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PubMed1992

Phosphoinositide metabolism in the developing conceptus. Effects of hyperglycemia and scyllo-inositol in rat embryo culture.

Culture of the postimplantation rat conceptus in hyperglycemic medium causes developmental abnormalities and is associated with a diminished water-soluble myo-inositol content. We investigated the effect myo-inositol depletion has on lipid-soluble phosphoinositides, precursors, and water-soluble inositol phosphates. Rat conceptuses were cultured from gestational day 9.5 (presomite, early head fold) to day 10.5 (7-15 somites) in 6.7-73.3 mM D-glucose. Significant decreases in the phosphoinositides of the embryo were observed with increased culture D-glucose concentrations. PI was reduced 15-34%, PIP 18-46%, and PIP2 26-46%. Yolk sac phosphoinositides also were reduced but to a lesser degree. Culture in hyperglycemic media also mediated significant reductions of conceptus inositol phosphates. To investigate whether effects similar to those induced by D-glucose could be mediated by another agent capable of decreasing myo-inositol content, we used scyllo-inositol, a transported but nonmetabolized isomer of myo-inositol. Conceptuses cultured in medium containing scyllo-inositol (0.06-16.7 mM) had dose-dependent decreases of myo-[3H]inositol in water-soluble and lipid-soluble fractions. Incorporation of myo-[3H]inositol into phosphoinositides and inositol phosphates was decreased concomitantly. Developmental effects of D-glucose and scyllo-inositol were assessed in rat conceptuses cultured from day 9.5 (presomite, early head fold) to day 11.5 (22-28 somites). Culture in 40.0-73.3 mM glucose and 0.06-33.3 mM scyllo-inositol impaired growth while increasing dysmorphogenesis in a dose-dependent manner. The results suggest that decreases in conceptus myo-inositol and associated diminution of phosphoinositides, which are the inositol/lipid cycle precursors, are dysmorphogenic and may contribute to the etiology of diabetic embryopathy.

Analysis of Variance↗

PubMed1990

Sorbinil prevents the hypergalactosemic-induced reduction in [3H]-myo-inositol uptake and decreased [3H]-myo-inositol incorporation into the phosphoinositide cycle in bovine lens epithelial cells in vitro.

The synthesis of phosphatidylinositol, phosphatidylinositol-4-phosphate and phosphatidylinositol-4-5-bisphosphate was studied using 3H-myo-inositol (3H-MI) as precursor in cultured bovine lens epithelial cells (BLECs) maintained in galactose-free, physiological medium or 40 mM galactose (Gal) +/- sorbinil for six days. The formation of inositol polyphosphates from phosphoinositides was also shown. Galactitol did not exceed 2mM in Gal-incubated cells after six days of exposure; no galactitol was observed in BLECs maintained in galactose-free, physiological medium or Gal supplemented with sorbinil. Uptake of 3H-myo-inositol(3H-MI) into BLECs was significantly reduced in cells exposed to Gal. A concomitant reduction in 3H-MI incorporation was observed in the phosphoinositides, as well as with the released inositol phosphates. The simultaneous addition of sorbinil to the Gal medium corrected the drop in 3H-MI uptake and normalized 3H-MI incorporation into the phosphoinositides and inositol phosphates. While an apparent decrease in the three inositol-containing lipids was observed with the Gal-incubated cells, based on 3H-MI incorporation, there was no change in total membrane phosphatidylinositol content when compared to cells maintained in physiological medium as determined by the microgram Pl PO4 per microgram total membrane PO4. The apparent loss of radiolabeled phosphoinositides was attributed to the decreased specific activity resulting from the lower internal pool of 3H-MI in the Gal-exposed cells available for incorporation into the phosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldehyde Reductase↗

PubMed2000

Specific interactions among transmembrane 4 superfamily (TM4SF) proteins and phosphoinositide 4-kinase.

