Choroid Plexus Calcification Correlates with Cortical Microglial Activation in Humans: A Multimodal PET, CT, MRI Study

BACKGROUND AND PURPOSE

The choroid plexus (CP) within the brain ventricles is well-known to produce cerebrospinal fluid (CSF). Recently, the CP has been recognized as critical in modulating inflammation. MRI-measured CP enlargement has been reported in neuroinflammatory disorders like MS as well as with aging and neurodegeneration. The basis of MRI-measured CP enlargement is unknown. On the basis of tissue studies demonstrating CP calcification as a common pathology associated with aging and disease, we hypothesized that previously unmeasured CP calcification contributes to MRI-measured CP volume and may be more specifically associated with neuroinflammation.

MATERIALS AND METHODS

We analyzed 60 subjects (43 healthy controls and 17 subjects with Parkinson's disease) who underwent PET/CT using 11C-PK11195, a radiotracer sensitive to the translocator protein expressed by activated microglia. Cortical inflammation was quantified as nondisplaceable binding potential. Choroid plexus calcium was measured via manual tracing on low-dose CT acquired with PET and automatically using a new CT/MRI method. Linear regression assessed the contribution of choroid plexus calcium, age, diagnosis, sex, overall volume of the choroid plexus, and ventricle volume to cortical inflammation.

RESULTS

Fully automated choroid plexus calcium quantification was accurate (intraclass correlation coefficient with manual tracing = .98). Subject age and choroid plexus calcium were the only significant predictors of neuroinflammation.

CONCLUSIONS

Choroid plexus calcification can be accurately and automatically quantified using low-dose CT and MRI. Choroid plexus calcification-but not choroid plexus volume-predicted cortical inflammation. Previously unmeasured choroid plexus calcium may explain recent reports of choroid plexus enlargement in human inflammatory and other diseases. Choroid plexus calcification may be a specific and relatively easily acquired biomarker for neuroinflammation and choroid plexus pathology in humans.

Anterior cingulate cortex γ-aminobutyric acid deficits in youth with depression

Abnormally low γ-aminobutyric acid (GABA) levels have been consistently reported in adults with major depressive disorder (MDD). Our group extended this finding to adolescents, and documented that GABA deficits were associated with anhedonia. Here we aimed to confirm our prior finding of decreased brain GABA in youth with depression and explore its associations with clinical variables. Forty-four psychotropic medication-free youth with MDD and 36 healthy control (HC) participants (12-21 years) were studied. Participants represent a combined sample of 39 newly recruited youth (MDD=24) and 41 youth from our previously reported study (MDD=20). GABA levels and the combined resonances of glutamate and glutamine (Glx) were measured in vivo in the anterior cingulate cortex using proton magnetic resonance spectroscopy. Youth with depression exhibited significantly lower GABA levels than HC in both the newly reported (P=0.003) and the combined (P=0.003) samples. When depressed participants were classified based on the presence of anhedonia, only the anhedonic MDD subgroup showed reduced GABA levels compared to HC (P=0.002). While there were no associations between any clinical measures and GABA or Glx levels in the new sample, GABA was negatively correlated with only anhedonia severity in the combined MDD group. Furthermore, in the combined sample, hierarchical regression models showed that anhedonia, but not depression severity, anxiety or suicidality, contributed significant variance in GABA levels. This report solidifies the evidence for a GABA deficit early in the course of MDD, which correlates specifically with anhedonia in the disorder.

Relationships among Cortical Glutathione Levels, Brain Amyloidosis, and Memory in Healthy Older Adults Investigated In Vivo with 1H-MRS and Pittsburgh Compound-B PET

BACKGROUND AND PURPOSE

Oxidative stress has been implicated as an important pathologic mechanism in the development of Alzheimer disease. The purpose of this study was to assess whether glutathione levels, detected noninvasively with proton MR spectroscopy, are associated with brain amyloidosis and memory in a community-dwelling cohort of healthy older adults.

MATERIALS AND METHODS

Fifteen cognitively healthy subjects were prospectively enrolled in this study. All subjects underwent 1H-MR spectroscopy of glutathione, a positron-emission tomography scan with an amyloid tracer, and neuropsychological testing by using the Repeatable Battery for the Assessment of Neuropsychological Status. Associations among glutathione levels, brain amyloidosis, and memory were assessed by using multivariate regression models.

