Blood-brain barrier taurine transport during osmotic stress and in focal cerebral ischemia

Protective Effects of 28-O-Caffeoyl Betulin (B-CA) on the Cerebral Cortex of Ischemic Rats Revealed by a NMR-Based Metabolomics Analysis

28-O-caffeoyl betulin (B-CA) has been demonstrated to reduce the cerebral infarct volume caused by transient middle cerebral artery occlusion (MCAO) injury. B-CA is a novel derivative of naturally occurring caffeoyl triterpene with little information associated with its pharmacological target(s). To date no data is available regarding the effect of B-CA on brain metabolism. In the present study, a 1H-NMR-based metabolomics approach was applied to investigate the therapeutic effects of B-CA on brain metabolism following MCAO in rats. Global metabolic profiles of the cortex in acute period (9 h after focal ischemia onset) after MCAO were compared between the groups (sham; MCAO + vehicle; MCAO + B-CA). MCAO induced several changes in the ipsilateral cortex of ischemic rats, which consequently led to the neuronal damage featured with the downregulation of NAA, including energy metabolism dysfunctions, oxidative stress, and neurotransmitter metabolism. Treatment with B-CA showed statistically significant rescue effects on the ischemic cortex of MCAO rats. Specifically, treatment with B-CA ameliorated the energy metabolism dysfunctions (back-regulating the levels of succinate, lactate, BCAAs, and carnitine), oxidative stress (upregulating the level of glutathione), and neurotransmitter metabolism disturbances (back-regulating the levels of γ-aminobutyric acid and acetylcholine) associated with the progression of ischemic stroke. With the administration of B-CA, the levels of three phospholipid related metabolites (O-phosphocholine, O-phosphoethanolamine, sn-glycero-3-phosphocholine) and NAA improved significantly. Overall, our findings suggest that treatment with B-CA may provide neuroprotection by augmenting the metabolic changes observed in the cortex following MCAO in rats.

Effect of Taurine on Hemodiafiltration in Patients With Chronic Heart Failure

Taurine, an important factor in the living body, is essential for cardiovascular function and development and function of skeletal muscle, retina and central nervous system. In the present study, its effect on cardiovascular function was specifically taken into consideration. In hemodiafiltration (HDF) patients, the effect of taurine on patients with chronic heart failure (CHF), in whom dry weight was difficult to control, was evaluated. All patients who were subjected to regular HDF for 4 h three times per week at Joban hospital were included in this study. Patients with chronic heart failure, in whom dry weight was difficult to control (N = 4), were included in the evaluation of clinical status. X-ray and echocardiography were determined before and after taurine treatment. Almost all patients were taking nitric acid, warfarin, anti-platelet agents and vasopressors. Because vital signs were unstable in chronic heart failure, all cases withheld antihypertensive drugs during HDF. For unstable vital signs during HDF, pulmonary congestion was chronically recognized. After taurine was started, vital signs stabilized and lowering of dry weight was possible. In addition, X-ray and cardiac diastolic failure on echocardiography improved. Taurine was effective for CHF patients on HDF in whom dry weight was difficult to control in spite of various medications.

Evolution of blood-brain barrier damage associated with changes in brain metabolites following acute ischemia

Stroke is a serious medical condition that requires emergency care. In the case of ischemic stroke, ischemia may lead to damage to the blood-brain barrier (BBB); the damage in turn may exacerbate the condition. Therefore, noninvasive detection of BBB damage represents a challenge for experimental and clinical researchers. In this study, we assessed the onset of BBB disruption by means of T1-weighted images with administration of the contrast enhancement agent gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) and related BBB breakdown to brain metabolite changes in proton magnetic resonance spectrum (H-MRS) in the infarcted site following middle cerebral artery occlusion (MCAO) in rats. It was shown that MCAO for 30 min and 1.5 h caused no Gd-DTPA signal change in the T1-weighted images, whereas MCAO for 1 h significantly altered some of H-MRS brain metabolites, suggesting that brain metabolite changes occurred earlier than BBB damage after ischemic stroke. MCAO for 2 h caused BBB breakdown, which was related to changes in the levels of some brain metabolites detected by H-MRS. Between the second and the third hour after MCAO, brain metabolite changes continued as the result of BBB breakdown and the concurrent overperfusion to the infarcted site, which may ameliorate the metabolite changes, thus compensating for the functional failures of the brain after stroke.