In earlier work we established that phosphoinositide 4-kinase (PI 4-kinase) may associate with transmembrane 4 superfamily (TM4SF, tetraspanin) proteins, but critical specificity issues were not addressed. Here we demonstrate that at least five different TM4SF proteins (CD9, CD63, CD81, CD151 and A15/TALLA1) can associate with a similar or identical 55 kDa type II PI 4-kinase. These associations were specific, since we found no evidence for other phosphoinositide kinases (e.g. phosphoinositide 3-kinase and phosphoinositide-4-phosphate 5-kinase) associating with TM4SF proteins, and many other TM4SF proteins (including CD82 and CD53) did not associate with PI 4-kinase. CD63-PI 4-kinase complexes were almost entirely intracellular, and thus are distinct from other TM4SF-PI 4-kinase complexes (e.g. involving CD9), which are largely located in the plasma membrane. These results suggest that a specific subset of TM4SF proteins may recruit PI 4-kinase to specific membrane locations, and thereby influence phosphoinositide-dependent signalling.

1-Phosphatidylinositol 4-Kinase↗

PubMed2000

Visualization of myocardial phosphoinositide turnover with 1-[1-(11)C]-butyryl-2-palmitoyl-rac-glycerol in rats with myocardial infarction.

UNLABELLED: Phosphoinositide turnover mediates the signaling of angiotensin II, which plays a pivotal role in ventricular remodeling after myocardial infarction (MI). We tested the hypothesis that phosphoinositide turnover can be visualized by 1-[1 -(11)C]butyryl-2-palmitoyl-rac-glycerol (11C-DAG) in both infarcted and noninfarcted myocardium after MI in rats. METHODS: Rats received an injection of 11C-DAG 7 d after left coronary artery ligation, and myocardial lipids were extracted from both infarcted and noninfarcted areas of myocardium (n = 3). Metabolites of 11C-DAG were determined by thin-layer chromatography. Quantitative autoradiography of hearts was performed to visualize myocardial phosphoinositide turnover in rats that received an injection of 11C-DAG 1 d (n = 3) and 7 d (n = 5) after MI and 7 d after a sham operation (n = 3). Quantitative autoradiography with 201TlCl was also performed to evaluate myocardial blood flow in rats 7 d after MI (n = 3). Cells occupying the infarcted myocardium were identified by immunohistochemistry. RESULTS: The radioactivity incorporated into the intermediates of phosphoinositide turnover was predominant in both the infarcted (67.1% +/- 5.2% of the total activity) and the noninfarcted (57.4% +/- 3.2%) myocardium. 11C-DAG radioactivity in the infarcted region normalized to that in the noninfarcted region was 1.09 +/- 0.04 in rats 7 d after MI, which was significantly higher than that in rats 1 d after MI (0.38 +/- 0.03, P < 0.001). 201Tl radioactivity in the infarcted region normalized to that in the noninfarcted region was only 0.19 +/- 0.01 7 d after MI. 11C-DAG radioactivity in the noninfarcted region normalized to that in the right ventricular free wall tended to be increased in rats 1 and 7 d after MI compared with the sham-operated rats; the differences, however, were not statistically significant (1.30 +/- 0.15, 1.20 +/- 0.07, and 1.13 +/- 0.02, respectively). Immunohistochemistry revealed that abundant fibroblasts, myofibroblasts, and macrophages occupied the infarcted myocardium 7 d after MI, but the cellularity was low during the first day after MI. CONCLUSION: These data suggest that 11C-DAG may be useful for visualizing regions with activated phosphoinositide turnover after MI. Because wound healing and fibrogenic processes are important factors of ventricular remodeling, 11C-DAG and PET may offer new information benefiting patient management after MI.

Animals↗

PubMed1999

Effect of removal of external calcium on phosphoinositide hydrolysis in cultured myotubes of embryonic chicken.

The effect of removal of external Ca2+ on phosphoinositide hydrolysis was investigated in cultured myotubes from 9-day-old Leghorn embryonic chicken. In the myotubes exposed to Ca(2+)-free Ringer's solution, the turnover of phosphoinositide was exponentially decreased with a time constant of about 26 min. In the presence of external Ca2+, the hydrolysis of phosphoinositide was significantly increased by exposure to 80 mmol/L K+ solution. After removal of external Ca2+, 80 mmol/L K+ exposure caused a slight decrease of phosphoinositides hydrolysis in comparison with the control (normal Ringer). It is indicated that hydrolysis of phosphoinositide in cultured myotubes can be enhanced by high K+ exposure. External Ca2+ is essential for this effect, which is different from mature muscle fibres.

