Effect of mouse calcineurin on induction and growth of rice callus transformed by the calcineurin gene

Calcineurin is a Ca²⁺/calmodulin dependent serine/threonine protein phosphatase, and has multiple functions in animal cells. In this work, mouse calcineurin was introduced into wild-type rice and the expression of calcineurin inhibited the induction and growth of rice calli. Inhibitor analysis showed that untransformed and CNAtr transgenic callus cultures had different sensitivity to cyclosporin A (CsA), a specific inhibitor of protein phosphatase calcineurin. When callus cultures were subject to 1 μM of CsA, the growth of calli induced from untransformed wild-type rice was inhibited. Interestingly, the growth inhibition of CNAtr transgenic calli was not detected in presence of 1 μM of CsA. Our findings showed that the heterologous calcineurin might be involved in the regulation of cell growth in plant cells.     

Inhibition of myogenesis by trifluoperazine and compound 48/80

When trifluoperazine (TFP), a calmodulin antagonist, was given to chick or rat myoblasts in cultures, formation of multinucleated myotubes was inhibited. The inhibition of cell fusion by TFP in rat cultures prevents the normal increase in the amount of acetylcholine receptors (AChR) and creatine kinase (CK), while the levels of these proteins in chick muscle cultures are hardly affected. Another calmodulin antagonist, compound 48/80, inhibits fusion at doses that correspond closely to its antagonistic effects on calmodulin. Thus, our results suggest a possible role for calmodulin in the regulation of myoblast fusion, but not on the appearance of muscle proteins.     

Metabolism of inositol 1- and 4-monophosphates in HL60 promyelocytic leukaemia cells

The metabolism of inositol 1- and 4-monophosphates in HL60 promyelocytic leukaemia cells was studied. LiCl, BeCl2 and NaF inhibited the hydrolysis of both monophosphates with half maximal inhibition occurring at 1.2 mM, 0.3 microM, 0.25 mM (Ins 1P) and 0.14 mM, 0.56 microM, 0.28 mM (Ins 4P) respectively. Lithium was an uncompetitive inhibitor with respect to both substrates. Ins 4P inhibited the hydrolysis of Ins 1P in a concentration dependent manner, suggesting that it acts as a competing substrate for the same enzyme. Half maximal inhibition occurred at 120 microM Ins 4P. The lithium sensitive activity responsible for the metabolism of both monophosphates was present in a soluble fraction made from the cells. Taken together these data suggest that Ins 1P and Ins 4P are hydrolysed by a single soluble enzyme activity which is sensitive to inhibition by lithium, beryllium and fluoride.     

Regulation of inositol monophosphatase in Saccharomyces cerevisiae

Inositol monophosphatase is a key enzyme in the de novo biosynthesis of inositol and in the phosphoinositide second-messenger signalling pathway. Inhibition of this enzyme is a proposed mechanism for lithium's pharmacological action in bipolar illness (manic depression). Very little is known about how expression of this enzyme is regulated. Because the yeast Saccharomyces cerevisiae has been shown to be an excellent model system in which to understand the regulation of inositol metabolism, we characterized inositol monophosphatase in this yeast. Lithium inhibited monophosphatase activity in vitro . Growth in the presence of inositol resulted in increased expression of the enzyme in vivo , although inositol had no effect on enzyme activity in vitro . The inositol effect was apparent when cells were grown in glucose but not in glycerol/ethanol. Monophosphatase activity was derepressed as cells entered stationary phase. This effect was apparent only during growth in glucose plus inositol. The results demonstrate that S. cerevisiae monophosphatase is inhibited by lithium and regulated by factors affecting phospholipid biosynthesis.     

Effects of trifluoperazine, a calmodulin antagonist, on rabbit T- and B-cell responses to mitogens and antigen

