Localization and turnover of phosphatidylinositol 4,5-bisphosphate in caveolin-enriched membrane domains

Caveolae are small, plasma membrane invaginations that have been implicated in cell signaling. In A431 cells, approximately half of the total cellular phosphatidylinositol 4,5-bisphosphate (PtdIns 4, 5-P2) was found to be localized in low density, Triton-insoluble membrane domains enriched in caveolin. Treatment of cells with either epidermal growth factor or bradykinin for 5 min at 37 degrees C resulted in approximately a 50% decrease in this caveolar PtdIns 4,5-P2 with no change in the levels of plasma membrane PtdIns 4,5-P2. These data suggest that the PtdIns 4,5-P2 present in cells is largely compartmentalized and that the caveolar PtdIns 4,5-P2 is subject to hydrolysis by hormone-stimulated phospholipase C. As growth factor receptors, seven transmembrane domain receptors, heterotrimeric G proteins, and the inositol trisphosphate receptor have all been shown to be enriched in caveolae, these findings suggest that both the generation and response to inositol trisphosphate is highly compartmentalized within the cell.     

Functional coupling of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP3R) plays a key role in intracellular Ca2+ signaling. InsP3R is activated by InsP3 produced from phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C cleavage. Using planar lipid bilayer reconstitution technique, we demonstrate here that rat cerebellar InsP3R forms a stable inhibitory complex with endogenous PIP2. Disruption of InsP3R-PIP2 interaction by specific anti-PIP2 monoclonal antibody resulted in 3-4-fold increase in InsP3R activity and 10-fold shift in apparent affinity for InsP3. Exogenously added PIP2 blocks InsP3 binding to InsP3R and inhibits InsP3R activity. Similar results were obtained with a newly synthesized water soluble analog of PIP2, dioctanoyl-(4,5)PIP2, indicating that insertion of PIP2 into membrane is not required to exert its inhibitory effects on the InsP3R. We hypothesize that the functional link between InsP3R and PIP2 described in the present report provides a basis for a local, rapid, and efficient coupling between phospholipase C activation, PIP2 hydrolysis, and intracellular Ca2+ wave initiation in neuronal and non-neuronal cells.     

Gelsolin binding to phosphatidylinositol 4,5-bisphosphate is modulated by calcium and pH

The actin cytoskeleton of nonmuscle cells undergoes extensive remodeling during agonist stimulation. Lamellipodial extension is initiated by uncapping of actin nuclei at the cortical cytoplasm to allow filament elongation. Many actin filament capping proteins are regulated by phosphatidylinositol 4,5-bisphosphate (PIP2), which is hydrolyzed by phospholipase C. It is hypothesized that PIP2 dissociates capping proteins from filament ends to promote actin assembly. However, since actin polymerization often occurs at a time when PIP2 concentration is decreased rather than increased, capping protein interactions with PIP2 may not be regulated solely by the bulk PIP2 concentration. We present evidence that PIP2 binding to the gelsolin family of capping proteins is enhanced by Ca2+. Binding was examined by equilibrium and nonequilibrium gel filtration and by monitoring intrinsic tryptophan fluorescence. Gelsolin and CapG affinity for PIP2 were increased 8- and 4-fold, respectively, by microM Ca2+, and the Ca2+ requirement was reduced by lowering the pH from 7.5 to 7.0. Studies with the NH2- and COOH-terminal halves of gelsolin showed that PIP2 binding occurred primarily at the NH2-terminal half, and Ca2+ exposed its PIP2 binding sites through a change in the COOH-terminal half. Mild acidification promotes PIP2 binding by directly affecting the NH2-terminal sites. Our findings can explain increased PIP2-induced uncapping even as the PIP2 concentration drops during cell activation. The change in gelsolin family PIP2 binding affinity during cell activation can impact divergent PIP2-dependent processes by altering PIP2 availability. Cross-talk between these proteins provides a multilayered mechanism for positive and negative modulation of signal transduction from the plasma membrane to the cytoskeleton.     

