Reduction in the rate of inositol 1,4,5-trisphosphate synthesis in rat parotid acini by lithium

Stimulation of muscarinic cholinergic receptors on rat parotid acinar cells causes a rapid production of inositol phosphates, with the key metabolic event being the breakdown of phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) and diacylglycerol. Here a high-performance liquid chromatographic technique was used to measure the effects of intracellular lithium ions on the amount of various inositol phosphates produced. When acini were stimulated maximally with acetylcholine (ACh), the sum of all inositol phosphates produced followed a monoexponential function with a production rate constant for Ins(1,4,5)P-3 of 0.07 +/- 0-01 solidus/sec. The presence of 23 mM LiCl intracellularly reduced the production rate constant of Ins(1,4,5)P-3 induced by ACh to 0.03 +/- 0.01 solidus/sec, resulting in a decrease in the Ins(1,4,5)P-3 production as well as in the magnitude of the rise in the intracellular free Ca2+ concentration. The lithium ion (Li+) did not affect the rate of conversion of Ins(1,4,5)P-3 to either inositol 1,4-bisphosphate or inositol 1,3,4,5-tetrakisphosphare. The rate of the inositol phosphate production after the addition of the Ca2+ ionophore ionomycin was unaffected by intracellular Li+ (23 mM), which implies that the action of Li+ was at the muscarinic cholinergic receptor, on G-protein or on the interactions between G-proteins and phospholipase C. Thus, in the early events after receptor stimulation with ACh, Li+ causes a reduction in the concentration of the cellular messengers Ins(1,4,5)P-3 and Ca2+. (C) 2001 Elsevier Science Ltd. All rights reserved.