Trifluoperazine (TFP), an inhibitor of the calcium-binding protein, calmodulin (CaM), was used to assess the role of calmodulin in the responses of rabbit lymphoid cells to stimulation with mitogen and antigen. After binding goat anti-rabbit Fab antibody, rabbit B cells lose their surface immunoglobulin (Ig) through endocytosis and then reexpress this protein during the next 24 hr. This reexpression was markedly inhibited by TFP. The brief and early addition of TFP markedly inhibited the increased [3H]thymidine (Tdr) uptake by rabbit T cells treated with concanavalin A and B cells exposed to anti-Fab. TFP greatly inhibited the induction by keyhole limpet hemocyanin (KLH) of the in vitro syntheses of antibody, Ig, and protein by KLH-primed lymph node cells (LNC). The earlier the TFP the greater was the inhibition of induction of these syntheses. However, once induced, synthesis and secretion of antibody were not inhibited by TFP. In striking contrast to the inhibition by TFP of the mitogenic and antigenic responses of lymphoid cells was the lack of effect of this drug on resting lymphocytes. Since TFP was not cytotoxic for either resting or mitogen- or antigen-stimulated LNC, it is highly unlikely that the observed inhibitory effects of this drug were due to its cytotoxicity. We postulate that an early signal for the activation of LNC proliferation, differentiation, and the syntheses of antibody, Ig, and protein involves a calcium-CaM-mediated reaction. Based on this work and that of others, the calcium-CaM complex may mediate an interaction between the ligand-occupied surface receptor and the cytoskeleton.     

<Emphasis Type="Italic">In vitro</Emphasis> selection of salt tolerant cell lines in <Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.

Cell lines able to grow on media containing 50, 100, 150 or 200 mM NaCl were established from potato callus cultures by direct recurrent selection or gradual selection. In callus subjected to direct selection only small clusters of cells survived on medium with 150 or 200 mM NaCl, whereas on 100 mM small cell portions appear necrotic. When cell lines were obtained by successive subcultures on media with increased concentrations of NaCl, salt-tolerant calli were more compact and developed a greenish colour free from necrotic areas. The response of calli lines grown on media with NaCl was compared to control line. The NaCl-tolerant calli showed a decrease in relative growth rate and water content, with higher reductions in the 150 mM tolerant callus. Lipid peroxidation was increased in 50 mM and 100 mM NaCl-tolerant calli, while in 150 mM tolerant callus remained similar to 100 mM values. There was a significant increase in ascorbic acid content in 100 mM and 150 mM NaCl-tolerant calli as compared to the 50 mM, that was two-fold the value found in the control. Also, the contents of soluble and insoluble proteins increased in salt-tolerant lines. SDS-PAGE of soluble proteins showed the synthesis of specific polypeptides in the presence of NaCl in culture medium and the synthesis of a new polypeptide.     

Control of inositol biosynthesis in Saccharomyces cerevisiae: properties of a repressible enzyme system in extracts of wild-type (Ino+) cells.

Inositol biosynthesis was studied in soluble, cell extracts of a wild-type (Ino) strain of Saccharomyces cerevisiae. Two reactions were detected: (i) conversion of D-glucose-6-phosphate to a phosphorylated form of inositol, presumably inositol-1-phosphate (IP synthethase, EC5.5.1.4), and (ii) conversion of phosphorylated inositol to inositol (IP phosphatase, EC3.1.3.25). The in vitro rate of conversion of glucose-6-phosphate to inositol was proportional to incubaion time and enzyme concentration. The pH optimum was 7.0. The synthesis of inositol required oxidized nicotinamide adenine dinucleotide (NAD) and was stimulated byNH4C1 and MgC12. NADP substituted poorly for NAD, and NADH inhibitedthe reaction. Phosphorylated inositol accumulated in the absence of MgC12, suggesting that inositol-phosphate is an intermediate in the pathway and that Mg ions stimulate the dephosphorylation of inositol-phosphate. IP synthetase was inhibited approximately 20% in the presence of inositol in the reaction mixture at concentrations exceeding 1 mM. The enzyme was repressed approximately 50-fold when inositol was present in the growth medium at concentrations exceeding 50 muM. IP synthetase reached the fully repressed level approximately 10 h after the addition of inositol to logarithmic cultures grown in the absence of inositol. The specific activity of the enzyme increased with time in logarithmically growing cultures lacking inositol andapproached the fully depressed level as the cells entered stationary phase.     

Growth interactions between calli and explants of rose plants in vitro

Growth of explants or calli of two rose cultivars ‘Sonia’ and ‘Golden Times’, was extensively promoted when they were grown on agar together with calli of rose rootstocks Rosa indica or Rosa canina, while growth of callus of a miniature rose cultivar was either not affected or inhibited. The growth of R. indica callus was inhibited when accompanied by explants of ‘Sonia’ or ‘Golden Times’. Promotion or inhibition of explants or callus growth was also observed when the agar medium was supplemented with conditioned liquid medium from cell suspension cultures of cv. Sonia or R. indica. Autoclaved conditioned medium from R. indica lost its promoting effect, while that from Sonia lost its inhibiting effect after autoclaving. The possible interaction between the rootstock and scion tissues is discussed.     