Animals↗

PubMed1992

Cyclic AMP differentiates two separate but interacting pathways of phosphoinositide hydrolysis in the DDT1-MF2 smooth muscle cell line.

The activation of adenosine A1 receptors in DDT1-MF2 smooth muscle cells resulted in both the inhibition of agonist-stimulated cAMP accumulation and the potentiation of norepinephrine-stimulated phosphoinositide hydrolysis. Pharmacological analysis indicated the involvement of an A1 adenosine receptor subtype in both of these responses. In the absence of norepinephrine, the activation of the adenosine receptor did not directly stimulate phosphoinositide hydrolysis. The adenosine receptor-mediated augmentation of norepinephrine-stimulated phosphoinositide hydrolysis was pertussis toxin sensitive and was selectively antagonized by agents that mimicked cAMP (8-bromo-cAMP) or raised cellular cAMP levels (forskolin). This initially suggested that cAMP might partially regulate the magnitude of the phospholipase C response to norepinephrine and that adenosine agonists might enhance the phospholipase C response by reducing cAMP levels. However, neither the reduction of cellular cAMP levels by other agents nor the inhibition of cAMP-dependent protein kinase was sufficient to replicate the action of adenosine receptor activation on phosphoinositide hydrolysis. Thus, in the presence of norepinephrine, adenosine receptor agonists appear to stimulate phosphoinositide hydrolysis via a pathway that is separate from, but dependent upon, that of norepinephrine. This second pathway can be distinguished from that which is stimulated by norepinephrine on the basis of its sensitivity to inhibition by both cAMP and pertussis toxin.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1992

The cloned platelet thrombin receptor couples to at least two distinct effectors to stimulate phosphoinositide hydrolysis and inhibit adenylyl cyclase.

Thrombin both stimulates phosphoinositide hydrolysis and inhibits adenylyl cyclase in a variety of cell types. Whether the cloned human platelet thrombin receptor accounts for both of these signaling events is unknown. We report that thrombin receptor agonist peptide causes both phosphoinositide hydrolysis and inhibition of adenylyl cyclase in naturally thrombin-responsive CCL-39 cells. To exclude the possibility that the agonist peptide or thrombin itself may activate these pathways via distinct receptors and to circumvent a lack of suitable thrombin receptor-null cells, we utilized a designed "enterokinase receptor," a thrombin receptor with its thrombin cleavage recognition sequence LDPR replaced by DDDDK, the enterokinase cleavage recognition sequence. Transfection of enterokinase-unresponsive cells with this construct conferred both enterokinase-sensitive phosphoinositide hydrolysis and inhibition of adenylyl cyclase. The phosphoinositide hydrolysis response was largely insensitive to pertussis toxin, whereas the adenylyl cyclase response was completely blocked by pertussis toxin. These data show that the cloned thrombin receptor can effect both phosphoinositide hydrolysis and inhibition of adenylyl cyclase via at least two distinct effectors, most likely Gq-like and Gi-like G-proteins.

Adenylyl Cyclase Inhibitors↗

PubMed1992

Seizures selectively impair agonist-stimulated phosphoinositide hydrolysis without affecting protein kinase C activity in rat brain.

The influence of seizures on phosphoinositide hydrolysis and protein kinase C activity was measured in rat hippocampus and cerebral cortex, primarily using a model in which generalized convulsive status epilepticus was induced by administration of LiCl (3 mmole/kg) 20 hr prior to pilocarpine (30 mg/kg). A short (5 min) period of seizures reduced phosphoinositide hydrolysis in hippocampal slices stimulated by norepinephrine or ibotenate, but did not alter the responses to carbachol, 50 mM K+, or NaF. Induction of seizures with diisopropylfluorophosphate caused a similar reduction in the response to norepinephrine without altering carbachol-stimulated phosphoinositide hydrolysis. The inhibition of norepinephrine-stimulated phosphoinositide hydrolysis after seizures generated by lithium plus pilocarpine administration was apparently not due to inhibitory influences of quisqualate or activation of protein kinase C since both of these treatments caused similar inhibitions in slices from control and treated rats. Seizures induced by lithium plus pilocarpine or by kainate did not alter the activity of protein kinase C or the distribution of protein kinase C between membrane and cytosolic fractions. Thus, seizures cause a neurotransmitter-selective impairment of phosphoinositide hydrolysis, and this response may play a role in the severity or duration of seizure activity.