RESULTS

Lower glutathione levels were associated with greater brain amyloidosis in the temporal (P = .03) and parietal (P = .05) regions, adjusted for apolipoprotein E ε4 carrier status. There were no significant associations between glutathione levels and cognitive scores.

CONCLUSIONS

This study found an association between cortical glutathione levels and brain amyloidosis in healthy older adults, suggesting a potential role for 1H-MR spectroscopy measures of glutathione as a noninvasive biomarker of early Alzheimer disease pathogenesis.

Increased ventricular cerebrospinal fluid lactate in depressed adolescents

BACKGROUND

Mitochondrial dysfunction has been increasingly examined as a potential pathogenic event in psychiatric disorders, although its role early in the course of major depressive disorder (MDD) is unclear. Therefore, the purpose of this study was to investigate mitochondrial dysfunction in medication-free adolescents with MDD through in vivo measurements of neurometabolites using high-spatial resolution multislice/multivoxel proton magnetic resonance spectroscopy.

METHODS

Twenty-three adolescents with MDD and 29 healthy controls, ages 12-20, were scanned at 3T and concentrations of ventricular cerebrospinal fluid lactate, as well as N-acetyl-aspartate (NAA), total creatine (tCr), and total choline (tCho) in the bilateral caudate, putamen, and thalamus were reported.

RESULTS

Adolescents with MDD exhibited increased ventricular lactate compared to healthy controls [F(1,41)=6.98, P=0.01]. However, there were no group differences in the other neurometabolites. Dimensional analyses in the depressed group showed no relation between any of the neurometabolites and symptomatology, including anhedonia and fatigue.

CONCLUSIONS

Increased ventricular lactate in depressed adolescents suggests mitochondrial dysfunction may be present early in the course of MDD; however it is still not known whether the presence of mitochondrial dysfunction is a trait vulnerability of individuals predisposed to psychopathology or a state feature of the disorder. Therefore, there is a need for larger multimodal studies to clarify these chemical findings in the context of network function.

Motor cortex glutathione deficit in ALS measured in vivo with the J-editing technique

This study compared in vivo levels of the antioxidant glutathione (GSH) in the motor cortex of 11 ALS patients with those in 11 age-matched healthy volunteers (HV). Using the standard J-edited spin-echo difference MRS technique, GSH spectra were recorded on a 3.0 T GE MR system from a single precentral gyrus voxel. GSH levels expressed as ratios to the unsuppressed voxel tissue water (W) were 31% lower in ALS patients than in HV (p=.005), and 36% lower in ALS than in HV (p=.02) when expressed as ratios to the total creatine peak (tCr), supporting a role for oxidative stress in ALS. Levels of the putative neuronal marker N-acetylaspartate (NAA) relative to W did not differ between ALS and HV (p=.26), but were lower by 9% in ALS than in HV (p=.013) when expressed as ratios relative to tCr. This discrepancy is attributed to small but opposite changes in NAA and tCr in ALS that, as a ratio, resulted in a statistically significant group difference, further suggesting caution in using tCr as an internal reference under pathological conditions.

Assessment of disease severity in late infantile neuronal ceroid lipofuscinosis using multiparametric MR imaging

BACKGROUND AND PURPOSE

LINCL is a uniformly fatal lysosomal storage disease resulting from mutations in the CLN2 gene that encodes for tripeptidyl peptidase 1, a lysosomal enzyme necessary for the degradation of products of cellular metabolism. With the goal of developing quantitative noninvasive imaging biomarkers sensitive to disease progression, we evaluated a 5-component MR imaging metric and tested its correlation with a clinically derived disease-severity score.

MATERIALS AND METHODS

MR imaging parameters were measured across the brain, including quantitative measures of the ADC, FA, nuclear spin-spin relaxation times (T2), volume percentage of CSF (%CSF), and NAA/Cr ratios. Thirty MR imaging datasets were prospectively acquired from 23 subjects with LINCL (2.5-8.4 years of age; 8 male/15 female). Whole-brain histograms were created, and the mode and mean values of the histograms were used to characterize disease severity.

RESULTS

Correlation of single MR imaging parameters against the clinical disease-severity scale yielded linear regressions with R2 ranging from 0.25 to 0.70. Combinations of the 5 biomarkers were evaluated by using PCA. The best combination included ADC, %CSF, and NAA/Cr (R2=0.76, P<.001).

