Inositol trisphosphates in carbachol-stimulated rat parotid glands

Synthesis of polyphosphoinositides in nuclei of Friend cells. Evidence for polyphosphoinositide metabolism inside the nucleus which changes with cell differentiation

Previous work demonstrated the existence of phosphatidylinositol kinase and phosphatidylinositol phosphate kinase in rat liver nuclei, with the suggestion that these activities are in the nuclear membrane [Smith & Wells (1983) J. Biol. Chem. 258, 9368-9373]. Here we show that highly purified nuclei from Friend cells, washed free of nuclear membrane by Triton, can incorporate radiolabel from [gamma-32P]ATP into phosphatidic acid, phosphatidylinositol phosphate and phosphatidylinositol 4,5-bisphosphate. The degree of radiolabelling of phosphatidylinositol bisphosphate is highly dependent on the state of differentiation of the cells, being barely detectable in growing cells and much greater after dimethyl sulphoxide-induced differentiation; this difference is mostly due to different amounts of phosphatidylinositol phosphate in the isolated nuclei. We suggest that polyphosphoinositides are made inside the nucleus and that they have a role in chromatin function; either the phospholipids themselves play a role, or there is a possibility of intranuclear signalling by inositide-derived molecules.

Inositol lipids and DNA replication

Control of DNA synthesis by growth factors seems to depend upon the generation of intracellular mitogenic signals, which are responsible for initiating the sequence of events leading to the onset of DNA synthesis. Many growth factors have tyrosine kinase activity suggesting the proteins phosphorylated on tyrosine might be likely candidates as intracellular signals. Other candidates are the calcium and hydrogen ions whose concentrations change dramatically during the action of most growth factors, many of which also stimulate the hydrolysis of inositol lipids. In particular, certain growth factors stimulate the hydrolysis of phosphatidylinositol 4,5-bisphosphate to give the two second messengers diacylglycerol and inositol 1,4,5-trisphosphate (Ins1,4,5P3). The former stimulates protein kinase C, which is responsible for increasing intracellular pH by switching on an Na+-H+ exchanger. The water-soluble Ins1,4,5P3 released to the cytosol can be metabolized along two separate pathways: it can either be dephosphorylated to free inositol or it can be converted into additional inositol polyphosphates such as Ins1,3,4,5P4 and Ins1,3,4P3. These inositol phosphates seem to play a key role in regulating intracellular calcium, with Ins1,4,5P3 functioning to release internal calcium, whereas Ins1,3,4,5P4 may function to regulate the entry of external calcium. There is evidence to suggest that these internal messengers may converge on certain key processes responsible for initiating the programme of cell growth. It is argued that an increase in intracellular calcium might be an important intracellular signal for activating both the transcription of a family of early genes, typified by fos, as well as the enzyme S6 kinase, which phosphorylates the ribosomal protein S6 which may regulate protein synthesis. The increase in pH seems to play a permissive role and may create the necessary ionic milieu for S6 phosphorylation and protein synthesis to occur. The onset of RNA and protein synthesis, which occur within the first few minutes after the arrival of a growth factor, represent the initial events of the programme of cell growth which culminates in DNA synthesis and cell division.

Formation of inositol phosphate isomers in GH3 pituitary tumour cells stimulated with thyrotropin-releasing hormone. Acute effects of lithium ions

With a h.p.l.c. system, the inositol mono-, bis- and tris-phosphate isomers found in [3H]inositol-labelled GH3 cells were resolved and identified. These cells possess at least ten distinct [3H]inositol-containing substances when acid-soluble extracts are analysed by anion-exchange h.p.l.c. These substances were identified by their co-elution with known inositol phosphate standards and, to a limited extent, by examining their chemical structure. Two major inositol monophosphate (InsP) isomers were identified, namely Ins1P and Ins4P, both of which accumulate after stimulation with the hypothalamic releasing factor (TRH) (thyrotropin-releasing hormone). Three inositol bisphosphate (InsP2) isomers were resolved, of which two were positively identified, i.e. Ins(1,4)P2 and Ins(3,4)P2. TRH treatment increases both of these isomers, with Ins(1,4)P2 being produced at a faster rate than Ins(3,4)P2. The third InsP2 isomer has yet to be fully identified, although it is co-eluted with an Ins(4,5)P2 standard. This third InsP2 is also increased after TRH stimulation. In common with other cell types, the GH3 cell contains two inositol trisphosphate (InsP3) isomers: Ins(1,4,5)P3, which accumulates rapidly, and Ins(1,3,4)P3, which is formed more slowly. The latter substance appears simultaneously with its precursor, inositol 1,3,4,5-tetrakisphosphate. We also examined the effects of acute Li+ treatment on the rates of accumulation of these isomers, and demonstrated that Li+ augments TRH-mediated accumulation of Ins1P, Ins4P, Ins(1,4)P2, the presumed Ins(4,5)P2 and Ins(1,3,4)P3. These results suggest that the effects of Li+ on inositol phosphate metabolism are more complex than was originally envisaged, and support work carried out by less sophisticated chromatographic analysis.

Stimulation by bradykinin, angiotensin II, and carbachol of the accumulation of inositol phosphates in PC-12 pheochromocytoma cells: differential effects of lithium ions on inositol mono- and polyphosphates

Rat PC-12 pheochromocytoma cells respond to stimulation with bradykinin, angiotensin II, and carbachol with an increased formation of labeled inositol phosphates after preincubation of the cells with [3H]inositol. Li+ potentiates greatly the agonist-induced increase in amount of inositol mono-, bis-, and trisphosphate but not the increase in amount of inositol tetrakisphosphate. Separation of the isomers of inositol trisphosphate shows that the lithium-induced increase in amount of inositol trisphosphate is due to potentiation evoked by lithium of the accumulation of inositol-1,3,4-trisphosphate.

Interactions between intracellular cyclic AMP and agonist-induced inositol phospholipid breakdown in isolated gastric mucosal cells of the rat

The interactions between putative second effector mechanisms for hydrogen ion secretion were studied in isolated gastric cell preparations of the rat containing 60-70% parietal cells. Dibutyryl-cAMP and the compounds which increased the level of cAMP (histamine plus rolipram and forskolin plus rolipram) inhibited the carbachol-induced accumulation of [3H]inositol tris-, bis- and monophosphate. There was both a temporal and quantitative correlation between the increase in cAMP and the inhibition of the accumulation of [3H]inositol phosphates. Cimetidine attenuated the inhibitory effect of histamine on the formation of [3H]inositol phosphates. The enhancement of the accumulation of [3H]inositol phosphates by various concentrations of carbachol affected neither the basal nor the histamine-stimulated cAMP levels. In contrast to dibutyryl-cAMP, dibutyryl-cGMP did not modify the carbachol-induced formation of [3H]inositol phosphates. The biologically active phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), which activates protein kinase C, inhibited both the basal and carbachol-induced accumulation of [3H]inositol phosphates. We suggest that the inhibition of the formation of inositol trisphosphate by the increase in the intracellular level of cAMP and by the activation of protein kinase C might be intracellular negative feedback systems which prevent the overreaction of the acid-secreting parietal cells under the simultaneous influence of the physiological gastric secretagogues.