Animals↗

PubMed1991

Phosphoinositide breakdown is associated with Fc-gamma RII-mediated activation of 5'-lipoxygenase in murine eosinophils.

In this report we present data on the ability of murine eosinophils to generate inositol phosphate derivatives, and their relationship with the activation of 5'-lipoxygenase by a Fc-gamma R-dependent mechanism. The addition of anti-IgG F(ab')2 to mouse eosinophils, previously sensitized with IgG, induces inositol phosphate generation after 2 min and after 10 min of stimulation. Maximal generation of inositol tris and inositol tetrakis phosphate has been detected after 15 min of stimulation, and the optimal concentration of anti-IgG F(ab')2 was found to be 25 micrograms. Inositol tris phosphate formation is also observed at 5 min after the addition of the calcium ionophore A23187 (5 microM). We also report that neomycin, an inhibitor of phosphoinositide-phospholipase C, inhibits Fc-gamma R-mediated phosphoinositide breakdown in a dose-dependent manner (88% inhibition at 150 microM of neomycin). The possible involvement of phosphoinositide breakdown in the activation of 5'-lipoxygenase has been investigated. Using streptolysin-O permeabilized cells and different doses of neomycin that inhibit phosphoinositide breakdown, we have demonstrated a parallel decrease in LTC4 released by these cells, using either A23187 (86% inhibition at 200 microM of neomycin) or anti-IgG F(ab')2 (82.4% inhibition at 100 microM of neomycin). [Ca2+]i elevation has been observed by loading the cells with the fluorescent calcium indicator Fura-2 penta-acetoxy methyl ester and after stimulating with the anti-Fc-gamma RII mAb (2.4G2). It is likely that the activation of murine eosinophils by a Fc-gamma R mechanism stimulates phosphoinositide breakdown as a primary step that leads to the activation of murine 5'-lipoxygenase, producing the formation of leukotriene C4.

Animals↗

PubMed1991

Preferential coupling of cell surface muscarinic receptors to phosphoinositide hydrolysis in human neuroblastoma cells.

The ability of muscarinic receptors, present in either the cell surface or sequestered compartments of intact human SK-N-SH neuroblastoma cells, to stimulate phosphoinositide hydrolysis has been examined. When cells were first exposed to carbachol for 1 h at 37 degrees C, approximately 50% of the cell surface receptors became sequestered, and this was accompanied by a comparable reduction in the subsequent ability of muscarinic agonists to stimulate phosphoinositide turnover, as monitored by the release of labeled inositol phosphates at 10 degrees C. At this temperature, muscarinic receptor cycling between the two cell compartments is prevented. Upon warming the carbachol-pretreated cells to 37 degrees C, receptor cycling is reinitiated and stimulated phosphoinositide turnover is fully restored within 5-8 min. When measured at 10 degrees C, the reduction of stimulated phosphoinositide turnover observed following carbachol pretreatment was similar in magnitude for both hydrophilic (carbachol, oxotremorine-M) and lipophilic (arecoline, oxotremorine-2, and L-670,548) agonists. The loss of response for both groups of agonists could be prevented if the incubation temperature was maintained at 37 degrees C, rather than at 10 degrees C. At the latter temperature carbachol pretreatment of SK-N-SH cells reduced the maximum release of inositol phosphates elicited by either carbachol or L-670,548 but not the agonist concentrations required for half-maximal stimulation. Radioligand binding studies, carried out at 10 degrees C, indicate that following receptor sequestration, significantly higher concentrations of carbachol were required to occupy the available muscarinic receptor sites. In contrast the lipophilic full agonist L-670,548 recognized receptors present in control and carbachol-pretreated cells with comparable affinities. Analysis of the inositol lipids present after carbachol pretreatment indicate that only a minimal depletion of the substrates necessary for phospholipase C activation had occurred. The results indicate that the agonist-induced sequestration of muscarinic receptors from the cell surface results in a loss of stimulated phosphoinositide hydrolysis when measured under conditions in which the return of the sequestered receptors to the cell surface is prevented. Thus, only those receptors present at the cell surface are linked to phospholipase C activation.