Rubidium chloride tolerant callus cultures of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) accumulate more potassium and cross tolerate to other salts

Callus cultures from salt tolerant (CSR-10) and susceptible (Swarnadhan) varieties of Oryza sativa L. were established in Murashige and Skoog’s (MS) medium containing lethal concentrations (50 mM) of rubidium chloride (RbCl) as a selective agent. While 95–100% cells were viable in callus cultures grown without RbCl, viability was 75% in 50 mM RbCl selected cultures. Growth of RbCl selected calli in presence of salt was comparable to that of callus grown without it. Cells tolerant to RbCl showed more vacuoles and accumulated more K⁺ in comparison with their corresponding controls. Suspension cultures were established and uptake of ⁸⁶Rb⁺ was measured at 10 and 20 min intervals, which revealed a linear relationship between the absorption of K⁺ and time. Callus cultures (560-day-old) tolerant to 50 mM RbCl regenerated shoots with 35–40% frequencies in both the varieties, but the same age-old callus grown in the medium devoid of RbCl did not show any organogenesis. Callus cultures that are tolerant to 50 mM RbCl when exposed to 25 mM LiCl, 50 mM NaCl, 50 mM KCl and 25 mM CsCl also exhibited cross tolerance in both the varieties. This is the first time that a callus line of rice resistant to RbCl was raised and shown to accumulate a major cation K⁺ and also an increased influx of it.     

Inhibition of calcium and calmodulin-dependent phosphodiesterase activity in rats by capsaicin

Capsaicin, reported to elevate hormone sensitive lipase (HSL), is also found to inhibit the Ca++ and calmodulin-dependent cAMP phosphodiesterase (PDE) activity in adipose tissue of rats, fed high fat diet. The dependence of the enzyme activity on Ca++ and calmodulin in vitro, in control rats, is shown by its substantial lowering in the presence of EGTA and inhibition by trifluoperazine (TFP) (IC50 between 10-20 microM). This enzyme activity is also inhibited by both red pepper extract (80% inhibition with 50 microliter) and capsaicin (IC50 between 0.3-1 microM) in a dose dependent manner. Capsaicin has been found to inhibit Ca++-dependent PDE activity by 60% in the test rats. Enzyme inhibition in vivo, due to capsaicin, was overcome by addition of calmodulin to the assay system. Inclusion of fluphenazine or capsaicin in assay inhibited not only the calmodulin-restored enzyme activity from test rats but also that of control rats. These results suggest a possible mechanism for the stimulation of lipolytic activity by capsaicin in vivo.     

Planarian regeneration: in vivo and in vitro effects of calcium and calmodulin on DNA synthesis

Regeneration does not occur when planarians are grown in Ca2+-free medium. The possible effect of calcium upon DNA synthesis was therefore studied using cultured planarian cells and regenerating planarian fragments. In the cultures, DNA synthesis was Ca2+-dependent and required a minimum of 10(-6) M Ca2+ in the medium. It was gradually decreased in cells grown in Ca2+-free medium. Addition of Ca2+ to these cultures raised DNA synthesis. The time lag between addition of Ca2+ and stimulation of DNA synthesis varied with culture age. The triggering effect of Ca2+ was amplified by ionophore A 23187. A calcium binding protein, ram testis calmodulin, intensified the stimulatory effect of calcium, but EGTA blocked this effect. In the presence of trifluoperazine (TFP), DNA synthesis was not stimulated by Ca2+. This inhibition by TFP was overcome by adding calmodulin to the medium. Ca2+ therefore triggered DNA synthesis in vitro, and this role might have been potentiated by calmodulin. In vivo, DNA synthesis was shown to be dependent on the Ca2+ concentration in the medium in which intact or regenerating planarians were grown. In 12-h regenerates, the Ca2+ concentration in the medium was no longer critical. Total calcium content decreased just after sectioning until completion of healing (at 6 h) and then rose significantly to a peak at 12 h which coincided with the first peak of DNA synthesis. The calmodulin content gradually diminished during the first 6 h after sectioning. After a transient rise at 12 h, calmodulin content further decreased until 48 h. The results demonstrate the crucial role of Ca2+ in triggering DNA synthesis in planarian cells in vitro and in regenerating fragments. Calmodulin, whose concentration is very low in planarians compared to vertebrates, might help to induce the first peak of DNA synthesis at 12 h after sectioning, but is probably not the main Ca2+-binding protein involved in the regeneration process.     

