Arterio-venous differences of choline and choline lipids across the brain of rat and rabbit

Reproducible and time-dependent modification of serum protein binding in Wistar Kyoto rats

INTRODUCTION

The theoretical basis of the influence of (alterations in) plasma protein binding on pharmacokinetics (PK) is well-established. In contrast, the impact of protein binding on pharmacodynamics has not been examined in a systematic manner. Here we present an experimental approach to modify serum protein levels and binding in the rat, in a robust, reproducible, and time-dependent manner.

METHOD

Male Wistar Kyoto rats were divided into three different groups. The control group (n=4) did not receive treatment. In the cannulation(-) group (n=6) the rats were instrumented with three permanent blood cannulas. The rats in the cannulation(+) group received in addition to the cannulation a subcutaneous injection of turpentine oil of 100 microl/100 g bodyweight. The effects were characterized in terms of 1) the time course of serum levels of albumin and alpha(1)-acid glycoprotein (AGP), and 2) the effect on the ex vivo serum protein binding of S(-)-propranolol.

RESULTS

In control rats the AGP serum concentration was stable at a value of 169+/-16 microg/ml. In the cannulation(-) group a maximum ten- to fifteen-fold increase in serum AGP concentration was observed at 48 h post surgery, followed by a gradual return back to baseline within 1 week. In the cannulation(+) group a similar concentration-time profile for AGP was found, but without a complete return to baseline within 1 week and with a much higher variability. Ex vivo, an increase in AGP serum concentration from 55 to 675 microg/ml resulted in a profound decrease in the free fraction of S(-)-propranolol from 14+/-0.6 to 1.9+/-0.3%.

CONCLUSIONS

In conclusion, through cannulation alone the serum protein levels and binding were modified in a robust, reproducible and time-dependent manner. Therefore this experimental approach is suitable for the investigation of the influence of protein binding on both pharmacokinetics and pharmacodynamics.

Selected pharmacodynamic and biochemical properties of selenonium choline

1. Five minutes after intravenous administration of 50 mg/kg of the novel choline analogue selenonium choline [(CH3)2Se + CH2CH2OH, SeCh], only 8% of the administered dose was accounted for in blood, brain, liver, heart, and kidney tissues. 2. SeCh was acetylated in vivo to acetylselenonium choline (ASeCh) in all of the tissues examined. 3. During postmortem incubation, brain concentrations of SeCh and ASeCh increased to 535% and to 425%, respectively. 4. K(m) and Vmax values for the phosphorylation of SeCh by choline kinase were higher and lower, respectively, compared to the phosphorylation of choline. 5. Acetylation of SeCh was described with K(m) and Vmax values that were both higher than the values for Ch. 6. The data suggest that SeCh is phosphorylated and incorporated into various lipids in brain tissue, and is acetylated to ASeCh by both nonneural and neural tissues.

Cerebrospinal fluid acetylcholine and choline in vascular dementia of Binswanger and multiple small infarct types as compared with Alzheimer-type dementia

The acetylcholine (ACh) and choline (Ch) concentrations in the cerebrospinal fluid were investigated in patients with vascular dementia of the Binswanger type (VDBT) or multiple small infarct type (MSID) as compared with patients with Alzheimer-type dementia (ATD). The ACh concentration in patients with ATD was found to be significantly lower than in controls (73%, p < 0.0001), and showed a significant positive correlation with dementia scale scores (rs = 0.63, p < 0.03). The Ch concentration in the CSF of ATD patients was approximately the same as in controls. In VDBT/MSID patients, the ACh concentration was significantly lower than in controls (p < 0.001) also showing a significant positive correlation with dementia scale scores (rs = 0.62, p < 0.02), but was significantly higher than in ATD patients (p < 0.001). Moreover, the Ch concentration in VDBT/MSID patients was significantly higher than in controls (p < 0.001) or ATD patients (p < 0.001). These results suggest that simultaneous determination of ACh and Ch concentrations in CSF may be useful for differentiating VDBT/MSID from ATD and that increasing the ACh level using cholinergic agents may be a beneficial therapeutic strategy for the treatment of ATD as well as VDBT/MSIT, and is worthy of further investigation.