Bridged Bicyclo Compounds, Heterocyclic↗

PubMed1991

NKR-P1, an activating molecule on rat natural killer cells, stimulates phosphoinositide turnover and a rise in intracellular calcium.

NKR-P1 is a 60-kDa homodimer expressed on all rat NK cells. Previous studies by others suggest that NKR-P1 may play a role in NK cell activation because antibody to NKR-P1 stimulates the release of granules from NK cells, and anti-NKR-P1 causes redirected lysis by activated NK cells against targets that express FcR. To examine the mechanism of transmembrane signaling by NKR-P1, we studied the rat NK cell line, RNK-16. We here demonstrate that F(ab')2 antibody to NKR-P1 stimulates phosphoinositide turnover and a rise in intracellular calcium within RNK-16 cells. The response is augmented by cross-linking the F(ab')2 antibody. The phosphoinositide/calcium pathway is also stimulated by NKR-P1 in activated rat NK cells, although no response is detectable in polymorphonuclear cells, which also express NKR-P1. We also demonstrate that RNK-16 cells kill the anti-NKR-P1 (3.2.3) hybridoma and that exposure to the hybridoma target cells stimulates phosphoinositide turnover in RNK-16 cells. Both killing and phosphoinositide turnover are inhibited by F(ab')2 anti-NKR-P1, implicating NKR-P1 in both responses. In contrast, neither cytotoxicity nor phosphoinositide turnover is appreciably blocked by F(ab')2 anti-NKR-P1 in response to YAC-1 targets. Thus, with either target, killing is linked to phosphoinositide turnover, but killing of YAC-1 involves pathways that differ from those that direct killing of the anti-NKR-P1 hybridoma. Our studies support the hypothesis that NKR-P1 may serve as an activating cell-surface receptor on NK cells, and they clarify the mechanisms by which it activates NK cells.

Animals↗

PubMed1990

Multiple effects of increases in phosphoinositide hydrolysis on islets and their relationship to changing patterns of insulin secretion.

Evidence continues to accumulate which suggests that increases in phosphoinositide hydrolysis play an integral role in beta cell insulin secretory response patterns. In the present report some of the methodological approaches utilized to monitor phosphoinositide-hydrolysis in islets are reviewed. The contribution of phosphoinositide-derived second messenger molecules to acute insulin release as well as the induction of time dependent potentiation are also discussed. Finally, the role of impaired phosphoinositide hydrolysis in the induction of time dependent suppression of insulin release is also explored. It is concluded that alterations in phosphoinositide hydrolysis contribute, at least in part, to both ordered and disordered patterns of insulin release noted under a variety of experimental conditions.

Animals↗

PubMed1990

Serotonin-2 receptors coupled to phosphoinositide hydrolysis in a clonal cell line.

A permanent line of cells has been established from the transplantable rat pituitary tumor 7315a. P11 cells have been cloned repeatedly, and after more than 60 passages their growth and characteristics are stable. Results of radioligand binding studies with 125I-lysergic acid diethylamide (125I-LSD) indicate that P11 cells express serotonin-2 (5-HT2) receptors. Analysis of the binding of 125I-LSD to membranes prepared from P11 cells revealed the presence of a single class of high affinity sites (Kd = 1.6 nM; Bmax = 211 fmol/mg of protein). The pharmacological profile of the inhibition of the binding of 125I-LSD by a panel of drugs was consistent with the expected profile of these drugs at 5-HT2 receptors. The affinity of the site for serotonin was in the low micromolar range and was decreased by GTP. Phosphoinositide hydrolysis in P11 cells, measured in the presence of lithium, was stimulated by serotonin. Increasing concentrations of the 5-HT2-selective antagonist ketanserin blocked phosphoinositide hydrolysis stimulated by serotonin, and Schild analysis was consistent with a simple competitive interaction. The Ki for ketanserin derived from Schild analysis was comparable to the Ki for ketanserin at the binding site for 125I-LSD. These results suggest that stimulation of phosphoinositide hydrolysis in P11 cells by serotonin is mediated by 5-HT2 receptors. Pretreatment of P11 cells with pertussis toxin caused ADP-ribosylation of Gi and Go, but did not affect the ability of serotonin to stimulate phosphoinositide hydrolysis. Therefore, the guaninine nucleotide-binding protein involved in the coupling of 5-HT2 receptors to phospholipase C in P11 cells is unlikely to be either Gi or Go. P11 cells expressing 5-HT2 receptors coupled to phosphoinositide hydrolysis will be a useful model system for future studies of the regulation and function of 5-HT2 receptors on cultured cells.

