Characteristics of [3H]hemicholinium-3 binding to rat striatal membranes: evidence for negative cooperative site-site interactions

Inhibitors of Acetylcholinesterase Derived from 7-Methoxytacrine and Their Effects on the Choline Transporter CHT1

BACKGROUND

Reversible acetylcholinesterase inhibitors are used in Alzheimer disease therapy. However, tacrine and its derivatives have severe side effects. Derivatives of the tacrine analogue 7-methoxytacrine (MEOTA) are less toxic.

METHODS

We evaluated new derivatives of 7-MEOTA (2 homodimers linked by 2 C4-C5 chains and 5 N-alkylated C4-C8 side chain derivatives) in vitro, using the rat hippocampal choline transporter CHT1.

RESULTS

Some derivatives were effective inhibitors of rat acetylcholinesterase and comparable with 7-MEOTA. All derivatives were able to inhibit CHT1, probably via quaternary ammonium, and this interaction could be involved in the enhancement of their detrimental side effects and/or in the attenuation of their promising effects. Under conditions of disrupted lipid rafts, the unfavorable effects of some derivatives were weakened. Only tacrine was probably able to stereospecifically interact with the naturally occurring amyloid-β isoform and to simultaneously stimulate CHT1. Some derivatives, when coincubated with amyloid β, did not influence CHT1. All derivatives also increased the fluidity of the cortical membranes.

CONCLUSION

The N-alkylated derivative of 7-MEOTA bearing from C4 side chains appears to be the most promising compound and should be evaluated in future in vivo research.

Identification and characterization of ML352: a novel, noncompetitive inhibitor of the presynaptic choline transporter

The high-affinity choline transporter (CHT) is the rate-limiting determinant of acetylcholine (ACh) synthesis, yet the transporter remains a largely undeveloped target for the detection and manipulation of synaptic cholinergic signaling. To expand CHT pharmacology, we pursued a high-throughput screen for novel CHT-targeted small molecules based on the electrogenic properties of transporter-mediated choline transport. In this effort, we identified five novel, structural classes of CHT-specific inhibitors. Chemical diversification and functional analysis of one of these classes identified ML352 as a high-affinity (K_i = 92 nM) and selective CHT inhibitor. At concentrations that fully antagonized CHT in transfected cells and nerve terminal preparations, ML352 exhibited no inhibition of acetylcholinesterase (AChE) or cholineacetyltransferase (ChAT) and also lacked activity at dopamine, serotonin, and norepinephrine transporters, as well as many receptors and ion channels. ML352 exhibited noncompetitive choline uptake inhibition in intact cells and synaptosomes and reduced the apparent density of hemicholinium-3 (HC-3) binding sites in membrane assays, suggesting allosteric transporter interactions. Pharmacokinetic studies revealed limited in vitro metabolism and significant CNS penetration, with features predicting rapid clearance. ML352 represents a novel, potent, and specific tool for the manipulation of CHT, providing a possible platform for the development of cholinergic imaging and therapeutic agents.

Molecular properties of the high-affinity choline transporter CHT1

This article summarizes molecular properties of the high-affinity choline transporter (CHT1) with reference to the historical background focusing studies performed in laboratories of the author. CHT1 is present on the presynaptic terminal of cholinergic neurons, and takes up choline which is the precursor of acetylcholine. The Na(+)-dependent uptake of choline by CHT1 is the rate-limiting step for synthesis of acetylcholine. CHT1 is the integral membrane protein with 13 transmembrane segments, belongs to the Na(+)/glucose co-transporter family (SLC5), and has 20-25% homology with members of this family. A single nucleotide polymorphism (SNP) for human CHT1 has been identified, which has a replacement from isoleucine to valine in the third transmembrane segment and shows the choline uptake activity of 50-60% as much as that of wild-type CHT1. The proportion of this SNP is high among Asians. Possible importance of choline diet for those with this SNP was discussed.

