Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “PHOSPHOINOSITIDES”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,117 records · Page 62Linked to original sources
PubMed1990

Effect of aging on alpha-1 adrenergic stimulation of phosphoinositide hydrolysis in various regions of rat brain.

The effects of aging were examined on the ability of alpha-1 adrenergic receptor agonists to stimulate phosphoinositide hydrolysis in three brain regions. Tissue minces of thalamus, cerebral cortex and hippocampus from 3-, 18- and 28-month-old male Fischer 344 rats were prelabeled with [3H]myoinositol. Exposure of these prelabeled minces to phenylephrine and (-)-norepinephrine revealed that accumulation of [3H]inositol phosphates was selectively reduced by 20 to 30% in the thalamus and cerebral cortex of the oldest age group. Analysis of concentration-response and competition binding curves indicated that this decrease was due to diminished agonist efficacy rather than diminished receptor affinity. The reduction in responsiveness to phenylephrine and (-)-norepinephrine in the cerebral cortex and the lack of any changes in the hippocampus parallel previously reported changes in the density of alpha-1 adrenergic receptors with aging. These data indicate that the ability of alpha-1 adrenergic receptor agonists to stimulate phosphoinositide hydrolysis is reduced in some, but not all, brain regions of aged Fischer 344 rats.

Adrenergic alpha-Agonists↗
PubMed1990

Purification from bovine liver membranes of a guanine nucleotide-dependent activator of phosphoinositide-specific phospholipase C. Immunologic identification as a novel G-protein alpha subunit.

Cholate-solubilized extracts from bovine liver plasma membranes preincubated with the nonhydrolyzable GTP analog guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) displayed enhanced phosphoinositide-specific phospholipase C activity compared with extracts from membranes incubated without nucleotide or with ATP or GDP analog. Resolution of the GTP gamma S-elicited activator of phospholipase C was achieved using heparin-Sepharose which bound the phospholipase C activity. Recombination of non-adsorbed extract with salt-eluted phospholipase C activity resulted in a stimulation of enzyme activity. The GTP gamma S-dependent activator was purified, on the basis of its ability to activate partially purified phospholipase C, by sequential chromatography on Q-Sepharose, Sephacryl S-300, octyl-Sepharose, and Mono Q. The presence of G-protein beta subunits and the alpha subunits of Gi1, Gi2, and Gi3 was detected, by immunoblot analysis, in Mono Q-purified phospholipase C activator preparations. Resolution of the activator from these alpha subunits was achieved by incubation with pertussis toxin in the presence of millimolar NAD+ followed by rechromatography on Mono Q. The phospholipase C activator, thus resolved from ADP-ribosylated alpha i subunits, possessed an approximate Mr of 42 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and copurified with a substoichiometric amount of beta subunit. Immunoblot analysis of fractions from the final Mono Q column revealed cross-reactivity of the 42-kDa phospholipase C activator with antipeptide antibodies raised against residues 160-169 of alpha i1 and a region of sequence common to all known G-protein alpha subunits. The 42-kDa activator was not recognized by other alpha subunit-specific or common antibodies. These findings identify the purified phospholipase C activator as a novel G-protein alpha subunit. This may represent the active subunit of the pertussis toxin-insensitive G-protein mediating receptor-stimulated phosphoinositide breakdown in mammalian liver.

Adenosine Diphosphate Ribose↗
PubMed1990

Role of phosphoinositide turnover and cyclic AMP accumulation in prostaglandin-stimulated noradrenaline release from cultured adrenal chromaffin cells.

