Increased transfer ofCa into brain and cerebrospinal fluid from plasma during chronic hypocalcemia in rats

Recent studies have shown regulation of central nervous system [Ca] after chronic hypo- and hypercalcemia. To investigate the mechanism of this regulation , 3-week-old rats were fed diets for 8 weeks that contained low or normal levels of Ca. Plasma [Ca] was 40% less in rats fed the low Ca diet than in animals fed normal diet. Unidirectional transfer coefficients for Ca (K_Ca) and Cl (K_Cl) into cerebrospinal fluid (CSF) and brain were determined from the 10 min uptake of intravenously injected⁴⁵Ca and³⁶Cl in awake animals. K_Ca for CSF was 68% greater in low-Ca rats than in normal rats. Likewise, the values of K_Ca for brain regions with areas adjacent to the ventricles like the hippocampus and pons-medulla were 50% higher than in normal animals. On the other hand, K_Cas for parietal cortex, a brain region distant from the choroid plexus and not expected to be influenced by Ca entry into CSF, were similar between the groups. Comparison of the regional ratios of K_Ca/K_Cl revealed that a selective increase of Ca transport occurred into CSF and all brain regions except the parietal cortex in Ca-deficient rats. The results suggest that Ca homeostasis of CSF and brain [Ca] during chronic hypocalcemia is due to increased transfer of Ca from blood to brain, and that the regulation occurs via the CSF, possibly at the choroid plexus, but not via the cerebral capillaries.     

Increased transfer of 45Ca into brain and cerebrospinal fluid from plasma during chronic hypocalcemia in rats

Recent studies have shown regulation of central nervous system [Ca] after chronic hypo- and hypercalcemia. To investigate the mechanism of this regulation, 3-week-old rats were fed diets for 8 weeks that contained low or normal levels of Ca. Plasma [Ca] was 40% less in rats fed the low Ca diet than in animals fed normal diet. Unidirectional transfer coefficients for Ca (KCa) and Cl (KCl) into cerebrospinal fluid (CSF) and brain were determined from the 10 min uptake of intravenously injected 45Ca and 36Cl in awake animals. KCa for CSF was 68% greater in low-Ca rats than in normal rats. Likewise, the values of KCa for brain regions with areas adjacent to the ventricles like the hippocampus and pons-medulla were 50% higher than in normal animals. On the other hand, KCas for parietal cortex, a brain region distant from the choroid plexus and not expected to be influenced by Ca entry into CSF, were similar between the groups. Comparison of the regional ratios of KCa/KCl revealed that a selective increase of Ca transport occurred into CSF and all brain regions except the parietal cortex in Ca-deficient rats. The results suggest that Ca homeostasis of CSF and brain [Ca] during chronic hypocalcemia is due to increased transfer of Ca from blood to brain, and that the regulation occurs via the CSF, possibly at the choroid plexus, but not via the cerebral capillaries.     

Saturable transport of Ca into CSF in chronic hypo- and hypercalcemia.

To further characterize possible saturable transport of Ca into CSF during chronic plasma [Ca] changes, weanling rats were fed diets differing in Ca for 10 weeks. Transfer coefficients for unidirectional uptake of 45Ca and 36Cl into CSF (Kcsf) were determined 3 or 10 min after intravenous tracer injection in unanesthetized animals. In rats fed low Ca diet, Kcsfs for 45Ca and 36Cl were elevated above control, but the 45Ca/36Cl ratio for Kcsf, a more specific measure of Ca transport, was also increased. In animals fed high Ca diet, Kcsfs of both radiotracers were not statistically different from control, but the 45Ca/36Cl ratio was decreased. Injection of CaCl2 into hypocalcemic rats elevated plasma [Ca], depressed 45Ca Kcsf, and returned the 45Ca/36Cl ratio to the control value. The inverse relationship between plasma ionized [Ca] and 45Ca Kcsf was fitted to saturation kinetics with Km less than or equal to 0.53 mumol/ml, maximal Ca influx for the saturable component between 27 and 67 x 10(-5) mumol.g-1.s-1, and the passive component of Kcsf less than or equal to 15 x 10(-5) ml.g-1.s-1. We conclude that Ca transport into CSF is saturable and this transport is important in the regulation of CSF [Ca].     

Response of infant and adult rat choroid plexus potassium transporters to increased extracellular potassium.