Formation of inositol 1,3,4,6-tetrakisphosphate during angiotensin II action in bovine adrenal glomerulosa cells

Angiotensin II stimulates the formation of several inositol polyphosphates in cultured bovine adrenal glomerulosa cells prelabelled with [3H] inositol. Analysis by high performance anion exchange chromatography of the inositol-phosphate compounds revealed the existence of two additional inositol tetrakisphosphate (InsP4) isomers in proximity to Ins-1,3,4,5-P4, the known phosphorylation product of Ins-1,4,5-trisphosphate and precursor of Ins-1,3,4-trisphosphate. Both of these new compounds showed a slow increase after stimulation with angiotensin II. The structure of one of these new InsP4 isomers, which is a phosphorylation product of Ins-1,3,4-P3, was deduced by its resistance to periodate oxidation to be Ins-1,3,4,6-P4. The existence of multiple cycles of phosphorylation-dephosphorylation reactions for the processing of Ins-1,4,5-P4 may represent a new aspect of the inositol-lipid related signalling mechanism in agonist-activated target cells.

Isolation of soluble metabolites of the phosphatidylinositol cycle from Samanea saman

An improved protocol for the separation of inositol phosphates by high performance liquid chromatography was used to resolve inositol phosphates from pulvini (motor organs) of the legume, Samanea saman. The pulvini contained inositol phosphate, inositol bisphosphate, and inositol trisphosphate isomers which co-migrated with those of mammalian red blood cells, and one or more other inositol metabolites which, to our knowledge, have not been previously noted in preparations of inositol phosphates. The finding of inositol phosphates in Samanea which comigrate with mammalian inositol phosphates supports the possibility that the phosphatidylinositol cycle may function in signal transduction in plants as well as in animals.

myo-Inositol phosphorothioates, phosphatase-resistant analogues of myo-inositol phosphates. Synthesis of DL-myo-inositol 1,4-bisphosphate and DL-myo-inositol 1,4-bisphosphorothioate

Syntheses of a metabolite of the second messenger myo-inositol 1,4,5-trisphosphate, myo-inositol 1,4-bisphosphate, and an analogue, the 1,4-bisphosphorothioate, are reported, by using phosphite chemistry on (+/-)-1,2:4,5-di-isopropylidene-myo-inositol. The synthesis of (+/-)-1,2:4,5-di-isopropylidene 3,6-bis[di-(2-cyanoethyl)]phosphite provides an intermediate that can be oxidized to either the corresponding bisphosphate or bisphosphorothioate. myo-Inositol phosphorothioates are proposed as novel analogues of myo-inositol phosphates; their resistance to phosphatase-catalysed breakdown is reported.

Activation by calmodulin of inositol-1,4,5-trisphosphate 3-kinase in guinea pig peritoneal macrophages

The activity of inositol-1,4,5-trisphosphate 3-kinase in the cytosol fraction of guinea pig macrophages was assayed with special reference to the dependence on the free Ca2+ concentration. The enzyme activity, as assessed by the production of inositol 1,3,4,5-tetrakisphosphate was reversibly activated by free Ca2+ concentrations ranging from 10(-7) to 10(-6)M. The calmodulin antagonists, W-7 and chlorpromazine, inhibited the Ca2+-activated enzyme activity in a dose-dependent fashion, thereby indicating that calmodulin may be involved in the activation by Ca2+. The content of calmodulin in the cytosol fraction (about 2.8 micrograms/mg of cytosol protein) was markedly reduced to less than 0.03 microgram/mg of proteins by subfractionation by ammonium sulfate, followed by an anion-exchange chromatography. The subfraction obtained by the chromatography showed no Ca2+ dependence in the enzyme activity, while an exogenous addition of calmodulin with 10(-6)M Ca2+ increased the enzyme activity. The enzyme activity was retained on a calmodulin-affinity column in the presence of Ca2+, and was eluted from the column by lowering the free Ca2+ concentration by adding ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid. These results clearly indicate that calmodulin activates the inositol-1,4,5-trisphosphate 3-kinase activity.

Formation and metabolism of inositol 1,4,5-trisphosphate in human platelets

1. myo-[3H]Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], when added to lysed platelets, was rapidly converted into [3H]inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4], which was in turn converted into [3H]inositol 1,3,4-trisphosphate [Ins(1,3,4)P3]. This result demonstrates that platelets have the same metabolic pathways for interconversion of inositol polyphosphates that are found in other cells. 2. Labelling of platelets with [32P]Pi, followed by h.p.l.c., was used to measure thrombin-induced changes in the three inositol polyphosphates. Interfering compounds were removed by a combination of enzymic and non-enzymic techniques. 3. Ins(1,4,5)P3 was formed rapidly, and reached a maximum at about 4 s. It was also rapidly degraded, and was no longer detectable after 30-60 s. 4. Formation of Ins(1,3,4,5)P4 was almost as rapid as that of Ins(1,4,5)P3, and it remained detectable for a longer time. 5. Ins(1,3,4)P3 was formed after an initial lag, and this isomer reached its maximum, which was 10-fold higher than that of Ins(1,4,5)P3, at 30 s. 6. Comparison of the intracellular Ca2+ concentration as measured with fura-2 indicates that agents other than Ins(1,4,5)P3 are responsible for the sustained maintenance of a high concentration of intracellular Ca2+. It is proposed that either Ins(1,3,4)P3 or Ins(1,3,4,5)P4 may also be Ca2+-mobilizing agents.

