Inositol trisphosphate formation and calcium mobilization in Swiss 3T3 cells in response to platelet-derived growth factor

Inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate formation in Ca2+-mobilizing-hormone-activated cells

The inositol trisphosphate liberated on stimulation of guinea-pig hepatocytes, pancreatic acinar cells and dimethyl sulphoxide-differentiated human myelomonocytic HL-60 leukaemia cells is composed of two isomers, the 1,4,5-trisphosphate and the 1,3,4-trisphosphate. Inositol 1,4,5-trisphosphate was released rapidly, with no measurable latency on hormone stimulation, and, consistent with its proposed role as an intracellular messenger for Ca2+ mobilization, there was good temporal correlation between its formation and Ca2+-mediated events in these tissues. There was a definite latency before an increase in the formation of inositol 1,3,4-trisphosphate could be detected. In all of these tissues, however, it formed a substantial proportion of the total inositol trisphosphate by 1 min of stimulation. In guinea-pig hepatocytes, where inositol trisphosphate increases for at least 30 min after hormone application, inositol 1,3,4-trisphosphate made up about 90% of the total inositol trisphosphate by 5-10 min. In pancreatic acinar cells, pretreatment with 20 mM-Li+ caused an increase in hormone-induced inositol trisphosphate accumulation. This increase was accounted for by a rise in inositol 1,3,4-trisphosphate; inositol 1,4,5-trisphosphate was unaffected. This finding is consistent with the observation that Li+ has no effect on Ca2+-mediated responses in these cells. The role, if any, of inositol 1,3,4-trisphosphate in cellular function is unknown.

A peptide that inhibits the mitogenic stimulation of Swiss 3T3 cells by bombesin or vasopressin

The synthetic peptide [D-Arg1,D-Pro2,D-Trp7,9,Leu1]substance P inhibits the stimulation of DNA synthesis induced in Swiss 3T3 cells by bombesin or vasopressin, but not that induced by a wide range of other growth factors and mitogens. The stimulation induced by 10 pM-3 nM-bombesin is inhibited by 1-30 microM-antagonist in a manner consistent with competition at the bombesin receptor. The inhibition by the antagonist of the stimulation induced by vasopressin suggests a previously unrecognized interaction of the antagonist with vasopressin receptors. The antagonist should be useful in studies of cell proliferation both in vivo and in vitro.

Induction of protein kinase C activation and Ca2+ mobilization by fibroblast growth factor in Swiss 3T3 cells

Addition of fibroblast growth factor (FGF) to quiescent cultures of Swiss 3T3 cells rapidly induced diacylglycerol formation, protein kinase C activation and Ca2+ mobilization. Protein kinase C-activating agents such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1-oleoyl-2-acetylglycerol (OAG) mimicked the action of FGF and stimulated DNA synthesis in the presence of insulin. Prolonged treatment of the cells with phorbol-12,13-dibutyrate (PDBu) led to the down-regulation and complete disappearance of protein kinase C. In these cells, TPA and OAG did not induce DNA synthesis any more. FGF still elicited Ca2+ mobilization and DNA synthesis, but the magnitude of DNA synthesis was reduced to almost half as compared with that in the control cells. These results clearly indicate that both diacylglycerol and Ca2+ may serve as second messengers for FGF and suggest that these messengers may be involved in the mitogenic action of this growth factor.

Involvement of the Ca2+/phospholipid-dependent protein kinase in the G1 transit of T51B rat liver epithelial cells

The G0----G1 and G1----S transitions (but not the intervening events) in the G1 phase of T51B rat liver epithelial cells in serum-stimulated confluent cultures required a high concentration of extracellular Ca2+ and were accompanied or immediately preceded by increases in the amount of EDTA-extractable protein kinase C, a Ca2+/phospholipid-dependent enzyme. Involvement of this Ca2+-dependent enzyme in the two Ca2+-dependent transitions was further indicated by the facts that 12-O-tetradecanoyl phorbol-13-acetate (TPA), a compound that stimulated protein kinase C from T51B cells even in the absence of Ca2+, enabled these cells to transit G1 in Ca2+-deficient medium, while a TPA analogue (4 alpha-phorbol-12, 13-didecanoate (4 alpha-PDD) that did not stimulate the enzyme in cell-free preparations did not promote G0----G1 or G1----S transit in Ca2+-deficient medium.