The choroid plexus (CP) has an important role in regulating ion concentrations in the cerebrospinal fluid (CSF). Maintenance of potassium concentration [( K]) in brain extracellular fluids lower than in serum is critical to proper neuronal function. We studied K (86Rb) transport by the isolated, lateral CP from infant and mature rats to determine the nature and development of CSF [K] regulation. Comparison of transport characteristics in media with 3-11 mM [K] showed that the adult CP increased uptake in a stepwise fashion with each 2 mM increase in [K], up to a 90% increase with 9 mM [K] over control (3 mM [K]). In contrast, 3-day CP increased uptake in 5 mM, but could not increase K uptake further with higher [K]. One-week tissues showed an intermediate response. Kinetic analysis of the ouabain-inhibitable component suggested that the immature tissues may express a different isoform of the catalytic (alpha) subunit of the Na,K-ATPase. These data may explain the previously established inability of neonatal rats to regulate CSF [K] when serum [K] is elevated, and they indicate that active K transport by the CP cells plays an integral role in CSF [K] regulation.     

Calcium influxes into brain and cerebrospinal fluid are linearly related to plasma ionized calcium concentration

Unidirectional Ca influxes into brain and cerebrospinal fluid (CSF) were measured at different plasma concentrations of ionized Ca ([Ca]i) in pentobarbital-anesthetized rats. Plasma [Ca]i was varied acutely from 0.6 to 3.0 mumol/ml by intravenous infusion of EGTA, NaCl or CaCl2 or by thyroparathyroidectomy. Ca influx was determined from the 15-min uptake of 45Ca after intravenous injection. There were significant regional differences in 45Ca uptake into the CNS, with a approximately 20-fold greater rate into ventricular CSF than into frontal cortex. Autoradiographs of 45Ca uptake demonstrated that uptake into frontal cortex reflects primarily transport across the cerebral capillaries, whereas uptake into ventricular CSF reflects transport across the choroid plexuses. At both sites, Ca influx was a linear function of plasma [Ca]i and extrapolated to zero at [Ca]i = 0. Infusion of EGTA or CaCl2 did not alter the integrity of the blood-brain barrier, as determined by the permeability to [14C]sucrose. These results indicate that Ca influx into the CNS is not regulated by a saturable mechanism that is sensitive to acute changes in plasma [Ca]i. The proportionality between influx and concentration is suggestive of passive diffusional transport. The brain is protected from acute changes in plasma [Ca]i by the low cerebrovascular permeability to Ca, approximately 5 X 10(-8) cm/s.     

Saturable uptake of [125I]L-triiodothyronine at the basolateral (blood) and apical (cerebrospinal fluid) sides of the isolated perfused sheep choroid plexus.

The uptake of 125I-labelled L-triiodothyronine (T3) was measured on the blood side of the isolated perfused choroid plexus of the sheep using steady-state and single-circulation paired tracer techniques. The steady-state uptake of T3 was 33.5% (perfusion fluid protein content was 0.05 g.dl-1) which could be reduced to 9.4% in the presence of 500 microM unlabelled T3 showing partial saturation. The CSF to blood steady-state [125I]T3 measurements gave plasma/CSF ratio, R%, of 24.6 +/- 4.8% which was reduced to 9.8 +/- 2.1% in the presence of 500 microM unlabelled T3 in the mock CSF. The transport of T3 across the blood face of the choroid plexus and the CSF to blood transport, failed to show sodium dependence. Using the single circulation paired tracer technique, the initial uptake in less than 60 s, Umax of [125I]T3 was 50.4 +/- 3.9% relative to the extracellular marker [3H]D-mannitol. However, when 250 microM unlabelled T3 was present, Umax was reduced by 66%, although further significant inhibition at higher concentrations was not observed. Uptake of T3 at the blood side of the choroid plexus was partially saturated in the presence of unlabelled reverse T3 and DT3, suggesting little uptake stereospecificity. Unlabelled thyroxine (T4) and the amino acid analogues cycloleucine (aminocyclopentane-1-carboxylic acid) and BCH (beta-2-aminobicyclo-[2,2.1]-heptane-2-carboxylic acid) each reduced [125I]T3 uptake significantly, but not to the same degree as T3 stereoisomers. The neutral amino acids alanine and phenylalanine, had no effect on uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

The uptake of delta9-tetrahydrocannabinol in choroid plexus and brain cortex in vitro and in vivo.