CONCLUSIONS

The multiparametric disease-severity score obtained from the combination of ADC, %CSF, and NAA/Cr whole-brain MR imaging techniques provided a robust measure of disease severity, which may be useful in clinical therapeutic trials of LINCL in which an objective assessment of therapeutic response is desired.

Metabolic response of glioblastoma to superselective intra-arterial cerebral infusion of bevacizumab: a proton MR spectroscopic imaging study

BACKGROUND AND PURPOSE

SIACI of bevacizumab has emerged as a promising novel therapy in the treatment of recurrent GB. This study assessed the potential of (1)H-MRS as an adjunctive technique in detecting metabolic changes reflective of antiproliferative effects of targeted infusion of bevacizumab in the treatment of GB.

MATERIALS AND METHODS

Eighteen patients enrolled in a phase I/II study of SIACI of bevacizumab for treatment of recurrent GB were included. Concurrent MR imaging and (1)H-MRS scans were performed before and after treatment. Five distinct morphologic ROIs were evaluated for structural and metabolic changes on MR imaging and (1)H-MRS, which included enhancing, nonenhancing T2 hyperintense signal abnormality, and multiple control regions. Pre- and post-SIACI of bevacizumab peak areas for NAA, tCho, tCr, as well as tCho/tCr and tCho/NAA ratios, were derived for all 5 ROIs and compared using the Wilcoxon signed-rank test.

RESULTS

A significant median decrease of 25.99% (range -55.76 to 123.94; P = .006) in tCho/NAA was found post-SIACI of bevacizumab relative to pretreatment values in regions of enhancing disease. A trend-level significant median decrease of 6.45% (range -23.71 to 37.67; P = .06) was noted in tCho/NAA posttreatment in regions of nonenhancing T2-hyperintense signal abnormality.

CONCLUSIONS

The results of this (1)H-MRS analysis suggest that GB treatment with SIACI of bevacizumab may be associated with a direct antiproliferative effect, as demonstrated by significant reductions of tCho/NAA after the intervention.

Natural history of MELAS associated with mitochondrial DNA m.3243A>G genotype

OBJECTIVE

To describe the natural history of clinical and laboratory features associated with the m.3243A>G mitochondrial DNA point mutation. Natural history data are needed to obtain prognostic information and for clinical trial planning.

METHODS

We included 85 matrilineal relatives from 35 families with at least 2 visits in this prospective cohort study. Thirty-one were fully symptomatic with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), and 54 were carrier relatives. Evaluations included standardized questionnaires (medical history and daily living functioning), physical examination, neuropsychological testing, and a battery of imaging and laboratory tests. We evaluated changes in clinical and laboratory features over time and survival. Outcomes are reported over a follow-up period of up to 10.6 years (mean 3.8 ± 2.2 years for patients and 5.5 ± 3.0 for carrier relatives).

RESULTS

Neurologic examination, neuropsychological testing, and daily living scores significantly declined in all patients with MELAS, whereas no significant deterioration occurred in carrier relatives. Cerebral MRI scores declined significantly in patients with MELAS. Magnetic resonance spectroscopy estimates of lactate in the lateral ventricles increased over time, and high lactate was associated with increased mortality. Symptom onset in childhood often was associated with worse outcome. Patients with MELAS had a greater death rate than carrier relatives.

CONCLUSIONS

Patients with MELAS carrying the m.3243A>G mutation show a measurable decline in clinical and imaging outcomes. It is hoped that these data will be helpful in anticipating the disease course and in planning clinical trials for MELAS.

A pilot study of the effects of chronic paroxetine administration on hippocampal N-acetylaspartate in generalized anxiety disorder

The neural basis of generalized anxiety disorder (GAD) is poorly characterized. The effect of chronic administration (12 weeks) of paroxetine, a selective serotonin reuptake inhibitor, on N-acetylaspartate (NAA), a marker of neuronal viability, was evaluated in adults with GAD using proton magnetic resonance spectroscopic imaging ((1)H MRSI) at 1.5 T. We hypothesized that, pretreatment abnormalities in hippocampal NAA/creatine (NAA/Cr) would normalize with symptomatic improvement. Nine GAD patients (mean age = 41.7 year; 4 females) received 12 weeks of open-label paroxetine treatment, flexibly dosed up to 60 mg/day. Clinical outcome was assessed with the Hamilton Anxiety Rating Scale (HAM-A). Multislice ( 1)H MRSI scans were performed at unmedicated baseline and following 6 and 12 weeks of treatment. Ten untreated healthy volunteers (HVs) (mean age = 37.1 year; 4 females) received scans at the same intervals. All patients achieved remission (HAM-A <or= 7) by week 12. Compared to HVs, GAD patients showed persistently lower levels of bilateral hippocampal NAA/Cr (17.7% mean decrease; Cohen's d = 1.29) that were maintained across all three time points, despite marked symptom improvement. This pilot study failed to support an association between a hippocampal neuronal marker and anxiolytic response to paroxetine, and suggests further investigation of potential trait-like hippocampal abnormalities in GAD.