Synaptojanin inhibition of phospholipase D activity by hydrolysis of phosphatidylinositol 4,5-bisphosphate

A 150-kDa protein that inhibits phospholipase D (PLD) activity stimulated by ADP-ribosylation factor and phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2) was previously purified from rat brain. The sequences of peptides derived from the purified PLD inhibitor now identify it as synaptojanin, a nerve terminal protein that has been implicated in the endocytosis of fused synaptic vesicles and shown to be a member of the inositol polyphosphate 5-phosphatase family. Further characterization of the enzymatic properties of synaptojanin now shows that it hydrolyzes only the 5-phosphate from inositol 1,4,5-trisphosphate (I(1,4,5)P3) and that it does not catalyze the dephosphorylation of either I(1,3,4)P3 or inositol 1, 4-bisphosphate. However, synaptojanin hydrolyzes both the 4- and 5-phosphates of PI(4,5)P2 and the 4-phosphate of phosphatidylinositol 4-phosphate, converting both compounds to phosphatidylinositol. Magnesium is required for the hydrolysis of I(1,4,5)P3, but not for that of phosphoinositides, by synaptojanin. The inhibition of PLD by synaptojanin is attributable to its ability to hydrolyze PI(4,5)P2. Synaptojanin did not inhibit PLD in the absence of PI(4,5)P2, and the extent of PLD inhibition was related to the extent of PI(4,5)P2 hydrolysis in substrate vesicles. It has been proposed that the biosynthesis of PI(4,5)P2 and the activation of PLD by ADP-ribosylation factor constitute a positive loop to increase rapidly the concentrations of PI(4,5)P2 and phosphatidic acid (PA) during membrane vesiculation. The PA thus produced, probably together with PI(4,5)P2, facilitates vesicle coat assembly. The hydrolysis of PI(4,5)P2, and consequent inhibition of PLD, by synaptojanin might therefore constitute a mechanism to halt the positive loop connecting PI(4,5)P2 and PA during the endocytotic cycle of synaptic vesicles and serve as a signal for uncoating.     

Syndecan-4 proteoglycan cytoplasmic domain and phosphatidylinositol 4,5-bisphosphate coordinately regulate protein kinase C activity

Phosphatidylinositol 4,5-bisphosphate (PIP2) is involved in the organization of the actin cytoskeleton by regulating actin-associated proteins. The transmembrane heparan sulfate proteoglycan syndecan-4 also plays a critical role in protein kinase C (PKC) signaling in the formation of focal adhesions and actin stress fibers. The cytoplasmic domain of syndecan-4 core protein directly interacts with and potentiates PKCalpha activity, and it can directly interact with the phos- phoinositide PIP2. We, therefore, investigated whether the interaction of inositol phosphates and inositol phospholipids with syndecan-4 could regulate PKC activity. Data from in vitro kinase assays using purified PKCalpha beta gamma show that in the absence of phosphatidylserine and diolein, PIP2 increased the extent of autophosphorylation of PKCalpha beta gamma and partially activated it to phosphorylate both histone III-S and an epidermal growth factor receptor peptide. This activity was dose-dependent, and its calcium dependence varied with PKC isotype/source. Addition of the cytoplasmic syndecan-4 peptide, but not equivalent syndecan-1 or syndecan-2 peptides, potentiated the partial activation of PKCalpha beta gamma by PIP2, resulting in activity greater than that observed with phosphatidylserine, diolein, and calcium. This study indicates that syndecan-4 cytoplasmic domain may bind both PIP2 and PKCalpha, localize them to forming focal adhesions, and potentiate PKCalpha activity there.     

Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, Galphas or Galphai1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Galpha conformation. To study the possibility of tubulin involvement in Galphaq-mediated phospholipase Cbeta1 (PLCbeta1) signaling, the m1 muscarinic receptor, Galphaq, and PLCbeta1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCbeta1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCbeta1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Galphaq and PLCbeta1, accompanied by guanine nucleotide transfer from tubulin to Galphaq, is suggested as a mechanism for the enzyme activation. The PLCbeta1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCbeta1 by tubulin, since high tubulin concentrations might prevent the access of PLCbeta1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Galphaq for tubulin. Involvement of tubulin in the articulation between PLCbeta1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.     