Organogenesis and Plantlet Formation from Organ- and Seedling-Derived Calli of Rice (Oryza sativa)

Callus was induced in different somatic organs of Oryza sativa L. Specific minimum 2,4-dichlorophenoxyacetic acid (2,4-D) concentrations in the medium were necessary for the induction of callus from different organs while high levels of 2,4-D (6–10 mg/l) induced callus formation in each organ tested. The optimum 2,4-D concentration for callus induction and growth for root-derived calli was 2 mg/l and for leaf-derived 6 mg/l. Root and shoot organogenesis were induced in both root- and leaf-derived calli by sub-culturing to a medium lacking 2,4-D. Root organogenesis occurred at a higher frequency than shoot organogenesis. Shoot organogenesis rarely occurred in calli without differentiated roots. Increased age of callus cultures almost completely inhibited shoot development. The addition of the cytokinin 6-γ,γ-dimethylallyl-amino purine partially restored the potential for shoot organogenesis. Whole plants were easily recovered from the calli and grown to maturity with some plants exhibiting phenotypic abnormalities.     

Trifluoperazine,a calmodulin antagonist,inhibits muscle cell fusion

We investigated the effect of trifluoperazine (TFP), a calmodulin antagonist, on the fusion of chick skeletal myoblasts in culture. TFP was found to inhibit myoblast fusion. This effect occurs at concentrations that have been reported to inhibit Ca2+-calmodulin in vitro, and is reversed upon removal of TFP. In addition, other calmodulin antagonists, including chlorpromazine, N-(6-aminohexyl)-5- chloro-1-naphthalene-sulfonamide (W7), and N-(6-aminohexyl)-1- naphthalene-sulfonamide (W5), inhibit fusion at doses that correspond closely to the antagonistic effects of these drugs on calmodulin. The expression of surface acetylcholine receptor, a characteristic aspect of muscle differentiation, is not impaired in TFP-arrested myoblasts. Myoblasts inhibited from fusion by 10 microM TFP display impaired alignment. In the presence of the Ca2+ ionophore {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187, the fusion block by 10 microM TFP is partially reversed and myoblast alignment is restored. The presence and distribution of calmodulin in both prefusional myoblasts and fused muscle cells was established by immunofluorescence. We observed an apparent redistribution of calmodulin staining that is temporally correlated with the onset of myoblast fusion. Our findings suggest a possible role for calmodulin in the regulation of myoblast fusion.     

Specific inositol phosphates inhibit basal and calmodulin-stimulated Ca(2+)-ATPase activity in human erythrocyte membranes in vitro and inhibit binding of calmodulin to membranes.

D-Myo-inositol 1,4,5-trisphosphate (Ins[1,4-,5]P3) inhibits rat heart sarcolemmal Ca(2+)-ATPase activity (T. H. Kuo, Biochem. Biophys. Res. Commun. 152: 1111, 1988). We have studied the effect and mechanism of action of Ins(1,4,5)P3 and related inositol phosphates on human red cell membrane Ca(2+)-ATPase (EC 3.6.1.3) activity in vitro. At 10(-6) M, Ins(1,4,5)P3 and D-myo-inositol 4,5-bisphosphate (Ins[4,5]P2) inhibited human erythrocyte membrane Ca(2+)-ATPase activity in vitro by 42 and 31%, respectively. D-Myo-inositol 1,3,4,5-tetrakisphosphate, D-myo-inositol 1,4-bisphosphate, and D-myo-inositol 1-phosphate were not inhibitory. Enzyme inhibition by Ins(1,4,5)P3 was blocked by heparin. Exogenous purified calmodulin also stimulated red cell membrane Ca(2+)-ATPase activity; this stimulation was inhibited by Ins(1,4,5)P3. Ins(4,5)P2 and Ins(1,4,5)P3, but not Ins(1,4)P2, inhibited the binding of [125I]calmodulin to red cell membranes. Thus, specific inositol phosphates reduce plasma membrane Ca(2+)-ATPase activity and enhancement of the latter in vitro by purified calmodulin. The mechanism of these effects may in part relate to inhibition by inositol phosphates of binding of calmodulin to erythrocyte membranes.     