Prostaglandins (PGs) E1, E2 and F2 alpha stimulated release of noradrenaline from chromaffin cells; the most potent was PGF2 alpha with an EC50 of about 0.1 microM. The rank order of potency for release, and the EC50 for each PG, was the same as that for stimulation of (poly)phosphoinositide turnover. PGE1-stimulated release was dependent on extracellular calcium and sensitive to dihydropyridine calcium channel agonists and antagonists at 1 microM, but unlike release stimulated by 50 mM extracellular potassium was not sensitive to verapamil or diltiazem at 10 microM. The PGs also enhanced the turnover of inositol phospholipids, but the PGE1-stimulated formation of inositol phosphates was small compared to that produced by bradykinin, which stimulates a similar degree of release. Unlike release, the stimulation of inositol phosphate formation by PGs was not dependent on the addition of calcium to the medium. In down-regulation experiments, involving 2 hr preincubation with 30 microM PGE1, release in response to the three PGs was attenuated, whereas the release response to bradykinin and nicotine was unaffected. However, the stimulation of (poly)phosphoinositide turnover by PG was not down-regulated by prior exposure to PGE1. This dissociation of the inositol phosphate response suggests that release in response to PGs is not downstream of stimulation of inositol phospholipid hydrolysis. A further series of experiments is reported which shows that release is not a consequence of increased rate of cyclic AMP synthesis. It seems likely that PG stimulation of noradrenaline release is a result of calcium entry through dihydropyridine-sensitive channels by a mechanism independent of these two second messenger systems.

Adrenal Medulla↗
PubMed1990

A central role for phosphoinositide hydrolysis in activating the lytic mechanism of human natural killer cells.

Using neomycin, which inhibits phosphoinositide breakdown, cytotoxicity mediated by natural killer (NK) cells was suppressed in a dose-dependent manner, with complete inhibition at 16 mM. Generation of inositol phosphates in effector cells after target cell binding was inhibited in the presence of neomycin. The formation of effector to target cell conjugates was not affected. Neomycin-induced inhibition of NK killing was abolished when TPA was added to the cytotoxic assays. This reconstitution was dependent upon extracellular Ca2+. When the intracellular free Ca2+ level in effector cells was reduced from 73 +/- 11 nM to 43 +/- 3 nM (n = 4) using the Ca2+ indicator dye, Quin 2, NK killing was markedly reduced. Inhibiting the enzyme diacylglycerol (DG) kinase in effector cells with 10 microM R59022 (DG kinase inhibitor) potentiates NK killing, suggesting an increase in protein kinase C (PKC) activity due to accumulation of DG. The PKC inhibitor, H-7, suppressed NK killing in a concentration-dependent manner. These results demonstrate that phosphoinositide metabolism is an early event and its derived second messengers play a central role in activating the lytic mechanism of NK cells.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1990

On the involvement of multiple muscarinic receptor subtypes in the activation of phosphoinositide metabolism in rat cerebral cortex.

We have examined the activation of phosphoinositide metabolism by muscarinic agonists in rat cerebral cortex, in an attempt to delineate the mechanisms by means of which some selective antagonists inhibit this response in a manner that deviates from simple mass action law. The accumulation of [3H]inositol phosphates induced by the full agonist carbamylcholine in cell aggregates preparations was inhibited by muscarinic antagonists with the following order of potency: telenzepine greater than atropine greater than 4-diphenylacetoxy-N-methyl-piperidine methbromide greater than pirenzepine greater than hexahydro-sila-difenidol greater than AF-DX 116. The same order of potency was found for the competition of these antagonists with [3H]telenzepine binding to M1 muscarinic receptors. The inhibition of the formation of [3H]inositol phosphates activated by acetylcholine, carbamylcholine, and oxotremorine-M by pirenzepine and telenzepine showed biphasic curves, with 62-73% of the response being inhibited with high affinity. Atropine, AF-DX 116, and pirenzepine shifted the concentration-response curves of oxotremorine-M to the right in a parallel manner. However, pirenzepine at micromolar concentrations showed deviation from linearity of the Schild regression. The blockade by high concentrations of pirenzepine and telenzepine showed less than additive dose ratios when assayed in the presence of atropine, suggesting deviation of their antagonism from simple competition. However, after alkylation with propylbenzilylcholine mustard in the presence of low concentrations of pirenzepine, the response to carbamylcholine and oxotremorine-M showed monophasic inhibition curves by pirenzepine and linear Schild regression for this antagonist. These results support the interpretation that the formation of [3H]inositol phosphates is activated by multiple muscarinic receptor subtypes in rat cerebral cortex. The profile of affinities of muscarinic antagonists indicates that a major component of the response is activated by an M1 receptor subtype and a minor component is probably mediated by M3 muscarinic receptors when acetylcholine, carbamylcholine, or oxotremorine-M are used to stimulate the response. Conversely, pirenzepine inhibited the response induced by methacholine and bethanechol in a monophasic manner with high affinity (Ki = 13 nM), suggesting that these agonists can selectively stimulate phosphoinositide metabolism through activation of M1 muscarinic receptors in rat cerebral cortex.

