Fluoride inhibits agonist-induced formation of inositol phosphates in rat cortex

Sodium fluoride inhibited carbachol, 5-hydroxytryptamine and noradrenaline stimulated formation of inositol phosphates in rat cerebral cortex. For example, carbachol (1 mM) induced a 337% increase of inositol phosphates above basal in 30 min which was reduced to 69% in the presence of NaF (10 mM). The IC50 for NaF was approximately 1.5 mM and inhibition was mediated by a decrease in maxima of the carbachol dose response curve rather than a shift to the right. This inhibitory action was not mimicked by NaBr or NaI, or by agents which increase cAMP. Inhibition did not appear to result from a toxic action of NaF since it had no effect on the formation of inositol phosphates by high K+; moreover, in higher concentrations NaF stimulated phospholipase C activity. Since fluoride ions are known to activate G-proteins in the concentrations used in this study, these results may indicate the existence of a novel G-protein linked to receptor inhibition of phospholipase C.

Stimulatory and inhibitory actions of excitatory amino acids on inositol phospholipid metabolism in rat cerebral cortex

1. The effects of excitatory amino acids on [3H]-inositol phosphate levels have been examined in rat cortical slices under basal conditions or following agonist stimulation. 2. Ibotenate and quisqualate provoked a substantial dose-dependent (EC50, 30 microM and 20 microM respectively) increase in inositol phosphates; these responses were not additive suggesting a common site of action for the two amino acids. The responses to maximally effective concentrations of ibotenate and quisqualate were not blocked by verapamil, tetrodotoxin or Cd2+, indicating that these effects are not indirect. Small, but significant, increases in inositol phosphates were also seen with glutamate and N-methyl-DL-aspartate (NMDLA); kainate and aspartate were ineffective. 3. Each excitatory amino acid tested reduced carbachol (1 mM) stimulated inositol phosphate formation. Kainate (IC50, 20 microM) and NMDLA (IC50, 20 microM) were the most effective inhibitors. Kainate also reduced the responses to noradrenaline, 5-hydroxytryptamine and 20 mM K+. 4. The inhibitory action of NMDLA, but not kainate, could be reversed with the NMDA antagonists, DL-2-amino-5-phosphonovalerate (APV) and MK-801; DL-2-amino-4-phosphonobutyrate (APB) was without effect. Since MK-801 blocks the ion channels associated with the NMDA receptor, it appears that inhibition requires the entry of ions into the cell. 5. APV and MK-801 potentiated the stimulatory response to ibotenate but had no effect on the response to quisqualate. Potentiation was presumably the result of blocking the inhibition by ibotenate mediated through NMDA receptors. 6. In conclusion, excitatory amino acids appear to reduce agonist-mediated inositol phosphate formation in rat cerebral cortex by a non-specific action, possibly including the influx of Na+ ions. In addition ibotenate and quisqualate substantially enhance inositol phosphate production: the pharmacology of the response suggests that it is mediated by a receptor distinct from previously defined excitatory amino acid receptor subtypes.

Lowering of the extracellular Na+ concentration enhances high-K+-induced formation of inositol phosphates in the guinea-pig ileum

1. Formation of inositol phosphates (InsPs) was measured in cross-chopped slices or dispersed cells, isolated by collagenase treatment, of guinea-pig ileum longitudinal smooth muscle pre-labelled with [3H]inositol. 2. Elevation of the extracellular K+ concentration by equimolar replacement of Na+ induced accumulation of InsPs in the dispersed cells and in the tissue slices. These effects were blocked by neither tetrodotoxin (1 microM) nor atropine (10 microM), and were approximately additive with carbachol-induced accumulation. 3. In the tissue slices, the response to K+ was partially inhibited by nifedipine (10 microM) and by CdCl2 (0.3 mM), but the carbachol-induced response was not altered. 4. Accumulation of InsPs induced by KCl-excess solution (high-K+ solution without Na+ replacement) was suppressed strongly by nifedipine and completely by CdCl2. The response to KCl excess was approx. 40% of that to high K+ with Na+ replacement. 5. Low-NaCl solution (replacement of NaCl with equimolar sucrose) also produced InsPs, and this was not blocked by either nifedipine (10 microM) or CdCl2 (0.3 mM). 6. The formation of InsPs by a maximally effective concentration of carbachol (1 mM) in the presence of KCl excess or low NaCl was greater than the additive effect of the two stimuli on their own. Enhancement of the carbachol-induced response by KCl excess disappeared in the presence of CdCl2 (0.3 mM). 7. These data suggest that formation of InsPs induced by high-K+ solution with equimolar replacement of Na+ consists of two components, i.e. high-K+-induced inositol-phospholipid hydrolysis by Ca2+ entry through voltage-sensitive channels, and low-Na+-induced formation of InsPs, insensitive to Ca2+ antagonists, but that both of them do not contribute significantly to the activation of phospholipase C by muscarinic stimuli.

