Acetylcholine leads to free radical production dependent on K(ATP) channels, G(i) proteins, phosphatidylinositol 3-kinase and tyrosine kinase.

Objective: Acetylcholine (ACh) mimics ischemic preconditioning (PC) and therefore protects the heart against lethal ischemia. Steps common to both ischemic and drug-induced PC are opening of mitochondrial K(ATP) channels (mito K(ATP)) and generation of reactive oxygen species (ROS). The aim of this study was to test whether ACh-induced ROS production could be seen in a vascular smooth muscle cell line, and, if so, to investigate the underlying signaling pathway.

Methods: Mitochondrial ROS generation was quantified by measuring changes in fluorescence of ROS-sensitive intracellular markers in vascular smooth muscle cells (A7r5).

Results: Fluorescence, and, therefore, ROS production, was increased to 197.5+/-8.5% of baseline after 45 min of exposure of cells to 2 mM ACh (P<0.001 vs. untreated controls). This effect was blocked by co-treatment with a muscarinic receptor antagonist (atropine 102.8+/-2.9%, 4-DAMP 92.6+/-7.4%) or by inhibition of G(i) with pertussis toxin (PTX) (90.5+/-4.4%), implicating a receptor-mediated rather than non-specific effect of ACh. The increased fluorescence induced by ACh was also abrogated by the free radical scavenger N-(2-mercaptopropionyl) glycine (104.2+/-10.1%), documenting that ROS were indeed the cause of the enhanced fluorescence. Both diazoxide, a K(ATP) channel opener, and valinomycin, a potassium ionophore, also significantly increased ROS production, and these effects were not blocked by PTX, while the K(ATP) channel closer 5-hydroxydecanoate blocked ACh-induced ROS production (92.3+/-3.8%). These results suggest ROS production is directly influenced by K(ATP) activity and K(+) movements in the cell. The tyrosine kinase inhibitor genistein (102.8+/-6.6%) and the phosphatidylinositol 3 (PI3)-kinase inhibitor wortmannin (90.7+/-4.1%) also inhibited the ability of ACh to increase ROS production.

Conclusion: The signaling pathway by which ACh leads to ROS generation in A7r5 cells involves a muscarinic surface receptor, a pertussis toxin-sensitive G protein, PI3-kinase, at least one tyrosine kinase, and a 5-hydroxydecanoate (5-HD)-dependent K(ATP) (presumably that in mitochondria).