Effects of hypoxia on choline exchange among organs

Cerebral blood flow (CBF) and the arteriovenous (A-V) difference for choline (Ch) across brain, lung, splanchnic territory, liver, kidney, and lower limb were studied in anesthetized, mechanically ventilated rats subjected to 10-20-min periods of hypoxia induced by lowering the inspired O2 concentration to 13%. A large, time-dependent increase in arterial blood Ch concentration occurred during hypoxia. This phenomenon coincided with a net rate of uptake of Ch by the brain during hypoxia (0.81 +/- 0.24 nmol/min, n = 10; p less than 0.05), which contrasted with a net rate of loss of Ch by this organ during the control period that preceded hypoxia (-0.20 +/- 0.08 nmol/min, n = 10; p less than 0.05). During hypoxia, lungs and splanchnic territory showed negative A-V differences for Ch levels (net Ch loss), whereas brain, liver, kidney, and lower limb showed positive A-V differences for Ch levels (net Ch uptake). Ch output from lungs was already detected at 5 min within the period of hypoxia and reversed rapidly after restoration of normal oxygenation. On the other hand, Ch output from the splanchnic territory became evident only 10 min after commencement of hypoxia and outlasted this experimental condition. It is concluded that extracerebral production of Ch during hypocapnic hypoxia raises the arterial concentration of this molecule and, by reversing the gradient across cerebral capillaries, prevents the cerebral loss of Ch in this condition.

Unsaturated fatty acids esterified in 2-acyl-l-lysophosphatidylcholine bound to albumin are more efficiently taken up by the young rat brain than the unesterified form

The aim of the present study was to investigate whether unsaturated 2-acyl-lysophosphatidylcholine bound to plasma albumin is a relevant delivery form of unsaturated fatty acids to the developing brain. Twenty-day-old rats were perfused for 30 s with labeled palmitic, oleic, linoleic, and arachidonic acids in either their unesterified form or esterified in 2-acyl-lysophosphatidylcholine labeled on the choline and fatty acid moieties. Both forms were bound to albumin. Incorporation in brain lipid classes was followed within 1 h. The brain uptake of the unesterified fatty acids reached a plateau at 5-15 min and was maximal for arachidonic acid (0.45% of the perfused dose). The brain uptake of palmitoyl-lysophosphatidylcholine was similar to that of palmitic acid, whereas that of other lysophosphatidylcholines increased with the degree of unsaturation (rate and maximal uptake) and was six- to 10-fold higher than that of the corresponding unesterified fatty acid. 2-Acyl-lysophosphatidylcholines were taken up without prior hydrolysis and reacylated into doubly labeled phosphatidylcholine, which was the most labeled lipid class, whereas lipid distribution of the unesterified fatty acid was more diversified. Partial hydrolysis of 2-acyl-lysophosphatidylcholine occurred in the brain tissue, and redistribution of the fatty acyl moiety into other phospholipid classes was also observed and was the highest for arachidonic acid. In this case, the percentage of esterification of this fatty acid in phosphatidylinositol (expressed as a percentage of the total lipid fraction) was relatively lower than that observed when the unesterified form was used.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-dependent changes in cerebral choline and acetylcholine induced by transient global ischemia in rats

We occluded the carotid and vertebral arteries of 12 rats for 15 minutes to measure the brain concentrations of choline and acetylcholine and cerebral blood flow at the end of the ischemic period or 15, 30, or 150 minutes after circulation was reestablished. The animals were sacrificed with microwave radiation focused to the head immediately after a brief infusion of [14C]iodoantipyrine with rapid sampling of arterial blood. Brain tissue samples were extracted with ether to separate the tracer, which was subsequently measured by liquid scintillation counting and used to calculate local cerebral blood flow. The aqueous phase was then processed for the measurement of choline and acetylcholine concentrations by gas chromatography/mass spectrometry. The results showed a large increase in tissue choline content and a decrease in tissue acetylcholine content during ischemia. During recirculation, choline levels progressively declined, reaching levels lower than those in four control rats after 150 minutes of recirculation for most brain regions. A reciprocal relation between the brain choline concentration and local cerebral blood flow was found. Acetylcholine levels showed an initial rebound to greater than control during recirculation, with subsequent normalization. Brain acetylcholine concentration was positively correlated with brain choline concentration, provided that cerebral blood flow was greater than 0.3 ml x g-1 x min-1. Because tissue free choline was depleted in most brain regions 150 minutes after transient ischemia, we speculate that prolonged ischemia may produce a greater depletion of tissue free choline with a resulting decline in tissue acetylcholine. This could play an important role in the cognitive deficit associated with vascular dementia.