Does the hydrolysis of inositol phospholipids lead to the opening of voltage operated Ca2+ channels in guinea-pig ileum? Studies with fluoride ions and caffeine

Fluoride ions (1-30 mM) stimulate phosphoinositide hydrolysis in guinea-pig ileum longitudinal smooth muscle slices, and this is not inhibited in the presence of indomethacin or nifedipine. This action is associated with a slow contractile response which peaks after approximately five minutes and then declines towards baseline; at this time the contractile response to a maximally effective concentration of carbachol is also inhibited. Fluoride-induced contractions are inhibited completely in the presence of nifedipine. Similarly, contractions induced by caffeine, which releases Ca2+ from intracellular stores, are also inhibited by nifedipine. These data are consistent with a model in which the activation of a G-protein by F- ions leads to the following sequential events: activation of phospholipase C, release of intracellular Ca2+, opening of voltage operated (i.e. dihydropyridine sensitive) Ca2+ channels and contraction. The transient nature of the fluoride contraction and the inhibition of the carbachol contraction may be due to a slow elevation of cAMP levels induced by F-.

Neurokinin3-receptors are linked to inositol phospholipid hydrolysis in the guinea-pig ileum longitudinal muscle-myenteric plexus preparation

1. Tachykinin-stimulated inositol phospholipid hydrolysis was examined in slices of longitudinal muscle from guinea-pig ileum. 2. Substance P, neurokinin A and neurokinin B induced a concentration-dependent accumulation of total [3H]-inositol phosphates in the presence of 12 mM lithium with similar maximal responses and EC50 values. 3. The selective NK1-receptor agonist, substance P methyl ester, and the selective NK3-receptor agonist succ-[Asp6, MePhe8]-SP(6-11) (senktide) also stimulated [3H]-inositol phosphate formation with maximum responses of 50.69 +/- 0.96 and 45.64 +/- 1.17% relative to 10 microM substance P, respectively. Substance P methyl ester was approximately equipotent with substance P, whereas senktide was approximately 100 times more potent. 4. When added together, maximally effective concentrations of substance P methyl ester and senktide gave responses that were fully additive. In contrast, responses to substance P and neurokinin B were not additive. 5. The stimulation of [3H]-inositol phosphate formation by substance P, neurokinin B and senktide was not affected by atropine (2 microM) or tetrodotoxin (TTX, 0.3 microM). 6. The contractile effect of senktide was inhibited completely by TTX and partially blocked by atropine. Contractions induced by substance P methyl ester were not changed in the presence of TTX or atropine. 7. [D-Pro4, D-Trp7,9,10]-SP(4-11) competitively antagonized the action of substance P methyl ester on inositol phospholipid hydrolysis and contraction, but had no significant effect on senktide-induced inositol phospholipid breakdown or contraction. 8. These results suggest that NK3-receptors in the guinea-pig ileum are coupled to inositol phospholipid hydrolysis.

The action of the protein kinase C inhibitor, staurosporine, on human platelets. Evidence against a regulatory role for protein kinase C in the formation of inositol trisphosphate by thrombin