Alkylating Agents↗
PubMed1990

Possible mechanism for preterm labor associated with bacterial infection. I: Stimulation of phosphoinositide metabolism by endotoxin in endometrial fibroblasts.

Growing evidence suggests an association between intra-amniotic infection and premature initiation of parturition. We recently demonstrated that some factor(s) including endotoxin produced by the organism stimulates endogenous phospholipase A2 resulting in liberation of arachidonic acid and prostaglandin formation (Arch. Gynecol. Obstet. 244: 1-6 (1988). The studies presented in this report were designated to evaluate the mechanism for endotoxin to stimulate phospholipase A2 using human endometrial fibroblasts. Exposure of the fibroblasts to endotoxin from Escherichia coli in the presence of [32P] phosphate increased 32P-labeling of phosphatidic acid (PA) and phosphatidyl-inositol (PI) in a dose-dependent and a time-dependent manners. The PA labeling occurred without a measurable lag time. These findings demonstrate that the endotoxin stimulates phosphoinositide metabolism in human endometrial fibroblasts by a receptor-mediated mechanism. Membrane phosphoinositide turnover stimulated by endotoxin results in cytosolic Ca2+ increment, liberation of arachidonic acid, which may be involved in the initiation of parturition.

Bacterial Infections↗
PubMed1990

Sucralfate protection of gastric mucosa against alcohol-induced injury: a phosphoinositide mediated process.

The mechanism of protection by sucralfate against gastric mucosal injury induced by ethanol was investigated. The experiments in vivo were conducted with groups of rats with and without indomethacin pretreatment, and the animals received sucralfate followed by ethanol. In the in vitro experiments, gastric mucosa was cultured in the presence of sucralfate, ethanol, or both. The in vivo experiments revealed that ethanol caused extensive gastric hemorrhagic lesions which were significantly reduced following sucralfate pretreatment and that this effect of sucralfate was not prevented by indomethacin. The data with gastric mucosal culture demonstrated that ethanol caused a 24% decrease in mucin synthesis, while mucin synthesis in the presence of sucralfate increased by 32%. This increase was accompanied by the enhanced metabolism of mucosal phosphoinositides, as reflected by a 22% decrease in PI, 1,2-fold increase in IP1 and 3.4-fold increase in IP3. In contrast, ethanol, caused 1.5-fold increase in IP1 and PIP2, and 35% decrease in PIP, 47% decrease in IP2 and 38% decrease in IP3. However, when the mucosal culture was carried out in the presence of both sucralfate and ethanol, the detrimental changes evoked by ethanol in mucin synthesis were prevented. The results suggest that the mucosal protective action of sucralfate involves the metabolism of phosphoinositide-derived messenger molecules.

Animals↗
PubMed1987

Dexamethasone inhibits receptor-activated phosphoinositide breakdown in rat basophilic leukemia (RBL-2H3) cells.

