LITHIUM STIMULATES ACCUMULATION OF 2ND-MESSENGER INOSITOL 1,4,5-TRISPHOSPHATE AND OTHER INOSITOL PHOSPHATES IN MOUSE PANCREATIC MINILOBULES WITHOUT INOSITOL SUPPLEMENTATION

Previous studies showed that lithium, beginning at therapeutic plasma concentrations in the treatment of manic depression, increased the accumulation of second-messenger inositol 1,4,5-triphosphate [Ins(1,4,5)P-3] in cerebral cortex slices of guinea pig and rhesus monkey [Lee, Dixon, Reichman, Moummi, Los and Hokin (1992) Biochem. J. 282, 377-385; Dixon, Lee, Los and Hokin (1992) J. Neurochem. 59, 2332-2335; Dixon, Los and Hokin (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 8358-8362]. These studies have now been extended to a peripheral tissue, mouse pancreatic minilobules. In the presence of carbachol, concentrations of lithium from 1 to 20 mM sharply and progressively increased the accumulation of Ins(1,4,5)P-3 and inositol 1,3,4,5-tetrakisphosphate, followed by a decrease. Assay of these inositol polyphosphates by either the prelabelling technique or mass assay gave similar results. Atropine quenching of cholinergically stimulated pancreatic minilobules led to a rapid disappearance of Ins(1,4,5)P-3. This disappearance was impeded by lithium. This suggested that the lithium-induced elevation in Ins(1,4,5)P-3 was due to inhibition of the 5-phosphatase and, on the basis of the markedly elevated concentrations of inositol 1,3,4-trisphosphate [Ins(1,3,4)P-3] and inositol 1,4-bisphosphate in the presence of lithium, probably by feedback inhibition by these latter two compounds. An additional mechanism, i.e. a stimulatory effect of lithium on phospholipase C, cannot, however, be ruled out. The other reaction product of phospholipase C, inositol cyclic 1:2,4,5-trisphosphate, also increased in the presence of lithium. This may also be due to inhibition of the 5-phosphatase, which is the exclusive mechanism for removal of this compound. The effects of lithium on the accumulation of other inositol phosphates paralleled that of Ins(1,4,5)P-3, with the exception of inositol 3,4-bisphosphate, which decreased. This was presumably due to the inhibition of Ins(1,3,4)P-3 1-phosphatase by lithium. Unlike mouse cerebral cortex slices [Lee, Dixon, Reichman, Moummi, Los and Hokin (1992) Biochem. J. 282, 377-385], inositol supplementation was not required to demonstrate lithium-stimulated Ins(1,4,5)P-3 accumulation in mouse pancreatic minilobules. This indicates that inositol depletion sufficient to impair lithium-stimulated Ins(1,4,5)P-3 accumulation does not occur in mouse pancreatic minilobules, even though an elevation of cytidine diphosphodiacylglycerol occurred, indicating some inositol depletion due to lithium. Elevation of Ins(1,4,5)P-3 by lithium may be a general phenomenon in the central nervous system and peripheral tissues under non-rate-limiting concentrations of inositol.