Assessing disease severity in late infantile neuronal ceroid lipofuscinosis using quantitative MR diffusion-weighted imaging

BACKGROUND AND PURPOSE

Late infantile neuronal ceroid lipofuscinosis (LINCL), a form of Batten disease, is a fatal neurodegenerative genetic disorder, diagnosed via DNA testing, that affects approximately 200 children in the United States at any one time. This study was conducted to evaluate whether quantitative data derived by diffusion-weighted MR imaging (DWI) techniques can supplement clinical disability scale information to provide a quantitative estimate of neurodegeneration, as well as disease progression and severity.

MATERIALS AND METHODS

This study prospectively analyzed 32 DWI examinations from 18 patients having confirmed LINCL at various stages of disease. A whole-brain apparent diffusion coefficient (ADC) histogram was fitted with a dual Gaussian function combined with a function designed to model voxels containing a partial volume fraction of brain parenchyma versus CSF. Previously published whole-brain ADC values of age-matched control subjects were compared with those of the LINCL patients. Correlations were tested between the peak ADC of the fitted histogram and patient age, disease severity, and a CNS disability scale adapted for LINCL.

RESULTS

ADC values assigned to brain parenchyma were higher than published ADC values for age-matched control subjects. ADC values between patients and control subjects began to differ at 5 years of age based on 95% confidence intervals. ADC values had a nearly equal correlation with patient age (R2=0.71) and disease duration (R2=0.68), whereas the correlation with the central nervous system disability scale (R2=0.27) was much weaker.

CONCLUSION

This study indicates that brain ADC values acquired using DWI may be used as an independent measure of disease severity and duration in LINCL.

Quantitative objective markers for upper and lower motor neuron dysfunction in ALS

OBJECTIVE

To investigate the value of objective biomarkers for upper (UMN) and lower (LMN) motor neuron involvement in ALS.

METHODS

We prospectively studied 64 patients with ALS and its subsets using clinical measures, proton MR spectroscopic imaging ((1)H MRSI), diffusion tensor imaging, transcranial magnetic stimulation, and the motor unit number estimation (MUNE) at baseline and every 3 months for 15 months and compared them with control subjects.

RESULTS

(1)H MRSI measures of the primary motor cortex N-acetyl-aspartate (NAA) concentration were markedly reduced in ALS (p = 0.009) and all UMN syndromes combined (ALS, familial ALS [fALS], and primary lateral sclerosis; p = 0.03) vs control values. Central motor conduction time to the tibialis anterior was prolonged in ALS (p < 0.0005) and combined UMN syndromes (p = 0.001). MUNE was lower in ALS (p < 0.0005) and all LMN syndromes combined (ALS, fALS, and progressive muscular atrophy; p = 0.001) vs controls. All objective markers correlated well with the ALS Functional Rating Scale-Revised, finger and foot tapping, and strength testing, suggesting these markers related to disease activity. Regarding changes over time, MUNE changed rapidly, whereas neuroimaging markers changed more slowly and did not significantly differ from baseline.

CONCLUSIONS

(1)H MR spectroscopic imaging measures of the primary motor cortex N-acetyl-aspartate (NAA) concentration and ratio of NAA to creatine, central motor conduction time to the tibialis anterior, and motor unit number estimation significantly differed between ALS, its subsets, and control subjects, suggesting they have potential to provide insight into the pathobiology of these disorders.

Dichloroacetate causes toxic neuropathy in MELAS: a randomized, controlled clinical trial

OBJECTIVE

To evaluate the efficacy of dichloroacetate (DCA) in the treatment of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS).