Nonoisotopic assay for the presynaptic choline transporter reveals capacity for allosteric modulation of choline uptake

Current therapies to enhance CNS cholinergic function rely primarily on extracellular acetylcholinesterase (AChE) inhibition, a pharmacotherapeutic strategy that produces dose-limiting side effects. The Na(+)-dependent, high-affinity choline transporter (CHT) is an unexplored target for cholinergic medication development. Although functional at the plasma membrane, CHT at steady-state is localized to synaptic vesicles such that vesicular fusion can support a biosynthetic response to neuronal excitation. To identify allosteric potentiators of CHT activity, we mapped endocytic sequences in the C-terminus of human CHT, identifying transporter mutants that exhibit significantly increased transport function. A stable HEK-293 cell line was generated from one of these mutants (CHT LV-AA) and used to establish a high-throughput screen (HTS) compatible assay based on the electrogenic nature of the transporter. We established that the addition of choline to these cells, at concentrations appropriate for high-affinity choline transport at presynaptic terminals, generates a hemicholinium-3 (HC-3)-sensitive, membrane depolarization that can be used for the screening of CHT inhibitors and activators. Using this assay, we discovered that staurosporine increased CHT LV-AA choline uptake activity, an effect mediated by a decrease in choline K(M) with no change in V(max). As staurosporine did not change surface levels of CHT, nor inhibit HC-3 binding, we propose that its action is directly or indirectly allosteric in nature. Surprisingly, staurosporine reduced choline-induced membrane depolarization, suggesting that increased substrate coupling to ion gradients, arising at the expense of nonstoichiometric ion flow, accompanies a shift of CHT to a higher-affinity state. Our findings provide a new approach for the identification of CHT modulators that is compatible with high-throughput screening approaches and presents a novel model by which small molecules can enhance substrate flux through enhanced gradient coupling.

Comparative effects of aluminum and ouabain on synaptosomal choline uptake, acetylcholine release and (Na+/K+)ATPase

Closing the gap between adverse health effects of aluminum and its mechanisms of action still represents a huge challenge. Cholinergic dysfunction has been implicated in neuronal injury induced by aluminum. Previously reported data also indicate that in vivo and in vitro exposure to aluminum inhibits the mammalian (Na(+)/K(+))ATPase, an ubiquitous plasma membrane pump. This study was undertaken with the specific aim of determining whether in vitro exposure to AlCl(3) and ouabain, the foremost utilized selective inhibitor of (Na(+)/K(+))ATPase, induce similar functional modifications of cholinergic presynaptic nerve terminals, by comparing their effects on choline uptake, acetylcholine release and (Na(+)/K(+))ATPase activity, on subcellular fractions enriched in synaptic nerve endings isolated from rat brain, cuttlefish optic lobe and torpedo electric organ. Results obtained show that choline uptake by rat synaptosomes was inhibited by submillimolar AlCl(3), whereas the amount of choline taken up by synaptosomes isolated from cuttlefish and torpedo remained unchanged. Conversely, choline uptake was reduced by ouabain to a large extent in all synaptosomal preparations analyzed. In contrast to ouabain, which modified the K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions, AlCl(3) induced reduction of stimulated acetylcholine release was only observed when rat synaptosomes were challenged. Finally, it was observed that the aluminum effect on cuttlefish and torpedo synaptosomal (Na(+)/K(+))ATPase activity was slight when compared to its inhibitory action on mammalian (Na(+)/K(+))ATPase. In conclusion, inhibition of (Na(+)/K(+))ATPase by AlCl(3) and ouabain jeopardized the high-affinity (Na(+)-dependent, hemicholinium-3 sensitive) uptake of choline and the Ca(2+)-dependent, K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions. The effects of submillimolar AlCl(3) on choline uptake and acetylcholine release only resembled those of ouabain when rat synaptosomes were assayed. Therefore, important differences were found between the species regarding the cholinotoxic action of aluminum. The variability of (Na(+)/K(+))ATPase sensitivity to aluminum of cholinergic neurons might contribute to their differential susceptibility to this neurotoxic agent.

