In Vivo Distribution of CGS-19755 Within Brain in a Model of Focal Cerebral Ischemia

The blood-brain barrier permeability of the competitive N-methyl-D-aspartate receptor antagonist CGS-19755 [cis-4-(phosphonomethyl)-2-piperidine carboxylic acid] was assessed in normal and ischemic rat brain. The brain uptake index of CGS-19755 relative to iodoantipyrine was assessed using the Oldendorf technique in normal brain. The average brain uptake index in brain regions supplied by the middle cerebral artery was 0.15 +/- 0.35% (mean +/- SEM). The unidirectional clearance of CGS-19755 from plasma across the blood-brain barrier was determined from measurements of the volume of distribution of CGS-19755 in brain. These studies were performed in normal rats and in rats with focal cerebral ischemia produced by combined occlusion of the proximal middle cerebral artery and ipsilateral common carotid artery. In normal rats the regional plasma clearance across the blood-brain barrier was low, averaging 0.015 ml 100 g-1 min-1. In ischemic rats this clearance value averaged 0.019 ml 100 g-1 min-1 in the ischemic hemisphere and 0.009 ml 100 g-1 min-1 in the nonischemic hemisphere. No significant regional differences in plasma clearance of CGS-19755 were observed in either normal or ischemic rats except in cortex injured by electrocautery where a 14-fold increase in clearance across the blood-brain barrier was measured. We conclude that CGS-19755 crosses the blood-brain barrier very slowly, even in acutely ischemic tissue.     

Saturable mechanism for delta sleep-inducing peptide (DSIP) at the blood-brain barrier of the vascularly perfused guinea pig brain

Cellular uptake of [125I] labelled DSIP at the luminal interface of the blood-brain barrier (BBB) was studied in the ipsilateral perfused in situ guinea pig forebrain. Regional unidirectional transfer constants (Kin) calculated from the multiple-time brain uptake analysis were 0.93, 1.33 and 1.66 microliter.min-1 g-1 for the parietal cortex, caudate nucleus and hippocampus, respectively. In the presence of 7 microM unlabelled DSIP the brain uptake of [125I]-DSIP (0.3 nM) was inhibited, the values of Kin being reduced to 0.23-0.38 microliter.min-1 g-1, values that were comparable with the Kin for mannitol. The rapidly equilibrating space of brain, measured from the intercept of the line describing brain uptake versus time on the brain uptake ordinate, Vi, was greater for [125I]-DSIP than for mannitol; in the presence of unlabelled DSIP this was reduced to that of mannitol, and it was suggested that the larger volume for [125I]-DSIP represented binding at specific sites on the brain capillary membrane. L-tryptophan, the N-terminal residue of DSIP, in concentrations of 7 microM and 1 mM, inhibited Kin without affecting Vi. A moderate inhibition of Kin was obtained by vasopressin ([Arg8]-VP), but only at a concentration as high as 0.2 mM. The results suggest the presence of a high affinity saturable mechanism for transport of DSIP across the blood-brain barrier, with subsequent uptake at brain sites that are highly sensitive to L-tryptophan, and may be modulated by [Arg8]-VP.     

Blood-Brain Transfer of l-Phenylalanine Declines After Peripheral but Not Central Nervous Administration of Vasopressin

To determine whether a previously reported effect of vasopressin on blood-brain transfer of leucine extends to other large neutral amino acids, we measured the regional blood-brain transfer of L-phenylalanine with the integral technique. Intravenous co-injection of L-phenylalanine and arginine vasopressin (30 nmol to 10 pmol) resulted in a decrease of the permeability-surface area (PaS) product of phenylalanine of between 11 and 39%. In addition, the peptide elicited a decrease of the cerebral blood flow of between 11 and 56% combined with a drastic decrease of the cardiac output (32-64%) and an elevation of the blood pressure to approximately 150% of control values. However, we found no changes of the cardiac output, the blood pressure, or the PaS product of phenylalanine after microdialysis (30 min, 5 microliters min-1) of arginine vasopressin (15 mumol L-1) into the dorsal hippocampus, but cerebral blood flow was decreased. The results support the hypothesis that arginine vasopressin receptors at the blood-brain barrier are involved in the regulation of large neutral amino acid transfer from blood to brain and indicate that these receptors are located at the luminal membrane of the endothelial cells.     