Metabolism of D-myo-inositol 1,3,4,5-tetrakisphosphate by rat liver, including the synthesis of a novel isomer of myo-inositol tetrakisphosphate

1. We have studied the metabolism of Ins(1,3,4,5)P4 (inositol 1,3,4,5-tetrakisphosphate) by rat liver homogenates incubated in a medium resembling intracellular ionic strength and pH. 2. Ins(1,3,4,5)P4 was dephosphorylated to a single inositol trisphosphate product, Ins(1,3,4)P3 (inositol 1,3,4-trisphosphate), the identity of which was confirmed by periodate degradation, followed by reduction and dephosphorylation to yield altritol. 3. The major InsP2 (inositol bisphosphate) product was inositol 3,4-bisphosphate [Shears, Storey, Morris, Cubitt, Parry, Michell & Kirk (1987) Biochem. J. 242, 393-402]. Small quantities of a second InsP2 product was also detected in some experiments, but its isomeric configuration was not identified. 4. The Ins(1,3,4,5)P4 5-phosphatase activity was primarily associated with plasma membranes. 5. ATP (5 mM) decreased the membrane-associated Ins(1,4,5)P3 5-phosphatase and Ins(1,3,4,5)P4 5-phosphatase activities by 40-50%. This inhibition was imitated by AMP, adenosine 5'-[beta gamma-imido]triphosphate, adenosine 5'-[gamma-thio]triphosphate or PPi, but not by adenosine or Pi. A decrease in [ATP] from 7 to 3 mM halved the inhibition of Ins(1,3,4,5)P4 5-phosphatase activity, but the extent of inhibition was not further decreased unless [ATP] less than 0.1 mM. 6. Ins(1,3,4,5)P4 5-phosphatase was insensitive to 50 mM-Li+, but was inhibited by 5 mM-2,3-bisphosphoglycerate. 7. The Ins(1,3,4,5)P4 5-phosphatase activity was unchanged by cyclic AMP, GTP, guanosine 5'-[beta gamma-imido]triphosphate or guanosine 5'-[gamma-thio]triphosphate, or by increasing [Ca2+] from 0.1 to 1 microM. 8. Ins(1,3,4)P3 was phosphorylated in an ATP-dependent manner to an isomer of InsP4 that was partially separable on h.p.l.c. from Ins(1,3,4,5)P4. The novel InsP4 appears to be Ins(1,3,4,6)P4. Its metabolic fate and function are not known.

Ca2+-mobilizing actions of platelet-derived growth factor differ from those of bombesin and vasopressin in Swiss 3T3 mouse cells

Addition of the mitogenic peptides bombesin and vasopressin to quiescent Swiss 3T3 mouse cells increased the cytosolic Ca2+ concentration without any measurable delay. In contrast, there was a significant lag period (16 +/- 1.2 s) before platelet-derived growth factor (PDGF) increased cytosolic Ca2+ concentration. This lag was not diminished at high concentrations of either porcine or human PDGF. Similar results were obtained in 3T3 cells loaded with quin-2 or fura-2. The differences in the effects of bombesin, vasopressin, and PDGF on Ca2+ movements were also substantiated by measurements of 45Ca2+ efflux and of cellular 45Ca2+ content. Activation of protein kinase C by phorbol esters inhibited Ca2+ mobilization induced by either bombesin or vasopressin. In contrast, phorbol esters had no effect on PDGF-induced cytosolic Ca2+ concentration increase or acceleration of 45Ca2+ efflux. Finally, bombesin and vasopressin caused a rapid increase in the production of inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate, whereas PDGF, even at a saturating concentration, exerted only a small effect. These results indicate that the signal transduction pathways activated by PDGF that lead to Ca2+ mobilization can be distinguished from those utilized by bombesin and vasopressin.

Muscarinic, alpha 1-adrenergic and peptidergic agonists stimulate phosphoinositide hydrolysis and regulate mucin secretion in rat submandibular gland cells

Three classes of agonists associated with Ca2+-mobilization--alpha 1-adrenergic (methoxamine), muscarinic (carbachol) and peptidergic (substance P, SP)--significantly stimulated the secretion of mucin from enzymatically-dispersed rat submandibular gland acinar cells. The same three secretagogues also caused the hydrolysis of membrane inositol phospholipids, resulting in elevated cellular levels of inositol phosphates, particularly inositol 1,4,5-trisphosphate (IP3). Exogenous IP3 elicited the dose-dependent release of mucin in dispersed cells suggesting that agonist-generated endogenous IP3 may provoke a secretory response. IP3 and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) in combination, stimulated an additive secretion of mucin in the model. The potential use of these two agents as specific probes of the IP3- and diacylglycerol-associated legs of the polyphosphoinositide (PPI) breakdown pathway is indicated. Although all three agonists shared a common action in stimulating PPI hydrolysis, their effects on the beta-adrenergic mucosecretory response were inconsistent. A brief preincubation of cells with carbachol or SP significantly reduced the subsequent isoproterenol (IPR)-provoked secretion of mucin, whereas methoxamine plus IPR stimulated an additive response. The mechanisms underlying these opposite effects are not known. Failure of IP3 or TPA to modify IPR responses suggests that modulation of the beta response may operate at a locus before the generation of diacylglycerol and IP3, possibly at the level of signal transduction. The study indicates a role for Ca2+-mobilizing agonists in controlling submandibular mucin secretion and provides evidence that receptor-linked phosphoinositide hydrolysis is an early stage in their stimulus-secretion coupling mechanism.

Inhibition of bombesin-induced mitogenesis by pertussis toxin: dissociation from phospholipase C pathway

Prior incubation of quiescent cultures of Swiss 3T3 cells with pertussis toxin selectively inhibited the stimulation of DNA synthesis induced by peptides of the bombesin family. While pertussis toxin blocked mitogenesis at an early stage in the action of the peptide, the toxin did not impair the rapid stimulation of polyphosphoinositide breakdown, Ca2+ mobilization or activation of protein kinase C promoted by bombesin. Thus, inhibition of bombesin-induced mitogenesis by pertussis toxin can be dissociated from inactivation of the phospholipase C signalling pathway.

Metabolism of inositol 1,4,5-trisphosphate in permeabilized rat aortic smooth-muscle cells. Dependence on calcium concentration

The metabolism of [3H]inositol 1,4,5-trisphosphate ([3H]Ins(1,4,5)P3) was studied in permeabilized rat aortic smooth-muscle cells. Addition of [3H]Ins(1,4,5)P3 to the leaky cells led to formation of several labelled metabolites. Amounts of [3H]inositol bisphosphate and [3H]inositol 1,3,4,5-tetrakisphosphate ([3H]InsP4) reached a maximum within 2 min of incubation, whereas production of [3H]inositol monophosphate and [3H]inositol 1,3,4-trisphosphate ([3H]Ins(1,3,4)P3) was delayed. Formation of InsP4 and Ins(1,3,4)P3 was Ca2+-sensitive in the physiological intracellular range (0.06-5 microM), showing a maximum at 1 microM-Ca2+. A correlation between the formation of InsP4 and that of Ins(1,3,4)P3 was observed, suggesting that the former is the precursor of the latter. These results suggest that, in vascular smooth-muscle cells, Ins(1,4,5)P3 is metabolized via two distinct pathways: (1) a dephosphorylation pathway, leading to formation of inositol bis- and mono-phosphate; and (2) a Ca2+-sensitive phosphorylation/dephosphorylation pathway, involving formation of InsP4 and leading to formation of Ins(1,3,4)P3.

