Marianne M. Hertz

Filtration and diffusion of water across the blood-brain barrier in man

Abstract Water transport across the blood-brain barrier was studied in 15 patients during hypertonic intracarotid injection. The filtration permeability coefficient (Pf) measured in 10 patients was 10.4 × 10−4 cm sec−1. The diffusion permeability coefficient (Pd) for tracer water measured in 12 patients was 2.4 × 10−4 cm sec−1, but this value is probably too small due to back diffusion. The P f P d ratio was 4.3 and might therefore be even smaller. The results indicate that pores or slits are not present in the blood-brain barrier and probably all water transport occurs by dissolving in the membrane phase.

Insulin increases glucose transfer across the blood-brain barrier in man.

The influence of insulin on unidirectional flux of glucose across the blood-brain barrier and on net uptake of glucose by the brain was investigated in seven fasting patients. The unidirectional extraction, E, of [14C]D-glucose was determined using 36Cl- as an intravascular reference, by the indicator dilution method. 0.4 U insulin/kg body wt was infused intravenously over 30 min while blood glucose was maintained constant by glucose infusion. Six determinations were made in each patient, two before, two during insulin infusion, and two after. In connection with each blood-brain barrier study, arterial and cerebral venous samples were taken for measurement of glucose, oxygen, insulin, K+, and phosphate. Cerebral blood flow (CBF) was measured in each patient. The main finding was an increased extraction of glucose from 14 to 21% and a highly significant increase in unidirectional flux (CBF X unidirectional extraction X arterial glucose concentration) from 0.46 to 0.66 mumol/g X min during insulin infusion (plasma insulin approximately 1,500 microU/ml). The net brain uptake of glucose (CBF X arterio-venous difference for glucose) as unaltered during the investigation period of 45 min, which is too short a time for insulin to penetrate the barrier. It follows that the backflux of glucose from the brain was increased during insulin application. The effect of insulin might be a speeding up of the glucose carrier in analogy to heart muscle.

Blood—brain barrier permeability during electroshock seizures in the rat

Abstract. The effect of electrically induced seizures on the permeability of the rat blood‐brain barrier was investigated. The small radioactive tracers sodium (24N+), chloride (36Cl‐), carbon labelled thiourea (14C‐thiourea) and glucose (14C‐D‐glucose) were studied in indicator dilution experiments with indium labelled diethylene‐triaminepenta‐acetic acid (113mIn‐DTPA) as reference substance. This method allows a quantitative estimate of the transcapillary loss of solutes, the extraction (E), during a single passage through the brain. Passage of macromolecules was studied using as marker substance Evans Blue which binds to plasma albumin.

Kinetic Analysis of the Human Blood-Brain Barrier Transport of Lactate and Its Influence by Hypercapnia

Blood–brain barrier permeability to L-lactate was studied in 18 patients with the double indicator technique. Venous outflow curves were obtained during normo- and hypercapnia and were analyzed by means of a model that takes tracer backflux and capillary heterogeneity of transit times into account. The average unidirectional extraction of L-lactate was 15%; the transport from the blood to the brain (PS1) was 0.081 ml g−1 min−1 and the transport from the brain to the blood (PS2) was on the same order of magnitude. In hypercapnia, arterial pH decreased from 7.39 to 7.26 and PS1 to L-lactate increased significantly by 110%. PS2 also increased although a statistically significant difference compared to the resting state was not reached. It is concluded that L-lactate is easily taken up by the human brain, and that the mechanism by which it crosses the blood–brain barrier is equilibrative. Furthermore, the brain permeability to lactate is enhanced by hypercapnia and the mechanism is believed to act through the decrease in pH.