The activation of rat basophilic leukemia cells for histamine release is accompanied by Ca2+ influx and arachidonic acid release. IgE receptor but not A23187 ionophore stimulation of these cells also resulted in phosphoinositide breakdown. In these experiments, the culture of these cells with dexamethasone inhibited IgE- and ionophore-mediated histamine release. The concentration for 50% of maximal inhibition was 12 nM, and prolonged exposure to the drug was required, with maximal effect observed in 8 to 15 hr. The inhibitory effect of dexamethasone was reversible (t1/2 for recovery was 16 hr). Dexamethasone blocked the IgE-mediated 45Ca2+ influx and the release of [14C]-arachidonic acid (IC50 of 1 nM and 10 nM respectively). Dexamethasone inhibited the IgE receptor-mediated phosphoinositide breakdown (IC50 of 5 nM). It also decreased arachidonic acid release after A23187 stimulation demonstrating an effect on phospholipase A2. Therefore, exposure of the cells to dexamethasone results in the inhibition of both phospholipase A2 and phospholipase C pathways of arachidonic acid generation.

Animals↗
PubMed1989

The human preproendothelin-1 gene: possible regulation by endothelial phosphoinositide turnover signaling.

Preproendothelin-1 (ppET-1) mRNA has previously been demonstrated to be markedly induced in cultured endothelial cells (EC) by the addition to the medium of thrombin, an agent known to stimulate phosphoinositide turnover in EC. In this study, the mechanism of regulation of ppET-1 mRNA expression was investigated in cultured human umbilical vein EC by RNA blot analysis with cloned ppET-1 gene as a probe. The mRNA for ppET-1 was rapidly upregulated by O-tetradecanoylphorbol-13-acetate (TPA) (0.5 microM) and by ionomycin (5 microM) within 10 min of addition to the medium, but not by forskolin (50 microM). The rapid induction of ppET-1 mRNA by TPA or ionomycin occurred even in the presence of cycloheximide, indicating that the mRNA induction does not require de novo protein synthesis. The ppET-1 mRNA was an extremely unstable species of mRNA with an apparent half-life of about 15 min. However, the half-life of ppET-1 mRNA was not appreciably affected by TPA or ionomycin, suggesting that the mRNA induction by these agents is mostly due to an activation of the transcription of the mRNA. These observations indicate that the production of ET-1 in human EC can be controlled by a transcriptional gene regulation directly coupled to the intracellular signals from the phosphoinositide-turnover pathway, i.e., activation of protein kinase C and increase in intracellular Ca2+. These mechanisms are discussed in relation to information on the primary structure of cloned ppET-1 gene.

Blotting, Northern↗
PubMed1989

Type-A cholecystokinin receptors in CHP212 neuroblastoma cells: evidence for association with G protein and activation of phosphoinositide hydrolysis.

125I-Bolton Hunter-cholecystokinin octapeptide (BH-CCK8) and (-)-[3H]L-364718 membrane binding assays were used to identify and characterize cholecystokinin (CCK) receptors in CHP212 human neuroblastoma cells. The ligand binding properties of CCK receptors in these cells are similar to those found in pancreas (CCK-A sites) and differ from the predominant type of CCK binding site found in brain (CCK-B sites). The specific binding of 125I-BH-CCK8 but not (-)-[3H]L-364718 was reduced by the metabolically stable GTP analog guanosine 5'-(beta-delta-imido)trisphosphate. A substantial difference in the Bmax for the radiolabeled agonist (125I-BH-CCK8) and antagonist [(-)-[3H]L-364718] was noted. These observations are consistent with CCK receptors existing in guanine nucleotide-binding protein-coupled and -uncoupled states. Similar to its action in pancreatic acinar cells, CCK8(S) stimulated the accumulation of [3H]inositol phosphates in cells prelabeled with [3H]myo-inositol (EC50 = 3.2 +/- 0.4 nM; maximum response = 4.5 +/- 0.4 x basal). The intrinsic activity of CCK analogues in stimulating phosphoinositide hydrolysis was substantially less than their reported intrinsic activity in stimulating phosphoinositide hydrolysis in pancreatic acinar cells. The CHP212 neuroblastoma cell may serve as a useful model for the recently reported CCK-A binding site found in the central nervous system.