Atrial natriuretic peptide response to endurance physical training in the rat

The effect of an endurance physical training programme on the plasma and atrial natriuretic peptides (ANP) and on renal glomerular ANP receptors was evaluated in male normotensive Wistar rats. Maximal O2 uptake was significantly greater in the endurance trained (117.1 ml O2.kg-1.min-1, SEM 6.18 versus the control rats 84.2 ml O2.kg-1.min-1, SEM 4.88, P less than 0.01. In addition, various muscle oxidative enzymes were also significantly higher in endurance trained animals. An increase in resting plasma [ANP] was observed after 11 weeks of physical training (40.02 pg.ml-1, SEM 7.07 vs 22.8 pg.ml-1, SEM 3.83, P less than 0.05). Glomerular ANP receptor density was lower in trained rats (272 fmol.mg-1 protein, SEM 3.1 vs 380 fmol.mg-1 protein, SEM 6.1, P less than 0.05), whereas atrial tissue [ANP] was not significantly different between controls and trained animals. However, in trained rats, circulating [ANP] was closely correlated with left atrial [ANP] (r = -0.92, P less than 0.05). Resting systolic blood pressure had not changed at the end of this physical training programme. It is considered that under physiological conditions ANP may be involved in long-term extracellular fluid volume homeostasis through the regulation of renal glomerular ANP receptors, and that the left atrium might play a significant role in this long term fluid volume control.     

Effect of Dexamethasone on Transport of α-Aminoisobutyric Acid and Sucrose Across the Blood-Brain Barrier

The effect of glucocorticoids on the blood-brain barrier (BBB) was studied in rats following a single injection or 3 days of dexamethasone administration. Tracers with a low permeability across the intact endothelium, [14C]sucrose and alpha-[3H]aminoisobutyric acid ([3H]AIB), were simultaneously injected intravenously in untreated rats or in rats treated with dexamethasone. Unidirectional blood-to-brain transfer constants (Ki) in 14 regions of the rat brain were determined. In regions of control brain, average Ki values for AIB and sucrose were approximately 0.0020 and 0.00060 ml g-1 min-1, respectively. The lowest transfer constants were found in caudate nucleus, hippocampus, white matter, and cerebellum. In dexamethasone-treated animals, Ki values for both sucrose and AIB markedly decreased by 30-50% in almost all brain regions. These results indicate that a single injection or 3 days of treatment with dexamethasone causes an apparent reduction in the normal BBB permeability, and dexamethasone may greatly interfere with drug delivery into brain. These observations may have an importance for the administration of drugs in brain disease in the presence of steroids.     

Influence of acute maximal exercise on lecithin:cholesterol acyltransferase activity in healthy adults of differing aerobic performance

To document the possible influence of a single episode of maximal aerobic stress on the serum lecithin:cholesterol acyltransferase (LCAT) activity in subjects with differing histories of training, two groups of healthy male adults [controls (C), n = 18, 28.6 years, SD 5.2, 50.1 ml.kg-1.min-1 maximal O2 uptake (VO2max), SD 5.3; endurance trained athletes (T), n = 18, 31.4 years, SD 8.8, 65.0 ml.kg-1.min-1 VO2max, SD 2.8] were examined in a maximal aerobic stress test. In addition to the routine assessment of lipid status, LCAT activity was measured immediately before and after exercise. At rest nearly identical LCAT activity values were found in both groups: C 64.4 nmol.ml-1.h-1, SD 16.7 vs T 65.0 nmol.ml-1.h-1, SD 20.9. The post-exercise LCAT values induced by the maximal stress test increased significantly to (C) 95.7 nmol.ml-1.h-1, SD 23.5, +48.6%, P less than 0.001; (T) 83.5 nmol.ml-1.h-1, SD 24.3, +29.1%, P less than 0.01. Neither the pre nor the postexercise individual LCAT activity values showed any significant correlation to the corresponding data on physical performance.     