Ethanol does not stimulate guanine nucleotide-induced activation of phospholipase C in permeabilized hepatocytes

Guanine nucleotides are thought to mediate the interaction of the receptors for calcium-mobilizing hormones and phosphoinositide-specific phospholipase C. In the present study the characteristics of guanine nucleotide-dependent phospholipase C activation were studied in [3H]inositol-labeled permeabilized hepatocytes. The nonhydrolyzable GTP analogs guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and guanyl-5'-yl imidodiphosphate stimulated the production of inositol phosphates by phospholipase C. The effect was concentration-dependent with half-maximal and maximal stimulation occurring with 0.6 and 10 microM GTP gamma S, respectively. The guanine nucleotide-induced stimulation of phosphoinositide breakdown was selective for phosphatidylinositol (4,5)-bisphosphate over phosphatidylinositol (4)-phosphate. The individual inositol phosphates formed after maximal GTP gamma S exposure were analyzed by high-performance liquid chromatography. Inositol 1,4,5-trisphosphate was rapidly produced, followed by the formation of inositol 1,3,4,5-tetrakisphosphate and inositol 1,3,4-trisphosphate. Ethanol is known to activate hormone-sensitive phospholipase C in intact rat hepatocytes. Ethanol (0.3 M) was ineffective in altering the characteristics of GTP gamma S-stimulated phospholipase C activation, in both digitonin-treated and sonicated hepatocytes. The metabolism of the various inositol phosphate isomers was unaffected by ethanol. The findings demonstrate the potential for the use of permeabilized hepatocytes in the analysis of phospholipase C activation by guanine nucleotides. Ethanol does not activate phospholipase C by altering this process.

Homologous desensitization of substance-P-induced inositol polyphosphate formation in rat parotid acinar cells

Maximal concentrations of substance P and methacholine induced a rapid increase in [3H]inositol trisphosphate ([3H]IP3) formation. After about 1 min, the [3H]IP3 in the substance-P-treated cells ceased to increase further, whereas in the methacholine-treated cells [3H]IP3 continued to increase. Addition of methacholine to the substance-P-treated cells caused a rapid increase in [3H]IP3, whereas a second addition of a 10-fold excess of substance P had no effect. Pretreatment of cells with substance P, followed by removal of the substance P by washing, resulted in a decreased response to a second application of substance P. A similar protocol involving pretreatment with methacholine had no effect on subsequent responsiveness to substance P. Analysis of [3H]substance P binding to substance-P-treated cells indicated that the number of receptors for substance P was decreased, but the affinity of the receptors for substance P was unaffected. After substance P pretreatment, a prolonged incubation (2 h) restored responsiveness of the cells to substance P, measured as [3H]IP3 formation, and restored the number of binding sites to control values. These findings indicate that, in the rat parotid gland, substance P induces a homologous desensitization of its receptor, which involves a slowly reversible down-regulation or sequestration of substance-P-binding sites.

Confirmation of the identities of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the use of one-dimensional and two-dimensional n.m.r. spectroscopy

Multinuclear n.m.r. spectroscopy, including the use of two-dimensional methodology, was used to confirm the identity of inositol 1,3,4-triphosphate and its metabolic precursor inositol 1,3,4,5-tetrakisphosphate. The cyclohexane ring in each molecule exhibits a chair conformation with all phosphate groups occupying equatorial positions.

Calmodulin activates inositol 1,4,5-trisphosphate 3-kinase activity in pig aortic smooth muscle

The effects of calmodulin (CaM) on inositol 1,4,5-trisphosphate (InsP3) 3-kinase activity in pig aortic smooth muscle were examined. The cytosol fraction of muscle cells, containing 1.2-2.0 micrograms of CaM/mg of cytosol protein (thus 0.12-0.2%, w/w), showed a Ca2+-dependent InsP3 3-kinase activity, and there was no further activation by exogenous addition of CaM purified from dog brain. (NH4)2SO4 fractionation of the cytosol fraction revealed that a 20-60%-satd.-(NH4)2SO4 fraction was rich in the enzyme activity, and the activity without exogenous CaM was still dependent on Ca2+, although the CaM content in this fraction was minute (0.013-0.016%, w/w). The kinase activity observed in the absence of exogenous CaM became insensitive to Ca2+ when a 20-60%-satd.-(NH4)2SO4 fraction was applied to a DEAE-cellulose column, but exogenous addition of CaM increased the enzyme activity from 80-120 to 450 pmol/min per mg of protein, with addition of 10 microM free Ca2+. A fraction separated by DEAE-cellulose chromatography was applied to a CaM affinity column. The kinase activity was retained on the column in the presence of Ca2+, and was eluted by lowering the free Ca2+ concentration by adding EGTA. These results directly show that CaM activates InsP3 3-kinase activity and the enzyme becomes sensitive to Ca2+.

Inositol phosphate metabolism in bradykinin-stimulated human A431 carcinoma cells. Relationship to calcium signalling

Stimulation of human A431 epidermoid carcinoma cells by bradykinin causes a very rapid release of inositol phosphates and a transient rise in cytoplasmic free Ca2+ concentration ([Ca2+]i). Bradykinin-induced inositol phosphate formation is half-maximal at a concentration of 4 nM and is not affected by pertussis toxin. H.p.l.c. analysis of the various inositol phosphates shows an immediate but transient accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which reaches a peak value of approx. 10 times the basal level within 15 s and slightly precedes the rise in [Ca2+]i, both parameters changing in parallel. After a lag period, bradykinin also induces a massive accumulation of Ins(1,3,4)P3 and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Our data support the view that part of the newly formed Ins(1,4,5)P3 is converted into Ins(1,3,4)P3 phosphorylation/dephosphorylation with Ins(1,3,4,5)P4 as intermediate. Furthermore, A431 cells were found to contain strikingly high basal levels of two other inositol phosphates, presumably inositol pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6), representing more than 50% of the total 3H radioactivity incorporated into inositol phosphates. The presumptive InsP5 and InsP6 are only slightly affected by bradykinin. Although Ins(1,3,4)P3 and InsP4 could function as second messengers, our results suggest that, unlike Ins(1,4,5)P3, neither Ins(1,3,4)P3 nor InsP4 are involved in Ca2+ mobilization.