The ability of several putative inhibitors of protein kinase C (PKC) to block dioctanoylglycerol (DC8)-induced phosphorylation of a 47 kDa protein (a recognized substrate for PKC) in human platelets was investigated. Staurosporine (1 microM) caused complete inhibition of phosphorylation, whereas the other reagents were either inactive (polymyxin B) or gave only partial inhibition (C-1, H-7, tamoxifen). Staurosporine (1 microM) fully inhibited the phosphorylation of the 47 kDa protein in platelets challenged with thrombin, but also inhibited the phosphorylation of a 20 kDa protein which is a substrate for myosin light-chain kinase. The inhibition of both kinases by staurosporine was associated with the inhibition of thrombin-induced secretion of ATP and 5-hydroxytryptamine and a slowing of the aggregation response; staurosporine, however, had no effect on the formation of phosphatidic acid and inositol phosphates induced by thrombin. Staurosporine also reversed the inhibitory action of phorbol esters on thrombin-induced formation of phosphatidic acid. These data are consistent with a role for these two kinases in secretion and aggregation (although there must be additional control signals, since aggregation was only slowed, not inhibited), but suggest that neither kinase is involved in the regulation of phosphoinositide metabolism. This latter conclusion contradicts previous observations that the activation of PKC by phorbol esters or membrane-permeable diacylglycerols alters the apparent activity of both phospholipase C and inositol trisphosphatase. Possible explanations for this discrepancy are discussed.

Evidence for neurokinin-3 receptor-mediated tachykinin release in the guinea-pig ileum

Contraction of longitudinal muscle strips of the guinea-pig ileum induced by the selective NK3 receptor agonist, succ-[Asp6,MePhe8]SP-(6-11) (senktide), were completely inhibited by tetrodotoxin and partially blocked by atropine. The atropine-resistant contraction was markedly reduced if the smooth muscle tachykinin receptors were either desensitized with the selective NK1 agonist substance P methyl ester, or blocked with the tachykinin receptor antagonist [D-Pro4,D-Trp7,9,10]SP-(4-11). These results suggest that activation of NK3 receptors on enteric neurones results in acetylcholine and tachykinin release.

A diacylglycerol kinase inhibitor, R59022, potentiates secretion by and aggregation of thrombin-stimulated human platelets

The diacylglycerol kinase inhibitor R59022 (10 microM) potentiates secretion and aggregation responses in human platelets challenged with sub-maximal concentrations of thrombin. Potentiation correlates closely with increased formation of diacylglycerol, increased phosphorylation of a 40 kDa protein, a known substrate for protein kinase C, and with decreased formation of phosphatidic acid, the product of diacylglycerol kinase. Phosphorylation of myosin light chains, formation of inositol phosphates and the mobilization of Ca2+ by thrombin are not affected by R59022 (10 microM). These data support a role for protein kinase C in platelet aggregation and secretion, and provide further evidence that endogenous diacylglycerols bring about the activation of this enzyme. These data also add further argument against a role for phosphatidic acid in platelet activation.

Neomycin cannot be used as a selective inhibitor of inositol phospholipid hydrolysis in intact or semi-permeabilized human platelets. Aminoglycosides activate semi-permeabilized platelets

High concentrations of neomycin (2-10 mM) inhibited aggregation, but not shape change, of intact platelets by collagen, ADP and the Ca2+ ionophore, A23187, the last two studies being carried out in the presence of the cyclo-oxygenase inhibitor indomethacin. In contrast, over the same range of concentrations neomycin inhibited both aggregation and shape change induced by thrombin. Under these conditions activation of platelets by collagen and by thrombin, but not by A23187 or by ADP, is believed to be dependent on the hydrolysis of membrane inositol phospholipids. These data therefore suggest that the inhibitory action of neomycin on intact platelets is not related to its previously reported inhibitory effect on phosphoinositide metabolism. The selective inhibition of thrombin-induced shape change indicates a second site of action of neomycin on intact platelets. On platelets rendered semi-permeable with saponin, neomycin and a second aminoglycoside antibiotic, streptomycin (each 0.06-2 mM), stimulated secretion and aggregation responses. These effects were inhibited by indomethacin and by EGTA. Activation of semi-permeabilized platelets by neomycin is associated with the formation of inositol phosphates and phosphatidic acid, indicating activation by phospholipase C. This effect is also inhibited by indomethacin, implying that it is secondary to the formation of prostaglandins and endoperoxides. These results are discussed in the context of the use of neomycin as a selective inhibitor of polyphosphoinositide metabolism.

