Inhibitory effect of diazepam on muscarinic receptor‐stimulated inositol 1,4,5‐trisphosphate production in rat parotid acinar cells

This study examined the effect of diazepam (DZP) on phosphoinositide turnover, which plays an important role in the regulation of salivary secretion, in rat parotid acinar cells. DZP (10 −9 M to 10 −5 M ), a potent agonist of both central‐ and peripheral‐type benzodiazepine receptors, dose‐dependently decreased inositol 1,4,5‐trisphosphate (IP 3 ) production stimulated by carbachol, a muscarinic receptor agonist, in the cells. DZP produced a maximum inhibitory response at a concentration of 10 −5 M , with IP 3 production decreased to 63% of maximal levels. The concentration inducing half maximal inhibition of IP 3 production was approximately 3.5×10 −8 M . An inhibitory response to DZP was produced by a short‐term pretreatment (<3 min) of the cells and prevented by antagonist and competing ligand for the central‐ and peripheral‐type benzodiazepine receptors, flumazenil and PK 11195, respectively. DZP showed a non‐competitive inhibition of carbachol‐stimulated IP 3 production. It did not directly inhibit the activities of GTP‐binding regulatory proteins and phosphatidylinositol 4,5‐bisphosphate‐specific phospholipase C (PLC) in the parotid gland membranes, though choline chloride inhibited PLC activity. DZP (10 −5 M ) attenuated the increase in the intracellular Ca 2+ concentration ([Ca 2+ ] i ) in the cells following stimulation of the muscarinic and α 1 ‐adrenoceptors. These results suggest that in the parotid acinar cells, DZP inhibits muscarinic receptor‐stimulated IP 3 production through benzodiazepine receptors and that PLC activity which produces IP 3 is inhibited by chloride. The decreases in IP 3 and [Ca 2+ ] i in the cells may be connected with the suppression of salivary secretion induced by DZP.British Journal of Pharmacology (2002) 137 , 945–952. doi: 10.1038/sj.bjp.0704968