Cellular Mechanisms of Acrolein-Induced Alteration in Calcium Signaling in Airway Smooth Muscle

Abstract

Acrolein, an unsaturated aliphatic aldehyde, is a potent respiratory irritant. We have previously observed that acrolein administered ex vivo to isolated airways alters subsequent airway responsiveness to muscarinic agonists in terms of both mechanical activity of rings and calcium signaling in isolated cells. In the present study, we have examined the mechanisms by which acrolein alters Ca2+ signaling. In freshly isolated rat tracheal smooth muscle cells, preexposure to acrolein increased the [Ca2+]i oscillation frequency in response to endothelin 1 (ET-1, 0.1 μM), a contractile agonist that acts via the activation of a receptor different from the muscarinic cholinoceptor. We then studied acrolein-induced alteration in cell signaling with special attention to the steps downstream of membrane receptor activation i.e., the inositol 1,4,5-trisphosphate (InsP3) signaling pathway. Pretreatment of cells with LiCl (20 mM), a modulator of InsP3 concentration, mimicked the effect of acrolein exposure on agonist-induced [Ca2+]i response, i.e., increased the amplitude of the first Ca2+ rise and the oscillation frequency in response to 0.1 and 10 μM acetylcholine (ACh), respectively. Moreover, in tracheal smooth muscle, preexposure to acrolein significantly increased carbachol-induced [3H]inositol-phosphates accumulation, up to 34 ± 11% above unexposed tissue values. Finally, in β-escin permeabilized cells, injection of InsP3 (0.1–10 μM) induced a concentration-dependent [Ca2+]i rise followed, for high InsP3 concentration, by [Ca2+]i oscillations, a calcium response whose pattern was similar to that induced by ACh. Exposure to acrolein did not alter the InsP3-induced [Ca2+]i response. These results indicate that the effect of acrolein exposure on Ca2+ responses in airway smooth muscle is not restricted to activation of the muscarinic cholinoceptor and is due to an enhancement in agonist-induced InsP3 production. Since acrolein does not modify InsP3 receptor channel sensitivity, we conclude that acrolein-induced alteration in calcium signaling can be ascribed to its sole effect on InsP3 production.