Ionophore A23187 stimulates phosphorylation of the 40,000 dalton protein in human platelets without phospholipase C activation

The Ca2+ ionophore A23187 (0.2-5 microM) stimulates the phosphorylation of the substrates of protein kinase C (40,000 dalton protein) and myosin light chain kinase (20,000 dalton protein) in the presence or absence of cyclooxygenase inhibitors. In the presence of cyclooxygenase inhibitors or millimolar Ca2+ there is no stimulation of phospholipase C by A23187. Fingerprints of the 32P-labeled 40,000 dalton protein isolated from platelets that have been stimulated with A23187, thrombin, phorbol 12,13-dibutyrate and 1,2-didecanoylglycerol were identical. Higher concentrations of A23187 (1-5 microM) induced the loss of polyphosphoinositides through phosphomonoesterase activity.

Inositol 1,4,5-trisphosphate induces aggregation and release of 5-hydroxytryptamine from saponin-permeabilized human platelets

Inositol 1,4,5-trisphosphate induces aggregation and the release of [3H]5-hydroxytryptamine from human platelets rendered permeable with saponin. This action of inositol 1,4,5-trisphosphate is associated with a significant formation of thromboxane B2, activation of phospholipase C, and phosphorylation of 20,000- and 40,000-dalton proteins, which are the substrates for myosin light chain kinase and protein kinase C, respectively. All of these responses are blocked by the cyclooxygenase inhibitors indomethacin and aspirin and the dual cyclooxygenase and lipoxygenase inhibitor 3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline (BW 755C). These data indicate that platelet activation by inositol 1,4,5-trisphosphate is initiated by the mobilization of Ca2+, which leads to phospholipase A2 activation. The thromboxanes and endoperoxides that are subsequently generated then induce activation via cell surface receptors.

Lack of association of epidermal growth factor-, insulin-, and serum-induced mitogenesis with stimulation of phosphoinositide degradation in BALB/c 3T3 fibroblasts

The hypothesis that inositol phospholipid degradation is a step in the mechanism by which epidermal growth factor (EGF) stimulates mitogenesis in confluent monolayers of quiescent BALB/c 3T3 fibroblasts was tested. The maximum mitogenic response (a nearly 30-fold increase in incorporation of [3H]thymidine) occurred at 1 ng/ml EGF (0.16 nM). This degree of stimulation corresponded to 60% of that elicited by 10% serum. To determine whether EGF stimulated formation of inositol phosphates via degradation of polyphosphoinositides, the intracellular levels of [3H] inositol phosphates and [3H]phosphoinositides were determined after EGF addition to BALB/c 3T3 fibroblasts prelabeled with [3H]inositol. These experiments were performed under conditions designed to mimic exactly those conditions used to study mitogenesis. The results demonstrated that 10% serum or 10 ng/ml of platelet-derived growth factor, but not as much as 50 ng/ml EGF or 10 micrograms/ml insulin, increased the levels of inositol phosphates via degradation of phosphoinositides in the presence of 10 mM Li+. The serum-induced effects occurred in 30 s, the earliest time investigated. Phorbol dibutyrate (100 nM), alone or in conjunction with EGF (10 ng/ml), failed to stimulate inositol phospholipid degradation. However, phorbol dibutyrate inhibited the serum-induced stimulation. Finally, fetal bovine serum dialyzed so as to retain peptide mitogens lost almost 70% of the capacity to stimulate degradation of inositol phospholipids while remaining as mitogenic as the control serum. Thus, stimulation of inositol phospholipid degradation is an unlikely component in the mechanism by which EGF and probably insulin and serum stimulate mitogenesis in BALB/c 3T3 fibroblasts.

Decanoyl lysophosphatidic acid induces platelet aggregation through an extracellular action. Evidence against a second messenger role for lysophosphatidic acid

Platelets rapidly convert 1,2-didecanoyl-sn-glycerol into its corresponding phosphatidic acid and lysophosphatidic acid derivatives, thereby providing a means of introducing these two compounds into platelets. 1-Decanoyl-2-lyso-3-sn-phosphatidic acid, when added directly to platelets, induced platelet aggregation and raised intracellular Ca2+ levels at concentrations of 0.3 microM upwards, but was without effect when formed intracellularly from 1,2-didecanoylglycerol at an estimated concentration of approx. 47 microM. This indicates that the site of platelet activation by lysophosphatidic acid is extracellular. A concentration of thrombin (0.2 unit/ml), which produced maximal platelet aggregation, caused an estimated intracellular formation of 20 microM-lysophosphatidic acid in the presence of 2 mM-Ca2+; however, there was no detectable release of lysophosphatidic acid into the bathing medium. Lysophosphatidic acid, therefore, may not be an intracellular second messenger involved in platelet aggregation by thrombin.