Cholecystokinin↗
PubMed1989

Chlorpromazine inhibits vesiculation, alters phosphoinositide turnover and changes deformability of ATP-depleted RBCs.

To delineate further the underlying mechanism by which amphiphilic drugs can modulate vesicle release from human RBCs, we studied the effect of chlorpromazine on erythrocyte vesiculation induced by ATP depletion. This was correlated with turnover of the phosphoinositides as well as RBC deformability during the process since phosphoinositide metabolism may be involved in shape regulation of RBCs. Echinocytic shape transformation and subsequent vesiculation of RBCs, which commonly occur during ATP depletion, were inhibited by chlorpromazine. Furthermore, with a newly developed two-dimensional thin-layer chromatography separation of RBC membrane phospholipids, we showed that chlorpromazine significantly decreased the dephosphorylation of phosphatidylinositol-4,5-bisphosphate (PIP2) in both ATP-depleted RBCs as well as in cells with partly maintained ATP levels. Concomitantly, there was a smaller increase in the relative amount of phosphatidylinositol. In addition, chlorpromazine also inhibited the decreased in RBC deformability as well as the shift of osmotic fragility that occurs during ATP depletion of erythrocytes.

Adenosine Triphosphate↗
PubMed1989

Monocyte-derived interleukin 1: effects on norepinephrine-stimulated aortic contraction and phosphoinositide turnover.

Medium conditioned by silica-stimulated human peripheral blood monocytes expresses vascular suppressive activity. Rat aortic rings, after incubation in conditioned medium, exhibited comprised contraction to stimulation by norepinephrine (NE). Maximal contraction (300 +/- 51 mg tension/mg tissue) and sensitivity (-5.91 +/- 0.23 M [log EC50]) were both reduced in comparison to contraction (762 +/- 66) and sensitivity (-7.42 +/- 0.11) displayed by rings after incubation in control medium. A polyvalent antibody (Ab) against human interleukin 1 (Il-1) neutralized the suppressive activity in conditioned medium. Rings incubated in conditioned medium containing Ab exhibited normal maximal contraction (722 +/- 46) and a partial restoration of sensitivity to NE (-6.91 +/- 0.13). In contrast, incubation of rings in control medium supplemented with recombinant human Il-1 resulted in a dose-dependent suppression of aortic contraction to NE that was analogous to the defects induced by monocyte-conditioned medium. No significant differences in NE-stimulated phosphoinositide hydrolysis were present between rings incubated in Ab-treated or untreated conditioned or control media. The data suggest that monocyte-derived Il-1 may have a significant influence on vascular contractile function and that the mechanism by which Il-1 induces vascular dysfunction cannot be demonstrated to involve inhibition of NE-stimulated phosphoinositide metabolism.

Acetylcholine↗
PubMed1989

Oxytocin regulates Ca2+ level in myometrium by influencing phosphoinositide metabolism.

The effect of oxytocin on phosphoinositide metabolism as well as on membrane protein phosphorylation in myometrial tissue was studied. Oxytocin enhanced the 32P incorporation into phospholipids in myometrial tissue. The effect of oxytocin on phosphoinositide metabolism was also detected in plasma membrane of 20 days pregnant rats. Phosphorylated membrane lipids have been analysed and phosphatidylinositol 4, 5-bisphosphate proved to be the main reaction product. Oxytocin enhanced the 32P incorporation into phospholipids measured in the first 30 sec then the labeling decreased more rapidly then in case of the control. The effect of oxytocin proved to be concentration dependent. The protein phosphorylation was also influenced by oxytocin. However the amount of alkylphosphate formed depended on the presence or absence of Ca2+, Ca2+-calmodulin and cyclic AMP, oxytocin influenced the protein phosphorylation in the presence of Ca2+-calmodulin only.