BACKGROUND

High levels of ventricular lactate, the brain spectroscopic signature of MELAS, correlate with more severe neurologic impairment. The authors hypothesized that chronic cerebral lactic acidosis exacerbates neuronal injury in MELAS and therefore, investigated DCA, a potent lactate-lowering agent, as potential treatment for MELAS.

METHODS

The authors conducted a double-blind, placebo-controlled, randomized, 3-year cross-over trial of DCA (25 mg/kg/day) in 30 patients (aged 10 to 60 years) with MELAS and the A3243G mutation. Primary outcome measure was a Global Assessment of Treatment Efficacy (GATE) score based on a health-related event inventory, and on neurologic, neuropsychological, and daily living functioning. Biologic outcome measures included venous, CSF, and 1H MRSI-estimated brain lactate. Blood tests and nerve conduction studies were performed to monitor safety.

RESULTS

During the initial 24-month treatment period, 15 of 15 patients randomized to DCA were taken off study medication, compared to 4 of 15 patients randomized to placebo. Study medication was discontinued in 17 of 19 patients because of onset or worsening of peripheral neuropathy. The clinical trial was terminated early because of peripheral nerve toxicity. The mean GATE score was not significantly different between treatment arms.

CONCLUSION

DCA at 25 mg/kg/day is associated with peripheral nerve toxicity resulting in a high rate of medication discontinuation and early study termination. Under these experimental conditions, the authors were unable to detect any beneficial effect. The findings show that DCA-associated neuropathy overshadows the assessment of any potential benefit in MELAS.

Objective tests for upper motor neuron involvement in amyotrophic lateral sclerosis (ALS)

OBJECTIVE

To develop objective markers for upper motor neuron (UMN) involvement in ALS, the value of single-voxel MR spectroscopy (MRS) and transcranial magnetic stimulation (TMS) was studied.

METHODS

Test results of 164 ALS patients who had MRS only (n = 91), TMS only (n = 13), or both (n = 60) were analyzed; also, 11 autopsy examinations were evaluated.

RESULTS

Abnormal test results consistent with UMN involvement were found in 134 patients with clinical UMN signs: 86% on MRS, 77% on TMS, and 70% on MRS and TMS together. Among 30 patients with solely LMN signs (progressive muscular atrophy), UMN results were found in 63% on MRS, 63% on TMS, and 46% on both tests together. There was a significant association of the degree of abnormal N-acetyl aspartate/creatine ratios with UMN signs (p = 0.01). The sensitivity to detect UMN involvement was 0.86 for MRS (specificity 0.37) and 0.77 for TMS (specificity 0.38). At autopsy, all 11 patients had pathologic UMN abnormalities, including 4 with normal MRS and 1 with normal TMS in life.

CONCLUSIONS

MRS is highly sensitive, somewhat more than TMS, and shows good correlation with clinical UMN signs. Combining MRS and TMS results in the same patient with further refinement may help in the early diagnosis of ALS.

Cerebral lactic acidosis correlates with neurological impairment in MELAS

OBJECTIVE

To evaluate the role of chronic cerebral lactic acidosis in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS).

METHODS

The authors studied 91 individuals from 34 families with MELAS and the A3243G point mutation and 15 individuals from two families with myoclonus epilepsy and ragged red fibers (MERRF) and the A8344G mutation. Subjects were divided into four groups. Paternal relatives were studied as controls (Group 1). The maternally related subjects were divided clinically into three groups: asymptomatic (no clinical evidence of neurologic disease) (Group 2), oligosymptomatic (neurologic symptoms but without the full clinical picture of MELAS or MERRF) (Group 3), and symptomatic (fulfilling MELAS or MERRF criteria) (Group 4). The authors performed a standardized neurologic examination, neuropsychological testing, MRS, and leukocyte DNA analysis in all subjects.

RESULTS

The symptomatic and oligosymptomatic MELAS subjects had significantly higher ventricular lactate than the other groups. There was a significant correlation between degree of neuropsychological and neurologic impairment and cerebral lactic acidosis as estimated by ventricular MRS lactate levels.

CONCLUSIONS

High levels of ventricular lactate, the brain spectroscopic signature of MELAS, are associated with more severe neurologic impairment.

