in canine tracheal epithelium

Summary

Furosemide inhibits Cl transport in a variety of epithelial and nonepithelial cells. To examine the mechanism of Cl secretion in canine tracheal epithelium, the effect of furosemide on transepithelial ion fluxes, membrane resistances, and electromotive forces was determined using intracellular microelectrodes and an equivalent electrical circuit analysis. There were six main observations: First, furosemide was only effective when added to the submucosal solution. Second, inhibition by furosemide (10⁻³ m) was specific for Cl secretion with no effect on Na absorption. Third, furosemide produced a half-maximal inhibition of Cl secretion at a concentration of 7×10⁻⁶ m. A Hill plot yielded a slope not different from unity, suggesting a one-for-one interaction of furosemide with the Cl transport process. Fourth, despite complete inhibition of Cl secretion, furosemide produced only small changes in transepithelial and apical membrane resistance, indicating that the primary effect was not an inhibition of Cl exit from the cell across the apical membrane. Fifth, basolateral membrane resistance and electromotive force were not altered by furosemide. This finding suggested that the effect of furosemide at the basolateral membrane was on an electrically neutral Cl entry process. Sixth, calculation of the intracellular Cl concentration from the electromotive force across the apical membrane indicated that furosemide decreased intracellular Cl concentration by 50%, consistent with an inhibition of Cl entry into the cell. These results indicate that Cl enters the epithelial cell via an electrically neutral process at the basolateral membrane and that furosemide selectively inhibits that process, resulting in a decreased intracellular Cl concentration and a decrease in the driving force for Cl exit across the apical membrane.