Platelet-derived growth factor stimulates rapid polyphosphoinositide breakdown in fetal human fibroblasts

The addition of human platelet-derived growth factor (PDGF) to confluent, quiescent cultures of human diploid fibroblasts induced the rapid breakdown of cellular polyphosphoinositides. The levels of 32P-labeled phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol (PI) decreased by 30 to 40% within 1 min after exposure of the cells to PDGF. The levels of PIP and PIP2 returned to their initial values within 3 and 10 min, respectively, after PDGF addition. The level of PI continued to increase after it had returned to control values and was up threefold within 30 min after PDGF addition. In cells prelabeled with myo-[3H]inositol PDGF caused an eightfold increase in the levels of inositol trisphosphate (IP3) within 2 min. Lesser increases, twofold and 1.3-fold, respectively, were seen in levels of inositol bisphosphate (IP2) and inositol monophosphate (IP). Within 10 min after PDGF addition the levels of all three inositol phosphates had decreased to control values. The levels of IP3 measured 2 min after PDGF addition depended on the PDGF concentration and were maximal at 5-10 ng/ml of PDGF. Similar concentrations of PDGF stimulate maximal cell growth and DNA synthesis in these cells.

Phosphoinositides in mitogenesis: neomycin inhibits thrombin-stimulated phosphoinositide turnover and initiation of cell proliferation

Thrombin stimulates 32Pi incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bis-phosphate (PIP2), and phosphatidylinositol (PI), and initiates DNA synthesis in hamster (NIL) fibroblasts at a half-maximal concentration of 125 ng/ml. Neomycin, which binds PIP2 and PIP, inhibits both thrombin-stimulated initiation of cell proliferation and 32P pI incorporation into at concentrations above 2 mM without affecting thrombin binding, thymidine uptake, or cellular protein synthesis. At lower concentrations, neomycin inhibits thrombin-stimulated release of inositol 1,4,5-trisphosphate (IP3), by selectively binding PIP2, but does not inhibit 32P incorporation into PI or initiation of DNA synthesis. Phosphoinositide recycling and diacylglycerol release therefore appear necessary for initiation of cell proliferation by thrombin. IP3-stimulated Ca++ mobilization may not be required for thrombin mitogenesis, however, since neomycin can block IP3 release without inhibiting initiation.

Synergism between diacylglycerols and calcium ionophore in the induction of human B cell proliferation mimics the inositol lipid polyphosphate breakdown signals induced by crosslinking surface immunoglobulin

Resting human tonsillar B cells were stimulated to divide by heat killed Staphylococcus aureus Cowan strain 1 which was shown to induce hydrolysis of phosphatidylinositol 4, 5-bisphosphate known to give rise to diacylglycerol and an increase in cytosolic free calcium. Addition of the diacylglycerols, 1-oleoyl-2 acetyl glycerol or sn-1, 2-dioctanoylglycerol, together with the calcium ionophore ionomycin to B cell cultures induced marked cell proliferation whereas these agents were ineffective when used alone. Both diacylglycerols were shown to compete with [3H] phorbol 12,13 dibutyrate in binding to protein kinase C. These data support the hypothesis that synergism between cytosolic calcium and endogenous diacylglycerol, which activates protein kinase C, is involved in signal transduction in the proliferation of human B cells.

Formation of inositol phosphates and calcium mobilization in Swiss 3T3 cells in response to prostaglandin F2 alpha

Signal transduction in the mitogenic action of prostaglandin F2 alpha on Swiss 3T3 cells has been studied. Confluent and quiescent Swiss 3T3 cells prelabeled with myo-[2-3H]inositol were stimulated with PGF2 alpha for 15 min at 37 degrees C in the presence of 5 mM LiCl, and the amount of total [3H]inositol phosphates, a sum of inositol tris-, bis-, and mono-phosphates, accumulated in the cells was determined. Addition of PGF2 alpha to the cells at 0.2 to 10 microM induced a 1.7 to 2.4-fold increase in [3H]-inositol phosphates. The accumulation was dose-dependent. Since assay of the agonist-dependent accumulation of inositol phosphates in the presence of LiCl has been used as a sensitive method for identifying those receptors that are coupled to the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns (4,5)P2], these results indicate that PGF2 alpha induces in Swiss 3T3 cells hydrolysis of inositol lipids by a phospholipase C. The receptor-stimulated hydrolysis of PtdIns(4,5)P2 is usually coupled with a rise in cytoplasmic free Ca2+ concentration ([Ca2+]i). The effect of PGF2 alpha on [Ca2+]i was studied in quin-2 loaded Swiss 3T3 cells. On addition of 0.1 microM and 1 microM PGF2 alpha, there was an immediate increase in quin-2 fluorescence by 16 to 19% indicating a 1.5 to 1.8-fold increase in [Ca2+]i. These results therefore indicate that PGF2 alpha at 0.1 to 1 microM induces in Swiss 3T3 cells the hydrolysis of inositol lipids and a rise in [Ca2+]i.