The role of calcium in the cyclic AMP response to histamine in rabbit cerebral cortical slices

The effect of calcium on the H1- and H2-receptor components of the cyclic AMP response to histamine in rabbit cerebral cortical slices has been investigated. Removal of calcium ions from the incubation medium during the preparation, preincubation and final incubation of brain slices significantly reduced the cyclic AMP responses to adenosine, histamine and the H2-selective agonist, impromidine. Removal of calcium ions from the incubation medium during only the final incubation with agonists did not influence the responses to adenosine, histamine, impromidine and the H1-selective agonist, 2-thiazolylethylamine. Final incubation of rabbit cerebral cortical slices in calcium-free buffer containing EGTA (1 mM) however, selectively reduced the cyclic AMP responses to the H1-agonists histamine and 2-thiazolylethylamine without affecting the response to impromidine or adenosine. These latter incubation conditions significantly reduced the maximal extent of the augmentation of impromidine- or adenosine-stimulated cyclic AMP accumulation produced by H1-receptor stimulation, without affecting the EC50 values of the H1-agonists. Calcium-free/EGTA conditions did not, however, alter the dose-response parameters for the response to the H2-agonist, impromidine. These data provide further evidence that the two histamine receptor systems affect cyclic AMP accumulation in rabbit cerebral cortical slices by different mechanisms.

EGF raises cytosolic Ca2+ in A431 and Swiss 3T3 cells by a dual mechanism. Redistribution from intracellular stores and stimulated influx

The changes in Ca2+ homeostasis and phosphoinositide hydrolysis induced by EGF were studied in human epidermoid carcinoma A431 cells both when attached to a substratum and after detachment and suspension. The cytosolic Ca2+ concentration was measured by the conventional fluorimetric technique, using the specific probe, quin2, as well as by a new microscopic technique in which single cells are investigated after loading with another probe, fura-2. EGF applied in the complete, Ca2+-containing medium caused a rapid rise in the cytosolic Ca2+ concentration, that remained elevated for several minutes. In Ca2+-free, EGTA-containing medium, part of this response persisted, as revealed by quin2 results in suspended cells and microscopic results with fura-2. The lack of Ca2+ rise seen in attached cells loaded with quin2 and treated with EGF in Ca2+-free medium was probably the result of a Ca2+ buffer artifact. Concomitantly to the Ca2+ signal, EGF induced phosphoinositide hydrolysis, with stimulated accumulation of inositol 1,3,4,trisphosphate and -1,3,4,5-tetrakisphosphate. These results, as well as additional microscopic fura-2 results in Swiss 3T3 fibroblasts, demonstrate that the Ca2+ signal elicited by EGF is due to two components: redistribution from an intracellular store (possibly mediated by generation of inositol trisphosphate) and stimulated influx across the plasmalemma. This latter process was not detected in 3T3 cells treated with either PDGF or bombesin (growth factors that cause much greater phosphoinositide hydrolysis and Ca2+ redistribution responses than EGF). It is therefore suggested that the Ca2+ influx effect of EGF is under the control of a separate, as yet unidentified mechanism.

Pathway for inositol 1,3,4-trisphosphate and 1,4-bisphosphate metabolism

We prepared [3H]inositol-,3-[32P]phosphate-and 4-[32P]phosphate-labeled inositol phosphate substrates to investigate the metabolism of inositol 1,3,4-trisphosphate and inositol 1,4-bisphosphate. In crude extracts of calf brain, inositol 1,3,4-trisphosphate is first converted to inositol 3,4-bisphosphate, then the inositol 3,4-bisphosphate intermediate is further converted to inositol 3-phosphate. Similarly, inositol 1,4-bisphosphate is converted to inositol 4-phosphate, and no inositol 1-phosphate is formed. We partially purified an enzyme that we tentatively name inositol polyphosphate 1-phosphatase. This cytosolic enzyme converts inositol 1,3,4-trisphosphate to inositol 3,4-bisphosphate and also converts inositol 1,4-bisphosphate to inositol 4-phosphate. The enzyme does not utilize inositol 1,3,4,5-tetrakisphosphate, inositol 1,4,5-trisphosphate, or inositol 1-phosphate as substrates. Thus we propose a new scheme for inositol phosphate metabolism. According to this pathway inositol 1,4,5-trisphosphate and inositol 1,4-bisphosphate are degraded to inositol 4-phosphate. Inositol 1-phosphate, which is the major inositol monophosphate formed in stimulated brain, is derived either from phospholipase C cleavage of phosphatidylinositol or from the degradation of inositol cyclic phosphates.

Inositol 1,2-cyclic 4,5-trisphosphate is not a product of muscarinic receptor-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis in rat parotid glands

We have employed a neutral-pH extraction technique to look for inositol 1,2-cyclic phosphate derivatives in [3H]inositol-labelled parotid gland slices stimulated with carbachol. The incubations were terminated by adding cold chloroform/methanol (1:2, v/v), the samples were dried under vacuum and inositol phosphates were extracted from the dried residues by phenol/chloroform/water partitioning. Water-soluble inositol metabolites were separated by h.p.l.c. at pH 3.7. 32P-labelled inositol phosphate standards (inositol 1-phosphate, inositol 1,2-cyclic phosphate, inositol 1,4,5-trisphosphate and inositol 1,2-cyclic 4,5-trisphosphate) were quantitively recovered through both extraction and chromatography steps. Treatment of inositol cyclic phosphate standards with 5% (w/v) HClO4 for 10 min prior to chromatography resulted in formation of the expected non-cyclic compounds. [3H]Inositol 1-phosphate and [3H]inositol 1,4,5-trisphosphate were both present in parotid gland slices and both increased during stimulation with 1 mM-carbachol. There was no evidence for significant quantities of [3H]inositol 1,2-cyclic phosphate or [3H]inositol 1,2-cyclic 4,5-trisphosphate in control or carbachol-stimulated glands. Parotid gland homogenates rapidly converted inositol 1,4,5-trisphosphate to inositol bisphosphate and inositol tetrakisphosphate, but metabolism of the inositol cyclic trisphosphate was much slower. The results suggest that inositol 1,4,5-trisphosphate, but not inositol 1,2-cyclic 4,5-trisphosphate, is the water-soluble product of muscarinic receptor-stimulated phospholipase C in rat parotid glands.