Suppression of phospholipase C beta, gamma, and delta families alters cell growth and phosphatidylinositol 4,5-bisphosphate levels

Phosphatidylinositol-specific phospholipase C (PLC) activity reflects a summation of the activities of three families, beta, gamma, and delta, each of which is regulated differently. In order to understand the contribution of each family to cell proliferation signaling, expression of each family was suppressed by use of an inducible expression vector for antisense PLC sequences in a single cell line, FTO-2B rat hepatocytes. Activation of second messengers of PLC [diacylglycerol (DAG) and inositol 1,4,5-tris(phosphate) (IP3)] was dramatically reduced, providing a strategy for probing the consequences of PLC deficiency on cell function. Importantly, while one PLC family was suppressed, the other PLCs actively responded to specific stimuli, suggesting parallel and independent signaling pathways for each PLC family in FTO-2B cells. Selective suppression of each PLC family altered cell growth markedly and differentially. The rank order for suppression of cell growth by loss of a PLC family was gamma > delta > beta. Exploration of down-stream growth regulators revealed that loss of beta and gamma, but not delta, families was associated with markedly reduced basal ras and protein kinase C activity. Moreover, suppression of each of the three PLC families caused remarkably reduced basal and stimulated MAP kinase activities. Interestingly, cellular levels of PIP2 were increased and dramatically correlated with growth inhibition rate in the clones with suppressed PLC activity, suggesting that PIP2 itself can serve as a second messenger of cell growth regulation.     

Role of human papillomaviruses in the malignant transformation of keratinocytes

Human papillomaviruses (HPVs) are considered the main etiological agent of cervical cancer and of its precursory lesions (CIN). The HPV transformation potential consists in the presence of two oncogenes, E6 and E7, which inactivate the tumour-suppressor proteins p53 and pRB of the host cell, inducing uncontrolled transformation and proliferation mechanisms. These effects were demonstrated by in vitro experiments.     

Group IV cytosolic phospholipase A2 binds with high affinity and specificity to phosphatidylinositol 4,5-bisphosphate resulting in dramatic increases in activity

The group IV cytosolic phospholipase A2 (cPLA2) exhibits a potent and specific increase in affinity for lipid surfaces containing phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) at physiologically relevant concentrations. Specifically, the presence of 1 mol% PtdIns(4,5)P2 in phosphatidylcholine vesicles results in a 20-fold increase in the binding affinity of cPLA2. This increased affinity is accompanied by an increase in substrate hydrolysis of a similar magnitude. The binding studies and kinetic analysis indicate that PtdIns(4,5)P2 binds to cPLA2 in a 1:1 stoichiometry. The magnitude of the effect of PtdIns(4,5)P2 is unique among anionic phospholipids and larger than that for other polyphosphate phosphatidylinositols. The effect of PtdIns(4,5)P2 on the activity of cPLA2 is at least an order of magnitude larger than the concomitant changes in the fraction of the enzyme associated with lipid membranes. Striking parallels between the interaction of cPLA2 with PtdIns(4,5)P2 and the interaction of the pleckstrin homology domain of phospholipase C delta 1 with PtdIns(4,5)2 combined with sequence analysis of cPLA2 lead us to propose the existence and location of a pleckstrin homology domain in cPLA2. We further show that the very nature of the interaction of proteins such as cPLA2 with multiple ligands incorporated into membranes follows a specific model which necessitates the use of an experimental methodology suitable for a membrane interface to allow for a meaningful analysis of the data.     

INP51, a yeast inositol polyphosphate 5-phosphatase required for phosphatidylinositol 4,5-bisphosphate homeostasis and whose absence confers a cold-resistant phenotype