[3H]delta9 Tetrahydrocannabinol (delta9-THC) was actively transported by the choroid plexus and cerebral cortical slices of the rabbit when incubated as a BSA-microsuspension in artificial rabbit CSF. The transport system for delta9-THC in choroid plexus had a V max of 174 nmoles/mg tissue/h, approximately 9-fold greater than that observed for cortical slices. In vivo experiments demonstrated a preferential distribution of delta9-THC in choroid plexus at 1 h after intravenous injection. These results indicate that delta9-THC is actively accumulated by choroidal epithelium and may also be transported across the epithelial stroma into the capillary circulation. This suggests that the choroid plexus participates in the regulation of delta9-THC concentration in CSF and indirectly in brain by means of the "sink" function of the CSF.     

Leptin transport at the blood–cerebrospinal fluid barrier using the perfused sheep choroid plexus model

Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood–brain and blood–cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood–CSF barrier) in isolation from the blood–brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [¹²⁵I]leptin (7.5 pmol l⁻¹) at the blood face of the choroid plexus was 21.1±5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [¹²⁵I]leptin (14.0±3.7%). In addition, trichloroacetic acid precipitable [¹²⁵I] was detected in newly formed CSF, indicating intact protein transfer across the blood–CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [¹²⁵I]Leptin uptake at the blood–plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis–Menten-type kinetics (K_m=16.3±1.8 nM, V_max=41.2±1.4 pmol min⁻¹ g⁻¹), and a non-saturable component (K_d=0.065±0.002 ml min⁻¹ g⁻¹). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood–CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.     

Leptin transport at the blood--cerebrospinal fluid barrier using the perfused sheep choroid plexus model

Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood-brain and blood-cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood-CSF barrier) in isolation from the blood-brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [(125)I]leptin (7.5 pmol l(-1)) at the blood face of the choroid plexus was 21.1+/-5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [(125)I]leptin (14.0+/-3.7%). In addition, trichloroacetic acid precipitable [(125)I] was detected in newly formed CSF, indicating intact protein transfer across the blood-CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [(125)I]Leptin uptake at the blood-plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis-Menten-type kinetics (K(m)=16.3+/-1.8 nM, V(max)=41.2+/-1.4 pmol min(-1) g(-1)), and a non-saturable component (K(d)=0.065+/-0.002 ml min(-1) g(-1)). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood-CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.     

Iron and transferrin uptake by brain and cerebrospinal fluid in the rat.

Iron and transferrin uptake into the brain, CSF and choroid plexus, and albumin uptake into the CSF and choroid plexus, were determined after the intravenous injection of [59Fe-125I]transferrin and [131I]albumin into control rats aged 15, 21 and 63 days and 21-day iron-deficient rats. Iron uptake by the brain was unidirectional, greatly exceeded that of transferrin and was equivalent to 39 and 36% of the plasma iron pool per day in the 15-day control and 21-day iron-deficient rats. The rate of transferrin catabolism in the rats was only about 20% of the plasma pool per day. Iron and transferrin uptake into the brain and CSF decreased with increasing age and was greater in the iron-deficient than in the control 21-day rats. The quantity of 125I-transferrin recovered in the CSF could account for only a small proportion of the iron taken up by the brain. Albumin transfer to the CSF also decreased with age but was lower than that of transferrin and was not affected by iron deficiency. Similarly, the plasma: CSF concentration ratios of transferrin and albumin, as determined immunologically, decreased with age and were greater for transferrin than albumin. It is concluded that iron uptake by the brain is dependent on iron release from transferrin at the cerebral capillary endothelial cells with recycling of transferrin to the plasma and transfer of the iron into the brain interstitium. Only a small fraction of the transferrin bound by brain capillaries is transcytosed into the brain and CSF, this being one source of CSF transferrin while other sources are local synthesis and transfer from the plasma by the choroid plexuses.     

