Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids

We have examined the effects of the macrocyclic lactone protein kinase C (PKC) activator bryostatin 1 on taxol-induced apoptosis and inhibition of clonogenicity in the human monocytic leukemia cell line U937. Exposure of cells to bryostatin 1 (10 nM; 15 hr) after (but not before) a 6-hr incubation with 0.5 microM taxol significantly increased apoptosis and resulted in an approximately 3 log reduction in clonogenicity. Cell cycle analysis revealed that the increase in apoptotic cells following bryostatin 1 treatment occurred primarily in the population undergoing taxol-mediated G2M arrest. The actions of bryostatin 1 were not attributable to potentiation of taxol-induced tubulin stabilization or to a reduction in the intracellular retention of taxol. Following exposure of cells to taxol, the Bcl-2 protein displayed an alteration in mobility that was not modified appreciably by bryostatin 1 treatment. The mobility shift in Bcl-2 protein from cells exposed to taxol followed by bryostatin 1 was eliminated by treatment of lysates with the protein phosphatase 2A (PP2A); the latter effect was blocked by okadaic acid. Treatment of cells with taxol followed by bryostatin 1 did not increase the amount of total Bax (compared with treatment with taxol alone), but did increase the amount of free Bax in the supernatant fraction. Finally, the ability of bryostatin 1 to potentiate taxol-induced apoptosis in U937 cells was mimicked closely by 2'-amino-3'-methoxyflavone (PD98059), a specific inhibitor of the mitogen-activated protein kinase (MAPK) kinase (MEK). Collectively, these findings indicate that bryostatin 1 increases the susceptibility of U937 cells to taxol-induced apoptosis and inhibition of clonogenicity. They also raise the possibility that this phenomenon may involve functional alterations in Bcl-2 and/or other proteins involved in regulation of the cell death pathway.     

Cellular mechanisms involved in brain ischemia

Cellular mechanisms, both destructive and protective, that are associated with cerebral ischemia are reviewed in this paper. Central to understanding the evolution of stroke are the concepts of ischemic core and surrounding penumbral region damage, delayed neuronal death, and neuronal rescue. The role of spreading depression in the evolution of subsequent ATP depletion, ion shifts, glutamate release, activation of glutamate receptors, intracellular Ca2+ changes, and generation of reactive oxygen species in the penumbra in relationship to neuronal and glial cell damage are discussed. We conclude that the most fruitful areas for future stroke research include traditional approaches as well as novel approaches. Traditional approaches include stroke prevention and examination of the effects of combinations of proven and promising effective therapeutic interventions. Novel approaches include delineating mechanisms whereby growth factors and compounds such as deprenyl and staurosporine afford neuroprotection, ultimately leading to direct manipulation of the signal transduction pathways that lead to neuronal dysfunction and death. This includes determining which genes are activated and repressed in specific response to hypoxia-ischemia and determining how such alterations in gene expression affect survival and function of neurons. We also suggest that advantage be taken of the blood-brain barrier compromise during stroke in designing neuroprotective therapies.     

Blood-brain barrier phenylalanine transport and individual vulnerability in phenylketonuria

In vivo nuclear magnetic resonance spectroscopy can be used to measure intracerebral phenylalanine (Phe) concentrations in patients with phenylketonuria (PKU). Stationary levels, obtained under free nutrition, as well as time courses after an oral Phe load (100 mg/kg) were investigated in 11 PKU patients and were correlated with the individual clinical outcome. At blood levels around 1.2 mmol/L, brain Phe was 0.41 to 0.73 mmol/L in clinically "typical" patients, but less than 0.15 mmol/L in three untreated, normally intelligent, adult women. Kinetic investigations revealed higher transport Michaelis constants and lower ratios of the brain influx and consumption rates in these women than in the "typical" control patients (Kt,app = 0.45 to 1.10 mmol/L versus 0.10 mmol/L; T(max)/v(met) = 2.55 to 3.19 versus 7.8 to 14.0). Such variations seem to be major causative factors for the individual vulnerability to PKU.     

Role of blood-brain barrier in cerebral homeostasis

A detailed cytogenetic map was constructed around the chromosomal breakpoint of t(8;13) observed in a patient with multiple exostoses. The order of seven loci defined by cosmid clones mapped to 8q23 was determined by means of two-color fluorescence in situ hybridization (FISH) on elongated prophase chromosomes, and localizations of these markers relative to the breakpoint were examined. The results indicated that loci defined by cC18-553 and cC18-1512 flank the breakpoint. By pulsed-field gel electrophoresis of DNA digested with BssHII and Southern hybridization with cC18-1512, DNA from the patient showed a band which was not observed in DNA isolated from either parent. As the normal size of this BssHII fragment is 600 kb, the chromosomal breakpoint probably lies less than 600 kb away from cC18-1512.     

