Effects of low concentrations of paraoxon on Ca2+ mobilization in a human parotid salivary cell-line HSY

The salivary gland is a target organ of organophosphate pesticides (OPs). I nhibition of acetylcholinesterase (AChE) by OPs leads to a decrease in acet ylcholine (ACh) breakdown that results in overstimulation of muscarinic cho linergic receptors (mChR). However, OPs may also directly interact with dow nstream elements of the phosphoinositide (PI) signalling pathway coupled wi th mChR. The present study examined the effects of exposure to low concentr ations of the OP paraoxon on inositol 1,4,5-trisphosphate (IP3) formation a nd Ca2+ mobilization in response to ACh or ATP in the human parotid cell-li ne HSY. Exposure to 0.1 and 1 nM, but not 10 nM, paraoxon for 24 hr signifi cantly elevated the basal cytosolic free Ca2+ ([Ca2+](i)). This increase wa s abolished by atropine. Ca2+ release from the IP3-sensitive store in respo nse to ACh or ATP, a P2Y-nucleotide agonist, was significantly increased in cells pre-exposed to 0.1 nM paraoxon, However, IP3 formation was inhibited by paraoxon but mChR expression was not altered. Although IP3 receptor exp ression was not changed, Ca2+ release elicited by IP3, in streptolysin O to xin-permeabilized cells was significantly larger in cells pre-exposed to 0. 1 nM paraoxon, suggesting that paraoxon increases the sensitivity of IP3 re ceptors. Paraoxon exposure also induced a concentration-dependent reduction in the total capacity of intracellular Ca2+ stores, whereas the capacity o f the IP3-sensitive Ca2+ store was not altered by paraoxon, as judged by di scharging of the IP3-sensitive Ca2+ store with thapsigargin (TC). Ca2+ infl ux stimulated by ACh or ATP was also enhanced by 0.1 nM, but not 1 and 10 n M, paraoxon. On the other hand, Ca2+ influx activated by TG was enhanced by exposure to all concentrations of paraoxon, indicating that paraoxon modul ates the Ca2+ entry pathway. These results suggest that low concentrations of paraoxon interact with elements of the PI pathway, enhancing Ca2+ releas e and influx mechanisms. (C) 2000 Elsevier Science Ltd. All rights reserved .