Cortical choline transporter function measured in vivo using choline-sensitive microelectrodes: clearance of endogenous and exogenous choline and effects of removal of cholinergic terminals

The capacity of the high-affinity choline transporter (CHT) to import choline into presynaptic terminals is essential for acetylcholine synthesis. Ceramic-based microelectrodes, coated at recording sites with choline oxidase to detect extracellular choline concentration changes, were attached to multibarrel glass micropipettes and implanted into the rat frontoparietal cortex. Pressure ejections of hemicholinium-3 (HC-3), a selective CHT blocker, dose-dependently reduced the uptake rate of exogenous choline as well as that of choline generated in response to terminal depolarization. Following the removal of CHTs, choline signal recordings confirmed that the demonstration of potassium-induced choline signals and HC-3-induced decreases in choline clearance require the presence of cholinergic terminals. The results obtained from lesioned animals also confirmed the selectivity of the effects of HC-3 on choline clearance in intact animals. Residual cortical choline clearance correlated significantly with CHT-immunoreactivity in lesioned and intact animals. Finally, synaptosomal choline uptake assays were conducted under conditions reflecting in vivo basal extracellular choline concentrations. Results from these assays confirmed the capacity of CHTs measured in vivo and indicated that diffusion of substrate away from the electrode did not confound the in vivo findings. Collectively, these results indicate that increases in extracellular choline concentrations, irrespective of source, are rapidly cleared by CHTs.

Diet restriction in mice causes a decrease in hippocampal choline uptake and muscarinic receptors that is restored by administration of tyrosine: interaction between cholinergic and adrenergic receptors influencing cognitive function

We have studied the effects of diet restriction (DR) to 60% and 40% of daily requirements, and tyrosine administration on cognitive function in mice, to define the nutritional-neurochemical interactions on autonomic tone involved in behavior and energy regulation. Cognitive function in the Morris Water maze was significantly impaired after 40% DR compared to both control and 60% DR. It was restored after tyrosine in association with increased M1 cholinergic and beta-adrenergic receptor function, and decreased alpha-adrenergic function. DR to 40% significantly decreased choline uptake (p <.05) and M1 receptor number (Bmax) (p <.05), without changes in affinity (Kd), choline acetyl transferase (ChAT) or acetyl cholinesterase (AChE) activity. Tyrosine administration significantly increased choline uptake (Bmax) (p <.05) and M1 density in the 40% DR (p <.01) without changes in affinity. ChAT activity was decreased after tyrosine--significantly after 40% DR (p <.05) while AChE was not affected. Both M1 mRNA and protein were not influenced by DR or tyrosine administration. Tyrosine hydroxylase mRNA was decreased significantly by 40% DR (p <.01). The effect of DR and tyrosine appeared to be both pre- and post-synaptic, indicating modulation of cholinergic activity by adrenergic tone. Nutritional effect on behavior and autonomic tone may have implications for the treatment of mood changes associated with weight loss and semi-starvation.

Inhibition of choline acetyltransferase by excitatory amino acids as a possible mechanism for cholinergic dysfunction in the central nervous system

Choline acetyltransferase (ChAT) activity was reduced by more than 85% in cultured retina cells after 16 h treatment with 150 microM kainate (T(1/2) : 3.5 h). Glutamate, AMPA and quisqualate also inhibited the enzyme in equivalent proportion. Cell lesion measured by lactate dehydrogenase (LDH) release, 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide - thiazolyl blue (MTT) reduction and microscopic observation was not detected even after 48 h with kainate. Other retina neurochemical markers were not affected by kainate and full recovery of the enzyme was achieved 9 days after kainate removal. Moreover, hemicolinium-3 sensitive choline uptake and hemicolinium-3 binding sites were maintained intact after kainate treatment. The immunoblot and immunohistochemical analysis of the enzyme revealed that ChAT molecules were maintained in cholinergic neurons. The use of antagonists showed that ionotropic and group 1 metabotropic receptors mediated the effect of glutamate on ChAT inhibition, in a calcium dependent manner. The quisqualate mediated ChAT inhibition and part of the kainate effect (30%) was prevented by 5 mM N(G)-nitro-L-arginine methyl ester (L-NAME). Veratridine (3 microM) also reduced ChAT by a Ca(2+) dependent, but glutamate independent mechanism and was prevented by 1 microM tetrodotoxin.