Iron and transferrrin uptake by brain and cerebrospinal fluid in the rat

Iron and transferrin uptake into the brain, CSF and choroid plexus, and albumin uptake into the CSF and choroid plexus, were determined after the intravenous injection of [⁵⁹Fe-¹²⁵]Itransferrin and [¹³¹I]albumin into control rats aged 15, 21 and 63 days and 21-day iron-deficient rats. Iron uptake by the brain was unidirectional, greatly exceeded that of transferrin and was equivalent to 39 and 36% of the plasma iron pool per day in the 15-day control and 21-day iron-deficient rats. The rate of transferrin catabolism in the rats was only about 20% of the plasma pool per day. Iron and transferrin uptake into the brain and CSF decreased with increasing age and was greater in the iron-deficient than in the control 21-day rats. The quantity of ¹²⁵I-transferrin recovered in the CSF could account for only a small proportion of the iron taken up by the brain. Albumin transfer to the CSF also decreased with age but was lower than that of transferrin and was not affected by iron deficiency. Similarly, the plasma: CSF concentration ratios of transferrin and albumin, as determined immunologically, decreased with age and were greater for transferrin than albumin. It is concluded that iron uptake by the brain is dependent on iron release from transferrin at the cerebral capillary endothelial cells with recycling of transferrin to the plasma and transfer of the iron into the brain interstitium. Only a small fraction of the transferrin bound by brain capillaries is transcytosed into the brain and CSF, this being one source of CSF transferrin while other sources are local synthesis and transfer from the plasma by the choroid plexuses.     

Stilbenes inhibit exchange of chloride between blood, choroid plexus and cerebrospinal fluid

Disulfonic stilbenes (DIDS and SITS) were used to probe the nature of Cl transport in choroid plexus (CP) of adult rats. DIDS was more effective than SITS in reducing in vitro choroid cell [Cl]. Intravenously- or intraventricularly-administered DIDS substantially decreased 36Cl transport from blood to CP to CSF. Rate of CSF renewal was diminished nearly 30% by DIDS. These inhibitory findings implicate an anion antiporter for mediating Cl flux (and thus water) between blood and CSF.     

In vitro studies on the uptake and incorporation of natural amino acids in rabbit choroid plexus

Isolated rabbit choroid plexus was incubated aerobically at 37 °C in artificial cerebrospinal fluid (CSF) medium containing 1 mg/ml glucose and trace amounts of [¹⁴C]leucine. The uptake of leucine occurred against a concentration gradient, was saturable, energy dependent and inhibited by natural amino acids. Of the total amount accumulated after 1 h, in the presence of trace levels of [¹⁴C]leucine, approximately 80% of the leucine was found to be in the bound form. A similar free to bound distribution was observed for isoleucine, phenylalanine, tyrosine, histidine and proline. Classic inhibitors of cytoplasmic protein synthesis inhibited the incorporation of leucine into protein but did not prevent [¹⁴C]leucine from being actively taken up into the choroid plexus tissue water. Our in vitro studies on leucine uptake are in general agreement with previous in vivo data and help support the view that the choroid plexus plays a role in the regulation of cerebrospinal fluid and cerebral amino acids.     

Autonomic nerves in the mammalian choroid plexus and their influence on the formation of cerebrospinal fluid

The choroid plexuses of all ventricles receive a well-developed adrenergic and cholinergic innervation reaching both the secretory epithelium and the vascular smooth muscle cells. Also peptidergic nerves, containing vasoactive intestinal polypeptide, are present but primarily associated only with the vascular bed. A sympathetic inhibitory effect on the plexus epithelium has been indicated in determinations of carbonic anhydrase activity and by studies of various aspects of active transport in isolated plexus tissue. Pharmacological analysis in vitro has shown the choroidal arteries to possess both vasoconstrictory alpha-adrenergic and vasodilatory beta-adrenergic receptors. Electrical stimulation of the sympathetic nerves, which originate in the superior cervical ganglia, induces as much as 30% reduction in the net rate of cerebrospinal fluid (CSF) production, while sympathectomy results in a pronounced increase, about 30% above control, in the CSF formation. There is strong reason to believe that the choroid plexus is under the influence of a considerable sympathetic inhibitory tone under steady-state conditions. From pharmacological and biochemical experiments it is suggested that the sympathomimetic reduction in the rate of CSF formation is the result of a combined beta-receptor-mediated inhibition of the secretion from the plexus epithelium and a reduced blood flow in the choroid plexus tissue resulting from stimulation of the vascular alpha-receptors. The choroid plexus probably also represents an important inactivation site and gate mechanism for sympathomimetic amines, as evidenced by considerable local activity of catechol-O-methyl transferase and monoamine oxidase, primarily type B. The CSF production rate is also reduced by cholinomimetic agents, suggesting the presence of muscarinic-type cholinergic receptors in the choroid plexus.     