Global in vivo replacement of choline by N-aminodeanol. Testing a hypothesis about progressive degenerative dementia: I. Dynamics of choline replacement

Severe disruption of certain cholinergic pathways is a characteristic feature of Alzheimer's disease. Attempts to establish animal models by interfering with cholinergic function have not been very successful. We now present data which show a substantial and progressive replacement of free and phospholipid-bound choline by the novel choline isostere N-amino-N,N-dimethylaminoethanol during its dietary administration in place of choline. Free choline in blood fell to approximately 20% of controls after 10 to 30 days on diet. Phospholipid-bound choline in plasma was reduced to less than 15%, and in erythrocytes to about 22%. After 120 days of diet free and bound choline were reduced in most tissues to approximately 30% of controls. Only liver retained more than 80% of free choline. Acetylcholine was decreased to 33 to 50% of control. Total true and false transmitter in experimental animals was in all tissues less that acetylcholine in controls, suggesting that muscarinic transmission would be impaired. Moderate reduction of choline acetyltransferase activity was seen in striatum and myenteric plexus, and of QNB-binding in hippocampus, striatum and myenteric plexus.

Small rises in plasma choline reverse the negative arteriovenous difference of brain choline

The concentrations of free choline in blood plasma from a peripheral artery and from the transverse sinus, in the CSF, and in total brain homogenate, have been measured in untreated rats and in rats after acute intraperitoneal administration of choline chloride. In untreated rats, the arteriovenous difference of brain choline was related to the arterial choline level. At low arterial blood levels (less than 10 microM) as observed under fasting conditions, the arteriovenous difference was negative (about -2 microM), indicating a net release of choline from the brain of about 1.6 nmol/g/min. In rats with spontaneously high arterial blood levels (greater than 15 microM), the arteriovenous difference was positive, implying a marked net uptake of choline by the brain (3.1 nmol/g/min). The CSF choline concentration, which reflects changes in the extracellular choline concentration, also increased with increasing plasma levels and closely paralleled the gradually rising net uptake. Acute administration of 6, 20, or 60 mg of choline chloride/kg caused, in a dose-dependent manner, a sharp rise of the arterial blood levels and the CSF choline, and reversed the arteriovenous difference of choline to markedly positive values. The total free choline in the brain rose only initially and to a quantitatively negligible extent. Thus, the amount of choline taken up by the brain within 30 min was stored almost completely in a metabolized form and was sufficient to sustain the release of choline from the brain as long as the plasma level remained low. We conclude that the extracellular choline concentration of the brain closely parallels fluctuations in the plasma level of choline.(ABSTRACT TRUNCATED AT 250 WORDS)

Utilization of high-density lipoprotein sphingomyelin by the developing and mature brain in the rat

Utilization of very long chain saturated fatty acids by brain was studied by injecting 20-day-old and adult rats with high-density lipoprotein containing [stearic or lignoceric acid-14C, (methyl-3H)choline]sphingomyelin. Labeling was followed for 24 h. Very small amounts of 14C were recovered in the brain of all rats, and there was no preferential uptake of lignoceric acid. Approximately 20% of the entrapped 14C was located in the form of unchanged sphingomyelin 24 h after injection. This result shows that the rat brain utilizes very little very long chain fatty acids (greater than or equal to 20 C atoms) from high-density lipoprotein sphingomyelin, even during the myelinating period. The [3H]choline moiety from sphingomyelin was recovered in brain phosphatidylcholine in a higher proportion in comparison with the 14C uptake. The brain 3H increased throughout the studied period in all experiments, but was much higher in the myelinating brain than in the mature brain. From the radioactivity distribution in liver and plasma lipids, it is clear that the choline 3H in the brain originates from either double-labeled phosphatidylcholine of lipoproteins or tritiated lysophosphatidylcholine bound to albumin, both synthesized by the liver.