Sequence analysis of Saccharomyces cerevisiae chromosome IX identified a 946 amino acid open reading frame (YIL002C), designated here as INP51, that has carboxyl- and amino-terminal regions similar to mammalian inositol polyphosphate 5-phosphatases and to yeast SAC1. This two-domain primary structure resembles the mammalian 5-phosphatase, synaptojanin. We report that Inp51p is associated with a particulate fraction and that recombinant Inp51p exhibits intrinsic phosphatidylinositol 4,5-bisphosphate 5-phosphatase activity. Deletion of INP51 (inp51) results in a "cold-tolerant" phenotype, enabling significantly faster growth at temperatures below 15 degreesC as compared with a parental strain. Complementation analysis of an inp51 mutant strain demonstrates that the cold tolerance is strictly due to loss of 5-phosphatase catalytic activity. Furthermore, deletion of PLC1 in an inp51 mutant does not abrogate cold tolerance, indicating that Plc1p-mediated production of soluble inositol phosphates is not required. Cells lacking INP51 have a 2-4-fold increase in levels of phosphatidylinositol 4,5-bisphosphate and inositol 1,4, 5-trisphosphate, whereas cells overexpressing Inp51p exhibit a 35% decrease in levels of phosphatidylinositol 4,5-bisphosphate. We conclude that INP51 function is critical for proper phosphatidylinositol 4,5-bisphosphate homeostasis. In addition, we define a novel role for a 5-phosphatase loss of function mutant that improves the growth of cells at colder temperatures without alteration of growth at normal temperatures, which may have useful commercial applications.     

Type I phosphatidylinositol-4-phosphate 5-kinases synthesize the novel lipids phosphatidylinositol 3,5-bisphosphate and phosphatidylinositol 5-phosphate

Inositol phospholipids regulate a variety of cellular processes including proliferation, survival, vesicular trafficking, and cytoskeletal organization. Recently, two novel phosphoinositides, phosphatidylinositol-3,5-bisphosphate (PtdIns-3,5-P2) and phosphatidylinositol- 5-phosphate (PtdIns-5-P), have been shown to exist in cells. PtdIns-3,5-P2, which is regulated by osmotic stress, appears to be synthesized by phosphorylation of PtdIns-3-P at the D-5 position. No evidence yet exists for how PtdIns-5-P is produced in cells. Understanding the regulation of synthesis of these molecules will be important for identifying their function in cellular signaling. To determine the pathway by which PtdIns-3,5-P2 and Ptd-Ins-5-P might be synthesized, we tested the ability of the recently cloned type I PtdIns-4-P 5-kinases (PIP5Ks) alpha and beta to phosphorylate PtdIns-3-P and PtdIns at the D-5 position of the inositol ring. We found that the type I PIP5Ks phosphorylate PtdIns-3-P to form PtdIns-3,5-P2. The identity of the PtdIns-3,5-P2 product was determined by anion exchange high performance liquid chromatography analysis and periodate treatment. PtdIns-3,4-P2 and PtdIns-3,4,5-P3 were also produced from PtdIns-3-P phosphorylation by both isoforms. When expressed in mammalian cells, PIP5K Ialpha and PIP5K Ibeta differed in their ability to synthesize PtdIns-3,5-P2 relative to PtdIns-3,4-P2. We also found that the type I PIP5Ks phosphorylate PtdIns to produce PtdIns-5-P and phosphorylate PtdIns-3,4-P2 to produce PtdIns-3,4,5-P3. Our findings suggest that type I PIP5Ks synthesize the novel phospholipids PtdIns-3,5-P2 and PtdIns-5-P. The ability of PIP5Ks to produce multiple signaling molecules indicates that they may participate in a variety of cellular processes.     

Biphasic activation of PKBalpha/Akt in platelets. Evidence for stimulation both by phosphatidylinositol 3,4-bisphosphate, produced via a novel pathway, and by phosphatidylinositol 3,4,5-trisphosphate

Stimulation of platelet thrombin receptors or protein kinase C causes fibrinogen-dependent aggregation that is a function of integrin alphaIIb beta3 activation. Such platelets rapidly and transiently form phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) and a small amount of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). After aggregation, a larger amount of PtdIns(3,4)P2 is generated. We report that this latter PtdIns(3,4)P2 arises largely through wortmannin-inhibitable generation of PtdIns3P and then phosphorylation by PtdIns3P 4-kinase (PtdIns3P 4-K), a novel pathway apparently contingent upon the activation of the Ca2+-dependent protease calpain. Elevation of cytosolic Ca2+ by ionophore, without integrin/ligand binding, is insufficient to activate the pathway. PtdIns3P 4-K is not the recently described "PIP5KIIalpha." Cytoskeletal activities of phosphatidylinositol 3-kinase and PtdIns3P 4-K increase after aggregation. Prior to aggregation, PtdIns3P 4-K can be regulated negatively by the beta gamma subunit of heterotrimeric GTP-binding protein. After aggregation, PtdIns3P 4-K calpain-dependently loses its susceptibility to Gbeta gamma and is, in addition, activated. Both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 have been shown to stimulate PKBalpha/Akt phosphorylation and activation by phosphoinositide-dependent kinase 1. We find that activation of PKBalpha/Akt in platelets is phosphorylation-dependent and biphasic; the initial phase is PtdIns(3,4,5)P3-dependent and more efficient, whereas the second phase depends upon PtdIns(3,4)P2 generated after aggregation. There is thus potential for both pre- and post-aggregation-dependent signaling by PKBalpha/Akt.     

