Changes in Regional Blood–Brain Transfer of l-Leucine Elicited by Arginine-Vasopressin

Arginine-vasopressin (AVP), injected into the carotid artery in physiological concentration together with L-leucine, changed kinetic constants of the blood-brain barrier (BBB) transport of this neutral amino acid without changing the cerebral blood flow (CBF). The maximum velocity of transport (Vmax), the half-saturation constant (Km), the nonsaturable transport constant (KD), and CBF were estimated in nine brain regions of male Wistar rats anesthetized with ether. In cerebral hemisphere, Vmax decreased from 21 nmol . min-1 . g-1 (control) to 7.6 nmol . min-1 . g-1 (AVP). Km decreased from 0.11 to 0.029 mM. Regional differences of the kinetic constants were found in controls as well as in AVP-treated animals. In all regions, the calculated constants Vmax and Km of animals coinjected with AVP were significantly decreased when compared to controls. A direct or indirect interaction of AVP with the transport system of large neutral amino acids is suggested.     

Effect of angiotensin II on liposome uptake by the rat brain in vivo

Studies have been performed to assess the possibility of using small unilamellar liposomes as therapeutic carriers to the brain of hypertensive rats. Rats were made temporal hypertensive by the infusion of angiotensin II (AII; 15 micrograms in 1 ml) through their right common carotid artery. Another control group was infused with physiological saline. Free 125iodine-BSA (125I-BSA) and 125I-BSA encapsulated liposomes (average diameter approximately equal to 100 nm) were injected in the tail vein 2 min after the infusion of AII or saline. Plasma radioactivity was monitored at different times up to 15 min when the cerebral uptake of 125I-BSA was determined. While a little variation in plasma clearance pattern of liposomes in hypertensive and control group was noticed, the uptake by cerebral tissues was markedly higher in hypertensive group. Analysis of pharmacokinetic parameters in relation to cerebral uptake indicated AII induced a short term opening of the blood-brain barrier (BBB) resulting in an increased cerebral uptake. Positively charged liposomes was found to be most effective in hypertensive state.     

N omega-nitro-L-arginine influences cerebral metabolism in awake sheep.

Cerebral vasodilation in hypoxia may involve endothelium-derived relaxing factor-nitric oxide (NO). An inhibitor of NO formation, N omega-nitro-L-arginine (LNA, 100 micrograms/kg i.v.), was given to conscious sheep (n = 6) during normoxia and again in hypocapnic hypoxia (arterial PO2 approximately 38 Torr). Blood samples were obtained from the aorta and sagittal sinus, and cerebral blood flow (CBF) was measured with 15-microns radiolabeled microspheres. During normoxia, LNA elevated (P < 0.05) mean arterial pressure from 82 +/- 3 to 88 +/- 2 (SE) mmHg and cerebral perfusion pressure (CPP) from 72 +/- 3 to 79 +/- 3 mmHg, CBF was unchanged, and cerebral lactate release (CLR) rose temporarily from 0.0 +/- 1.9 to 13.3 +/- 8.7 mumol.min-1 x 100 g-1 (P < 0.05). The glucose-O2 index declined (P < 0.05) from 1.67 +/- 0.16 to 1.03 +/- 0.4 mumol.min-1 x 100 g-1. Hypoxia increased CBF from 59.9 +/- 5.4 to 122.5 +/- 17.5 ml.min-1 x 100 g-1 and the glucose-O2 index from 1.75 +/- 0.43 to 2.49 +/- 0.52 mumol.min-1 x 100 g-1 and decreased brain CO2 output, brain respiratory quotient, and CPP (all P < 0.05), while cerebral O2 uptake, CLR, and CPP were unchanged. LNA given during hypoxia decreased CBF to 77.7 +/- 11.8 ml.min-1 x 100 g-1 and cerebral O2 uptake from 154 +/- 22 to 105.2 +/- 12.4 mumol.min-1 x 100 g-1 and further elevated mean arterial pressure to 98 +/- 2 mmHg (all P < 0.05), CLR was unchanged, and, surprisingly, brain CO2 output and respiratory quotient were reduced dramatically to negative values (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Methylene blue inhibits hypoxic cerebral vasodilation in awake sheep.

