Amiloride Antagonizes β-Adrenergic Stimulation of cAMP Synthesis and Cl− Secretion in Human Tracheal Epithelial Cells

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Abstract
Amiloride, a potent blocker of the sodium channel in airway epithelium, has been administered by aerosol as a therapeutic agent for cystic fibrosis. Because amiloride in high concentration has been reported to interfere with cell functions, including adrenergic responses, we tested the ability of amiloride to inhibit β-adrenergic responses in human tracheal epithelial cells. Amiloride (10−4 M), applied from the basolateral surface of a cell monolayer, inhibited the changes in transepithelial potential and short circuit current to isoproterenol (10−6 M). The stimulation of cyclic adenosine monophosphate (cAMP) synthesis by isoproterenol was inhibited in dose-dependent fashion by amiloride (P = 0.007 by multivariate ANOVA with multiple samples correction). Amiloride did not affect baseline transepithelial potential, short circuit current, basal cAMP levels, cAMP response to prostaglandin E2, or basal adenylate cyclase activity measured directly in membrane preparations. Therefore, it is unlikely that amiloride exerts a nonspecific toxic effect on adenylate cyclase, receptor-cyclase coupling, or substrate or cofactor supply. The binding of [125I]iodocyanopindolol (ICYP), a β-adrenergic receptor antagonist, to membranes from human tracheal epithelial cells could be displaced by amiloride with IC50 = 410 µM; displacement was 70% at 10−3 M amiloride. These data are most consistent with the hypothesis that amiloride inhibits β-adrenergic responses in airway epithelial cells by occupying β-adrenergic receptor sites. Therapeutic administration of amiloride should take into account its affinity for adrenergic receptors.