Effect of nerve block on response of forearm blood flow to local temperature

To determine the role of neurotransmitter in the response of forearm blood flow (ABF) to local (forearm) skin temperature (Tsk) we measured ABF of six subjects at Tsk from 25 to 40 degrees C before (control) and after brachial plexus block (BPB). Control experiments were conducted in an ambient temperature of 27-29 degrees C, adjusted to minimize the subject's overall thermal sensation. Tsk was regulated by blowing a controlled-temperature airstream through a plastic bag enclosing the arm. We first lowered Tsk to 25 degrees C and after 20 min began to measure ABF with Whitney strain gauges. We then raised Tsk by 2.5 degrees C steps to 40 degrees C and measured ABF every 30 s for at least 10 min at each level of Tsk. Mean ABF rose from 1.1 ml X 100 ml-1 X min-1 at Tsk of 25 degrees C to 2.1 ml X 100 ml-1 X min-1 at 32.5 degrees C to 13.7 ml X 100 ml-1 X min-1 at 40 degrees C in control experiments and from 2.8 to 4.4 to 14.8 ml X 100 ml-1 X min-1 after BPB. The effect of Tsk on ABF was highly significant (P less than 0.0001) but the effect of BPB was not (P approximately equal to 0.2). At thermoneutrality, the effect of Tsk on ABF is largely independent of neural activity, since this effect is unaffected by nerve block.     

Ineffectiveness of dimethyl sulfoxide in altering the permeability of the blood-brain barrier

The failure of DMSO to alter the permeability of the blood-brain barrier has been studied using several polar, nonpolar, hydrophilic, and hydrophobic compounds labeled with selected radioactive isotopes. The metabolites were Na¹³¹I, ¹³¹I-iodinated human serum albumin, l-[³⁵S]methionine, dl-[ring-2-¹⁴C]tryptophan, [U-¹⁴C]sucrose, d-[6-¹⁴C]glucose, and [4-¹⁴C]cholesterol. DMSO was injected intraperitoneally at a dose of 1 g/kg followed after 1 hr by the intracarotid injection of the labeled metabolite. An appropriate volume of saline was substituted for the DMSO in control animals. The brain and one gastrocnemius muscle were removed at selected intervals up to 30 min and the uptake into these tissues was measured.It was found that the permeability of neither the blood-brain barrier nor skeletal muscle was altered by this concentration of DMSO. This dose of DMSO, administered intravenously, frequently caused death and, intraperitoneally, caused muscular twitching, lethargy, and hematuria.     

The role of posture on the changes in plasma atrial natriuretic factor and arginine vasopressin levels during immersion

In seven healthy male volunteers we investigated changes in plasma atrial natriuretic factor [( ANF]), arginine vasopressin [( AVP]) and plasma volume (PV) during supine immersion. Twenty minutes head-out water immersion in a supine position in a thermo-neutral water bath attenuated the increase in PV induced by 20 min in a supine position in air, but increased the mean plasma [ANF] from 32.0 pg.ml-1, SEM 5.1 to 53.3 pg.ml-1, SEM 3.6 and decreased the mean plasma [AVP] from 1.4 pg.ml-1, SEM 0.1 to 0.9 pg.ml-1, SEM 0.04. Simultaneously, diuresis and natriuresis increased markedly. During a 20-min control period in the supine posture without immersion, PV, plasma [ANF] and [AVP] remained unaffected while diuresis and natriuresis did not increase to the same extent. These data suggest that an increase in the central blood volume induced by a weak external hydrostatic pressure during supine immersion triggered the changes in plasma [ANF] and [AVP] and that the increase was probably due to a shift of blood volume from peripheral to central vessels. The changes in plasma [ANF] contributed to the changes in natriuresis.     

Effects of training on lactate production and removal during progressive exercise in humans.