Measurement of intramyocellular lipid levels with 2-D magnetic resonance spectroscopic imaging at 1.5 T

Intramyocellular lipid (IMCL) plays an important role in the study of metabolism in vivo. Magnetic resonance spectroscopy (MRS) studies of IMCL are usually performed with clinical 1.5-T magnetic resonance imaging (MRI) systems and have employed the single-voxel MRS technique. The present study reports the results of our systematic evaluation of the ability of single- and multi-voxel MRS to yield high-quality, contamination-free IMCL levels from the tibialis anterior (TA) muscle. A clinical, 1.5-T, whole-body MRI scanner was used to measure IMCL with a standard knee coil, head coil, or a 3-cm receive-only surface coil with a body coil transmit. Excellent IMCL spectra were obtained in healthy males in only 8 min from multiple 0.25-cm(3) voxels using the surface coil receive/body coil transmit in conjunction with the standard PRESS spectroscopic imaging (SI) technique. This approach provided the spatial resolution and voxel placement flexibility permitting optimal separation of IMCL and extramyocellular lipid. Our findings demonstrate the potential of the SI approach.

Metabolism of alternative substrates and the bioenergetic status of EMT6 tumor cell spheroids

In order to evaluate the ability of EMT6/Ro multicellular spheroids to utilize various pathways of energy production, (13)C and (31)P MRS have been employed to monitor the metabolism of glucose, glutamine, acetate and propionate. EMT6/Ro spheroids perfused with culture medium containing 5.5 mM glucose maintain stable levels of nucleotide triphosphates (NTP) and phosphocreatine (PCr) for up to 48 h, even in the absence of glutamine. The metabolism of 1-(13)C-glucose was almost entirely to 3-(13)C-lactate (88 +/- 12%, n = 7), even though the perfusion medium was equilibrated with 95% O(2). Labeling was also observed in other glycolytic metabolites, primarily alanine and alpha-glycerolphosphate. A low level of (13)C labeling in glutamate, indicative of mitochondrial oxidative metabolism (TCA cycle), was consistently detected when spheroids were perfused with 1-(13)C-glucose, almost exclusively in the C4 position of glutamate. Labeling of glutamate C2 and C3 was always less than 20% of the labeling in C4 and was usually undetectable. No evidence of adjacent carbon labeling in individual glutamate molecules (indicative of multiple cycles of label incorporation) was found, even in high-resolution (13)C NMR spectra of extracts from cells or spheroids. Despite the predominantly glycolytic metabolism of glucose, the mitochondrial substrate glutamine (2 mM, in the presence of < or =0.5 mM glucose from fetal bovine serum), supported stable levels of NTP and PCr in the tumor cells for up to 12 h. In the presence of 2.5 mM acetate, the bioenergetic status of cells in EMT6 spheroids declined slowly but measurably, and no incorporation of label from 2-(13)C-acetate into other metabolites was detected either in intact perfused spheroids or in high-resolution spectra of extracts. In contrast, when the anaplerotic TCA cycle substrate 3-(13)C-propionate replaced acetate, the high-energy phosphate levels in EMT6/Ro spheroids were somewhat reduced, but stabilized at a new lower level. Incubation of spheroids with 3-(13)C-propionate (with natural abundance glucose and glutamine) resulted in label detectable in the C2 and C3 of glutamate, but the primary labeled compound was methylmalonate, an intermediate in propionate metabolism. Addition of vitamin B(12), a cofactor for methylmalonyl CoA reductase, to the growth medium 24 h prior to perfusion with propionate resulted in the elimination of the methylmalonate resonance. A variety of 2- and 3-labeled metabolites were detected, including succinate, malate and glutamate. Labeling of C2 and C3 of lactate implicated cytoplasmic malic enzyme activity.

Hippocampal pathology in schizophrenia: magnetic resonance imaging and spectroscopy studies

The hippocampus is a site of previously reported structural and functional abnormalities in schizophrenia. We used magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy (MRS) to measure gray matter volumes, the neuronal marker N-acetylaspartate (NAA), and the combination of glutamate (Glu), glutamine (Gln), and gamma-aminobutyric acid (GABA), designated Glx. Measurements were obtained of the medial temporal lobe, centered on the hippocampus, in 10 male patients with schizophrenia (3 neuroleptic-medicated and 7 medication-free), and 10 matched normal volunteers. MRI volumetric measurements and MRS data obtained with short echo time (TE=20 ms) one-dimensional STEAM chemical shift imaging (CSI) on a GE 1.5 Tesla Signa system were analyzed. A laterality index ¿(L-R)/(L+R) was generated from the ratio of Glx to choline-containing compounds (Cho) to test asymmetry changes. Reliability of the MRS measures was assessed with five test-retest studies of healthy volunteers and showed coefficients of variation (CV) in the range of 36-44% for the MRS ratios and standard deviations (S.D.) of 0.15-0.17 for the laterality indices. The Glx/Cho laterality index showed a relative right-sided excess in this region in the patients (-0.23+/-0.20) compared to the controls (+0.06+/-0.20), which was not confounded by tissue composition or placement variability of the MRS voxels. Hippocampal volume deficit and asymmetry were not significant, and other MRS measures showed no differences between patients and controls. The preliminary finding of a lateralized abnormality in Glx is consistent with postmortem findings of asymmetric neurochemical temporal lobe abnormalities in schizophrenia.