Parthenogenetic activation decreases the polyphosphoinositide content of frog eggs

Polyphosphoinositides were quantified in metaphase II-arrested eggs of the amphibian Xenopus laevis and 8-10 min later in eggs activated by pricking. The content of phosphatidylinositol 4,5-biphosphate (PIP2) was remarkably high in metaphase II-arrested eggs with respect to that of phosphatidylinositol 4-phosphate (PIP). It was found to drop dramatically at activation. In contrast PIP content did not change significantly.

Involvement of Ca2+ in platelet-derived growth factor-induced expression of c-myc oncogene in Swiss 3T3 fibroblasts

Addition of platelet-derived growth factor (PDGF) to quiescent cultures of Swiss 3T3 fibroblasts rapidly increased the cytoplasmic free Ca2+ concentrations and afterwards the c-myc mRNA levels. The concentrations of PDGF needed for both reactions were roughly the same. Moreover, the Ca2+ ionophores, A23187 and ionomycin, mimicked the actions of PDGF and increased the c-myc mRNA levels. These results suggest that Ca2+ may be involved in the regulation of PDGF-induced expression of the c-myc gene.

B lymphocyte receptors and polyphosphoinositide degradation

Resting B lymphocytes can be activated and induced to proliferate by antibodies against their antigen receptors (anti-lg). We demonstrate an early increase in the level of [3H]inositol trisphosphate in [3H]inositol-labeled murine B cells, which suggests breakdown of phosphatidylinositol bisphosphate by phospholipase C. In line with this, the level of [3H]1,2-diacylglycerol was also elevated after incubation of [3H]arachidonic-acid-labeled B cells with anti-Ig. Anti-lg also caused a rapid increase in the level of cytosolic Ca2+ in B cells. In contrast, two other polyclonal B cell activators, lipopolysaccharide and phorbol myristate acetate, failed to induce any of these effects. Our results suggest that anti-lg may induce B cell growth via phosphoinositide degradation and Ca2+ mobilization, and that phorbol myristate acetate, and possibly lipopolysaccharide, bypass these initial events.

Inositol trisphosphate does not release Ca2+ from permeabilized cardiac myocytes and sarcoplasmic reticulum

The possibility that inositol 1,4,5-trisphosphate (IP3) may act as a Ca2+-mobilizing second messenger in cardiac muscle in a manner analogous to its actions in other cell types has been examined using saponin-permeabilized myocytes and isolated cardiac sarcoplasmic reticulum. Myocytes permeabilized in the presence of MgATP2- sequestered Ca2+ to a level of about 200 nM, similar to the cytosolic free Ca2+ concentration of intact cells, but addition of IP3 was ineffective in causing Ca2+ release from intracellular stores. Similarly, IP3 (up to 50 microM) was unable to inhibit Ca2+ uptake or cause Ca2+ release from isolated canine cardiac sarcoplasmic reticulum vesicles in the presence of either EGTA or sodium vanadate. These results indicate that IP3 is unlikely to mediate mobilization of intracellular Ca2+ stores in myocardial cells.

GTP enhances inositol trisphosphate-stimulated Ca2+ release from rat liver microsomes

Low concentrations of GTP (10-50 microM) greatly enhance the inositol 1,4,5-trisphosphate stimulated Ca2+ release from rat liver microsomal vesicles. The effect of GTP depends on the presence of low concentrations of polyethylene glycol in the incubation medium. Guanylyl imidodiphosphate is ineffective at mimicking the GTP effect and inhibits the action of GTP added subsequently.