Animals↗

PubMed1990

Suramin alters phosphoinositide synthesis and inhibits growth factor receptor binding in HT-29 cells.

Initiation of cell growth frequently involves activation of growth factor receptor-coupled tyrosine kinases and stimulation of the phosphoinositide second messenger system. The antitrypanosomal and antifiliarial drug suramin has been shown to exert antiproliferative activities by inhibition of growth factor receptor binding. We therefore investigated the effect of suramin on epidermal growth factor receptor-binding characteristics and, additionally, searched for effects on basal or cholinergically stimulated phospholipid metabolism in HT-29 cells. Suramin caused a dose-dependent and noncompetitive inhibition of 125I-epidermal growth factor binding (concentration producing 50% inhibition, 44.2 micrograms/ml) but did not alter muscarinic receptor binding. Suramin did not affect the basal 32P incorporation into phosphoinositides at concentrations of less than 200 micrograms/ml suramin. In contrast, the carbachol-stimulated enhancement of 32P incorporation into phosphatidic acid, phosphatidylinositol, and polyphosphoinositides was reduced by 48-95% in the presence of 100 micrograms/ml suramin. Thus, phosphoinositide and diacylglycerol kinases involved in basal and receptor-stimulated phosphoinositide metabolism may be localized in different subcellular compartments, which can be dissociated by the use of suramin. Direct measurements of phosphatidylinositol kinase and diacylglycerol kinase activities showed a potent inhibition when treated with suramin. Suramin did not affect the stimulation of phospholipase C by carbachol, determined by release of [3H]inositol phosphates in [3H]myoinositol-prelabeled cells. Our data indicate that suramin potently inhibits phosphoinositide resynthesis under stimulated conditions. Additionally, we confirm the inhibitory effects of suramin on epidermal growth factor receptor binding in a human intestinal cell line. The inhibitory effects of suramin on phospholipid metabolism may play a role in the antiproliferative actions of this drug.

1-Phosphatidylinositol 4-Kinase↗

PubMed1988

Tachykinin-induced phosphoinositide breakdown in airway smooth muscle and epithelium: relationship to contraction.

We have studied the contractile response and phosphoinositide hydrolysis induced by substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and Alp-Phe-Phe(R)-Gly[ANC-2]-Leu-Met-NH2 (L 363851), a selective NK2-receptor agonist, in guinea pig tracheal smooth muscle. The four tachykinins elicited a concentration-dependent contraction in tracheal smooth muscle devoid of epithelium, with the following order of potency: NKA greater than L 363851 greater than NKB greater than SP, (EC50 1.0 x 10(-9) M, 3.2 x 10(-9) M, 7.5 x 10(-9) M and 1.2 x 10(-7) M, respectively), which suggests that NK2 receptors predominate in airway smooth muscle. In the presence of epithelium, the sensitivity of airway smooth muscle to tachykinins was decreased, and the concentration response curves to tachykinins were shifted rightward by 30-fold for SP, 9-fold for NKA, and 5-fold for NKB. The concentration response curve to L 363851 was not significantly shifted in the presence of epithelium. This suggests that epithelium may release a relaxant factor in response to tachykinins via an NK1 receptor. In airway smooth muscle, we found that tachykinins elicited phosphoinositide breakdown with an order of potency similar to that for contractile response (EC50 2.2 x 10(-5) M, 3.6 x 10(-5) M, 4.4 x 10(-5) M, and 5.9 x 10(-5) M). In epithelium, SP alone elicited a significant phosphoinositide breakdown, suggesting that epithelial receptors to tachykinins may be of the NK1 subtype. Since it is established that phosphoinositide derivatives can elicit mobilization of intracellular calcium, our results suggest that phosphoinositide breakdown is the coupling mechanism for tachykinin-induced contraction of airway smooth muscle.

Animals↗

PubMed1985

Pirenzepine distinguishes between muscarinic receptor-mediated phosphoinositide breakdown and inhibition of adenylate cyclase.