Cerebral vasodilation in hypoxia may involve endothelium-derived relaxing factor-nitric oxide. Methylene blue (MB), an in vitro inhibitor of soluble guanylate cyclase, was injected intravenously into six adult ewes instrumented chronically with left ventricular, aortic, and sagittal sinus catheters. In normoxia, MB (0.5 mg/kg) did not alter cerebral blood flow (CBF, measured with 15-microns radiolabeled microspheres), cerebral O2 uptake, mean arterial pressure (MAP), heart rate, cerebral lactate release, or cerebral O2 extraction fraction (OEF). After 1 h of normobaric poikilocapnic hypoxia (arterial PO2 40 Torr, arterial O2 saturation 50%), CBF increased from 51 +/- 5.8 to 142 +/- 18.8 ml.min-1 x 100 g-1, cerebral O2 uptake from 3.5 +/- 0.25 to 4.7 +/- 0.41 ml.min-1 x 100 g-1, cerebral lactate release from 2 +/- 10 to 100 +/- 50 mumol.min- x 100 g-1, and heart rate from 107 +/- 5 to 155 +/- 9 beats/min (P < 0.01). MAP and OEF were unchanged from 91 +/- 3 mmHg and 48 +/- 4%, respectively. In hypoxia, 30 min after MB (0.5 mg/kg), CBF declined to 79.3 +/- 11.7 ml.min-1 x 100 g-1 (P < 0.01), brain O2 uptake (4.3 +/- 0.9 ml.min-1 x 100 g-1) and heart rate (133 +/- 9 beats/min) remained elevated, cerebral lactate release became negative (-155 +/- 60 mumol.min-1 x 100 g-1, P < 0.01), OEF increased to 57 +/- 3% (P < 0.01), and MAP (93 +/- 5 mmHg) was unchanged. The sheep became behaviorally depressed, probably because of global cerebral ischemia. These results may be related to interference with a guanylate cyclase-dependent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blood-brain barrier to hydrogen ion during acute metabolic acidosis in piglets

The present study investigates the integrity of the blood-brain barrier to H+ or HCO3- during acute plasma acidosis in 35 newborn piglets anesthetized with pentobarbital sodium. Cerebrospinal fluid acid-base balance, cerebral blood flow (CBF), and cerebral oxygenation were measured after infusion of HCl (0.6 N, 0.191-0.388 ml/min) for a period of 1 h at a constant arterial PCO2 of 35-40 Torr. HCl infusion resulted in decreased arterial pH from 7.38 +/- 0.01 to 7.00 +/- 0.02 (P less than 0.01). CBF measured by the tracer microsphere technique was decreased by 12% from 69 +/- 6 to 61 +/- 4 ml.min-1.100 g-1 (P less than 0.05). Infusion of 0.6 N NaCl as a hypertonic control had no effect on CBF. Cerebral metabolic rate for O2 and O2 extraction was not significantly changed from control (3.83 +/- 0.20 ml.min-1.100 g-1 and 5.7 +/- 0.6 ml/100 ml, respectively) during acid infusion. Cerebral venous PO2 was increased from 41.6 +/- 2.1 to 53.8 +/- 4.0 Torr by HCl infusion (P less than 0.02) associated with a shift in O2-hemoglobin affinity of blood in vivo from 38 +/- 2 to 50 +/- 1 Torr. Cisternal cerebrospinal fluid pH decreased from 7.336 +/- 0.014 to 7.226 +/- 0.027 (P less than 0.005), but cerebrospinal fluid HCO3- concentration was not changed from control (25.4 +/- 1.0 meq/l). These data suggest that there is a functional blood-brain barrier in newborn piglets, that is relatively impermeable to HCO3- or H+ and maintains cerebral perivascular pH constant in the face of acute severe arterial acidosis. (ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Vasopressin on Blood-Brain Transfer of Methionine in Dogs

We used a simplified probe detection system for positron-emitting radionuclides in order to measure blood-brain barrier transport of amino acids in anesthetized dogs. Plasma and brain time-activity curves were recorded after intravenous bolus injection of L-[11C]methionine before and after administration of 1 microgram of vasopressin. Three-compartment models with three or four transfer coefficients were used to derive the kinetics of L-[11C]methionine uptake in brain. The blood-brain clearance of the tracer (K1) was 0.075 ml ml-1 min-1 before and 0.041 ml ml-1 min-1 after injection of vasopressin. The partition volume and the initial distribution (plasma) volume of methionine were unchanged and within the expected limits. The net accumulation rate of methionine (K), estimated by both the four-parameter (kinetic) and three-parameter (graphic) approaches, decreased after vasopressin injection in all six studies.     

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.     