Effects of Al3+ on Ca2+-ATPase Activity on Chloroplasts of Rice and Relation to Calmodulin

The relationship between aluminium phytotoxicity and calmodulin has been studied with. calcium-dependent ATPase in chloroplasts of rice. This enzyme could be activated by extrinsic calmodulin. It showed that the activity of Ca2+-ATPase in chloroplasts was regulated by calmodulin. The activation of calmodulin to the enzyme might be inhibited by calmodulin antagonists, TFP and CPZ. The effects of A13+ on the activation of calmodulin was similar to that of the calmodulin antagonists. Calcium could reduce the inhibition of aluminiutn. It seems that there is a model of toxic responses in plants to aluminium: Al3+→calmodulin→target enzy mes→metabolism.     

Cadmium stress in sugar cane callus cultures: Effect on antioxidant enzymes

Catalase (CAT) and superoxide dismutase (SOD) are antioxidant enzymes which are important in the metabolism of reactive oxygen species (ROS), and can be induced by environmental stresses including cadmium (Cd), a heavy metal toxic to living organisms. Sugar cane (Saccharum officinarumL.) in vitro callus cultures were exposed to CdCl₂ and the activities of CAT and SOD were analysed. Lower concentrations of CdCl₂, such as 0.01 and 0.1 mM caused a significant increase in callus growth, whereas 0.5 and 1 mM CdCl₂ strongly inhibited growth of the callus cultures, but only after 9 days of CdCl₂ treatment. Red-brown patches were also observed in calluses exposed to 0.5 and 1 mM CdCl₂. Calluses grown in 0.01 and 0.1 mM CdCl₂ did not exhibit any changes in CAT activity even after 15 days of growth in the presence of CdCl₂. However, for calluses grown in higher concentrations of CdCl₂ (0.5 and 1 mM), a rapid increase in CAT activity was detected, which was 14-fold after 15 days. Furthermore, up to five CAT isoforms were observed in callus tissue. Total SOD activity did not exhibit any major variation. One Mn-SOD and two Cu/Zn-SOD isoenzymes were observed in callus cultures and none exhibited any variation in response to the CdCl₂ treatments. The results suggested that in sugar cane callus cultures, CAT may be the main antioxidant enzyme metabolizing H₂O₂.     

Lithium-mediated suppression of morphogenesis and growth in Candida albicans

Hyphal development in Candida albicans contributes to virulence, and inhibition of filamentation is a target for the development of antifungal agents. Lithium is known to impair Saccharomyces cerevisiae growth in galactose-containing media by inhibition of phosphoglucomutase, which is essential for galactose metabolism. Lithium-mediated phosphoglucomutase inhibition is reverted by Mg(2+). In this study we have assessed the effect of lithium upon C. albicans and found that growth is inhibited preferentially in galactose-containing media. No accumulation of glucose-1-phosphate or galactose-1-phosphate was detected when yeasts were grown in the presence of galactose and 15 mM LiCl, though we observed that in vitro lithium-mediated phosphoglucomutase inhibition takes place with an IC(50) of 2 mM. Furthermore, growth inhibition by lithium was not reverted by Mg(2+). These results show that lithium-mediated inhibition of growth in a galactose-containing medium is not due to inhibition of galactose conversion to glucose-6-phosphate but is probably due to inhibition of a signaling pathway. Deletion of the Ser-Thr protein phosphatase SIT4 and treatment with rapamycin have been shown to inhibit filamentous differentiation. We observed that C. albicans filamentation was inhibited by lithium in solid medium containing either galactose as the sole carbon source or 10% fetal bovine serum. These results suggest that suppression of hyphal outgrowth by lithium could be related to inhibition of the target of rapamycin (TOR) pathway.     