To determine whether the reduced blood lactate concentrations [La] during submaximal exercise in humans after endurance training result from a decreased rate of lactate appearance (Ra) or an increased rate of lactate metabolic clearance (MCR), interrelationships among blood [La], lactate Ra, and lactate MCR were investigated in eight untrained men during progressive exercise before and after a 9-wk endurance training program. Radioisotope dilution measurements of L-[U-14C]lactate revealed that the slower rise in blood [La] with increasing O2 uptake (VO2) after training was due to a reduced lactate Ra at the lower work rates [VO2 less than 2.27 l/min, less than 60% maximum VO2 (VO2max); P less than 0.01]. At power outputs closer to maximum, peak lactate Ra values before (215 +/- 28 mumol.min-1.kg-1) and after training (244 +/- 12 mumol.min-1.kg-1) became similar. In contrast, submaximal (less than 75% VO2max) and peak lactate MCR values were higher after than before training (40 +/- 3 vs. 31 +/- 4 ml.min-1.kg-1, P less than 0.05). Thus the lower blood [La] values during exercise after training in this study were caused by a diminished lactate Ra at low absolute and relative work rates and an elevated MCR at higher absolute and all relative work rates during exercise.     

Release of Endogenous and Accumulated Exogenous Amino Acids from Slices of Normal and Climbing Fibre-Deprived Rat Cerebellar Slices

Efflux of various amino acids from slices of rat cerebellar hemispheres was determined under resting or depolarizing conditions. It was increased under high K+(50 mM) as compared to low K+ (5 mM) conditions by 1258 pmol/mg protein for aspartate, 478 for gamma-aminobutyric acid (GABA), 44,693 for glutamate, and 615 for glycine. These were significantly higher than the corresponding values obtained under low-Ca2+ (0.1 mM), high-Mg2+ (12 mM) conditions, whereas for 11 other amino acids the K+-induced efflux was similar under normal and low-Ca2+ concentrations. The K+-induced efflux of exogenously accumulated L-[3H]aspartate, D-[3H]aspartate, and L-[3H]glutamate was higher by factors of 2, 5.8, and 6.3, respectively, under normal Ca2+ conditions, as compared with low-Ca2+, high-Mg2+ conditions. After climbing fibre degeneration induced by destruction of the inferior olive with 3-acetylpyridine, release of endogenous aspartate and exogenous L-[3H]glutamate and D-[3H]aspartate was significantly reduced, by 26%, 38%, and 27%, respectively. These results support the hypothesis that climbing fibres may use aspartate or a related compound as a neurotransmitter. In rat cerebellar tissue, L-[3H]glutamate and L-[3H]aspartate differ in several aspects: (1) L-[3H]glutamate uptake was 4 times higher than that of L-[3H]aspartate; (2) fractional rate constant of K+- evoked release of L-[3H]aspartate was 7% X 2.5 min-1, and of L-[3H]glutamate 36% X 2.5 min-1; and (3) specific activity of L-[3H]glutamate in the eluate collected during K+ stimulation was 3.5 times the value in the tissue, whereas for L-[3H]aspartate, specific activities in the eluate and tissue were similar.     

A comparative PET study using different 11C-labelled amino acids in Walker 256 carcinosarcoma-bearing rats

In Walker 256 carcinosarcoma-bearing rats, the dynamic distribution of l-[1-^11C]tyrosine, l-[methyl-¹¹C]methionine, l-[1-¹¹C]methionine and d-[1-¹¹C]methionine has been measured by PET. An equivalent tumor-imaging potential was observed for each of the three l-amino acids. Thirty minutes after injection, the tumors accumulated 57% (P < 0.01) more ¹¹C-activity from l-[1-¹¹C]methionine than from l-[methyl-¹¹C]methionine. At the same point of time, the livers showed a 33% (P < 0.001) higher ¹¹C-uptake with l-[methyl-¹¹C]methionine than with l-[1-¹¹C]methionine. The dynamic tissue data are in agreement with the findings in experiments with ¹⁴C-analogs.     