Iron deficiency reduces the hydrolysis of cell membrane phosphatidyl inositol-4,5-bisphosphate during splenic lymphocyte activation in C57BL/6 mice

Iron deficiency impairs lymphocyte proliferation in humans and laboratory animals by unknown mechanisms. In this study, we investigated whether this alteration can be attributed in part to impaired hydrolysis of cell membrane phosphatidyl inositol-4, 5-bisphosphate (PIP2), a required early event of T-lymphocyte activation. The study involved 46 iron-deficient (ID), 26 control (C) and 23 pair-fed (PF) mice, and ID mice that were repleted for 3 (n = 16), 7 (n = 17) or 14 d (n = 18). Mice were killed after 40-63 d (mean, 48 d) of consuming the test diet (0.09 mmol/kg iron) or the control diet (0.9 mmol/kg). The mean (+/-SEM) hemoglobin concentrations were 57 +/- 16.7, 176 +/- 2.6 and 181 +/- 9.7 g/L for ID, C and PF groups, respectively. After splenic lymphocytes were labeled in vitro with 3H-myoinositol for 3 h, PIP2 hydrolysis was estimated by measuring the radioactivity recovered as a mixture of inositol mono-, di- and triphosphate (IP) from concanavalin A (0, 1, 2.5, 5 and 10 mg/L) activated cells. Although cells from ID mice and those from mice repleted for 3 d incorporated slightly more radioactivity in cellular phospholipids than did cells from C or PF mice, less (P < 0.005) was recovered as IP than in controls, suggesting impaired conversion of the precursor to PIP2. At almost all incubation periods (10-120 min) and mitogen concentrations, the rate of PIP2 hydrolysis expressed as the ratio of radioactivity obtained in Con A-treated to untreated cells was significantly (P < 0.05) reduced in cells from ID mice compared with those obtained from C and PF mice. For cells that were activated for 60 min or less, iron repletion for 14 d significantly (P < 0.05) improved the rate of PIP2 hydrolysis. PIP2 hydrolysis positively and significantly (P < 0.05) correlated (r = 0.27-0.56) with indicators of iron status. Mitogenic response was also significantly (P < 0.05) reduced in ID but not PF mice, and it was corrected by iron repletion for 3, 7 or 14 d. Lymphocyte proliferation positively (r = 0.27-0.37, P < 0.01) correlated with indices of iron status and IP ratios. The data suggest that reduced PIP2 hydrolysis contributes to impaired blastogenesis in iron deficiency.     

Assembly of an abundant endogenous major histocompatibility complex class II/peptide complex in class II compartments

To identify the intracellular site(s) of formation of an endogenous class II/peptide complex in a human B cell line, we employed kinetic pulse-chase labeling experiments followed by subcellular fractionation by Percoll density gradient centrifugation and immunogold labeling on ultrathin cryosections. For direct demonstration of assembly of such complexes, we used the monoclonal antibody YAe, which detects an endogenous complex of the mouse class II molecule I-Ab with a 17-amino acid peptide derived from the alpha chain of HLA-DR (DR alpha52-68). We show that in human B lymphocytes, these class II/peptide complexes assemble and transiently accumulate in major histocompatibility complex class II-enriched compartments before reaching the cell surface.     