Effect of hypertension on the integrity of blood brain and blood CSF barriers,cerebral blood flow and CSF secretion in the rat

Hypertension has been related to the development of brain damage, dementia and other CNS dysfunctions. Disruption of the blood-brain barrier (BBB) is thought to contribute to these disorders. In this study, the integrity of both blood-brain and blood-CSF barriers during chronic hypertension was investigated. For this, the entry of [14C]sucrose and of lanthanum into brain tissue, choroid plexus, and CSF was studied. Also brain regional blood flow and brain [14C]sucrose volume of distribution were measured using indicator fractionation and ventriculo-cisternal perfusion methods, respectively. The above measurements were performed in normotensive (WKY) rats and in the spontaneously hypertensive rats (SHR). Choroid plexus and CSF uptakes of [14C]sucrose were found to be significantly greater in SHR compared to WKY rats (P<0.05). Intercellular entry of lanthanum was observed in choroidal tissue of SHR but not in that of WKY rats and at the BBB. Choroid plexus blood flow was significantly greater in SHR, 2.82+/-0.21 ml g(-1) min(-1), compared to 2.4+/-0.08 ml g(-1) min(-1) in WKY (P<0.05). There were no significant differences (P>0.05) in brain % water content and extracellular fluid [14C]sucrose volume of distribution between SHR and WKY rats. However, choroid plexus showed greater % water content in SHR (85.7+/-1.9%) compared to the WKY rats (81.5+/-1.7%). These results suggest that chronic hypertension in SHR may cause more pronounced defects in the integrity of the blood-CSF barrier than in the BBB.     

Choline and PAH transport across blood-CSF barriers: The effect of lithium

The components of the blood-CSF barrier responsible for the transport of p-aminohippuric acid (PAH) and choline from CSF to blood were identified using in vitro preparations of frog choroid plexus and arachnoid membranes. Choline was transported out of CSF across the arachnoid, while PAH was transported out across the choroid plexus. Probenecid and ouabain blocked both processes. The effect of Li on these transport processes was tested by the addition of 5 mM LiCl to the incubation media. Li increased, by a factor of two, choline transport across the arachnoid, but there was no effect of Li on PAH transport across the plexus. Lithium was passively transported across the choroid plexus, and we suggest that the major transport pathway is through the tight junctions. The steady-state distribution of Li between the choroidal epithelium and the incubation medium was only half that expected for passive distribution. This suggests the existence of sodium/lithium countertransport in these epithelial cell membranes.     

Cerebral calcium fluxes and calcium homeostasis in the rat: A minimal model

Abstract

Disturbance of calcium homeostasis and unregulated increase in intracellular calcium have been implicated in. cell damage and cell death in the central nervous system in particular. To determine the specific pathway(s) of cerebral Ca transport of importance in a pathophysiological situation, we have measured long term Ca flux in brain in vivo in rat, and developed a kinetic model incorporating physiologically relevant pathways of cerebral Ca transport. 45Ca was injected into a tail vein in conscious rats. Plasma ⁴⁵Ca was monitored up to 4 days post-injection and ⁴⁵Ca uptake determined in samples of cerebrospinal fluid (CSF) and at several sites of brain at euthanasia. Uptake of the tracer by tissues peaked after 1 h, isotopic equilibration taking longer. The uptake at hippocampus was the highest. Computer simulation of the kinetics of the plasmal CSF and tissue data was performed using a compartmental model, which included two subcompartments (intra- and extra-cellular) and two pathways from plasma to the brain: directly across the blood-brain barrier and via CSF, which included a delay. The analysis based on this model enabled estimation of the fractional rates of transport of Ca to cerebral and noncerebral tissues across all the barriers of the model. [Neural Res 1997; 19: 403-408]     

Transport of 5-aminolevulinic acid between blood and brain

Little is known about the movement of 5-aminolevulinic acid (delta-aminolevulinic acid; ALA) between blood and brain. This is despite the fact that increases in brain ALA may be involved in generating the neuropsychiatric symptoms in porphyrias and that systemic administration of ALA is currently being used to delineate the borders of malignant gliomas. The current study examines the mechanisms involved in the movement of [(14)C]ALA across the blood-brain and blood-CSF barriers in the rat. In the adult rat, the influx rate constant (K(i)) for [(14)C]ALA movement into brain was low ( approximately 0.2 microl/g per min), was unaffected by increasing plasma concentrations of non-radioactive ALA or probenecid (an organic anion transport inhibitor) and, therefore, appears to be a diffusional process. The K(i) for [(14)C]ALA was 3-fold less than that for [(14)C]mannitol, a molecule of similar size. This difference appears to result from a lower lipid solubility rather than saturable [(14)C]ALA transport from brain to blood. The K(i) for [(14)C]ALA for uptake into the neonatal brain was 7-fold higher than in the adult. However, again, this was unaffected by increasing plasma ALA concentrations suggesting a diffusional process. In contrast, at the blood-CSF barrier, there was evidence of carrier-mediated [(14)C]ALA transport from blood to choroid plexus and blood to CSF. Both processes were inhibited by administration of non-radioactive ALA and probenecid. However, experiments in choroid plexus epithelial cell primary cultures indicated that transport in these cells was polarized with [(14)C]ALA uptake from the apical (CSF) side being about 7-fold greater than uptake from the basolateral (blood) side. In total, these results suggest that the brain is normally fairly well protected from changes in plasma ALA concentration by the very low blood-brain barrier permeability of this compound and by a saturable efflux mechanism present at the choroid plexus.     