The formation of [3H]inositol phosphates in human platelets by palmitoyl lysophosphatidic acid is blocked by indomethacin

The intracellular Ca2+ thresholds for platelet shape change and aggregation by A23187 and palmitoyl lysophosphatidic acid were approximately 350 and 750 nM, respectively, as estimated using quin2. The similar thresholds for these two agonists imply they activate platelets through a similar mechanism. In the absence of cyclooxygenase inhibitors, both agents induce the formation of [3H]inositol phosphates, reflecting the activation of phospholipase C. This activation of phospholipase C is blocked by the cyclooxygenase inhibitor indomethacin. It is suggested that platelet activation by palmitoyl lysophosphatidic acid involves an initial mobilization of intracellular Ca2+ with subsequent activation of phospholipase A2; the arachidonic acid metabolites formed then stimulate phospholipase C.

Collagen stimulates [3H]inositol trisphosphate formation in indomethacin-treated human platelets

The present study investigates the pathway of metabolism of inositol phospholipids in human platelets exposed to collagen. Platelet activation by collagen was preceded by a lag phase usually lasting 10-20 s. Formation of [3H]inositol trisphosphate (IP3) was not observed during this period, but occurred in parallel with the onset of aggregation, release of ATP and phosphorylation of a 20 000 Da and a 40 000 Da protein. Indomethacin treatment partially inhibited all of these responses. Aggregation and ATP release, but not IP3 formation, were further inhibited in indomethacin-treated platelets loaded with the fluorescent Ca2+ indicator, quin2. Under these conditions there was no detectable mobilization of Ca2+. These results demonstrate that activation of platelets by collagen is associated with rapid hydrolysis of polyphosphoinositides by phospholipase C, thereby producing IP3. This observation is discussed in relation to IP3 as a possible Ca2+-mobilizing agent.

The action of substance P on contraction, inositol phospholipids and adenylate cyclase in rat small intestine

The actions of substance P and eledoisin on contraction, [3H]inositol 1-phosphate and cAMP formation in the rat ileum have been compared. Eledoisin was considerably more potent than substance P on both contraction and [3H]inositol 1-phosphate production. Neither peptide altered the cAMP levels in the tissue. These results are discussed in relation to the substance P receptor sub-type present in the rat ileum, and its second messenger.

myo-Inositol 1,4,5-trisphosphate stimulates protein phosphorylation in saponin-permeabilized human platelets

In an attempt to establish a system with physiological substrates and phospholipid surfaces to investigate Ca2+- and 1,2-diacylglycerol-dependent protein kinase C activation, saponized platelets were used. Saponin, through interaction with plasma membrane cholesterol, makes cells permeable without major disruption of organelles. Washed platelets, prelabeled with 32P, were treated with 1-50 micrograms of saponin per ml. Permeabilization was evident at a concentration of 10 micrograms of saponin per ml, as indicated by the action of extracellular Ca2+ on the phosphorylation of the 20,000- and 40,000-Da proteins. These proteins are, respectively, the substrates for myosin light chain kinase and protein kinase C. Activation of these enzymes occurred when the estimated free [Ca2+] was changed from approximately equal to 80 nM to 300 nM. The effect of Ca2+ on kinase C-induced phosphorylation was potentiated by 1,2-didecanoylglycerol (1 microM). myo-Inositol 1,4,5-trisphosphate (5-20 microM) increased phosphorylation of the 20,000- and 40,000-Da proteins. This action was time and concentration dependent. The effect of myo-inositol 1,4,5-trisphosphate on the activation of kinase C was additive with 1,2-didecanoylglycerol. The action of myo-inositol 1,4,5-trisphosphate could be due to mobilization of Ca2+ from platelet organelles and/or to a direct effect on protein kinases.