Blood-brain barrier transport of amino acids in healthy controls and in patients with phenylketonuria

Blood-brain barrier permeability to phenylalanine and leucine in four patients with phenylketonuria and in four volunteers was measured five times by the double-indicator method at increasing plasma concentrations of phenylalanine. Based on the permeability-surface area product (PS) from blood to brain (PS1) and on plasma phenylalanine levels, Vmax and the apparent Km for phenylalanine were determined. Statistically significant relationships between plasma phenylalanine and PS1 were established in three out of four volunteers, the average Vmax value being 46.7 nmol/g per min and the apparent Km 0.328 mmol/L. Owing to saturation of the carrier, such a relationship could not be established in the patients. In phenylketonuria, PS1 for phenylalanine and leucine decreased significantly by 55% and 46%, respectively. Transport from brain back to blood, PS2, decreased significantly and cerebral large neutral amino acid net uptake was generally decreased in patients with phenylketonuria. In conclusion, the transport of L-phenylalanine across the human blood-brain barrier follows Michaelis-Menten kinetics. In phenylketonuria, brain permeability to large neutral amino acids is reduced by about 50% and net uptake appears decreased.     

Non-diuretic-based antihypertensive therapy and potassium homeostasis in elderly patients

Aging and hypertension are associated with a progressive decline in renal blood flow and renal function. As a result, physicians planning therapeutic strategies to control blood pressure need to consider these changes and how they relate to potassium homeostasis, particularly in elderly patients. Commonly used antihypertensive drugs such as beta-blockers, angiotensin converting enzyme inhibitors and potassium-sparing diuretics need to be used with increasing caution in patients with declining renal function. This is especially important in patients with diabetes who may also have type IV renal tubular acidosis, and in patients given concomitant therapy with non-steroidal anti-inflammatory drugs. Other therapies such as calcium channel blockers, particularly those that gate atrioventricular nodal conduction, also need to be used with care in people with significant renal insufficiency and hyperkalemia, as this clinical scenario may result in a greater risk of complete heart block.     

Blood-brain barrier permeability during the development of experimental bacterial meningitis in the rat

A 60-year-old man presented with progressive large fiber sensory loss in the right first three fingers and, to a lesser extent, in both fourth and fifth fingers. Electrophysiologic studies were characteristic of chronic sensory demyelinating polyneuropathy, a variant of chronic inflammatory demyelinating polyneuropathy. Plasma exchange was unsuccessful, but intravenous immunoglobulin (IVIG) led to complete recovery of sensation for 2 months, although neurophysiologic abnormalities persisted. A battery of noninvasive tests to measure hand grip strength, tactile sensation at the fingertips, and motor control of prehension during precision grip revealed marked abnormalities in the right hand before IVIG. One month after IVIG, all test results had normalized, but they returned to pretreatment levels after 3 months. Functional evaluation of the hand may be a sensitive method to objectively quantify loss of and changes in cutaneous mechanoreceptor function of the fingers in large fiber sensory neuropathy.     

Study of calcium homeostasis in feline hyperthyroidism

Thirty cats with untreated hyperthyroidism were blood sampled and their calcium homeostatic mechanisms and renal function assessed. The results were compared with those obtained from 38 age-matched control cats. The hyperthyroid group of cats were found to have significantly lower blood ionised calcium and plasma creatinine concentrations and significantly higher plasma phosphate and parathyroid hormone concentrations. Hyperparathyroidism occurred in 77 per cent of hyperthyroid cats, with parathyroid hormone concentrations reaching up to 19 times the upper limit of the normal range. The aetiology, significance and reversibility of hyperparathyroidism in feline hyperthyroidism remains to be established but could have important implications for both bone strength and renal function.     

The regulation of calcium homeostasis in nerve cells

We have reviewed the major cellular elements related to the release and buffering of calcium in neurons. Voltage-operated, chemical-operated calcium channels and mechanisms of stability of intercellular calcium homeostasis (mitochondria, endoplasmic reticulum, calcium binding proteins, calcium exchange and calcium pump) are demonstrated in normal and pathological condition (105 ref.).     