Indications for a brain uptake of labelled vasopressin and ocytocin and the problem of the blood-brain barrier.

Fifteen seconds after intracarotid injection of [125J]-lysine vasopressin, [3H]-ocytocin, tritiated water or [3H]-inulin, the distribution of radioactivity in 18 regions of the rat brain and in the anterior pituitary was determined. Comparing the concentration of the different tracers (in % of injected radioactivity per g brain tissue), it is assumed that a small amount of the labelled neurohormones is taken up by the brain, indicating a penetration of the blood-brain barrier and/or an accumulation within the structures of the barrier system.     

Metabolism of methionine and biosynthesis of caffeine in the tea plant (Camellia sinensis L.).

1. Caffeine biosynthesis was studied by following the incorporation of 14C into the products of L-[Me-14C]methionine metabolism in tea shoot tips. 2. After administration of a 'pulse' of L-[Me-14C]methionine, almost all of the L-[Me-14C]methionine supplied disappeared within 1 h, and 14C-labelled caffeine synthesis increased throughout the experimental periods, whereas the radioactivities of an unknown compound and theobromine were highest at 3 h after the uptake of L-[Me-14C]methionine, followed by a steady decrease. There was also slight incorporation of the label into 7-methylxanthine, serine, glutamate and aspartate, disappearing by 36 h after the absorption of L-[Me-14C]methionine. 3. The radioactivities of nucleic acids derived from L-[Me-14C]methionine increased rapidly during the first 12 h incubation period and then decreased steadily. Sedimentation analysis of nucleic acids by sucrose-gradient centrifugation showed that methylation of nucleic acids in tea shoot tips occurred mainly in the tRNA fraction. The main product among the methylated bases in tea shoot tips was identified as 1-methyladenine. 4. The results indicated that the purine ring in caffeine is derived from the purine nucleotides in the nucleotide pool rather than in nucleic acids. A metabolic scheme to show the production of caffeine and related methylxanthines from the nucleotides in tea plants is discussed.     

Sex difference in maximal oxygen uptake. Effect of equating haemoglobin concentration

Ten men and 11 women were studied to determine the effect of experimentally equating haemoglobin concentration ([Hb]) on the sex difference in maximal oxygen uptake (VO2max). VO2max was measured on a cycle ergometer using a continuous, load-incremented protocol. The men were studied under two conditions: 1) with normal [Hb] (153 g X L-1) and 2) two days following withdrawal of blood, which reduced their mean [Hb] to exactly equal the mean of the women (134 g X L-1). Prior to blood withdrawal, VO2max expressed in L X min-1 and relative to body weight and ride time on the cycle ergometer test were greater (p less than .01) in men by 1.11 L X min-1 (47%), 4.8 ml X kg-1 min-1 (11.5%) and 5.9 min (67%), respectively, whereas VO2max expressed relative to fat-free weight (FFW) was not significantly different. Equalizing [Hb] reduced (p less than .01) the mean VO2max of the men by 0.26 L X min-1 (7.5%), 3.2 ml X kg-1 min-1 (6.9%) or 4.1 ml X kg FFW-1 min-1 (7.7%), and ride time by 0.7 min (4.8%). Equalizing [Hb] reduced the sex difference for VO2max less than predicted from proportional changes in the oxygen content of the arterial blood and arteriovenous oxygen content difference during maximal exercise. It was concluded that the sex difference in [Hb] accounts for a significant, but relatively small portion of the sex difference in VO2max (L X min-1). Other factors such as the dimensions of the oxygen transport system and musculature are of greater importance.     