Actions of sex steroids and corticosteroids on rabbit choroid plexus as shown by changes in transport capacity and rate of cerebrospinal fluid formation

The effect of betamethasone on choroid plexus transport and CSF formation in rabbits was studied. Following 5 days of daily treatment with betamethasone the CSF production rate was reduced by 43% as measured by ventriculo-cisternal perfusion with radioactive inulin. Accordingly, the Na(+)-K(+)-ATPase activity and the transport capacity in the choroid plexus, measured in terms of choline (10(-5) M) uptake and accumulation in vitro, decreased (in the lateral ventricles by 31% in both cases). Isolated choroid plexuses from rabbits were also used to determine uptake and accumulation of choline and the activities of various types of ATPases following pretreatment of the animals with 17-beta-oestradiol, alone or in combination with progesterone. The combined treatment reduced the choline uptake by 35% and also lowered the activity of Na(+)-K(+)-ATPase by 31% without influencing tissue wet weight. Thus, the demonstrated influences of glucocorticoids and sex steroids on the transport capacity in the choroid plexus seem to be important components in their postulated effects on intracranial hypertension.     

Saturable uptake of [I] -triiodothyronine at the basolateral (blood) and apical (cerebrospinal fluid) sides of the isolated perfused sheep choroid plexus

The uptake of ¹²⁵I-labelled -triiodothyronine (T₃) was measured on the blood side of the isolated perfused choroid plexus of the sheep using steady-state and single-circulation paired tracer techniques. The steady-stake uptake of T₃ was 33.5% (perfussion fluid protein content was 0.05 g·dl⁻¹) which could be reduced to 9.4% in the presence of 500 μM unlabelled T₃ showing partial saturation. The CSF to blood steady-state [¹²⁵I]T₃ measurements gave plasma/CSF ratio, R%, of 24.6 ± 4.8% which was reduced to 9.8 ± 2.1% in the presence of 500 μM unlabelled T₃ in the mock CSF. The transport of T₃ across the blood face of the choroid plexus and the CSF to blood transport, failed to show sodium dependence. Using the single circulation paired tracer technique, the initial uptake in less than 60 s, U_max of [¹²⁵I]T₃ was 50.4 ± 3.9% relative to the extracellular marker [³H] -mannitol. However, when 250 μM unlabelled T₃ was present, U_max was reduced by 66%, although further significant inhibition at higher concentrations was not observed. Uptake of T₃ at the blood side of the choroid plexus was partially saturated in the presence of unlabelled reverse T₃ and DT₃, suggesting little uptake stereospecificity. Unlabelled thyroxine (T₄) and the amino acid analogues cycloleucine (aminocyclopentane-1-carboxylic acid) and BCH (β-2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid) each reduced [¹²⁵I]T₃ uptake significantly, but not to the same degree as T₃ stereoisomers. The neutral amino acids alanine and phenylalanine, had no effect on uptake. The [¹²⁵I]T₃ unidirectional flux was calculated from U_max vlues and the resultant kinetic curve could be resolved into two components; a non-saturable process with a slope of 1.2 ml·min⁻¹·g⁻¹, and a saturable process with K_m = 66 ± 22 μM and V_max = 0.44 ± 0.11 μmol·min⁻¹·g⁻¹. These data suggest that [¹²⁵I]T₃ uptake at the blood face of the choroid plexus, is mediated by both saturable and non-saturable uptake processes, which lack stereospecificity, sodium dependence, and exhibit cross competition both with T₄ and the large neutral amino acid analogues, cycloleucine and BCH. Transport from CSF was also partially saturable, did not exhibit sodium dependence.