Subtypes of muscarinic cholinergic receptors have been proposed to exist, but the biochemical responses mediated by the putative subtypes are unknown. In the present study, muscarinic receptor-mediated phosphoinositide breakdown and inhibition of adenylate cyclase activity were characterized in rat brain as well as rat parotid and heart. To study whether these responses are mediated by separate subtypes of muscarinic receptors, the potencies of agonists and antagonists were determined in both assays. Antagonist potencies were calculated by Schild analysis. In the brain, the putatively selective muscarinic receptor antagonist, pirenzepine, exhibited Ki values of 21 nM in the assay of phosphoinositide breakdown and 310 nM in the assay of adenylate cyclase activity. Similarly, using radioligand binding techniques, it distinguished two binding sites with Kd values of 12 and 168 nM. The antagonist, atropine, on the other hand, was equipotent in the two biochemical assays and the radioligand binding assay with Ki values of approximately 1 to 2 nM. In peripheral tissues with robust muscarinic receptor-mediated phosphoinositide (parotid) and adenylate cyclase (heart) responses, pirenzepine exhibited a similar selectivity (19-fold) for the phosphoinositide assay that was seen in the brain, but it was 6- to 7-fold less potent in both peripheral tissues than in the central nervous system. In addition, the potencies of pirenzepine in binding and functional studies in each tissue were not as well correlated as in the brain. Atropine and other antagonists were 4- to 9-fold selective for inhibiting oxotremorine-stimulated phosphoinositide breakdown in the peripheral tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

(4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethy↗

PubMed1985

Quantitation of adenosine-5'-triphosphate used for phosphoinositide metabolism in human erythrocytes.

The human erythrocyte actively phosphorylates and dephosphorylates phosphatidylinositol present in the membrane in an apparent "futile cycle." Recent reports have proposed that this phosphorylation/dephosphorylation cycle is a significant consumer of adenosine-5'-triphosphate (ATP) in the erythrocyte. This study details two independent techniques for quantitating the ATP consumed by this phosphoinositide futile cycle. With the first technique a quasi-steady-state labeling of erythrocyte ATP with 32P-phosphate was obtained, and the rate of synthesis of 32P-phosphoinositides was then monitored. The second technique used a novel labeling strategy that allowed only ATP to be labeled with 32P; the transfer of 32P from ATP to phosphoinositides was then an independent measure of the ATP consumed for phosphoinositide synthesis. These two techniques documented that 0.5% to 1.0% of net ATP produced by the erythrocyte is used for phosphoinositide synthesis.

Adenosine Triphosphate↗

PubMed1985

Arachidonic acid stimulates phosphoinositide hydrolysis and human placental lactogen release in an enriched fraction of placental cells.

Previous investigations in this laboratory have indicated that arachidonic acid stimulates a rapid, dose-dependent, and reversible increase in human placental lactogen (hPL) release which is not dependent on cyclooxygenase or lipoxygenase metabolism. To investigate further the mechanism by which arachidonic acid stimulates the release of hPL, the effects of arachidonic acid on phosphoinositide hydrolysis were examined in an enriched cell culture population of term human syncytiotrophoblast. Phosphoinositide hydrolysis was assayed by three methods: the release of 3H from perfused cells prelabeled with [3H]myoinositol, the measurement of inositol phosphate accumulation, and the distribution of radioactivity in phospholipids separated by two-dimensional thin layer chromatography after exposure of 32P-labeled placental cells to arachidonic acid. Arachidonic acid stimulated a concentration-dependent, rapid, and reversible increase in the release of both [3H]myoinositol and hPL from perfused placental cells. This effect was not inhibited by prior incubation of cells with indomethacin (20 microM). In contrast, palmitic acid and oleic acid stimulated phosphoinositide hydrolysis only at a high concentration (100 microM). Arachidonic acid also stimulated the rapid appearance of inositol monophosphate in placental cells. The effect of arachidonic acid was specific for hydrolysis of phosphoinositides and phosphatidylserine and did not involve other phospholipids. Since phosphoinositide hydrolysis is associated with hormone release in a variety of secretory systems, these results suggest that the stimulation of hPL release by arachidonic acid may be mediated, at least in part, by the activation of phospholipase C.

Arachidonic Acid↗