Heterogeneity of Cerebral Capillary Flow in Man and Its Consequences for Estimation of Blood-Brain Barrier Permeability

Blood-brain barrier permeability studies made in man using the indicator dilution method revealed that the extraction of the test substance increases during the upslope of the venous (outflow) dilution curve. The present study aimed to obviate the possibility that this could result from intravascular phenomena, such as interlaminar diffusion (the result of differences in molecular size) and erythrocyte carriage. Several reference substances were employed for the determination of the extraction in order that careful correction could be made for differences in intravascular behavior of the test and reference substance. The test substances studied were D-glucose, L-phenylalanine, water, propranolol, and benzodiazepines, representing both carrier-transported and lipophilic substances. In-diethylenetriamine pentaacetic acid, Na+, Cl-, L-glucose, and L-lysine were employed as reference substances. For all the substances tested, and after correction for intravascular phenomena, the extractions were found to increase during the initial part of the dilution curve. This increasing extraction can be ascribed to heterogeneity of the cerebral circulation; the higher extraction corresponds to longer contact with the blood-brain barrier and indicates a longer transit time. Signs of heterogeneity were also present when blood flow was elevated above normal. Any influence that heterogeneity might have on the mean extraction value can be minimized by using an appropriate calculation of the extraction of the test substance.

Cerebral blood flow and oxygen consumption during ethanol withdrawal in the rat.

The ethanol withdrawal syndrome in man and animals is characterized by signs of CNS hyperactivity although a direct measurement of a physiological variable reflecting this CNS hyperactivity has never been performed in untreated man or in animals. We induced ethanol dependence in the rat by means of intragastric intubation with a 20% w/v ethanol solution, thus keeping the animals in a state of continuous severe intoxication for 3--4 days; during the subsequent state of withdrawal characterized by tremor, rigidity, stereotyped movements and general seizures a 25% increase in cerebral oxygen consumption (CMRO2) could be measured; this increase was not due to catecholamines originating from adrenal medulla as adrenomedullectomized animals showed a similar increase in CMRO2 (28%); the withdrawing animals showed a corresponding cerebral blood flow (CBF) increase. The elevated CMRO2 and CBF could be reduced to normal by administration of a beta-adrenergic receptor blocker (propranolol 2 mg/kg i.v.), and hence the increased CMRO2 during ethanol withdrawal could be related to catecholaminergic systems in the brain, e.g. the noradrenergic locus coeruleus system which is anatomically well suited as a general activating system. This interpretation is supported by the earlier neurochemical finding of an increased cerebral noradrenaline turnover during ethanol withdrawal. The exact mechanism underlying the increased cerebral oxygen consumption during ethanol withdrawal and the effect of propranolol on cerebral function during this condition remains to be clarified.

Kinetic analysis of blood-brain barrier transport of d-glucose in man: Quantitative evaluation in the presence of tracer backflux and capillary heterogeneity

The present study deals with the analysis of double-indicator curves for blood-brain barrier studies. Two mathematical models which provide for the estimation of backflux of tracer from brain to blood in conjunction with heterogeneity of the cerebral capillary and large-vessel transit times were used for the analysis of D-glucose transport on the basis of cerebral venous outflow curves. The two models, non-mixed and well mixed, arise from differing assumptions regarding the effective region surrounding the capillary lumen. An approximate solution for the well-mixed model was developed to increase computation speed. Fourteen D-glucose outflow curves and their reference curves were obtained from nine patients and subsequently analyzed by the two models. Further, in five patients data were obtained under different physiological conditions: normal, decreased, and increased cerebral blood flow rates. The results support the appropriateness of the well-mixed model and heterogeneity of the cerebral capillary transit times. The median value for the average extraction was 0.18 and the median distribution space was 0.14. The latter value is similar to the brain extracellular space that has been estimated by other methods. The extraction values calculated from the peak of the venous outflow curves were significantly smaller than the whole-brain average extraction values estimated with the well-mixed model (0.157 vs 0.178, P less than 0.0005). In summary: (a) capillary heterogeneity is present in the human brain and changes with cerebral blood flow; (b) after crossing the blood-brain barrier, D-glucose distributes in the brain extracellular fluid; and (c) the extraction curve is significantly influenced by backflux.