Leukoencephalopathy and raised brain lactate from heroin vapor inhalation ("chasing the dragon")

BACKGROUND

Inhalation of heated heroin vapor ("chasing the dragon"), which is gaining popularity among drug users seeking to avoid the risks of parenteral drug administration, can produce progressive spongiform leukoencephalopathy.

METHODS

We studied the clinical phenotype and course, MRI, MRS, and brain pathology in the first American patients described with this syndrome.

RESULTS

Two of the three heroin users studied inhaled heroin pyrolysate together daily over the course of 2 weeks. They developed ataxia, dysmetria, and dysarthria. Patient 1 progressed to an akinetic mute state with decorticate posture and subsequent spastic quadriparesis. Patient 2 developed a mild spastic quadriparesis and gait freezing. Patient 3 was asymptomatic following less heroin exposure. Brain MRI showed diffuse, symmetrical white matter hyperintensities in the cerebellum, posterior cerebrum, posterior limbs of the internal capsule, splenium of the corpus callosum, medial lemniscus, and lateral brainstem. MRS showed elevated lactate. Brain biopsy (Patient 1) showed white matter spongiform degeneration with relative sparing of U-fibers; electron microscopy revealed intramyelinic vacuolation with splitting of intraperiod lines. Progressive deterioration occurred in Patients 1 and 2 over 4 weeks. Both were treated with antioxidants including oral coenzyme Q, and clinical improvement occurred. Patient 1 recovered nearly completely over 24 months. Patient 2 improved, but developed a delayed-onset cerebellar hand tremor. Both still have white matter abnormalities on MRI and MRS.

CONCLUSIONS

Elevated lactate in white matter and the possible response to antioxidants suggests mitochondrial dysfunction in progressive spongiform leukoencephalopathy following inhalation of heated heroin vapor.

Motor neuron diseases: comparison of single-voxel proton MR spectroscopy of the motor cortex with MR imaging of the brain

PURPOSE

To evaluate single-voxel proton magnetic resonance (MR) spectroscopy in detection of abnormality of the upper motor neuron in patients with motor neuron diseases.

MATERIALS AND METHODS

In 43 of 50 patients with motor neuron disease and 14 of 14 control subjects, matching sets of MR spectra were obtained in the left and right motor cortex. The ratio of N-acetylaspartate (NAA) to creatine (Cr) was derived from peak area measurements. Mean ratios were calculated for control subjects and several patient groups, including patients with amyotrophic lateral sclerosis (ALS) or primary lateral sclerosis (PLS). MR images were evaluated for corticospinal tract hyperintensity and central sulcus dilatation.

RESULTS

Mean NAA/Cr values were significantly different between control subjects and the ALS or PLS groups (P < .05). With an optimal cutoff of 2.5, NAA/Cr values were abnormal in 15 (79%) of 19 patients with ALS, 12 (67%) of 18 patients with PLS, and one (7%) of 14 control subjects. Corticospinal tract hyperintensity, central sulcus enlargement, or both were found in 43% of the ALS group, 24% of the PLS group, and 7% of the control group.

CONCLUSION

NAA/Cr values determined at single-voxel proton MR spectroscopy are more sensitive than are standard findings at MR imaging in the detection of upper motor neuron disease.