Serotonin Synthesis Rate Measured in Living Dog Brain by Positron Emission Tomography

In vivo measurements by positron emission tomography of the brain serotonin synthesis rates in the normal dog, in the dog with increased plasma tryptophan concentration, and in the dog under different arterial oxygen tensions are described. The method described here permits repeated measurements in the same brain for the first time. An increase in the plasma tryptophan concentration from 16.6 to 191.5 and then to 381 microM resulted in close to a linear increase in the brain serotonin synthesis rate. When PaO2 was raised from 76 +/- 2 to 106 +/- 1 mm Hg, the rate of serotonin synthesis in the dog brain increased from 39 +/- 8 to 54 +/- 10 pmol g-1 min-1. The estimates of the Michaelis-Menten constants, Kappm and Vmax, for the transport of tryptophan through the blood-brain barrier are 303 +/- 54 microM and 63 +/- 10 nmol g-1 min-1, respectively.     

Blood–Brain Barrier Transport of Kynurenines: Implications for Brain Synthesis and Metabolism

To evaluate the potential contribution of circulating kynurenines to brain kynurenine pools, the rates of cerebral uptake and mechanisms of blood-brain barrier transport were determined for several kynurenine metabolites of tryptophan, including L-kynurenine (L-KYN), 3-hydroxykynurenine (3-HKYN), 3-hydroxyanthranilic acid (3-HANA), anthranilic acid (ANA), kynurenic acid (KYNA), and quinolinic acid (QUIN), in pentobarbital-anesthetized rats using an in situ brain perfusion technique. L-KYN was found to be taken up into brain at a significant rate [permeability-surface area product (PA) = 2-3 x 10(-3) ml/s/g] by the large neutral amino acid carrier (L-system) of the blood-brain barrier. Best-fit estimates of the Vmax and Km of saturable L-KYN transfer equalled 4.5 x 10(-4) mumol/s/g and 0.16 mumol/ml, respectively. The same carrier may also mediate the brain uptake of 3-HKYN as D,L-3-HKYN competitively inhibited the brain transfer of the large neutral amino acid L-leucine. For the other metabolites, uptake appeared mediated by passive diffusion. This occurred at a significant rate for ANA (PA, 0.7-1.6 x 10(-3) ml/s/g), and at far lower rates (PA, 2-7 x 10(-5) ml/s/g) for 3-HANA, KYNA, and QUIN. Transfer for KYNA, 3-HANA, and ANA also appeared to be limited by plasma protein binding. The results demonstrate the saturable transfer of L-KYN across the blood-brain barrier and suggest that circulating L-KYN, 3-HKYN, and ANA may each contribute significantly to respective cerebral pools. In contrast, QUIN, KYNA, and 3-HANA cross the blood-brain barrier poorly, and therefore are not expected to contribute significantly to brain pools under normal conditions.     

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)     

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.     

Slow Penetration of Thyrotropin-Releasing Hormone Across the Blood-Brain Barrier of an In Situ Perfused Guinea Pig Brain

Transport of 3H-labelled thyrotropin-releasing hormone (TRH) across the blood-brain barrier was studied in the ipsilateral perfused in situ guinea pig forebrain. The unidirectional transfer constant (Kin) calculated from the multiple time brain uptake analysis ranged from 1.14 X 10(-3) to 1.22 X 10(-3) ml min-1 g-1, in the parietal cortex, caudate nucleus, and hippocampus. Regional Kin values for [3H]TRH were significantly reduced by 43-48% in the presence of an aminopeptidase and amidase inhibitor, 2 mM bacitracin, suggesting an enzymatic degradation of tripeptide during interaction with the blood-brain barrier. In the presence of unlabelled 1 mM TRH and 2 mM bacitracin together, a reduction of [3H]TRH regional Kin values similar to that obtained with 2 mM bacitracin alone was obtained . L-Prolinamide, the N-terminal residue of tripeptide, at a 10 mM level had no effect on the kinetics of entry of [3H]TRH into the brain. The data indicate an absence of a specific saturable transport mechanism for TRH presented to the luminal side of the blood-brain barrier. It is concluded that intact TRH molecule may slowly penetrate the blood-brain barrier, the rate of transfer being some three times higher than that of D-mannitol.     