Stereoselectivity of the inhibition of [3H]hemicholinium-3 binding to the sodium-dependent high-affinity choline transporter by the enantiomers of alpha- and beta-methylcholine

In a previous report, we showed that the enantiomers of alpha- and beta-methylcholine inhibited choline uptake with stereoselectivity, but that their transport by the choline carrier of nerve terminals showed stereospecificity. The present experiments used the same choline analogues to determine if either of the above characteristics pertains to their ability to interact with the [3H]-hemicholinium-3 binding site present on striatal membranes and synaptosomes. [3H]Hemicholinium-3 binding to striatal membranes could be inhibited stereoselectively by the enantiomers of beta-methylcholine, but R(+)-alpha-methylcholine was little better than its enantiomer in this test. However, [3H]hemicholinium-3 binding to striatal synaptosomes was inhibited stereoselectively by the enantiomers of both alpha- and beta-methylcholine. This difference between the properties of [3H]hemicholinium-3 binding to membranes or to synaptosomes appears related to the presence of two ligand binding states. The [3H]hemicholinium-3 binding site could be shifted to a low-affinity state by ATP treatment and to a high-affinity state by EDTA washing. When the [3H]hemicholinium-3 binding site existed in its low-affinity state, binding was inhibited stereoselectively by the enantiomers of both alpha- and beta-methylcholine, but when shifted to its high-affinity state, it was inhibited stereoselectively only by the enantiomers of beta-methylcholine. We conclude that hemicholinium-3 interacts with the substrate recognition site of the high-affinity choline transporter, but that the stereoselectivity of this site changes depending on its affinity state.

Modulation of high-affinity choline carrier activity following incubation of rat hippocampal synaptosomes with hemicholinium-3

Membrane carriers display structural and functional asymmetry with a substrate binding site which can be oriented alternately, but not simultaneously, to the extracellular and intracellular environment. Hemicholinium-3 is an inhibitor of the high-affinity choline carrier in cholinergic nerve terminals which binds to the transporter at the outer membrane surface but is not taken up into the cell. In the present study, we investigated the decline in choline transport which occurs during the first few minutes cholinergic nerve terminals are incubated in physiological salt solutions. Following incubation of rat hippocampal synaptosomes with hemicholinium-3, samples were washed free of the inhibitor and high-affinity choline uptake was measured. Choline uptake into hemicholinium-treated nerve terminals was significantly greater than control (132 +/- 4%). This effect appeared not to be due to an increase in uptake of choline above initial values in the hemicholinium-treated synaptosomes, but to a decrease in choline carrier activity in control samples by more than 25% during the first few minutes of incubation. Addition of hemicholinium-3 to samples after the preincubation induced decrease in choline uptake, followed by a wash period to remove the inhibitor resulted in elevation of choline uptake levels to initial levels. The effect of hemicholinium-3 was concentration-dependent, requiring near saturating concentrations of the inhibitor to elicit the effect. Measurement of acetylcholine content of synaptosomes at different points during the incubation procedure revealed that there was a trend for transmitter levels to vary inversely compared to choline uptake activity, but the differences were not statistically significant during treatments when significant changes in transport activity were measured.

High-affinity choline transport sites: use of [3H]hemicholinium-3 as a quantitative marker

High-affinity choline transport (HAChT), the rate-limiting and regulatory step in acetylcholine (ACh) synthesis, is selectively localized to cholinergic neurons. Hemicholinium-3 (HC3), a potent and selective inhibitor of HAChT, has been used as a specific radioligand to quantify HAChT sites in membrane binding and autoradiographic studies. Because both HAChT velocity and [3H]HC3 binding change as in vivo activity of cholinergic neurons is altered, these markers are also useful measures of cholinergic neuronal activity. Evidence that [3H]HC3 is a specific ligand for HAChT sites on cholinergic terminals is reviewed. The ion requirements of HAChT and [3H]HC3 binding indicate that sodium and chloride are required for recognition of both choline and [3H]HC3. A common recognition site is also indicated by the close correspondence of the potency of HC3 and choline analogues for inhibiting both HAChT and [3H]HC3 binding. The parallel regional distributions of both markers in adult brain, during development and after specific lesions, all indicate specific cholinergic localization. The close association of HAChT and [3H]HC3 binding sites is also supported by parallel regulatory changes occurring after in vivo drug treatments and in vitro depolarization. Overall, the data indicate a close association between HAChT and [3H]HC3 binding and are consistent with the sites being identical. Methodologic considerations in using [3H]HC3 as a ligand and considerations in interpretation of results are also discussed.