Animals↗
PubMed1989

Growth factor signaling pathways: phosphoinositide metabolism and phosphorylation of phospholipase C.

Recent demonstrations of growth factor-stimulated increases in cellular phosphoinositide metabolism suggest that regulatory enzymes of this important signaling pathway may be substrates for growth factor receptor tyrosine kinases. Studies employing phosphotyrosine antibodies, specific phospholipase C antibodies, and purified phospholipase C proteins support the conclusion that the 145-kD phospholipase C-gamma 1 isoenzyme is rapidly and selectively phosphorylated by the activated epidermal growth factor and platelet-derived growth factor receptors. The selective interaction of these receptors with phospholipase C-gamma 1 suggests a novel, direct mechanism for agonist stimulation of phosphoinositide metabolism.

Growth Substances↗
PubMed1987

Purification and characterization of a phosphoinositide-specific phospholipase C from guinea pig uterus. Phosphorylation by protein kinase C in vivo.

Two peaks of phosphoinositide-specific phospholipase C (PI-PLC) activity were resolved when guinea pig uterus cytosolic proteins were chromatographed on a DEAE-Sepharose column. The first peak of enzyme activity eluting from the DEAE-Sepharose column (PI-PLC I) was further purified to homogeneity, whereas the second peak of enzyme activity was enriched 300-fold. PI-PLC I migrated as a 62-kDa protein on sodium dodecyl sulfate-polyacrylamide gels. Antibodies prepared against PI-PLC I failed to react with PI-PLC II. PI-PLC I hydrolyzed all three phosphoinositides, exhibiting a greater Vmax for phosphatidylinositol 4,5-bisphosphate greater than phosphatidylinositol 4-phosphate greater than phosphatidylinositol. Hydrolysis of phosphatidylinositol was calcium-dependent, whereas significant hydrolysis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate occurred in the presence of 2.5 mM EGTA. At physiological concentrations of calcium, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were the preferred substrates. Antibodies specific for PI-PLC I reacted with a 62-kDa protein in both the cytosol and membrane fractions from guinea pig uterus. Quantitation of the immunoblots revealed that 25% of the 62-kDa protein was membrane-associated, whereas only 5% of the total enzyme activity was membrane-associated. Approximately 20% of the membrane-bound phospholipase C activity and immunoreactive material were loosely bound, whereas the remainder required detergent extraction for complete solubilization. The 62-kDa protein associated with the membrane fractions did not bind lectin affinity columns, suggesting that it was not glycosylated. PI-PLC I was identified as a phosphoprotein in [32P]orthophosphate-labeled rat basophilic leukemia (RBL-1) cells by two-dimensional gel electrophoresis followed by immunoblotting. In untreated cells, 32P-labeled PI-PLC I was found in the cytosolic fraction. Treatment of RBL-1 cells with those phorbol esters which are known to activate the Ca2+/phospholipid-dependent enzyme protein kinase C, resulted in a time-dependent increase in the phosphorylation of both membrane-bound and cytosolic PI-PLC I. Thus, in RBL-1 cells, protein kinase C may play an important role in the regulation of phospholipase C through protein phosphorylation.

Animals↗
PubMed1987

Involvement of a guanine-nucleotide-binding component in membrane IgM-stimulated phosphoinositide breakdown.