Regional blood flows and cardiac hemodynamics in renovascular and mineralocorticoid hypertensive rats

Regional blood flows and cardiac hemodynamics were studied in 3 models of hypertensive rats: one-kidney DOC-saline, one-kidney, one-clip and two-kidney, one-clip hypertension and in normotensive control rats. All hypertensive models were characterized by increased peripheral vascular resistance and normal cardiac output. Coronary and cerebral blood flows varied among the hypertensive models but did not significantly differ from the normotensive rats. However, coronary blood flow of one-kidney, one-clip rats (8.4 +/- 1.3 ml X min-1 X g-1) was significantly higher than that of the two-kidney one-clip rats (6.5 +/- 1.2 ml X min.-1 X g-1, P less than 0.05). Cerebral blood flow of DOC-saline rats was lower than that of two-kidney one-clip or one-kidney one-clip renovascular rats. Renal blood flows of the unclipped kidney of two-kidney renovascular rats (3.77 +/- 0.85 ml X min-1 X g-1) and DOC-saline rats (2.95 +/- 0.83 ml X min-1 X g-1) were significantly lower than those of normotensive rats (5.92 +/- 1.16 ml X min-1 X g-1, P less than 0.05). In conclusion, although vascular resistance becomes elevated in all models of experimental hypertension, regional vascular resistance and blood flow distribution may differ depending on the vasoconstrictor mechanisms that participate in each model.     

Renal function studies in an experimental model of papillary necrosis in the rat

Twenty four hours after i.v. injection of bromoethylamine-hydrobromide (BEA) in rats, a uniform papillary necrosis is observed. The present study investigates the renal functional and the papillary haemodynamics in response to acute volume expansion (12% of body weight) in this model. Renal function studies were performed in hydropenic and volume expanded sham- or BEA-injected rats. In hydropenic normal animals a GFR of 1.97 +/- 0.14 ml/min, an urinary osmolarity (UOsm) of 1 011 +/- 94.5 mOsm/kg and a fractional sodium excretion (FENa) of 0.18 +/- 0.026% were obtained. In contrast, BEA-treated hydropenic animals showed a lower GFR (1.16 +/- 0.14 ml/min), UOsm (469 +/- 30.31 mOsm/kg) and a higher FENa (0.37 +/- 0.06%). In volume expansion a similar UOsm and FENa were obtained in both groups. The papillary plasma flow (PPF) was measured in each of the experimental groups by the albumin accumulation technique. The mean value in hydropenic normal animals was 50.65 +/- 2.12 m 100 g-1 min-1 and increased to 66.02 +/- 2.00 ml 100 g-1 min-1 after volume expansion (P less than 0.001). In BEA rats the PPF was 58.86 +/- 2.33 ml 100 g-1 min-1 in hydropenia (P less than 0.01 vs. control animals) and remained unchanged after volume expansion. Thus, during hydropenia, BEA-induced papillary necrosis results with a salt wasting state and an urinary concentration defect. After volume expansion no disturbance in sodium excretion capacity was observed. These results are compatible with the nephron-heterogeneity concept in the regulation of sodium excretion. The histological lesions cannot be explained by a decreased renal papillary plasma flow.     

Kinetic Analysis of Cerebrovascular Isoleucine Transport from Saline and Plasma

The concentration dependence of regional isoleucine transport across the blood-brain barrier was determined in anesthetized rats with the in situ brain perfusion technique of Takasato et al. [Am. J. Physiol. 247, H484-493 (1984)]. This technique allows, for the first time, accurate measurements of cerebrovascular amino acid transport in the absence of competing amino acids using saline perfusate, and in the presence of physiological concentrations of amino acids using plasma perfusate. Cerebrovascular isoleucine transport from saline perfusate followed Michaelis-Menten saturation kinetics where Vmax = 9 - 11 X 10(-4) mumol X s-1 X g-1 and Km = 0.054-0.068 mumol X ml-1 in six brain regions. A component of nonsaturable transport was not detected in any brain region even though perfusate isoleucine concentration was increased to greater than or equal to 150 times the normal plasma concentration. Isoleucine influx during plasma perfusion was only 8% of that predicted from the saline perfusion data due to transport inhibition by competing amino acids in plasma. Competitive inhibition increased the apparent Km for isoleucine transport from plasma by greater than or equal to 24-fold to 1.5-1.7 mumol X ml-1. These data provide accurate new estimates of the kinetic constants that describe amino acid transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer-site affinity (1/Km) for isoleucine is approximately fivefold greater than previously reported with the brain uptake index technique.     