Hypoxia induces different responses of striatal high- and low- affinity dopamine uptake sites

The dopamine (DA) uptake over a concentration range from 0.03 to 100 microM was studied in S1 fractions of the rat striatum prepared from control rats and those exposed for 14 h to hypobaric hypoxia. The uptake exhibited non-Michaelis-Menten kinetics, which were evaluated by applying an equation assuming two transport sites. The high-affinity uptake site was characterized by an apparent Michaelis-Menten constant of 0.47 microM and an apparent maximal transport rate of 113 pmol/mg protein/30 s. The respective constants of the low-affinity uptake site were 52.8 microM and 1490 pmol/mg protein/30 s. One hour after hypoxia kinetic constants of the high-affinity uptake were unchanged but the maximal transport rate of the low-affinity uptake was increased by 50%. The elevated low-affinity uptake capacity may represent a means of adaptation to hypoxia allowing a faster removal of high extracellular concentrations of DA.

Lasting effects of postnatal hypoxia and saline injection on the striatal dopamine transport and their modification by gangliosides

Hypobaric hypoxia (10 h daily, pO2 10 kPa) and saline administration (2.5 microliters/g body wt) from the 2nd till the 11th day of life both induced a long-lasting increase of the low-affinity dopamine (DA) uptake capacity in S1-fractions of the rat striatum. Additionally, the potassium-stimulated DA release was enhanced in adult control rats postnatally injected with saline. The administration of a mixture of bovine brain gangliosides (30 micrograms/g body wt) was found to prevent these effects. However, the kinetic constants of the DA uptake of hypoxic rats treated with gangliosides were reduced in comparison to untreated controls. Thus, the effects of gangliosides appear to differ between hypoxic and control conditions. The modification of the dopaminergic activity during brain development is discussed as a possible mechanism of the preventive effects of gangliosides against long-term cerebral dysfunctions following hypoxia or stress.

Regulation of hemicholinium binding sites in isolated nerve terminals

High-affinity uptake of choline, the rate-limiting, regulatory step for the synthesis of acetylcholine is regulated via presynaptic auto- and heteroreceptors. Binding studies using tritiated hemicholinium-3 ([3H]HCh-3) as the specific ligand for the choline carrier revealed that the number of hemicholinium binding sites in nerve terminals isolated from insect brain changes corresponding to the activity of synaptosomal kinase A and kinase C. Activation of kinase A apparently increases the total number of hemicholinium binding sites by recruiting additional occult carriers, whereas the effect of kinase C activity is most appropriately explained by preventing a down-regulation of carrier proteins. The kinase-mediated regulation of choline transporters is obviously due to a phosphorylation of the carrier protein itself.

Solubilization and partial characterization of [3H]choline mustard-labeled high-affinity choline carrier from presynaptic plasma membrane of Torpedo electric organ

The objective of the present study was to characterize the hydrodynamic properties of the detergent-solubilized high-affinity choline carrier from presynaptic plasma membranes of electric organ of Torpedo marmorata following radioaffinity labelling with [3H]choline mustard aziridinium ion [( 3H]ChM Az). Membrane proteins were solubilized with 0.8% CHAPS, then analyzed by gel permeation chromatography and equilibrium density sedimentation in sucrose gradients made in H2O and D2O. The radiolabeled protein eluted from the gel filtration column with a distribution coefficient of 0.36 allowing calculation of a Stokes radius of 4.4 nm. Distribution of the [3H]ChM Az-labeled proteins differed in the H2O and D2O sucrose gradients, with apparent sedimentation coefficients of 5.2 and 4.7, respectively, indicating that detergent may be bound to hydrophobic regions of the protein. The calculated partial specific volume for the protein complex was 0.76. The estimate of molecular mass for the complex was 115,000 Da, with the protein portion having a molecular mass of about 83,000 Da. Analysis of the peak fractions from the gel filtration column and sucrose gradients by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of a 3H-labeled polypeptide at 0,000-42,000 Da. The choline carrier may exist as a dimer of this polypeptide.