Neuroprotective Mechanisms of Taurine against Ischemic Stroke

Ischemic stroke exhibits a multiplicity of pathophysiological mechanisms. To address the diverse pathophysiological mechanisms observed in ischemic stroke investigators seek to find therapeutic strategies that are multifaceted in their action by either investigating multipotential compounds or by using a combination of compounds. Taurine, an endogenous amino acid, exhibits a plethora of physiological functions. It exhibits antioxidative properties, stabilizes membrane, functions as an osmoregulator, modulates ionic movements, reduces the level of pro-inflammators, regulates intracellular calcium concentration; all of which contributes to its neuroprotective effect. Data are accumulating that show the neuroprotective mechanisms of taurine against stroke pathophysiology. In this review, we describe the neuroprotective mechanisms employed by taurine against ischemic stroke and its use in clinical trial for ischemic stroke.

Pharmacokinetics and brain uptake of AM-36, a novel neuroprotective agent, following intravenous administration to rats

The plasma pharmacokinetics and brain uptake of the novel neuroprotective agent AM-36 (1-(2-(4-chlorophenyl)-2-hydroxy)ethyl-4-(3,5-bis-(1,1dimethylethyl)-4-hydroxyphenyl) methylpiperazine) were assessed over 72 h following i.v. administration to male Sprague-Dawley rats. At nominal i.v. doses of 0.2, 1 and 3mg kg(-1), AM-36 exhibited an extremely large volume of distribution (18.2-24.6 L kg(-1)) and a long terminal elimination half-life, ranging from 25.2 to 37.7 h. Over this dose range, AM-36 exhibited linear pharmacokinetics, with no apparent change in clearance, volume of distribution or dose-normalised area under the plasma concentration - time curve. AM-36 was very highly bound to plasma proteins (> 99.6%); however, this did not appear to affect the ability of AM-36 to permeate the blood-brain barrier. Following a single i.v. dose of AM-36 at 3mg kg(-1) to rats, brain concentrations were detected for up to 72 h, and the brain-to-plasma ratios were high at all time points (ranging from 8.2 at 5 min post-dose to 0.9 at 72 h post-dose). The very high brain uptake of AM-36 supports previous in-vivo efficacy studies demonstrating the neuroprotective effects of this compound when administered to rats with middle cerebral artery occlusion.

Absorption, tissue distribution, metabolism and elimination of taurine given orally to rats

Three biodisposition studies with taurine were performed in male and female adult rats at dosages of 30 and 300 mg/kg. A single oral dose of (14)C-taurine was rapidly absorbed, distributed to tissues and excreted unchanged in urine. Elimination of radioactivity from intracellular pools was slow. Pre-treatment of animals for 14 days with unlabelled taurine did not significantly affect the fate of (14)C-taurine. At the higher dose there was more extensive excretion combined with a lower percentage of the dose in the carcass, indicating the possibility of saturation of the tubular reabsorption mechanism for taurine. Daily administration of unlabelled taurine for 14 days did not result in an increase in total taurine in the brain. The data indicate that exogenous taurine rapidly equilibrates with endogenous body pools and that any excess is rapidly eliminated by the kidneys.