Disruption by Lithium of Phosphoinositide Signalling in Cerebellar Granule Cells in Primary Culture

Abstract: Mild depolarisation (20 m M KCI) synergistically enhances the ability of a muscarinic agonist to activate phosphoinositide turnover and to elevate inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] in cerebellar granule cells in primary culture. The effects of lithium on this intense stimulation of phosphoinositide turnover was studied. Lithium causes depletion of cytoplasmic inositol and phosphoinositides, which results in the inhibition of phosphoinositide turnover within 15 min and the return of Ins(1,4,5)P₃ to basal levels at this time. This inhibition could not be reversed by culturing and preincubating cerebellar granule cells in concentrations of inositol similar to those detected in the CSF. Inositol concentrations substantially in excess of those in the CSF not only reversed the effects of lithium on stimulated Ins(1,4,5)P₃ levels, but significantly enhanced this level in comparison with stimulation in the absence of lithium. sn -1,2-Diacylglycerol elevation during stimulated phosphoinositide turnover was also disrupted by lithium, but in contrast to Ins(1,4,5)₃, the presence of lithium resulted in a transient enhancement of the elevation evoked by carbachol plus mild KCI depolarisation, which was reversed by 500 µ M inositol, but not by 200 µ M inositol. The implications of these phenomena in relation to the mechanism of action of lithium in the treatment of manic depression are discussed.     

Purification and biochemical characterization of Mycobacterium tuberculosis SuhB, an inositol monophosphatase involved in inositol biosynthesis

Phosphatidylinositol is an essential component of mycobacteria, and phosphatidylinositol-based lipids such as phosphatidylinositolmannosides, lipomannan, and lipoarabinomannan are major immunomodulatory components of the Mycobacterium tuberculosis cell wall. Inositol monophosphatase (EC 3.1.3.25) is a crucial enzyme in the biosynthesis of free myo-inositol from inositol-1-phosphate, a key substrate for the phosphatidylinositol synthase in mycobacteria. Analysis of the M. tuberculosis genome suggested the presence of four M. tuberculosis gene products that exhibit an inositol monophosphatase signature. In the present report, we have focused on SuhB, which possesses the highest degree of homology with human inositol monophosphatase. SuhB gene was cloned into an E. coli expression vector to over-produce a His-tagged protein, which was purified and characterized. SuhB required divalent metal ions for functional inositol monophosphatase activity, with Mg(2+) being the strongest activator. Inositol monophosphatase activity catalyzed by SuhB was inhibited by the monovalent cation lithium (IC(50) = 0.9 mM). As anticipated, inositol-1-phosphate was the preferred substrate (K(m) = 0.177 +/- 0.025 mM; k(cat) = 3.6 +/- 0.2 s(-)(1)); however, SuhB was also able to hydrolyze a variety of polyol phosphates such as glucitol-6-phosphate, glycerol-2-phosphate, and 2'-AMP. To provide further insight into the structure-function relationship of SuhB, different mutant proteins were generated (E83D, D104N, D107N, W234L, and D235N). These mutations almost completely abrogated inositol monophosphatase activity, thus underlining the importance of these residues in inositol-1-phosphate dephosphorylation. We also identified L81 as a key residue involved in sensitivity to lithium. The L81A mutation rendered SuhB inositol monophosphatase activity 10-fold more resistant to inhibition by lithium (IC(50) = 10 mM). These studies provide the first steps in the delineation of the biosynthesis of the key metabolite inositol in M. tuberculosis.     

Differential Effects of Lithium on Muscarinic Receptor Stimulation of Inositol Phosphates in Rat Cerebral Cortex Slices

The accumulation of labelled inositol mono-, bis-, and trisphosphate in rat cerebral cortex slices was examined following preincubation with [3H]inositol. The muscarinic receptor agonist carbachol produced a rapid and sustained increased accumulation of each labelled inositol phosphate both in the presence and absence of 5 mM lithium. Lithium potentiated carbachol-stimulated accumulation of inositol monophosphate (EC50 0.5 mM) and inositol bisphosphate (EC50 4 mM) in a concentration-dependent manner. However, exposure to lithium in the presence of the muscarinic agonist produced a concentration- and time-dependent inhibition of inositol trisphosphate accumulation that was not related to receptor desensitisation. Although the present data do suggest that polyphosphoinositides are substrates for agonist-stimulated phospholipase C in brain, these results may not be entirely consistent with the production of inositol mono- and bisphosphate through inositol trisphosphate dephosphorylation. Furthermore, these data suggest site(s) additional to inositol monophosphatase that are affected by lithium.