Ca2(+)-dependent, ATP-induced conversion of the [3H]hemicholinium-3 binding sites from high- to low-affinity states in rat striatum: effect of protein kinase inhibitors on this affinity conversion and synaptosomal choline transport

Tritium-labeled hemicholinium-3 ([3H]HC-3) was used to characterize the sodium-dependent high-affinity choline carrier sites in rat striatal preparations. In an earlier study, we had shown that [3H]HC-3 labels choline carrier sites with high and low affinities and had suggested that the low-affinity sites represent "functional" carrier sites. The objective of the present study was to examine the mechanisms involved in the regulation of the two affinity states of [3H]HC-3 binding. Here, we demonstrate that these two affinity states are totally interconvertible; addition of 0.1 mM ATP in the binding assay medium quantitatively converted all the binding sites to the low-affinity state, whereas addition of 1 mM beta,gamma-methylene 5'-ATP quantitatively converted all the binding sites to the high-affinity state. Preincubation of the tissue (for 15 min at 37 degrees C) before the binding assay also converted the binding sites to the high-affinity state, whereas supplementation of the assay medium with ATP (0.5 mM) again induced expression of the low-affinity state of the binding sites. This effect of ATP was found to be selective for this nucleotide. Neither ADP (1 mM) nor cyclic AMP could mimic such an effect. Other nucleotide triphosphates--CTP (0.5 mM) and GTP (0.5 mM)--also could not substitute for ATP. GTP, however, caused nearly a 35% reduction in the number of binding sites, accompanying a loss of the low-affinity component of binding. This effect of GTP was also shared by 5'-guanylylimidodiphosphate but not by GDP or cyclic GMP. This ATP-dependent low-affinity conversion of [3H]HC-3 binding sites requires divalent metal ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of [3H]hemicholinium-3 to the high-affinity choline transporter in electric organ synaptosomal membranes

Sodium-dependent binding of [3H]hemicholinium-3 was observed to be 10-fold higher with presynaptic membranes from the electric organ than with electroplaque membranes and this binding site copurified with synaptosomal membranes. The KD for specific [3H]hemicholinium-3 binding was found to be 31 +/- 4 nM and the Bmax, 5.0 +/- 0.2 pmol/mg protein; a Ki of 16 nM was estimated for hemicholinium-3 as a competitive inhibitor of high-affinity choline transport in electric organ synaptosomes. Choline and choline analogues were equally potent as inhibitors of [3H]choline uptake and [3H]hemicholinium-3 binding. Tubocurarine and oxotremorine also inhibited uptake and binding, but carbachol was without effect in both tests. These findings suggest that [3H]hemicholinium binds to the high-affinity choline transporter present at the cholinergic nerve terminal membrane. A comparison of maximal velocities for choline transport and the maximal number of hemicholinium-3 binding sites indicated that the high-affinity choline transporter has an apparent turnover number of about 3s-1 at 20 degrees C under resting conditions. The high transport rates observed in electric organ synaptosomes are likely due to the high density of high-affinity choline transporters in this tissue, estimated on the basis of [3H]hemicholinium-3 binding to be of the order of 100/micron2 of synaptosomal membrane.

Involvement of phospholipase A2 in the regulation of [3H]hemicholinium-3 binding

We have examined the effects of exogenous phospholipase A2 (PLA2) on the sodium-dependent high-affinity choline uptake mechanism as assessed by the specific binding of [3H]hemicholinium-3 ([3H]HCh-3). Incubation of striatal synaptic membranes with bee venom PLA2 resulted in a concentration-dependent increase in the specific binding of [3H]HCh-3. The effect of PLA2 on [3H]HCh-3 binding was inhibited by quinacrine, a PLA2 inhibitor, and by removal of calcium. Scatchard analysis revealed that the observed changes in binding reflected a 2-fold increase in both the capacity and affinity of [3H]HCh-3 for its binding site. Choline and N-butylcholine inhibited the specific binding of [3H]HCh-3 in both control and PLA2-treated membranes with similar potency. When a low concentration of PLA2 was incubated with the striatal synaptosomes, a small but significant increase in high-affinity [3H]choline uptake was observed. However, higher concentrations of PLA2, which further increased the specific binding of [3H]HCh-3, caused a reduction of [3H]choline uptake, apparently due to disruption of synaptosomal integrity by PLA2. Finally, potassium depolarization- and PLA2-induced increases in specific [3H]HCh-3 binding were not additive. These results suggest a possible role for endogenous PLA2 in the calcium-dependent regulation of sodium-dependent high-affinity choline uptake.