The role of taurine in neuronal protection following transient global forebrain ischemia

Osmoregulation and post ischemic glutamate surge suppression (PIGSS) are important mechanisms in the neuroprotective properties of taurine. We studied the role of taurine in PIGSS following transient global forebrain ischemia (TGFI). A group of gerbils received a high dose of continuous intracerebral taurine during the peri-ischemic period. Beta-alanine was given similarly to a negative control group. The control group consisted of animals undergoing only TGFI. On the fourth day following commencement of drug administration, TGFI was induced. Concurrently, half the animals from each group receiving an agent had intracerebral microdialysis. All animals underwent histological assessment at day 7. The microdialysis and histological data was analyzed. Our results showed that taurine treatment did not cause PIGSS. The histological difference between the three groups was statistically insignificant. We conclude that intracerebral taurine in the dosage administered during peri-ischemic period, does not result in PIGSS or histologically evident neuroprotection.

Effect of intracerebral and subdural hematomas on energy-dependent transport across the blood-brain barrier

Although both intracerebral and subdural hematomas induce brain edema, previous studies have indicated that they may have different cerebrovascular effects. Our own investigations have demonstrated that while subdural hematomas (SDH) are associated with ischemia this is not the case following intracerebral hematomas (ICH). Previous studies have demonstrated a decrease in energy-dependent transport of glutamine across the blood-brain barrier (BBB) following focal cerebral ischemia. The present study investigates this further by examining the effects of SDH, ICH, and intracerebral thrombin injections, an agent involved in ICH-induced injury, on blood to brain glutamine transport. The injection of 200 microL of blood into the subdural space induced a marked reduction in glutamine transport (Ki, influx rate constant) into the cerebral cortex at 4 and 24 h following SDH (sham, 105+/-4% of contralateral cortex; SDH 4 h, 63+/-5%, p<0.01; SDH 24 h, 47+/-12%, p<0.05). There were no significant changes in glutamine Ki in subcortical areas following SDH. Following ICH (200-microL clot); however, there were only modest decreases in glutamine Ki in subcortical areas (sham, 98+/-2% of right cortex; ICH 4 h, 91+/-5%, p<0.01; ICH 24 h, 91+/-2%, p<0.05). Intracerebral injection of thrombin (5U) had minimal effect on glutamine Ki, in subcortical areas, at 4 h and induced a modest decrease in transport at 24 h (sham, 98+/-2% of right cortex; thrombin 4 h, 98+/-2%; thrombin 24 h, 86+/-2%, p<0.05). The present studies demonstrate marked differences in the effects of ICH and SDH on BBB function.

Osmolyte contents of cultured astrocytes grown in hypoosmotic medium

Primary rat cerebral astrocyte cultures were grown for 2 weeks in isoosmotic medium (305 mosmol) and then placed in similar medium with a reduced NaCl concentration. During the first hour of growth in this moderately hypoosmotic medium (240 mosmol), the cells lose 88% of their taurine contents, 62% of their alanine contents, and 54% of their aspartate contents while regaining normal volume. Loss of these amino acids accounts for 43% of observed volume regulation. Contents of these amino acids remain decreased during 24 h of growth in hypoosmotic medium. In contrast, potassium, glutamate, glutamine, and asparagine contents are not changed, relative to cells in isoosmotic medium, at time points between 1 h and 24 h of hypoosmotic exposure. The data suggest astrocytes contribute to net loss of amino acids, but not potassium, from brains exposed to hypoosmotic conditions in situ.

Blood-brain barrier glutamine transport during normoglycemic and hyperglycemic focal cerebral ischemia