Choline increases acetylcholine release and protects against the stimulation-induced decrease in phosphatide levels within membranes of rat corpus striatum

This study examined the possibility that membrane phospholipids might be a source of choline used for acetylcholine (ACh) synthesis. Slices of rat striatum or cerebellum were superfused with a choline-free or choline-containing (10, 20 or 40 microM) physiological solution with eserine, for alternating 20 min periods of rest or electrical stimulation. Superfusion media were assayed for choline and ACh, and slice samples taken before and after stimulation were assayed for choline, ACh, various phospholipids, protein and DNA. The striatal slices were able to sustain the stimulation-induced release of ACh, releasing a total of about 3 times their initial ACh contents during the 8 periods of stimulation and rest. During these 8 cycles, 885 pmol/micrograms DNA free choline was released from the slices into the medium, an amount about 45-fold higher than the initial or final free choline levels in the slices. Although repeated stimulation of the striatal slices failed to affect tissue levels of free choline or of ACh, this treatment did cause significant, dose-related (i.e., number of stimulation periods) stoichiometric decreases in tissue levels of phosphatidylcholine (PC) and of the other major phospholipids; tissue protein levels also declined significantly. Addition of exogenous choline to the superfusion medium produced dose-related increases in resting and evoked ACh release. The choline also fully protected the striatal slices from phospholipid depletion for as many as 6 stimulation periods. Cerebellar slices liberated large amounts of free choline into the medium but did not release measurable quantities of ACh; their phospholipid and protein levels did not decline with electrical stimulation. These data show that membrane phospholipids constitute a reservoir of free choline that can be used for ACh synthesis. When free choline is in short supply, ACh synthesis and release are sustained at the expense of this reservoir. The consequent reduction in membrane PC apparently is associated with a depletion of cellular membrane. The use of free choline by cholinergic neurons for two purposes, the syntheses of both ACh and membrane phospholipids, may thus impart vulnerability to them in situations where the supply of free choline is less than that needed for acetylation.

Focal ischemia enhances choline output and decreases acetylcholine output from rat cerebral cortex

Choline concentration is rate limiting in the synthesis of acetylcholine. There is a negative arteriovenous difference for choline concentration across the brain, indicating the steady output of choline from this organ. Cerebral ischemia may increase extracellular choline concentration by interfering with its removal by the circulation and by enhancing its net production from phospholipids. We tested this hypothesis in six rats subjected to middle cerebral artery occlusion. We determined choline and acetylcholine output from the ischemic cerebral cortex by analyzing their concentrations in the fluid contained in cortical cups by gas chromatography-mass spectrometry. Mean +/- SEM choline output over 40 minutes before ischemia (baseline value) was 31.1 +/- 1.6 pmol/min/cm2. During ischemia, mean +/- SEM choline output rose to 100.8 +/- 13, 97.3 +/- 12.7, 100 +/- 22.4, and 93.1 +/- 16.9 pmol/min/cm2 in four consecutive 10-minute periods, respectively. Mean +/- SEM acetylcholine output was 15.6 +/- 1.1 before and 5.9 +/- 1.2, 8.3 +/- 2.6, 8.6 +/- 2.1, and 13.7 +/- 4.6 pmol/min/cm2 in the four 10-minute collection periods during ischemia. All four choline values and the first acetylcholine value during ischemia were significantly different from their respective baseline values. We conclude that ischemia induces an increase in extracellular choline concentration with possible implications for acetylcholine metabolism. The attending transient decline in acetylcholine output may be due to impaired release due to local hypoxia or to decreased acetylcholine synthesis.

Developmental and regional quantitation of glycerophosphorylcholine phosphodiesterase activities in rat brain

The activity of glycerophosphorylcholine cholinephosphodiesterase was quantified in the diencephalon, mesencephalon, cerebral hemispheres, olfactory bulb and cerebellum postnatally for P5 until P70 of rat brain. The initially low activities gradually increase to adult levels by P30. The patterns of regional development are reminescent of those previously described for choline acetyltransferase activity. It is suggested that these may be functionally linked in neuronal cells. The activity of glycerophosphorylcholine phosphocholine phosphodiesterase was also determined and found to be similar although only one half as active as the enzyme liberating choline. The present experiments show that both the GPC phosphocholine phosphodiesterase and the GPC choline phosphodiesterase are regionally and developmentally regulated in rat brain.

Kinetics of choline uptake into isolated rat forebrain microvessels: evidence of endocrine modulation