Metabolism of inositol 1,4,5-trisphosphate in guinea-pig hepatocytes

Metabolism of inositol 1,4,5-trisphosphate was investigated in permeabilized guinea-pig hepatocytes. The conversion of [3H]inositol 1,4,5-trisphosphate to a more polar 3H-labelled compound occurred rapidly and was detected as early as 5 s. This material co-eluted from h.p.l.c. with inositol 1,3,4,5 tetrakis[32P]phosphate and is presumably an inositol tetrakisphosphate. A significant increase in the 3H-labelled material co-eluting from h.p.l.c. with inositol 1,3,4-trisphosphate occurred only after a definite lag period. Incubation of permeabilized hepatocytes with inositol 1,3,4,5-tetrakis[32P]phosphate resulted in the formation of 32P-labelled material that co-eluted with inositol 1,3,4-trisphosphate; no inositol 1,4,5-tris[32P]phosphate was produced, suggesting the action of a 5-phosphomonoesterase. The half-time of hydrolysis of inositol 1,3,4,5-tetrakis[32P]phosphate of approx. 1 min was increased to 3 min by 2,3-bisphosphoglyceric acid. Similarly, the rate of production of material tentatively designed as inositol 1,3,4-tris[32P]phosphate from the tetrakisphosphate was reduced by 10 mM-2,3-bisphosphoglyceric acid. In the absence of ATP there was no conversion of [3H]inositol 1,4,5-trisphosphate to [3H]inositol tetrakisphosphate or to [3H]inositol 1,3,4-trisphosphate, which suggests that the 1,3,4 isomer does not result from isomerization of inositol 1,4,5-trisphosphate. The results of this study suggest that the origin of the 1,3,4 isomer of inositol trisphosphate in isolated hepatocytes is inositol 1,3,4,5-tetrakisphosphate and that inositol 1,4,5-trisphosphate is rapidly converted to this tetrakisphosphate. The ability of 2,3-bisphosphoglyceric acid, an inhibitor of 5-phosphomonoesterase of red blood cell membrane, to inhibit the breakdown of the tetrakisphosphate suggests that the enzyme which removes the 5-phosphate from inositol 1,4,5-trisphosphate may also act to convert the tetrakisphosphate to inositol 1,3,4-trisphosphate. It is not known if the role of inositol 1,4,5-trisphosphate kinase is to inactivate inositol 1,4,5-trisphosphate or whether the tetrakisphosphate product may have a messenger function in the cell.

Evidence that inositol 1-phosphate in brain of lithium-treated rats results mainly from phosphatidylinositol metabolism

In cerebral cortex of rats treated with increasing doses of LiCl, the relative concentrations of Ins(1)P, Ins(4)P and Ins(5)P (when InsP is a myo-inositol phosphate) are approx. 10:1:0.2 at all doses. In rats treated with LiCl followed by increasing doses of pilocarpine a similar relationship occurs. myo-Inositol-1-phosphatase (InsP1ase) from bovine brain hydrolyses Ins(1)P, Ins(4)P and Ins(5)P at comparable rates, and these substrates have similar Km values. The hydrolysis of Ins(4)P is inhibited by Li+ to a greater degree than is hydrolysis of Ins(1)P and Ins(5)P. D-Ins(1,4,5)P3 and D-Ins(1,4)P2 are neither substrates nor inhibitors of InsP1ase. A dialysed high-speed supernatant of rat brain showed a greater rate of hydrolysis of Ins(1)P than of D-Ins(1,4)P2 and a lower sensitivity of the bisphosphate hydrolysis to LiCl, as compared with the monophosphate. That enzyme preparation produced Ins(4)P at a greater rate than Ins(1)P when D-Ins(1,4)P2 was the substrate. The amount of D-Ins(3)P [i.e. L-Ins(1)P, possibly from D-Ins(1,3,4)P3] is only 11% of that of D-Ins(1)P on stimulation with pilocarpine in the presence of Li+. DL-Ins(1,4)P2 was hydrolysed by InsP1ase to the extent of about 50%; both Ins(4)P and Ins(1)P are products, the former being produced more rapidly than the latter; apparently L-Ins(1,4)P2 is a substrate for InsP1ase. Li+, but not Ins(2)P, inhibited the hydrolysis of L-Ins(1,4)P2. The following were neither substrates nor inhibitors of InsP1ase; Ins(1,6)P2, Ins(1,2)P2, Ins(1,2,5,6)P4, Ins(1,2,4,5,6)P5, Ins(1,3,4,5,6)P5 and phytic acid. myo-Inositol 1,2-cyclic phosphate was neither substrate nor inhibitor of InsP1ase. We conclude that the 10-fold greater tissue contents of Ins(1)P relative to Ins(4)P in both stimulated and non-stimulated rat brain in vivo are the consequence of a much larger amount of PtdIns metabolism than polyphosphoinositide metabolism under these conditions.

Effect of gastric secretagogues on the formation of inositol phosphates in isolated gastric cells of the rat

The effects of compounds affecting gastric acid secretion were studied on the formation of inositol phosphates after prelabelling with [3H]-inositol in enriched gastric parietal cells of the rat, prepared by isopycnic centrifugation with Percoll. In cell preparations with 60 to 70% parietal cells, carbachol (10(-6)-10(-2) M) enhanced the accumulation of [3H]-inositol monophosphate ([3H]-IP1), [3H]-inositol bisphosphate ([3H]-IP2) and [3H]-inositol trisphosphate ([3H]-IP3) in a concentration-dependent manner, an effect which was antagonized by 10(-8) M atropine. Li+ (0.5-30 mM) enhanced the basal and carbachol-induced accumulation of all three [3H]-inositol phosphates, the formation of [3H]-IP1 being more sensitive to Li+ than those of [3H]-IP2 and [3H]-IP3. The concentration of Ca2+ in the incubation medium did not affect the relative stimulation of the accumulation of [3H]-inositol phosphates by carbachol, although the basal formation was higher in the presence of Ca2+ in the medium. In the absence of added Ca2+, the incorporation of [3H]-inositol into phospholipids was increased--an effect which was further enhanced by the addition of EGTA to the medium. Gastrin and pentagastrin (10(-8)-10(-5) M) enhanced the formation of [3H]-inositol phosphates, although they were clearly less effective than carbachol. Histamine (10(-6)-10(-3) M) had no effect of its own, but slightly attenuated the effect of carbachol. Cholecystokinin octapeptide (10(-9)-10(-6) M) slightly increased the formation of [3H]-inositol phosphates. Indomethacin (10(-4) M) had no consistent effect on the basal and carbachol-induced accumulation of [3H]-inositol phosphates, nor did prostaglandin E2 (10(-5) M) modify it. Adrenaline (10(-3) M), 5-hydroxytryptamine (10(-3) M), forskolin (10(-5) M), vasopressin (10(-5) M), angiotensin II (10(-5) M) and bombesin (10(-9)-10(-6) M) were all without effect. We suggest that the hydrolysis of inositol phospholipids may be involved in the signal transduction mechanism by which the activation of the muscarinic and gastrin receptors on the parietal cells leads to Ca2+ mobilization and the stimulation of hydrogen ion secretion.