Functional relevance of carnitine transporter OCTN2 to brain distribution of l-carnitine and acetyl-l-carnitine across the blood–brain barrier

Transport of L-[3H]carnitine and acetyl-L-[3H]carnitine at the blood-brain barrier (BBB) was examined by using in vivo and in vitro models. In vivo brain uptake of acetyl-L-[3H]carnitine, determined by a rat brain perfusion technique, was decreased in the presence of unlabeled acetyl-L-carnitine and in the absence of sodium ions. Similar transport properties for L-[3H]carnitine and/or acetyl-L-[3H]carnitine were observed in primary cultured brain capillary endothelial cells (BCECs) of rat, mouse, human, porcine and bovine, and immortalized rat BCECs, RBEC1. Uptakes of L-[3H]carnitine and acetyl-L-[3H]carnitine by RBEC1 were sodium ion-dependent, saturable with K(m) values of 33.1 +/- 11.4 microM and 31.3 +/- 11.6 microM, respectively, and inhibited by carnitine analogs. These transport properties are consistent with those of carnitine transport by OCTN2. OCTN2 was confirmed to be expressed in rat and human BCECs by an RT-PCR method. Furthermore, the uptake of acetyl-L-[3H]carnitine by the BCECs of juvenile visceral steatosis (jvs) mouse, in which OCTN2 is functionally defective owing to a genetical missense mutation of one amino acid residue, was reduced. The brain distributions of L-[3H]carnitine and acetyl-L-[3H]carnitine in jvs mice were slightly lower than those of wild-type mice at 4 h after intravenous administration. These results suggest that OCTN2 is involved in transport of L-carnitine and acetyl-L-carnitine from the circulating blood to the brain across the BBB.     

Production of [14C]fumonisin B1 by Fusarium moniliforme MRC 826 in corn cultures.

Kinetics of growth and fumonisin production by Fusarium moniliforme MRC 826 in corn "patty" cultures were investigated, and a technique was developed for the production of [14C]fumonisin B1 ([14C]FB1) by using L-[methyl-14C]methionine as the precursor. A significant (P < 0.01) correlation exists between fungal growth and FB1 (r = 0.89) and FB2 (r = 0.87) production in corn patties, beginning after 2 days and reaching the stationary phase after 14 days of incubation. [14C]FB1 was produced by adding L-[methyl-14C]methionine daily to cultures during the logarithmic phase of production. Incorporation of the isotope occurred at C-21 and C-22 of the fumonism molecule and was enhanced in the presence of unlabeled L-methionine. Although the concentration of exogenous unlabeled methionine is critical for incorporation of the 14C label, optimum incorporation was achieved by adding 50 mg of unlabeled L-methionine and 200 mu Ci of L-[methyl-14C]methionine to a corn patty (30 g) over a period of 9 days, yielding [14C]FB1 with a specific activity of 36 mu Ci/mmol.     

Measurement of Solute Transport Across the Blood–Brain Barrier in the Perfused Guinea Pig Brain: Method and Application to N-Methyl-α-Aminoisobutyric Acid

A technique for the vascular perfusion of the guinea pig head in vivo, suitable for measurements of blood-to-brain transport under controlled conditions of arterial inflow, has been developed. With a perfusion pressure ranging between 13 and 18 kPa and PCO2 in the arterial inflow of 5 and 5.5 kPa, cerebral blood flow, measured with [14C]butanol, was about 1 ml min-1 g-1 in the cerebral cortex, hippocampus, and caudate-putamen of the ipsilateral hemisphere; in the cerebellum and pontine white matter it was considerably less, and much higher perfusion pressures were required to establish equal blood flow throughout the whole brain. Regional water content, Na+/K+ ratio, ATP, energy charge potential, and lactate content of the ipsilateral side of perfused and nonperfused brain were not significantly different after 10 min perfusion. The D-[3H]mannitol space did not exceed 1% after 30 min of perfusion, indicating the integrity of the barrier. Over this period, EEG, ECG, and respiratory waveform remained normal. When [14C]N-methyl-alpha-aminoisobutyric acid (MeAIB), and D-[3H]mannitol were perfused together over periods extending to 30 min progressive uptakes of both solutes by the parietal cortex could be measured, and the unidirectional transfer constants estimated from multiple time-uptake data. The Kin for MeAIB (0.75 X 10(-3) ml min-1 g-1) was some three times that for mannitol. It is concluded that the technique provides a stable, well-controlled environment in the cerebral microvasculature of the ipsilateral perfused brain hemisphere suitable for examining the transport of slowly penetrating solutes into the brain.     