The active uptake of [methyl-3H]choline into isolated rat brain microvessel suspension was studied as a likely guide to the transport of choline across the blood-brain barrier. The method consisted primarily of incubation of the suspension with a fixed concentration of labeled choline in the presence of increasing concentrations of unlabeled choline or any other inhibitor (I) of active uptake, defined as the difference in uptake at 37 degrees and 0 degrees C. From the linear regression of (1/V) against [I], the following values of Vmax (nmol g-1 min-1) and Km (microM) were obtained for choline: 2-month-old males, 10.6 +/- 3.8 and 6.1 +/- 0.9; 3-month old random females, 28.4 +/- 5.9 and 12.6 +/- 4.0; females at metaestrus, 17.8 +/- 10.3 and 8.3 +/- 5.0; at diestrus, 31.1 +/- 9.3 and 13.0 +/- 2.6; at proestrus, 54.9 +/- 2.2 and 14.0 +/- 1.5; at estrus, 19.2 +/- 2.2 and 2.6 +/- 1.7. The differences between males and random females (p less than 0.018) and between females at proestrus and estrus (p less than 0.005) are significant. It is suggested that these inter- and intrasex variations in choline uptake reflect a dynamic adjustment of supply in accordance with brain demand for choline at the time of assay. Hemicholinium-3 was an effective inhibitor of choline uptake, Ki = 14.0 +/- 8.5 microM; dimethylaminoethanol was much less effective; and imipramine had no measurable effect.

Choline transport and metabolism in soman- or sarin-intoxicated brain

The metabolism and blood-brain transport of choline (Ch) were investigated in perfused canine brain under control conditions and for 60 min after inhibition of brain cholinesterases by the organophosphorus (OP) compounds soman (pinacolylmethylphosphonofluoridate). Ch and acetylcholine (ACh) in blood and brain samples were analyzed using gas chromatography-mass spectrometry methods. Net transport of Ch was determined by Ch analysis in arterial and venous samples. Unidirectional transport of [3H]Ch was determined using the indicator dilution method. During control perfusion periods of 90 min, net efflux of brain Ch occurred at a rate of 1.6 +/- 0.4 nmol/g/min, and the Ch content of the recirculated perfusate increased 10-fold to approximately 8 microM. Brain Ch content increased in proportion to the increase in perfusate Ch level, but brain ACh was unaltered. Rapid administration of soman (100 micrograms) or sarin (400 micrograms) into the arterial perfusate after a 40-min control period resulted in a greater than 10-fold increase in ACh content in cerebral cortex, brainstem, and hippocampus. The ACh content of cerebellum increased only slightly. The Ch level in all four brain regions studied also increased two- to fourfold above control levels. Ch efflux from brain, however, decreased to 0.2 +/- 0.1 nmol/g/min during the 60 min after OP exposure. Unidirectional influx of [3H]Ch was 0.49 +/- 0.07 nmol/g/min before and did not change significantly 10 or 40 min after OP exposure, thus indicating that the Ch transporter of the brain endothelial cell is not directly inhibited.2+ Based on these results, it is proposed that (a) efflux of brain Ch occurs from the extracellular compartment, which becomes depleted when ACh breakdown is inhibited;(ABSTRACT TRUNCATED AT 250 WORDS)

Choline biosynthesis in sheep. Evidence for extrahepatic synthesis

1. Choline production by various tissues of the sheep was measured by determining venous and arterial free choline concentrations in blood samples taken from various vessels in conscious multicannulated sheep. 2. Significant production of free choline occurred in the upper and lower body regions, and specifically in the heart, brain and hind-limb muscles of sheep, but there was no significant uptake or output of phosphatidylcholine across these tissues, as determined by arterio-venous differences. 3. In contrast, in the rat there were no significant arterio-venous differences in the concentrations of free choline or phosphatidylcholine across the hind-body. 4. Synthesis of phosphatidylcholine from endogenous phosphatidylethanolamine and S-adenosyl-L-[methyl-14C]methionine was measured in experiments in vitro using microsomal preparations from a variety of sheep and rat tissues. 5. The biosynthetic activity was highest in liver from sheep and rats, although the activity in sheep microsomal preparations was about one-quarter of that in rat microsomal preparations. 6. Microsomal preparations from sheep lung, kidney, gut epithelium, brain, heart and skeletal muscles also showed considerable biosynthetic activity, but in the rat the activity was virtually confined to the liver. 7. Overall, the results show a significant production of choline in extrahepatic tissues of the sheep, with skeletal muscle contributing some 60% of this extrahepatic activity. Thus the extrahepatic production of choline in the sheep, together with the extensive reutilization of bile choline, can explain the maintenance of the large endogenous body pool of choline in this species.