The rapid formation of inositol phosphates in human platelets by thrombin is inhibited by prostacyclin

The biochemical events underlying the ability of thrombin to enhance the metabolism of inositol phospholipids in human platelets have been investigated using platelets prelabeled with [3H]inositol. Thrombin treatment caused rapid formation of radioactive inositol monophosphate (IP), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) with less marked and more variable changes in the levels of radioactive inositol phospholipids. Formation of IP2 and IP3 could be detected 5 s after exposure to thrombin and before IP levels increased. Low doses of thrombin which produced only shape change in human platelets also caused significant formation of IP2 and IP3 but not IP. These results suggest that thrombin-induced platelet activation may be mediated through hydrolysis of polyphosphoinositides. The majority of IP formed presumably arises from the hydrolysis of IP2. Prostacyclin inhibited thrombin-induced formation of all three inositol phosphates.

Are the proposed substance P receptor sub-types, substance P receptors?

Recently, a number of laboratories have postulated the existence of receptor sub-types for substance P. This review is intended to represent a critical appraisal of these reports. In the majority of cases, the evidence for the existence of receptor sub-types has been obtained from observed potency differences of agonists. The problems with this approach are discussed. In addition, information obtained through substance P antagonists, binding studies and investigations of second messenger systems is presented and discussed in relation to the above receptor subdivisions. It is concluded that the present results are consistent with the existence of three receptor sub-types; however, it is suggested that substance P is the natural agonist for only one of these receptors, and that substance K and tuftsin may be the transmitters for the other two receptor sub-types.

3H-substance P binding to guinea-pig ileum longitudinal smooth muscle membranes

Specific binding of 3H-substance P was studied in guinea-pig ileum longitudinal smooth muscle membranes. A single population of non-interacting sites with an apparent dissociation constant of 1.8 nM was observed. The relative potencies of some structural analogues of substance P, notably eledoisin and substance P (5-11), in competing for 3H-substance P binding sites, however, had little resemblance to their potencies in contracting the intact muscle or in eliciting the breakdown of inositol phospholipids in this tissue. The results are discussed in the light of other binding studies for substance P.

1,2-Diacylglycerols do not potentiate the action of phospholipases A2 and C in human platelets

1,2-Diacylglycerol has recently been reported to potentiate the ability of phospholipases A and C to hydrolyze phospholipids in a cell-free system. The present study has been undertaken to investigate whether 1,2-diacylglycerol can also perform this function in intact cells using the platelet as a test system. Exogenous 1-oleoyl-2-acetyl-glycerol ( OAG ) and 1,2- didecanoylglycerol , at concentrations sufficient to produce maximal phosphorylation of a 40,000 dalton protein, caused no significant formation of [3H]inositol phosphates and [32P]phosphatidic acid (products of phospholipase C activation) or [14C]arachidonic acid metabolites and lysophosphatidyl[3H]inositol (products of phospholipase A2 activation). These data therefore imply that 1,2-diacylglycerols do not potentiate the actions of phospholipases A2 and C in intact platelets at concentrations that are physiologically relevant.

Substance P induced hydrolysis of inositol phospholipids in guinea-pig ileum and rat hypothalamus

The recent development of a sensitive assay procedure for monitoring the hydrolysis of inositol phospholipids allowed the detection of substance P-induced lipid hydrolysis in tissue slices of rat hypothalamus and guinea-pig ileum longitudinal smooth muscle. The EC50 values observed for substance P were similar on both tissues and corresponded well with the value previously reported for the rat parotid. Furthermore, the structure-activity profile for this response on all three tissues was similar and closely resembled that for ileum contraction. We therefore suggest that these three functionally distinct tissues possess the same subtype of substance P receptor coupled to inositol phospholipid hydrolysis, and that this receptor has the characteristics of the previously reported SP-P subtype.

Pharmacological characterization of a substance P antagonist, [D-Arg1,D-Pro2,D-Trp7,9,Leu11]-substance P

The substance P antagonist [D-Arg1,D-Pro2,D-Trp7,9,Leu11]-substance P produced parallel log dose-response curve shifts to both substance P and eledoisin on five in vitro smooth muscle preparations. The slope values of Arunlakshana-Schild plots were not significantly different from unity suggesting that it acts as a simple competitive antagonist on all five preparations with an association constant (Ka) in the range of 0.3-1.5 X 10(6)M-1. The Ka value of the antagonist was always slightly greater when tested against eledoisin than against substance P; however, this difference appears too small to suggest that these two agonists are acting on different receptor sub-types.