Neurobiological mechanisms involved in antidepressant therapies

Selective serotonin reuptake inhibitors (SSRIs) are effective in alleviating the symptoms of depression. However, clinical improvement is only obtained after several weeks of treatment. SSRIs, when administered acutely to animals, have little effect on synaptic levels of serotonin. This suggests the existence of one or more regulatory mechanisms controlling serotonergic neurotransmission. The firing rate of dorsal raphe serotonergic neurons is under the control of somatodendritic 5-hydroxytryptamine 1A (5-HT1A) autoreceptors, the release of serotonin from nerve terminals is under the control of 5-HT autoreceptors (5-HT1B subtype in rodents, 5-HT1D in other species), whereas the control of the activity of tryptophan hydroxylase, the rate-limiting enzyme of serotonin synthesis, is complex, involving 5-HT1A but possibly other 5-HT receptors including the 5-HT1B/D subtype. During prolonged administration with a SSRI, these three feedback systems become desensitized and their regulatory effects on serotonergic neurotransmission are weakened or lost. This has the effect of allowing the synaptic levels of serotonin to rise with a consequently increased stimulation of one or more types of postsynaptic 5-HT receptor. Thus, it is only after prolonged administration that the pharmacological activity of SSRI is fully expressed in terms of synaptic serotonin levels. This may explain the latency of antidepressant action seen with these drugs in humans. Various other classes of antidepressant therapies (tricyclic antidepressants and monoamine oxidase inhibitor drugs, electroconvulsive therapy) have long-term effects on one or more of the feedback mechanisms such that an increase in synaptic concentrations of serotonin may be a common mechanism of many antidepressant therapies.     

Rat brain salsolinol and blood-brain barrier

Behavioral and anatomical consequences of particularly large intrastriatal injections of ibotenic acid are described. Only in the rat with the largest injection, which encompassed almost the entire frontal lobe, were enduring aphagia and adipsia observed; epileptic attacks were, however, not detectable in this or in any other of the rats. In spite of the massiveness of the lesion, neither remote lesions nor damage to passing fibers was observed. It is therefore suggested to substitute kainic acid by ibotenic acid for the production of local, discrete brain lesions.     

Influence of nonsteroidal anti-inflammatory drugs on calcium efflux in isolated rat renal cortex mitochondria and aspects of the mechanisms involved

In the present study we investigated the influence of several nonsteroidal anti-inflammatory drugs on calcium efflux in isolated rat renal cortex mitochondria in order to assess their potential to disrupt cell calcium homeostasis, as well as aspects of the mechanisms associated with oxidation of mitochondrial pyridine nucleotides (NAD(P)H) and with inhibition of the process by cyclosporin A (CsA). Calcium efflux was estimated with arsenazo III as an indicator and the redox state of NAD(P)H was monitored fluorimetrically at the 366/450 nm excitation/emission wavelength pair. Dipyrone, paracetamol and ibuprofen did not induce calcium efflux even at 1 mM, piroxicam and salicylate were poor inducers, while diclofenac sodium and mefenamic acid were potent inducers releasing calcium even at 20 microM and 10 microM, respectively. In the presence of 10 microM calcium, CsA had no appreciable effect while in the presence of 30 microM calcium it delayed calcium efflux. Oxidation of mitochondrial NAD(P)H, concomitant with calcium efflux and inhibited by CsA, was observed only in the presence of 30 microM calcium. The results suggest that diclofenac sodium and mefenamic acid induce calcium efflux in mitochondria through both a mechanism intrinsic to the mitochondrial membrane permeability transition and a mechanism including the electroneutral Ca2+/nH+ porter.     

Modulation of calcium and potassium currents by lamotrigine

In a prospective noninterventional study of 75 consecutive patients (mean age 71 +/- 12 years) undergoing surgery for colorectal cancer, standard postoperative energy intake was evaluated. Seventeen patients expended 40%-60% of estimated basal energy during hospitalization, 33 patients 60%-80%, 22 patients 80%-100% and three patients 100%-125%. Weight loss was observed in 67 patients (mean loss 4.7 +/- 4.4%) during hospitalization. Men had a significantly higher mean total calorie deficit (p < 0.001), and mean weight loss percentage (p < 0.01), compared to women. Preoperative nutritional status, nutrition-associated complications and length of hospital stay did not change the nutritional support and intake. Correlation analyses resulted in significant associations between gender and total calorie deficit (rs = 0.41, p < 0.01), postoperative weight loss and total calorie deficit (rs = -0.32, p < 0.01), and between postoperative weight loss and length of stay (rs = 0.27, p < 0.05). We concluded that the patients' energy intake was insufficient compared to estimated basal energy expenditure. These results suggest a need for individualized nutritional care, based on each patient's energy needs and on registration of daily calorie intake, all with the aim of increasing energy intake postoperatively in standard hospital care.