Synaptosomal phospholipase D potential role in providing choline for acetylcholine synthesis

The phospholipase D of the rat brain synaptic membrane possesses the highest activity of this enzyme of any mammalian tissue examined. The synaptic phospholipase D activity is latent and barely detectable in the absence of 4 mM sodium oleate. Several other fatty acids were either less effective or ineffective as stimulators of activity compared to this monounsaturated fatty acid. The activity was decreased by hemicholinium-3, an inhibitor of choline uptake and slightly activated by neostigmine, an acetylcholinesterase inhibitor. Incubation of synaptosomes in the presence of sodium oleate and acetyl-coenzyme A resulted in the formation of a product chromatographing with acetylcholine. Acetylcholine formation was nearly undetectable in the absence of sodium oleate or acetyl-coenzyme A. These results implicate synaptosomal phospholipase D in releasing choline from phosphatidylcholine for acetylcholine formation.

Formation of unesterified choline by rat brain

Two preparations of rat brain (ischemic intact brain and homogenized whole brain) formed large amounts of unesterified (free) choline when incubated at 37 degrees C. The accumulation of choline was inhibited by microwave irradiation of brain, or by heating of brain to 50 degrees C, and was maximal at 37 degrees C at pH 7.4-8.5. Choline formation was only observed in subcellular fractions of brain that contained membranes. In homogenates of brain, choline accumulated at a rate exceeding 10 nmol/mg protein per h. There was a significant decrease in brain phosphatidylcholine concentration (of 50 nmol/mg protein) during incubation for 1 h at 37 degrees C. Concentrations of phosphocholine rose (by 2.3 nmol/mg protein), and concentrations of glycerophosphocholine and sphingomyelin did not change during this period. We used radiolabeled phospholipids to trace the fate of phosphatidylcholine and sphingomyelin during incubations of homogenates of brain. Phosphatidylcholine was degraded to form phosphocholine, glycerophosphocholine and free choline. No lysophosphatidylcholine accumulated. Sphingomyelin was degraded to form phosphocholine and a small amount of free choline. Magnesium ions stimulated choline production, while zinc ions were a potent inhibitor. Other divalent cations (calcium, manganese) had little effect on choline accumulation. ATP concentrations in brain homogenates were less than 5 nmol/mg protein (rapidly microwaved brain contained 27 nmol/mg protein). Addition of ATP or ADP to brain homogenates increased ATP concentrations and significantly inhibited choline accumulation. ATP diminished the formation of choline from added phosphatidylcholine, lysophosphatidylcholine, phosphocholine and glycerophosphocholine. The effects of ATP, zinc ion, or magnesium ion upon choline accumulation were not mediated by changes in the rates of utilization of choline for formation of phosphocholine or phosphatidylcholine. In summary, we showed that there was enhanced formation of choline when ATP concentrations within brain were low. This choline was derived, in part, from the degradation of phosphatidylcholine, and we suggest that phospholipase A activity was the primary initiator of choline release from this phospholipid.

Synaptosomal phospholipase D: potential role in providing choline for acetylcholine synthesis

The synaptic membrane (SM) prepared from rat brain synaptosomes displayed the highest activity of phospholipase D of mammalian tissue thus far observed. In common with the previously described latency of the microsomal enzyme, the SM phospholipase D (PLD) activity is barely detectable in the absence of 4 mM sodium oleate. Incubation of synaptosomes in the presence of choline labelled lecithin, sodium oleate and acetyl CoA resulted in the formation of a product co-chromatographing with acetylcholine. The potential role of SM PLD liberating choline from lecithin into the synaptic cleft is discussed in relationship to acetylcholine formation.

The fate of choline in the circulating plasma of the rat

Labelled free choline injected into the peritoneum failed to enter the brain but preferentially entered the liver. Subsequently labelled phospholipid was found in the plasma with a concurrent increase in the brain. This labelled plasma injected by cardiac puncture caused a rapid incorporation of the choline labelled phospholipid into the brain.

The release of free ethanolamine in rat brain homogenates incubated in Krebs ringer

Ethanolamine in mammalian brain is found chiefly in a lipid-bound form either as the diacyl phospholipid, phosphatidylethanolamine or as the plasmalogen, I-alk-11-enyl-2-acyl glycerophosphoethanolamine and to a lesser extent as the saturated ether analogue. The level of free ethanolamine in brain is very low, probably less than 40 nmol/g brain (Spanner & Ansell, 1977b) while that of phosphoethanolamine is about 1.0 mumol/g brain. Some time ago we found that if brain tissue was incubated in Krebs Ringer bicarbonate (pH 7.4) at 37 degrees, there was a steady release of free ethanolamine by the tissue. The following account is a summary of the findings from experiments designed to determine the source of the ethanolamine liberated.