Manganese Stimulates the Incorporation of [³H]Inositol into a Pool of Phosphatidylinositol in Brain That Is Not Coupled to Agonist‐Induced Hydrolysis

Abstract

Mn2+ greatly increases the incorporation of myo-[3H]inositol into phosphatidylinositol (PI) of brain and other tissues by stimulating the activity of a PI-myo-inositol exchange enzyme. This study examined the ability of norepinephrine (NE) and carbachol to stimulate the hydrolysis of [3H]PI formed in the absence and presence of Mn2+-stimulated [3H]inositol exchange. Rat cerebral cortical slices were incubated with myo-[3H]inositol for 60 min in an N-2-hydroxyethyl piperazine-N'-2-ethanesulfonic acid (HEPES) buffer without or with MnCl2 (1 mM). The tissue was washed and further incubated with unlabeled myo-inositol and LiCl (10 mM). Prelabeled slices were then incubated with NE (0.1 mM) or carbachol (1 mM) to induce agonist-stimulated [3H]PI hydrolysis. Mn2+ treatment resulted in eight- and sixfold increases in control levels of [3H]PI and [3H]inositol monophosphate [( 3H]IP), respectively. Both NE and carbachol stimulated [3H]IP formation in tissue prelabeled without or with manganese. However, the degree of stimulation (percentage of control values) was greatly attenuated in the presence of Mn2+. In the absence of Mn2+ treatment, NE decreased [3H]PI radioactivity in the tissue to 80% of control values. However, NE did not decrease [3H]PI radioactivity in the Mn2+-treated tissue. These data demonstrate that Mn2+ stimulates incorporation of myo-[3H]inositol into a pool of PI in brain that has a rapid turnover but is not coupled to agonist-induced hydrolysis.

Accumulation of Inositol Phosphates Induced by Chlorpromazine in C6 Glioma Cells

“…This suggests that the pool of phosphoinositides stimulated by Mn2+ is hydrolyzed in the presence of CPZ. This observation is interesting in view of the fact that this pool is not available for agonist-stimulated breakdown in brain slices (Gonzales and Crews, 1985;Schoepp, 1985).…”

Modulation of NMDA effects on agonist‐stimulated phosphoinositide turnover by memantine in neonatal rat cerebral cortex

“…Mn2" has been shown to enhance greatly [3H]-inositol incorporation into inositol phospholipids in different neuronal preparations (Yandrasitz & Segal, 1979;Schoepp, 1985). However, although the increased phospholipid radiolabelling is reflected in increased basal [3HI-InsPx accumulations in cerebral cortex slices, agonist-stimulated responses are largely unaffected suggesting that Mn2" facilitates [3H]-inositol incorporation into a phospholipid pool inaccessible to agonist-stimulated phospholipase C activity (Schoepp, 1985). Another important observation regarding the possible actions of memantine has been made from the detailed analysis of individual inositol (poly)phosphate isomers generated under basal and agonist-stimulated conditions.…”

“…In both neonatal and adult (Baird & Nahorski, 1991) (Yandrasitz & Segal, 1979;Schoepp, 1985 (Schoepp, 1985). Another important observation regarding the possible actions of memantine has been made from the detailed analysis of individual inositol (poly)phosphate isomers generated under basal and agonist-stimulated conditions.…”

Excitatory Amino Acid Agonist‐Antagonist Interactions at 2‐Amino‐4‐Phosphonobutyric Acid‐Sensitive Quisqualate Receptors Coupled to Phosphoinositide Hydrolysis in Slices of Rat Hippocampus

“…tant was decanted and analyzed for ['HIIP using anion exchange column chromatography as previously described (Schoepp, 1985). The protein content of the resultant pellet from each sample was determined by the biuret method (Layne, 1957).…”