Cross-linking of membrane immunoglobulin, the B cell receptor for antigen, activates the phosphoinositide signal transduction pathway. The initial event in this pathway is the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) by phospholipase C. This reaction yields two intracellular second messengers, diacylglycerol, which activates protein kinase C, and inositol trisphosphate, which causes an increase in cytoplasmic Ca2+. The experiments reported here demonstrate that activation of phospholipase C by membrane IgM (mIgM) involves a guanine nucleotide-dependent step. Saponin was used to permeabilize WEHI-231 B lymphoma cells and permit direct manipulation of nucleotide and Ca2+ concentrations. Very high levels of Ca2+ (greater than 100 microM) activated the phospholipase maximally without a requirement for cross-linking of mIgM. However, at much lower, physiologically relevant Ca2+ concentrations (100 to 500 nM), receptor-stimulated PtdInsP2 hydrolysis could be demonstrated. The ability of anti-IgM antibodies to activate phospholipase C in permeabilized WEHI-231 cells was greatly increased by nonhydrolyzable guanosine 5'-triphosphate (GTP) analogues (guanosine-5'-O-(3-thiotriphosphate) or 5'-guanylylimidodiphosphate), but not by guanosine diphosphate or guanosine diphosphate analogues or by a nonhydrolyzable analogue of adenosine triphosphate. This specificity for GTP analogues is consistent with the hypothesis that a GTP-binding regulatory protein analogous to those that couple receptors to adenylate cyclase is involved in the activation of phospholipase C by mIgM in WEHI-231 B lymphoma cells. In order to characterize this putative GTP-binding component, we examined the ability of pertussis toxin and cholera toxin to affect anti-IgM-stimulated inositol phosphate production. These bacterial toxins covalently modify and modulate the activity of various GTP-binding regulatory proteins and in some cell types can block receptor-stimulated PtdInsP2 breakdown. In WEHI-231 B lymphoma cells, neither toxin blocked signaling by mIgM. Thus mIgM appears to be coupled to the phosphoinositide signaling pathway by a GTP-dependent component that is insensitive to both pertussis toxin and cholera toxin.

Adenylate Cyclase Toxin↗
PubMed1987

Role of phosphoinositide metabolites in the prolongation of afterhyperpolarizations by alpha 1-adrenoceptors in rat dorsal raphe neurons.

Afterhyperpolarizations and outward tail currents of rat dorsal raphe neurons were measured by intracellular recording and single-electrode voltage clamping in the brain slice preparation. The alpha 1-agonist phenylephrine, and (in the presence of propranolol) norepinephrine, elicited an increase in the duration, but not of the initial amplitude, of afterhyperpolarizations and associated outward tail currents which followed depolarizing pulses. These effects were antagonized by prazosin, indicating that they were mediated by alpha 1-adrenoceptors. The outward tail currents were sensitive to apamin, a blocker of certain Ca2+-activated K+ currents. A prolongation of afterhyperpolarizations would offset the major excitatory alpha 1 effects, which were associated with suppression of resting K+ currents and of the A-current. Since polyphosphoinositide metabolites have been reported to be second messengers for Ca2+-dependent receptor actions, we compared their effects with those of alpha 1-receptor stimulation on these cells. Intracellular ejection of the putative second messenger myo-inositol-1,4,5-trisphosphate from the recording electrode transiently mimicked the actions of alpha 1-agonists on the afterhyperpolarization. Superfusion with 1 mM LiCl, simulating therapeutic levels of lithium, had no effect on the rate of recovery from inositol trisphosphate ejection. Superfusion with water-soluble phorbol esters (which mimic actions of another phosphoinositide metabolite, 1,2-diacylglycerol) suppressed rather than mimicked the activation of raphe cell firing by phenylephrine; this occurred with a rank-order potency consistent with activation of protein kinase C and was associated with suppression of a slow inward current and of the outward tail current. Our results suggest that phosphoinositide turnover is more likely to mediate modulatory or negative-feedback effects of alpha 1-adrenoceptors than to mediate the major excitatory effects.

Action Potentials↗
PubMed1986

The role of phosphoinositides in cell physiology.

The phosphoinositides are minor phospholipids present in all eukaryotic cells. They are storage forms for messenger molecules that function in the transmission of signals across the cell membrane and evoke responses to extracellular agonists. The phosphoinositides break down to liberate messenger molecules or precursors of messenger molecules. Many different compounds are formed, although the functions of only a few are understood currently.

Animals↗