Does Seizure Activity Produce Purkinje Cell Loss

Summary: Eight Wistar rats were exposed to 140 electroconvulsive seizures over 50 days. Ten rats served as controls. The density of Purkinje cells in cerebellum ranged from 15.3 to 18.5/mm in the treated rats and from 15.2 to 19.1/mm in the controls. No Purkinje cell loss was disclosed in the rats subjected to electroconvulsive seizures. Twenty‐five Mongolian gerbils of the seizure‐susceptible strain were selected according to seizure score with five animals in each group. Five Mongolian gerbils of a seizure‐resistant strain served as controls. The density of the Purkinje cells ranged from 21.4 to 29.8/mm in the seizure‐susceptible animals and from 27.6 to 31.5/mm in the controls, with a lower density in the gerbils with seizures compared with the controls (p <0.05). There was no relation to type or number of seizures. Eight gerbils of the seizure‐susceptible strain were included as a supplementary group, to disclose any possible genetic trait as an explanation of the lower Purkinje cell density. The Purkinje cell density in these animals ranged from 24.8 to 30.9/mm and did not differ from the density in the seizure‐resistant gerbils. Thus the lower density of Purkinje cells in the seizure‐susceptible Mongolian gerbils is a result of seizure activity. The excessive epileptic input with stimulation of the glutamatergic innervation of the Purkinje cells resulting in a persistent elevated 7‐amino‐butyric acid (GABA) tone may explain the damage to the Purkinje cells in the gerbils and the loss of Purkinje cells found in patients with severe epilepsy.

Asymmetrical Transport of Amino Acids across the Blood—Brain Barrier in Humans

Blood–brain barrier permeability to four large neutral and one basic amino acid was studied in 30 patients with the double indicator technique. The resultant 64 venous outflow curves were analyzed by means of two models that take tracer backflux and capillary heterogeneity into account. The first model considers the blood–brain barrier as a double membrane where amino acids from plasma enter the endothelial cell. When an endothelial cell volume of 0.001 ml/g was assumed, permeability from the blood into the endothelial cell was, for most amino acids, about 10–20 times larger than the permeability for the reverse direction. The second model assumes that the amino acids, after intracarotid injection, cross a single membrane barrier and enter a well-mixed compartment, the brain extracellular fluid, i.e., the endothelial cell is assumed to behave as a single membrane. With this model, for large neutral amino acids, the permeability out of the extracellular fluid space back to the blood was between 8 to 12 times higher than the permeability from the blood into the brain. Such a difference in permeabilities across the blood–brain barrier can almost entirely be ascribed to the effect of a nonlinear transport system combined with a relatively small brain amino acid metabolism. The significance of the possible presence of an energy-dependent A system at the abluminal side of the blood–brain barrier is discussed and related to the present findings. For both models, calculation of brain extraction by simple peak extraction values underestimates true unidirectional brain uptake by 17–40%. This raises methodological problems when estimating blood to brain transfer of amino acids with this traditional in vivo method.

Blood-brain barrier transfer and cerebral uptake of antiepileptic drugs.

The permeability across the blood‐brain barrier of phenobarbital, phenytoin, clonazepam, and diazepam was determined in a total of 29 patients with the double‐indicator dilution method. Cerebral blood flow was measured with the i33Xe intra‐arterial injection method. The unidirectional extraction (E) of the four drugs was 0.07, 0.11, 0.42, and 0.42, respectively. Permeability surface area products (PS) calculated for the drugs depended on E as well as on the plasma protein binding of the drugs and the cerebral blood flow and was calculated as 0.1, 0.5, 0.5, and 2.6 ml gm−1 min−1, respectively. A mathematic model of cerebral uptake and concentration is presented. The brain concentration of each drug is then calculated for two different states, one with a sudden rise from zero to an arterial concentration, which remains constant, and the other with the arterial concentration, which is achieved after rapid intravenous injection. The cerebral uptake rate of clonazepam and diazepam was much more rapid than that of phenobarbital and phenytoin. After intravenous clonazepam or diazepam injection, half‐maximal gray matter concentration is reached about 15 sec after the drug arrives at the brain.