Simultaneous measurement of regional blood flow and glucose extraction in rat brain

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Autoradiographic measurement of cerebral lactate transport rate constants in normal and activated conditions

We used quantitative autoradiography to measure the regional rate constants of blood-to-brain transport of lactate in normal rats and rats treated with kainic acid. Mean cerebral values of lactate transport rate constants were not significantly different between the normal and treated rats, being 0.13 and 0.14 min-1 (ml/g), respectively. Regional values were also generally similar between the groups, but structures that are known to be activated by kainic acid showed increased values in the treated rats compared with rates in the controls. Our measured values of lactate transport rate constants are approximately 50% as great as those published for glucose, indicating that blood-brain transfer of lactate can be significant. This observation supports the hypothesis that radiolabel derived from glucose can leave the brain as radiolabeled lactate in conditions in which intracerebral lactate concentration rises, a hypothesis that has previously been presented to explain differences between rates of accumulation of radiolabel derived from deoxyglucose and glucose in such conditions.

Nervous tissue thiamine metabolism in vivo. I. Transport of thiamine and thiamine monophosphate from plasma to different brain regions of the rat

The transport of thiamine (T) and thiamine monophosphate (TMP) across the blood-brain barrier was measured in vivo in the rat. Different doses of [14C]T (15-550 nmol) and [14C]TMP (11-110 nmol) were injected into the femoral vein. The content of T and its phosphoesters in blood and brain tissue (cerebellum, pons, medulla and cerebral cortex) 20 s after the injection was determined radiometrically after electrophoretic separation. Blood flow and blood volume in the same regions of the brain was also determined. Both T and TMP entered rapidly the cerebral tissue, where they were found chemically unmodified. The cerebral tissue extracted less than 7% of plasma T. At physiological plasma T concentrations, the rate of transport ranged from 0.43 to 0.65 nmol X g-1 X h-1 with only minor differences among the various regions. T was transported into the nervous tissue by two separate mechanisms: one saturable, that at physiological plasma T levels accounted for 95% (cerebellum) to 91% (cerebral cortex) of the total T taken up, and one non-saturable, that was most efficient in the cerebral cortex. The Km (half-saturation constant) of the former transport mechanism ranged from 1.95 to 2.75 nmol X ml-1 in the 4 areas investigated. Vmax (maximal transport rate) values ranged from 6 to 9 nmol X g-1 X h-1, the highest value being found in the cerebellum. The overall transport rate of TMP was on average 5-10 times as low as that of T and also showed a saturable and a non-saturable component. Both components were slower than those observed for T.

Ischemia reduces blood-to-brain glucose transport in the gerbil

The effect of carotid occlusion on cerebral blood flow (CBF), brain plasma volume for sucrose (Vplsuc), and unidirectional transport of glucose from blood to brain was measured in four regions of gerbil brain. Unilateral common carotid artery occlusion caused a variable decrease in CBF to the ipsilateral cerebral cortex and basal ganglia, with no change in CBF to the contralateral structures; cerebellum, or brainstem. One hour of bilateral carotid artery occlusion reduced flow to near zero in the cerebral cortex and to 30% of control in the basal ganglia, while increasing CBF to the cerebellum and brainstem. There was a significant decrease in the Vplsuc of the cerebral cortex and basal ganglia after 1 h of ischemia, perhaps due to compression of the intravascular space by edema fluid. Blood-to-brain glucose transport, 1 min after release from 1 h of bilateral carotid occlusion, was decreased in the cerebral cortex and basal ganglia, but not in the cerebellum or brainstem. These data indicate that 1 h of complete or incomplete ischemia reduces the rate of unidirectional glucose transport from blood to brain.

Regional brain blood flow, blood volume, and haematocrit values in the adult rat

Measurements of red cell volume, plasma volume, and tissue haematocrit (Hct) were made in 14 brain regions in adult rats using 51Cr-tagged red cells and 125I-labeled human serum albumin. The mean large vessel (systemic artery) Hct was 41.8, total body Hct was 35.3, and of the brain regions, the lowest value (septal nucleus) was 25.91 and the highest (visual cortex) was 32.05. The lowest blood volume was 6.29 microliters g-1 (caudate putamen) and the highest was 14.44 microliters g-1 (inferior colliculus). There was a significant difference between regions in both blood volume and tissue blood Hct. When brain regions were ranked in order of blood volume, this did not coincide with the order for blood flow.

Simultaneous determination of regional cerebral blood flow and blood--brain glucose transport kinetics in the gerbil

Cerebral blood flow (CBF) and unidirectional transport of glucose from blood to brain were measured simultaneously in four brain regions of the pentobarbital-anesthetized gerbil. The method consisted of the intravenous injection of a bolus containing [14C]butanol and [3H]glucose, followed by continuous withdrawal of arterial blood and sampling of brain 25 s later. CBF was lowest in the cerebral cortex (50 ml 100 g-1 min-1), highest in the brainstem (89 ml 100 g-1 min-1), and intermediate in the basal ganglia and cerebellum (66 and 69 ml 100 g-1 min-1, respectively). The kinetics of blood-to-brain glucose transport were measured in animals whose blood glucose concentration had been altered by glucose or insulin injections. The half-saturation constant for glucose transport (Km) was similar in all brain regions (7.37-8.14 mM), while the maximal rate of transport (Vmax) was lowest in the cerebral cortex (1.55 mumol g-1 min-1) and significantly higher in the basal ganglia, cerebellum, and brainstem (1.81-2.02 mumol g-1 min-1). These values for CBF and glucose transport are similar to those reported in the literature for other pentobarbital-anesthetized animals. The method provides a simple and rapid technique for determining the effect of ischemia and alterations in CBF on blood-to-brain glucose transport.