Effects of platelet-derived growth factor on Ca2+ in 3T3 cells

The effect of platelet-derived growth factor (PDGF) on cellular Ca2+ was examined in BALB/c-3T3 cells. PDGF induced: A decrease in cell 45Ca2+ content. An apparent increased rate of efflux of preloaded 45Ca2+. A decrease in residual intracellular 45Ca2+ remaining after rapid efflux. When added after the rapid phase of efflux of 45Ca2+ had occurred, an immediate decrease in post-efflux residual intracellular 45Ca2+. All of the observed changes in 45Ca2+ induced by PDGF are consistent with a rapid release of Ca2+ from an intracellular Ca2+ pool that has the slowest efflux and is relatively inaccessible to extracellular EDTA. When incubated with chlortetracycline (CTC), a fluorescent Ca2+ probe, 3T3 cell mitochondria became intensely fluorescent. Addition of PDGF resulted in a rapid decrease in CTC fluorescence intensity in both adherent and suspended 3T3 cells. The effects of PDGF on 3T3 cell Ca2+ stores and CTC fluorescence intensity were identical with the effects of the Ca2+ ionophore A23187 and of the proton ionophore carbonyl cyanide m-chlorophenyl hydrazone. Serum, which contains PDGF, also altered intracellular Ca2+ stores, but platelet-poor plasma, which does not contain PDGF, had no effect. EGF, insulin, and tetradecanoyl phorbol acetate (TPA), other factors which stimulate 3T3 cell growth, did not alter 3T3 cell Ca2+ stores. Release of Ca2+ from intracellular sequestration sites may be a mechanism by which PDGF stimulates cell growth.

Platelet-derived growth factor mimics phorbol diester action on epidermal growth factor receptor phosphorylation at threonine-654

Addition of platelet-derived growth factor (PDGF) to quiescent WI-38 human fetal lung fibroblasts mimics the effect of tumor-promoting phorbol diesters to inhibit the high-affinity binding of 125I-labeled epidermal growth factor (125I-EGF). PDGF, like phorbol diesters, was found to increase the phosphorylation state of EGF receptors immunoprecipitated from intact fibroblasts that were labeled to equilibrium with [32P]phosphate. Phosphoamino acid analysis of the EGF receptors indicated that both PDGF and phorbol diesters increased the level of [32P]phosphoserine and [32P]phosphothreonine. Phosphopeptide mapping of the EGF receptor demonstrated that PDGF increased the phosphorylation of several sites and induced the phosphorylation of a site that was not observed to be phosphorylated on EGF receptors isolated from control cells. This latter phosphorylation site on the EGF receptor was identified as threonine-654, previously shown to be phosphorylated in response to phorbol diesters in intact cells or by purified protein kinase C in vitro. Further, it was observed that PDGF mimicked the action of phorbol diesters to inhibit the EGF-dependent tyrosine phosphorylation of the EGF receptor in [32P]phosphate-labeled fibroblasts. These results are consistent with the hypothesis that increases in diacylglycerol and Ca2+ levels caused by addition of PDGF to fibroblasts activate protein kinase C and that this kinase, at least in part, mediates the effect of PDGF on the phosphorylation of the EGF receptor. The data further suggest that protein kinase C may play an important role in the regulation of cellular metabolism and proliferation by PDGF.

Inositol cyclic phosphates are produced by cleavage of phosphatidylphosphoinositols (polyphosphoinositides) with purified sheep seminal vesicle phospholipase C enzymes

Previous studies have shown that metabolism of phosphatidylinositol by phospholipase C produces a mixture of two water-soluble products: inositol 1-phosphate and inositol 1,2-(cyclic)phosphate. In the present study, we demonstrate that the water-soluble products of phosphatidylphosphoinositol (polyphosphoinositide) cleavage by purified ram seminal vesicle phospholipase C enzymes also contain cyclic phosphates. Inositol cyclic phosphates were detected by 18O labeling. In the presence of acid, cyclic phosphates are rapidly hydrolyzed to phosphomonoesters, and when the hydrolysis is carried out in H2 18O, the resultant phosphomonoesters will contain 18O. The 18O content of the phosphomonoesters was measured following alkaline phosphatase treatment and conversion of the inorganic phosphate to a volatile derivative for gas chromatography/mass spectrometry. Inositol cyclic phosphates were found in the phospholipase C cleavage products of all three phosphoinositides, but the ratio of cyclic to noncyclic product was found to decrease in the order phosphatidylinositol greater than phosphatidylinositol 4-phosphate greater than phosphatidylinositol 4,5-bisphosphate. The formation of myo-inositol 1,2(cyclic)-4-bisphosphate was further substantiated by anion-exchange HPLC of the water-soluble products of [32P]phosphatidylinositol 4-phosphate metabolism by phospholipase C. Two peaks were detected one of which, on acid treatment, incorporated 18O from H2 18O into phosphate groups, consistent with this peak containing the cyclic phosphate product. These results suggest that polyphosphoinositide breakdown in stimulated cells may occur via a cyclic phosphate intermediate, as has been described for phosphatidylinositol. These cyclic phosphates contain a reactive bond that may play a role in phosphoinositide-derived signal transduction.