Neuroprotection by tempol in a model of iron-induced oxidative stress in acute ischemic stroke

Studies suggest iron exacerbates the damage caused by ischemic stroke. Our aim was to elucidate the effect of iron overload on infarct size after middle cerebral artery occlusion (MCAO) and to evaluate the efficacy of tempol, a superoxide dismutase mimetic, as a neuroprotective agent. Rats were administered iron +/- tempol before MCAO; control rats received saline. The middle cerebral artery was occluded for 24 h, and the size of the resultant infarct was assessed and expressed as the percentage of the hemisphere infracted (%HI). Iron treatment increased infarct size compared with control (51.83 +/- 3.55 vs. 27.56 +/- 3.28%HI iron treated vs. control, P = 0.01); pretreatment with tempol reversed this (51.83 +/- 3.55 vs. 26.09 +/- 9.57%HI iron treated vs. iron + tempol treated, P = 0.02). We hypothesized that reactive oxygen species (ROS) were responsible for the iron-induced damage. We measured ROS generated by exogenous iron in brain and peripheral vasculature from rats that had not undergone MCAO. There was no increase in ROS production in the brain of iron-treated rats or in brain slices incubated with iron citrate. However, ROS generation in carotid arteries incubated with iron citrate was significantly increased. ROS generation from the brain was assessed after MCAO by dihydroethidine staining; there was a dramatic increase in the ROS generation by the brain in the iron-treated rats compared with control 30 min after MCAO. We propose that iron-induced ROS generation in the cerebral vasculature adds to oxidative stress during an ischemic episode after the disruption of the blood-brain barrier.     

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.     

Permeability of the Blood-Brain Barrier to the Immunosuppressive Cyclic Peptide Cyclosporin A

Uptake of the immunosuppressive lipophilic peptide cyclosporin A has been measured by a number of techniques. The brain uptake index (BUI) technique in the rat yields only a small BUI value that is not significantly different from that of sucrose and mannitol and is comparable to other published BUI values for this compound. Brain perfusion studies in the guinea pig produce a unidirectional cerebrovascular permeability constant (Kin) of 1.2 +/- 0.28 microliter g-1 min-1 for the hippocampus. Intravenous bolus injection techniques also in the guinea pig characteristically produce a larger Kin value of 2.53 +/- 0.38 microliter g-1 min-1 for the same brain region, even after a correction for the inulin space of the tissue has been made. Apparent penetration of cyclosporin A into the cerebrospinal fluid (CSF) determined with the intravenous bolus injection technique is small with a Kin of 0.79 +/- 0.07 microliter g-1 min-1. However it is suggested that the radioactivity present in CSF is largely tritiated water. Studies with cultured cerebral endothelial cells from the rat have also been carried out and show that the cultured cells take up and accumulate cyclosporin A in vitro, achieving a tissue-to-medium ratio of 20 after 25 min of incubation. It is suggested that cyclosporin A is primarily taken up from lipoprotein at the blood-brain interface but, because of tight junctions at the blood-brain and blood-CSF barriers, becomes effectively trapped in the cerebral endothelial cells and the choroid plexus.     

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.     

Heat production, blood flow, O2 uptake and CO2 output in the secretary process of the dog submandibular gland (author's transl)

1. Under normal circulation of the dog submandibular gland, the electrical stimulation induced a massive salivary secretion (about 0.35 ml . min-1.g-1 gland weight) and an increase in the glandular temperature (about 0.2 degrees C). The heat production was calculated of about 60 mW.g-1. 2. Clamping of the glandular artery made both of secretion and heat production to be transient. The early peak of secretion was about 0.12 ml.min-1.g and that of heat production was 7 approximately 10mW,g-1. Then each 1 ml secretion followed about 4.6 J heat production. 3. Under constant blood flow in the glandular circulation, the secretory process was divided clearly into 2 phases of peak and plateau. The glandular temperature increased about 0.12 degrees C with an early temperature drop. In the secretory plateau phase, the secretary rate was about 0.043 ml.min-1.g-1, the heat production was about 5 approximately 7 mW.g-1 and each 1 ml secretion caused about 8.2 J heat production. 4. The rate of oxygen uptake was about 20.9 microl.min-1g-1 at the resting state. The maximum during secretion was about 192 microliter.min-1.g-1. THe half time of the recovery process of O2 uptake tended slightly longer than that of heat production. 5. THe rate of CO2 output was about 21.8 microliter.min-1.g-1 at resting. The maximum during secretion was about 142 microliter.min-1.g-1 R. Q. were about 1 at resting and about 0.74 under secretion.