Glucose availability to individual cerebral structures is correlated to glucose metabolism

Regional cerebral glucose influx was measured using quantitative autoradiography after the intravenous infusion of [2-14C]glucose for a period of 10 or 20 s. Glucose influx varied considerably among structures over an almost threefold range. When compared with rates of regional glucose utilization, a significant correlation by region was found between glucose influx and utilization, demonstrating that the glucose supply to individual cerebral structures is closely matched to their metabolic needs.

Cerebral extraction of N-13 ammonia: its dependence on cerebral blood flow and capillary permeability -- surface area product

13N-labeled ammonia was used to investigate 1) the cerebral extraction and clearance of ammonia, 2) the mechanism by which capillaries accommodate changes in cerebral blood flow (CBF) and 3) its use for the measurement of CBF. The unidirectional extraction of 13NH3 in rhesus monkeys was measured during PaCO2 induced changes in CBF and dog studies were performed using in vitro tissue counting techniques to examine 13NH3 extraction in gray and white matter, mixed tissue and cerebellum during variations in CBF produced by combinations of embolization, local brain compression, and changes in PaCO2. The single pass extraction fraction of 13NH3 varied from about 70 to 20% over a CBF range of 12 to 140 cc/min/100 g. Capillary permeability-surface area product (PS) estimates with a Renkin/Crone model show PS increasing with CBF. The magnitude and rate of increase in PS with CBF was highest in gray matter greater than mixed tissue greater than white matter. Tissue extraction of 13NH3 vs CBF relationship was best described by a unidirectional transport model in which CBF increases by both recruitment of capillaries and by increases of blood velocity in open capillaries. This saturable-recruitment model provides a possible explanation for the mechanism of flow changes at the capillary level. The net 13NH3 extraction subsequent to an i.v. injection increases non-linearly with CBF. Doubling or halving basal CBF produced from 35 to 50% changes in the 13N tissue concentrations with further increases in CBF associated with progressively smaller changes in 13N concentrations.

Simultaneous measurement of cerebral blood flow and unidirectional movement of substances across the blood-brain barrier: theory, method, and application to leucine

The uptake of compounds by the brain depends upon cerebral blood flow. To determine the normal blood flow-cerebral extraction relationship, a method for rapid, simultaneous measurement of cerebral blood flow and brain extraction was developed and applied to blood-brain leucine transfer. Awake rats were injected intravenously with a mixture of n-[(14)C]butanol and [(3)H]leucine. The quantities of indicators accumulated over the following 5-12 s in brain and in a sample of arterial blood withdrawn at a know rate were used to determine the flux of butanol and leucine into brain. Butanol extraction was assessed independently by measuring arterial and cerebral venous concentrations of the indicator after a bolus injection. Cerebral blood flow was equal to the ratio of butanol flux into brain to butanol extraction by brain; leucine extraction was then calculated as the ratio of leucine influx to cerebral blood flow. Leucine extraction by brain and cerebral blood flow were shown to be related exponentially. The maximum velocity of active leucine transport was virtually the same at flows of 150 and 400 ml/100 g/min. The present method is theoretically applicable to the measurement of the extraction of any compound from blood by brain. By measuring the normal blood flow-extraction relationship, one can differentiate changes in extraction secondary to altered flow from changes intrinsic to pathologic conditions with inconstant cerebral blood flow.

Gray and white matter brain-blood transfer constants by steady-state tissue clearance in cat

Capillary transfer constants for gray matter have been measured by others from steady-state tissue clearance during ventriculocisternal perfusion. Similar studies in white matter, however, are complicated by the bulk flow of interstitial fluid (ISF). Recently we determined the velocity of bulk flow of ISF under normal conditions. We now report capillary transfer constants in gray and white matter by steady-state tissue clearance in the cat. Adults cats underwent a 2, 3, or 4 h ventriculocisternal perfusion with artificial cerebrospinal fluid containing [3H]sucrose and either [14C]urea or [14C]ethylene glycol. Diffusion coefficients and velocity of bulk flow were determined from tissue concentrations of the extracellular marker, sucrose. Steady-state tissue concentrations of urea and ethylene glycol were used to calculate transfer of those compounds from the brain to the blood. Urea reached steady-state by the third hour; capillary transfer constants were similar in gray and white matter. Ethylene glycol reached steady-state by the second hour; however, capillary transfer was more rapid in the gray matter than in white.

Starvation-induced changes in transport of ketone bodies across the blood-brain barrier

The permeability of the blood brain barrier (BBB) to beta-hydroxybutyrate (beta-HB) was computed in fed and starved (five days) rats by the simultaneous measurement of cerebral blood flow (diffusible indicator method-123I-iodoantipyrine) and brain uptake of 14C-beta-HB (relative to a 3H2O reference). The results from the present study demonstrate that the movement of beta-HB across the BBB in rat is by a carrier-mediated process. During starvation, total movement (carrier-mediated and diffusionary) of this ketone body into brain was observed to be enhanced because of an increase in the diffusionary loss across the cerebral capillary. The calculated transport kinetics also suggest that the beta-HB molecule has a greater affinity for the transport (mediator) protein during starvation, although the maximal rate of uptake by brain due to a carrier processes mediated Vmax is decreased either because there is a smaller quantity of the mediating molecule or because there is trans inhibition by a high cellular concentration of beta-HB or some analog.