Dephosphorylation of myo-inositol 1,4,5-trisphosphate and myo-inositol 1,3,4-triphosphate

We have augmented our previous studies [Storey, Shears, Kirk & Michell (1984) Nature (London) 312, 374-376] on the subcellular location and properties of Ins(1,4,5)P3 (inositol 1,4,5-trisphosphate) phosphatases in rat liver and human erythrocytes. We also investigate Ins(1,3,4)P3 (inositol 1,3,4-trisphosphate) metabolism by rat liver. Membrane-bound and cytosolic Ins(1,4,5)P3 phosphatases both attack the 5-phosphate. The membrane-bound enzyme is located on the inner face of the plasma membrane, and there is little or no activity associated with Golgi apparatus. Cytosolic Ins(1,4,5)P3 5-phosphatase (Mr 77,000) was separated by gel filtration from Ins(1,4)P2 (inositol 1,4-bisphosphate) and inositol 1-phosphate phosphatases (Mr 54,000). Ins(1,4,5)P3 5-phosphatase activity in hepatocytes was unaffected by treatment of the cells with insulin, vasopressin, glucagon or dibutyryl cyclic AMP. Ins(1,4,5)P3 5-phosphatase activity in cell homogenates was unaffected by changes in [Ca2+] from 0.1 to 2 microM. After centrifugation of a liver homogenate at 100,000 g, Ins(1,3,4)P3 phosphatase activity was largely confined to the supernatant. The sum of the activities in the supernatant and the pellet exceeded that in the original homogenate. When these fractions were recombined, Ins(1,3,4)P3 phosphatase activity was restored to that observed in unfractionated homogenate. Ins(1,3,4)P3 was produced from Ins(1,3,4,5)P4 (inositol 1,3,4,5-tetrakisphosphate) and was metabolized to a novel InsP2 that was the 3,4-isomer. Ins(1,3,4)P3 phosphatase activity was not changed by 50 mM-Li+ or 0.07 mM-Ins(1,4)P2 alone, but when added together these agents inhibited Ins(1,3,4)P3 metabolism. In Li+-treated and vasopressin-stimulated hepatocytes, Ins(1,4)P2 may reach concentrations sufficient to inhibit Ins(1,3,4)P3 metabolism, with little effect on Ins(1,4,5)P3 hydrolysis.

Separation of multiple isomers of inositol phosphates formed in GH3 cells

A high-performance-liquid-chromatography (h.p.l.c.) separation was developed, which resolves isomers of inositol monophosphate (IP), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) in a single run. In GH3 cells labelled with [3H]inositol, treated with Li+ and thyrotropin-releasing hormone (TRH), radiolabelled components identified as inositol 1-phosphate (I1P), inositol 2-phosphate (I2P), inositol 4-phosphate (I4P), inositol 1,4-bisphosphate [I(1,4)P2], inositol 1,3,4-trisphosphate [I(1,3,4)P3] and inositol 1,4,5-trisphosphate [I(1,4,5)P3] are present, as are multiple unidentified IP2 peaks. After TRH stimulation, both I1P and I4P increase, the increase in I4P preceding that of I1P; I(1,4)P2 and an unknown IP2 increase; and both I(1,3,4)P3 and I(1,4,5)P3 increase, the increase in I(1,4,5)P3 being rapid and transient, whereas the increase in I(1,3,4)P3 is slower and more sustained. The most rapidly appearing inositol phosphates produced after TRH stimulation are I(1,4)P2 and I(1,4,5)P3.

Inositol(3,4)bisphosphate and inositol(1,3)bisphosphate in GH4 cells--evidence for complex breakdown of inositol(1,3,4)trisphosphate

Analysis of inositol bisphosphates in GH4 cells labelled with [3H]myo-inositol shows that these cells contain three detectable inositol bisphosphates: inositol(1,4)bisphosphate, and two novel inositol bisphosphates. These latter inositol bisphosphates were degraded by periodate oxidation, borohydride reduction and alkaline phosphatase dephosphorylation; each yielded single non-cyclic alditols, ribitol and threitol, indicating that they must be respectively inositol(1,3)bisphosphate and inositol(3,4) bisphosphate. These two inositol bisphosphates are putative breakdown products of inositol(1,3,4)trisphosphate, and their occurrence suggests a complex route of hydrolysis of inositol(1,3,4)trisphosphate in intact cells.

Rapid increases in inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and cytosolic free Ca2+ in agonist-stimulated pancreatic acini of the rat. Effect of carbachol, caerulein and secretin

We compared the time course of increases in isomers of inositol trisphosphate [Ins(1,4,5)P3] and Ins(1,3,4)P3] and the tetrakisphosphate [Ins(1,3,4,5)P4] with changes in cytosolic free Ca2+ [( Ca2+]i) in dispersed pancreatic acini of the rat. There were rapid (5s) increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in response to carbachol, caerulein and secretin, whereas Ins(1,3,4)P3 increased more slowly. All three secretagogues induced increases in [Ca2+]i, which reached a peak at 15-20 s. Our results indicate that the very rapid formation of Ins(1,4,5)P3 is compatible with its second-messenger role in the initial elevation of [Ca2+]i.

Mobilization of intracellular calcium by methacholine and inositol 1,4,5-trisphosphate in rat parotid acinar cells

In the rat parotid acinar cell, methacholine caused an increase in [Ca2+]i as determined by quin-2 fluorescence. The increase in [Ca2+]i was initially independent of, and subsequently dependent on, the presence of extracellular Ca2+, indicating mobilization of intracellular Ca2+, as well as activation of Ca2+ entry. Methacholine mobilization of the internal Ca2+ pool and stimulation of the initial transient phase of K+ efflux have similar concentration dependencies; the EC50 value for Ca2+ mobilization is 80 nmol/L, the EC50 value for K+ efflux is 200 nmol/L. In a permeable parotid cell preparation, inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, and inositol 4,5-bisphosphate were able to release Ca2+ from an ATP-dependent, oligomycin-insensitive pool. These observations, when taken with the previous finding that methacholine stimulates Ca-independent inositol trisphosphate formation, support the view that inositol 1,4,5-trisphosphate acts as a second messenger mediating the release of an intracellular Ca2+ pool following muscarinic receptor activation in the parotid gland.