This study examines the effects of middle cerebral artery (MCA) occlusion in the rat on blood to brain glutamine transport, a potential marker of early endothelial cell dysfunction. It also examines whether the effects of ischemia on glutamine transport are exacerbated by hyperglycemia. In pentobarbital-anesthetized rats, 4 hours of MCA occlusion resulted in a marked decline in the influx rate constant for [14C]L-glutamine from 16.1+/-1.2 microL.g(-1).min(-1) in the contralateral hemisphere to 7.3+/-2.5 microL.g(-1).min(-1) in the ischemic core (P < 0.001). This reduction was even greater in xylazine-ketamine-anesthetized rats in which the influx decreased to 2.6+/-1.1 microL.g(-1) min(-1). This greater reduction appears related to the hyperglycemia induced by xylazine-ketamine anesthesia. Glucose injection in pentobarbital-anesthetized rats also resulted in a greater decline in [14C]L-glutamine influx in the ischemic core but had no effect on the contralateral tissue. The effects of hyperglycemia on glutamine transport in the ischemic tissue were associated with a decline in plasma volume, which may reflect either endothelial cell swelling or plugging of the microvasculature. The reduction in glutamine transport during ischemia was progressive, but even as early as 1 hour, there was a 60% and 40% decline in influx in hyperglycemic and normoglycemic rats, respectively. The fall in [14C]L-glutamine influx may reflect a dissipation of the endothelial cell [Na+] gradient. A decline in this gradient would affect many blood-brain barrier transporters with potentially deleterious effects on the ischemic brain.

Taurine synthesis and cysteine metabolism in cultured rat astrocytes: effects of hyperosmotic exposure

We investigated mechanisms controlling taurine synthesis in cultured rat cerebral astrocytes. The mean +/- SE rate of taurine synthesis from extracellular cysteine was 21.2 +/- 2.0 pmol.mg protein-1.min-1, whereas taurine degradation was < 1.3% of this rate. Eliminating cellular glutathione and inhibiting glutathione biosynthesis increased taurine synthesis from extracellular cysteine by 39%. In cell homogenates, cysteine dioxygenase (CDO) and cysteine-sulfinate decarboxylase activities were 2.4 +/- 0.2 and 8.3 +/- 2.8 nmol.mg protein-1.min-1, respectively. CDO activity was strongly dependent on cysteine concentration over physiological and pathophysiological ranges of intracellular cysteine concentration. Growth in hyperosmotic medium caused a greater increase in culture medium taurine content than that measured from cells in isosmotic growth medium. Hyperosmotic treatment transiently increased the rate of cysteine accumulation and cellular cysteine and glutathione contents but had no effect on the synthesis rate of taurine from extracellular cysteine. Thus cysteine is accumulated and then metabolized to taurine through CDO, whose activity depends on the intracellular cysteine concentration and appears to be rate limiting for taurine synthesis. Hyperosmotic exposure increases net taurine production yet has no effect on taurine synthesis from exogenously applied cysteine. Availability of substrate from intracellular pools must contribute to maintenance of high intracellular taurine during hyperosmotic exposure.

Choroid plexus taurine transport

The putative osmoregulatory agent, taurine, is lost from the brain during hypo-osmotic stress or ischemia, but the regulatory mechanisms involved in this loss have not been fully elucidated. In this study, we have examined taurine transport by the isolated rat choroid plexus, one element of the brain-blood interface, and examined how it may be regulated as part of brain volume regulation. Choroid plexus taurine uptake was Na- and Cl-dependent with a Vmax and Km of 6.5 +/- 0.3 pmol/mg/min and 232 +/- 33 microM. The latter is substantially greater than the normal CSF taurine concentration and this may be important in removing taurine released into the CSF during parenchymal cell swelling. Taurine uptake also appears calmodulin dependent as it was reduced by 84 and 91% in the presence of 25 microM trifluoperazine and 100 microM W-7, two calmodulin inhibitors. Taurine efflux from choroid plexus was stimulated by trifluoperazine, taurine, and hypo-osmotic stress. The latter two effects were reduced by niflumic acid, suggesting that taurine and hypo-osmotic stress act on the same pathway. The stimulation of efflux by hypo-osmotic stress decreased with time, whereas the effect of external taurine was sustained. If this efflux pathway is involved in the movement of taurine from choroid plexus to blood, these results suggest that changes in extracellular taurine may be more important than the direct effect of hypo-osmolality in the long-term loss of taurine from the brain.