Blood–Brain Barrier Transport of Valproic Acid

Valproic acid distribution in brain is less than that of other anticonvulsants such as phenytoin or phenobarbital. Possible mechanisms for this decreased distribution space in brain include (a) increased plasma protein binding of valproate relative to the other anticonvulsants and (b) asymmetric blood-brain barrier (BBB) transport of valproate such that the brain-to-blood flux exceeds the blood-to-brain flux. These mechanisms are investigated in the present studies using the intracarotid injection technique in rats and rabbits. In the rat, the brain uptake index (BUI) of [14C]valproate relative to [3H]water is 51 +/- 6%, indicating the blood-to-brain transport of water is twofold greater than that of valproate. However, the BUI of [14C]valproate relative to [3H]water decreased with time after carotid injection during a 4-min washout period, which indicates that brain-to-blood transport of valproate is greater than that of water. This suggests that the permeability of the BBB to valproate is polarized, with antiluminal permeability being much greater than luminal permeability. In rabbits, the BUI of [14C]valproate is 47 +/- 7% in newborns and 17 +/- 6% in adult animals. However, the high drug extraction in newborns may be attributed to decreased cerebral blood flow in the neonate as the BBB permeability-surface area (PS) products are unchanged (e.g., PS = 0.13 and 0.11 ml min-1 X g-1 in the newborn and adult rabbit, respectively). With regard to plasma protein binding effects on valproate transport, brain valproate uptake was also measured in the presence of human, lamb, pig, rat, horse, goat, hamster, dog, and mouse sera. Higher brain uptakes were observed when the unbound fraction of drug increased. However, our data indicate that a fraction of the valproic acid entering the capillaries bound to plasma proteins had the capacity to equilibrate with brain because of enhanced drug dissociation from albumin in the brain microcirculation. Since plasma protein-bound valproate is available for uptake by brain, the major factor underlying the diminished distribution of the drug in brain appears to be the asymmetric transport properties of the BBB to valproic acid.     

Transport of Leucine-Enkephalin Across the Blood-Brain Barrier in the Perfused Guinea Pig Brain

Transport of [tyrosyl-3,5-3H]enkephalin-(5-L-leucine) [( 3H]Leu-enkephalin) across the blood-brain barrier was studied in the adult guinea pig, by means of vascular perfusion of the head in vivo. The unidirectional transfer constant (Kin) estimated from the multiple-time uptake data for [3H]Leu-enkephalin ranged from 3.62 X 10(-3) to 3.63 X 10(-3) ml min-1 g-1 in the parietal cortex, caudate nucleus, and hippocampus. Transport of [3H]Leu-enkephalin was not inhibited by unlabelled L-tyrosine (the N-terminal amino acid) at a concentration as high as 5 mM, or by the inhibitor of aminopeptidase activity bacitracin (2 mM), suggesting that there was no enzymatic degradation of peptide at the blood-brain barrier. By contrast, 2 mM unlabelled Leu-enkephalin strongly inhibited the unidirectional blood-to-brain transport of [3H]Leu-enkephalin by 74-78% in the parietal cortex, caudate nucleus, and hippocampus. The tetrapeptide tyrosyl-glycyl-glycyl-phenylalanine (without the C-terminal leucine of Leu-enkephalin), at a concentration of 5 mM, caused a moderate inhibition ranging from 15 to 29% in the brain regions studied, whereas the tetrapeptide glycyl-glycyl-phenylalanyl-leucine (without the N-terminal tyrosine) at 5 mM was without effect on Leu-enkephalin transport. Unidirectional brain uptake of Leu-enkephalin was not altered in the presence of naloxone at a concentration as high as 3 mM (1 mg/ml), suggesting that there is no binding of Leu-enkephalin to opioid receptors at the blood-brain barrier. It is concluded that there is a specific transport mechanism for Leu-enkephalin at the blood-brain barrier in the guinea pig.(ABSTRACT TRUNCATED AT 250 WORDS)