Mitogen-stimulated release of inositol phosphates in human fibroblasts

Mitogenic stimulation of quiescent human fibroblasts (HSWP) with a growth factor mixture (consisting of epidermal growth factor (EGF), insulin, bradykinin, and vasopressin) rapidly induces an increase in Na influx via a Ca-mediated activation of an amiloride-sensitive Na/H exchanger. Inositol phosphates (specifically inositol-1',4',5'-phosphate) have been implicated in mediating the mobilization of intracellular Ca stores in other cell types and we have now completed a detailed analysis of the mitogen-induced release of inositol phosphates in HSWP cells. Stimulation of inositol trisphosphate release is rapid (within 5 s) and reaches a maximum level (416-485% basal) within 10-15 s after the addition of growth factor mixture. Inositol bisphosphate and inositol monophosphate reach maximum levels by 30 s (1257% basal) and 60 s (291% basal), respectively. Levels of all three compounds then decay toward basal levels but remain elevated (150-350% of basal levels) after 10 min of incubation with mitogens. The effects of different combinations of these growth factors and of the bee venom peptide, melittin, have also been determined. We have also found that 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate, which prevents the mitogen-induced rise in intracellular calcium activity and activation of Na influx, does not alter the mitogen-stimulated accumulation of inositol trisphosphate. In addition, the calcium ionophore A23187, which increases cytosolic Ca activity and induces a Na influx, does not stimulate the release of inositol trisphosphate. Assays performed in the presence of lithium, which inhibits inositol phosphate monophosphatase, promotes the prolonged and enhanced accumulation of inositol monophosphate. Treatment with the phospholipase inhibitor mepacrine or pretreatment with dexamethasone reduces the amount of inositol phosphates released upon mitogenic stimulation. Hence mitogenic stimulation of HSWP cells leads to the rapid stimulation of inositol phosphate release via a calcium-independent mechanism and suggests inositol trisphosphate as a candidate to mediate the release of intracellular calcium stores which is involved in the processes responsible for the activation of the Na/H exchanger.

Adrenergic regulation of formation of inositol phosphates in rat submandibular acini

Formation of inositol phosphates in response to adrenergic secretagogues was studied in rat submandibular acini labelled with myo-[2-3H]inositol. Noradrenaline rapidly (within 5 s) increased radioactivity incorporated into inositol 1,4,5-trisphosphate and inositol tetrakisphosphate, with less rapid (within 1 min) increases in inositol 1,3,4-trisphosphate being observed. Inositol polyphosphate formation was less sensitive to noradrenaline than was stimulation of mucin secretion and was mediated by stimulation of alpha- but not beta-adrenergic receptors. The beta-adrenergic agonist isoproterenol, which is a potent stimulator of mucin secretion [McPherson & Dormer (1984) Biochem. J. 224, 473-481] did not increase formation of inositol mono-, bis- or polyphosphates during a 15 min incubation. The results suggest that inositol phosphates do not mediate beta-adrenergic stimulation of mucin secretion in rat submandibular acini. In addition, rat submandibular acinar cells contain a Ca2+ pool which can be mobilized by isoproterenol [McPherson & Dormer (1984) Biochem. J. 224, 473-481], without involvement of inositol polyphosphates as second messengers.

Angiotensin-induced formation and metabolism of inositol polyphosphates in bovine adrenal glomerulosa cells

The actions of angiotensin II (AII) on inositol polyphosphate production and metabolism were analyzed in cultured bovine adrenal glomerulosa cells. In cells labeled for 24 hr with [3H]inositol, AII caused a rapid and prominent rise in formation of Ins-P3 (mainly the Ins-1,3,4,-P3 isomer) and of Ins-P4, with marked increases in two isomers of Ins-P2 and Ins-P. These findings are consistent with rapid formation and turnover of Ins-1,4,5-P3, partly via conversion to Ins-1,3,4,5-P4 with subsequent metabolism to Ins-1,3,4-P3 and lower inositol phosphates. The demonstration of a cytosolic Ins-P3-kinase gave further evidence for the presence of the tris/tetrakisphosphate pathway and Ins-P4 synthesis during AII action in the bovine adrenal cortex.

Regulation of inositol trisphosphate accumulation by muscarinic cholinergic and H1-histamine receptors on human astrocytoma cells. Differential induction of desensitization by agonists

Although activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a linear accumulation of inositol phosphates for up to 60 min in the presence of LiCl [Masters, Quinn & Brown (1985) Mol. Pharmacol. 27, 325-332], activation of H1-histamine receptors resulted in an increase in total inositol phosphate formation that was maintained for less than 5 min. The effects of stimulation of these two receptors on accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] were also examined. Incubation of 1321N1 cells with carbachol resulted in a rapid accumulation of all three inositol phosphates, reaching a maximum within 30 s; this elevated value was maintained for up to 60 min. The rate of disappearance of Ins(1,3,4)P3 from carbachol-treated cells after the addition of atropine paralleled or exceeded the rate of disappearance of Ins(1,4,5)P3. Although the initial rates of accumulation of Ins(1,4,5)P3, Ins(1,3,4)P3 and Ins(1,3,4,5)P4 in the presence of histamine were similar to that observed with carbachol, the amounts of these inositol phosphates had returned to control values within 5 min after the addition of histamine. The results indicate that, although the acute effects of muscarinic receptor and H1-histamine receptor stimulation on phosphoinositide hydrolysis are very similar, the histamine receptor is desensitized rapidly, whereas the muscarinic receptor is not. This effect on histamine-receptor function is apparently homologous, since preincubation of 1321N1 cells with histamine did not decrease the subsequent response to carbachol.

Denervation supersensitivity of 5-HT-1c receptors in rat choroid plexus

Serotonin (5-HT)-stimulated phosphoinositide hydrolysis in the choroid plexus is mediated by the 5-HT-1c receptor. The current study demonstrates that treatment of rats with the serotonergic neurotoxin, 5,7-dihydroxytryptamine, caused a marked depletion of 5-HT and a supersensitive 5-HT-1c mediated phosphoinositide hydrolysis response. These data suggest that the 5-HT-1c site in choroid plexus receives tonic serotonergic input.