Inhibition of mSOS-activity by binding of phosphatidylinositol 4,5-P2 to the mSOS pleckstrin homology domain

There are several recently reported examples of inositol phospholipids binding to pleckstrin homology (PH) domains of proteins. The PH domain of SOS, a guanine nucleotide exchange factor for Ras, binds to phosphatidylinositol 4,5 bisphosphate (PtdIns4,5P2). We found that binding of PtdIns4,5P2 to 6-his-tagged recombinant mSOS in vitro inhibits the ability of SOS to catalyze the association of GTP on p21RAS. This inhibition was specific for PtdIns4,5P2: a number of other phosphatidylinositols and phosphatidylserine failed to inhibit Ras GTP-association. We confirmed that the specificity of binding of PtdIns's to recombinant GST-SOS-PH domain is the same as the specificity of PtdIns's for inhibition of SOS activity: namely, that only PtdIns4,5P2 binds significantly to the SOS-PH domain. In addition, the inhibition of Ras GTP-binding is not blocked by excess free inositols suggesting that SOS binds to PtdIns4,5P2 with higher affinity than it binds to free inositols. Addition of SOS-PH domain protein prevented the inhibition of SOS by PtdIns4,5P2 as did addition of the high affinity PtdIns4,5P2-binding drug neomycin. This confirmed that SOS inhibition is mediated by the SOS-PH domain binding to the inositol moiety of PtdIns4,5P2. Binding of Grb2 to SOS did not prevent the inhibition of SOS by PtdIns4,5P2 suggesting that there must be another mechanism for regulating this inhibition. These findings show that the phospholipid PtdIns4,5P2 can suppress the activity of an enzyme involved in signal transduction and suggest that this inhibitory effect must be relieved when SOS is activated.     

Phosphatidylinositol (4,5)-bisphosphate-dependent activation of dynamins I and II lacking the proline/arginine-rich domains

Dynamins comprise a family of GTPases that participate in the early stages of endocytosis. The GTPase activity of neuronal specific dynamin I is stimulated by microtubules, negatively charged phospholipid vesicles, and Src homology 3-containing proteins, including Grb2. These activators were previously shown to bind to a proline/arginine-rich domain (PRD) in the carboxyl-terminal region of the enzyme. Dynamin II, which is ubiquitously expressed, had not been purified or characterized previously. In this study, the enzymatic properties of rat dynamin II and of D746, a dynamin II truncation mutant lacking the PRD, have been characterized. Dynamin II has a higher basal activity than dynamin I, but the two types of dynamin are stimulated similarly by microtubules, Grb2, and phospholipids. D746 is not activated by microtubules or Grb2, highlighting the significance of the PRD for these interactions, but it is activated by phospholipid vesicles containing phosphatidylserine or phosphatidylinositol-4,5- bisphosphate. Moreover, in contrast to previous reports, the PRD appears not to be required for phospholipid-stimulated self-assembly of dynamin, which is a key element in the regulation of its activity. Similar results were obtained with bovine brain dynamin I that had been subjected to limited proteolytic digestion to remove the PRD. Our data highlight the potential involvement of dynamin pleckstrin homology domains in the regulation of GTPase activity by phospholipids.     

Role of surgery in the treatment of malignant gliomas

The mucicrin stimulating the hexosamine gastrointestinal secretion and extracted form porcine duodenum, was concentrated in the zone 7 using sephadex G 100. The molecular weight of 4000-5000 corresponds to this zone. An opposite activity was detected in zone 3. The factor from digestive mucous origin being able to diminish the hexosamine gastric secretion and not influencing the arterial pressure was called mucimitigin. The molecular weight of about 35000 corresponds to this factor.     

Role of polymorphic transformation in the hot pressing of alumina

Conclusions At temperatures of up to 1400°C hot pressing under isothermal conditions enables higher (by 5%) densities to be obtained in compacts from-Al₂O₃ powders than in compacts from-Al₂O₃ powders. With rise in temperature the difference between the densification rates of- and -Al₂O₃ powders diminshes. The difference in density between- and-Al₂O₃ compacts is preserved up to a temperature of 1600°C in hot pressing at a constant heating rate of 5 deg C/min. In the temperature range 1300–18QO°C the energy of activation for hot pressing Al₂O₃ varies from 50 to 75 kcal/g· form. Estimated values of viscosity and effective diffusion coefficient are in accord with values determined in creep and sintering experiments.Translated from Poroshkovaya Metallurgiya, No. 9(165), pp. 76–80, September, 1976.