Stimulation of connective tissue cell growth by substance P and substance K

Connective tissue cells proliferate actively when cultured in the presence of serum. Platelet-derived growth factor (PDGF), a basic protein of relative molecular mass approximately 30,000, has been identified as the major serum mitogen for these cells; its main physiological/pathophysiological role may be to initiate wound healing in connection with tissue injury. However, growth of cultured cells is also influenced by several other factors, including epidermal growth factor, fibroblast growth factor, insulin and somatomedins. Furthermore, Rozengurt and Sinnett-Smith recently showed that bombesin, a neuroendocrine peptide isolated from frog skin, stimulates DNA synthesis and cell division in cultures of a specific subtype of 3T3 cells. Substance P and substance K (also known as neurokinin A or neuromedin L) are mammalian peptides belonging to the tachykinin family. Substance P has been studied extensively; it is distributed widely throughout the central and peripheral nervous system, including primary sensory neurones, and can be released in the periphery from axon collaterals of stimulated pain fibres and contribute to the inflammatory response. Substance K is a member of the tachykinin family isolated from mammalian spinal cord; Nawa et al. determined the primary structure of two types of substance P precursors, one of which contained a sequence homologous to substance K, as well as the sequence of substance P. We report here that substance P and substance K stimulate DNA synthesis in cultured arterial smooth muscle cells and human skin fibroblasts, and that this stimulation is inhibited by the substance P-antagonist spantide.

The digitonin-permeabilized pancreatic islet model. Effect of myo-inositol 1,4,5-trisphosphate on Ca2+ mobilization

Glucose-induced insulin secretion is thought to be mediated by submicromolar increases in intracellular Ca2+, although the intracellular processes are not well understood. We have used the previously characterized digitonin-permeabilized insulin-secreting pancreatic islet model to study the role of myo-inositol 1,4,5-trisphosphate (IP3), a putative second messenger for mobilization of intracellular Ca2+. Ca2+ efflux from the endoplasmic reticulum was studied with or without vanadate present to inhibit Ca2+ reuptake. IP3 (10 microM), at a free Ca2+ level of 0.06 microM, increased Ca2+ release by 30% and, when vanadate was present, by 50%. Maximal and half-maximal Ca2+ release was observed at 10 microM- and 2.5 microM-IP3, respectively. IP3 provoked a rapid release that was followed by slow reuptake. Reuptake was diminished in the presence of vanadate. Inositol 1,4-bisphosphate, inositol 1-phosphate and other phosphoinositide metabolites did not have any significant effect. Because increases in Ca2+ levels in the submicromolar range have been previously shown to induce insulin release in digitonin-permeabilized islets, our results are consistent with the concept of IP3 serving as a second messenger for insulin secretion.

Intracellular levels and distribution of Ca2+ in digitonin-permeabilized cells

Digitonin and other saponins can be used to selectively permeabilize the plasma membrane of a wide variety of cells without significantly affecting the gross structure and function of Ca2+-sequestering organelles such as mitochondria and endoplasmic reticulum. These characteristics have allowed digitonin to be used in the determination of the intracellular levels and distribution of Ca2+, as well as the measurement of Ca2+ fluxes by organelles "in situ". Studies conducted with several different types of digitonin-permeabilized cells indicate that the endoplasmic reticulum functions as a high affinity and low-capacity intracellular Ca2+ buffer, whereas mitochondria operate as a relatively low affinity but high capacity Ca2+ buffering system. However, recent findings suggest that mitochondria have a comparable affinity for net Ca2+ uptake in the presence of physiological concentrations of polyamines. The use of permeabilized cells has also been important in the identification of the endoplasmic reticulum as a site at which the recently discovered second messenger inositol trisphosphate acts to bring about an increase in the cytosolic free Ca2+ concentration. Thus, the selective permeabilization of cells with digitonin and its analogues has been a powerful yet simple tool in the study of intracellular Ca2+ homeostasis.

The influence of extracellular calcium on the distribution of protein kinase C in non-neoplastic and neoplastic rat liver cells

Non-neoplastic T51B rat liver epithelial cells cannot proliferate in Ca2+-deficient medium. This proliferative inhibition in Ca2+-deficient medium is accompanied by a large reduction in the amount of cellular EDTA-extractable Ca2+/phospholipid-dependent protein kinase (protein kinase C) activity. By contrast, tumorigenic epithelial cells from several Morris hepatomas proliferate in Ca2+-deficient medium and either maintain or greatly increase their content of EDTA-extractable protein kinase C.

Inositol 1,4,5-triphosphate microinjection triggers activation, but not meiotic maturation in amphibian and starfish oocytes

Inositol 3,4,5-triphosphate (InsP3) brought about cortical granule exocytosis and elevation of a fertilization membrane, due to a rapid increase of free calcium in cytoplasm, when injected into oocytes of the amphibian Xenopus laevis arrested at second meiotic metaphase. The same result was observed when injection was performed into oocytes of the starfish Marthasterias glacialis arrested either at the first meiotic prophase or after completion of meiosis. Although meiotic maturation was induced in both animals by specific hormones which have been previously shown to release Ca2+ within cytoplasm, InsP3 microinjection into prophase-arrested oocytes did not release them from prophase block.

Inositol trisphosphate, a novel second messenger in cellular signal transduction

There has recently been rapid progress in understanding receptors that generate intracellular signals from inositol lipids. One of these lipids, phosphatidylinositol 4,5-bisphosphate, is hydrolysed to diacylglycerol and inositol trisphosphate as part of a signal transduction mechanism for controlling a variety of cellular processes including secretion, metabolism, phototransduction and cell proliferation. Diacylglycerol operates within the plane of the membrane to activate protein kinase C, whereas inositol trisphosphate is released into the cytoplasm to function as a second messenger for mobilizing intracellular calcium.

Phorbol ester and diacylglycerol mimic growth factors in raising cytoplasmic pH

There is now good evidence that cytoplasmic pH (pHi) may have an important role in the metabolic activation of quiescent cells. In particular, growth stimulation of mammalian fibroblasts leads to a rapid increase in pHi (refs 3-6), due to activation of a Na+/H+ exchanger in the plasma membrane, and this alkalinization is necessary for the initiation of DNA synthesis. However, the mechanism by which mitogens activate the Na+/H+ exchanger to raise pHi is not known, although an increase in cytoplasmic free Ca2+ ([Ca2+]i) has been postulated as the primary trigger. We now present data suggesting that the Na+/H+ exchanger is set in motion through protein kinase C, a phospholipid- and Ca2+-dependent enzyme normally activated by diacylglycerol produced from inositol phospholipids in response to external stimuli. Using newly developed pH microelectrodes and fluorimetric techniques, we show that a tumour promoting phorbol ester and synthetic diacylglycerol, both potent activators of kinase C (refs 12-15), mimic the action of mitogens in rapidly elevating pHi in different cell types. Furthermore, we demonstrate that, contrary to previous views, an early rise in [Ca2+]i is not essential for the activation of Na+/H+ exchange and the resultant increase in pHi. Finally, we suggest that an alkaline pHi shift, mediated by Na+/H+ exchange, may be a common signal in the action of those hormones which elicit the breakdown of inositol phospholipids.

Specificity of inositol trisphosphate-induced calcium release from permeabilized Swiss-mouse 3T3 cells

Swiss-mouse 3T3 cells permeabilized with saponin were used to study the specificity of the inositol trisphosphate-induced release of 45Ca2+ from their intracellular stores. Inositol 1,4,5-trisphosphate was the most potent compound studied (dose giving half-maximal effect 0.3 microM). 45Ca2+ was also released by inositol 2,4,5-trisphosphate, glycerophosphoinositol 4,5-bisphosphate and inositol 4,5-bisphosphate, with doses giving half-maximal effect of respectively 1.6 microM, 1.6 microM and 20 microM, but not by inositol 1,4-bisphosphate (50 microM). These data suggest that the trans-vicinal phosphates on the 4- and 5-positions are essential for the Ca2+-mobilizing effect of inositol trisphosphate, and that in addition there is a requirement for a phosphate group on the opposite side of the molecule, with a preference for the 1-position.