In vivo chemical shift imaging of gamma-aminobutyric acid in the human brain

A gradient-based multiple quantum filtering method is presented for in vivo chemical shift imaging of gamma-aminobutyric acid (GABA) in the human brain, which provides effective suppression of the overlapping creatine singlet with close to optimal detection efficiency. It is shown by product operator calculations and coherence pathway analysis that under conditions of no B1 and B0 inhomogeneity gradient filtering retains 75% of the two outer resonance lines of the GABA-4 triplet with no creatine contamination. A variation of the method with 100% retention of the GABA-4 outer resonance lines but higher sensitivity to B1 inhomogeneity is also discussed. By using a localized version of the sequence with an 8-cm surface coil for transmission and detection, it was found in phantom experiments at 2.1 T that a 69% signal retention of the two outer resonance lines of the GABA-4 triplet was achieved relative to a spin echo sequence with inhibition of GABA J modulation. A creatine suppression ratio of 2000:1 was measured. The use of the method for chemical shift imaging of GABA is demonstrated by coronal images obtained from phantoms and from the occipital lobe of a healthy volunteer.

CADASIL in a North American family: clinical, pathologic, and radiologic findings

OBJECTIVE

To expand the reported phenotypic range of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL).

BACKGROUND

Despite numerous patient reports, our knowledge of the phenotypic range of CADASIL remains incomplete.

METHOD

We performed clinical, pathologic, and radiologic examinations on members of a family with CADASIL.

RESULTS

The proband is a 61-year-old man with a history of migraine and depression who has experienced multiple subcortical infarctions resulting in a stepwise decline. Neuropsychological testing documented a dementia syndrome with frontal lobe features and neurologic examination noted a left hemiparesis and a right-sided palmomental reflex. Brain biopsy with light microscopy revealed a nonatherosclerotic small-vessel angiopathy with periodic acid-Schiff positive granular changes in the media and white matter gliosis, with unremarkable cortex. Genetic testing confirmed a Notch3 mutation. The proband's first cousin has a history of depression, one seizure possibly resulting from an acute stroke, and a learning disorder. Neuropsychological testing demonstrated deficits in executive function and neurologic examination noted persistent extraneous adventitial movements, poor coordination, and primitive reflexes. Skin biopsy with electron microscopy demonstrated granular osmiophilic material within the basement membrane of vascular smooth muscle cells, which is considered to be pathognomonic of CADASIL. The proband's older son and younger son have histories of migraine and depression, respectively, and both also had learning disorders. MRI revealed diffuse white matter disease extending into the temporal lobes, and lacunar infarctions in these four nonhypertensive patients. Other family members have experienced migraine, recurrent stroke, dementia, and depression.

CONCLUSIONS

CADASIL is a genetic basis for vascular dementia that may be manifest earlier in life than previously reported.

Detection of tumor response to chemotherapy by 1H nuclear magnetic resonance spectroscopy: effect of 5-fluorouracil on lactate levels in radiation-induced fibrosarcoma 1 tumors

The aim of this study was to evaluate the ability of noninvasive 1H magnetic resonance spectroscopic imaging to detect the response of radiation-induced fibrosarcoma 1 tumors to treatment with 5-fluorouracil (5-FU). Parallel magnetic resonance studies of tumor extracts and assays of apoptosis and necrosis in tumor sections were performed to elucidate the mechanism underlying the changes detected in spectra in vivo. Cell death in tumors after a single dose of 5-FU (165 mg/kg, i.p.) was characterized by increased apoptosis, decreased necrotic fraction, and tumor shrinkage within 48 h. No significant change in normalized trimethylamine and lactate levels was observed during 3 days of untreated tumor growth. Following treatment with 5-FU, normalized intensities of both trimethylamine and lactate decreased significantly from pretreatment levels within 24 h and continued to decline at 48 h. The decrease in lactate levels determined by spectroscopic imaging in vivo was also observed in perchloric acid extracts of radiation-induced fibrosarcoma 1 tumors. Possible mechanisms for the decrease of tumor lactate levels include increased blood flow and decreased glycolytic rate. Unlike lactate, changes in normalized trimethylamine levels observed in vivo were not observed in tumor extracts. The mechanism underlying the anomalous decrease in the in vivo trimethylamine level is under investigation. These findings demonstrate that lactate is a reliable and sensitive indirect indicator of response to 5-FU in at least one tumor model and point to the possible clinical utility of this resonance as an index of clinical tumor response to chemotherapy.

Delayed posthypoxic demyelination. Association with arylsulfatase A deficiency and lactic acidosis on proton MR spectroscopy

Delayed demyelination is a rare and poorly understood complication of hypoxic brain injury. A previous case report has suggested an association with mild-to-moderate deficiency of arylsulfatase A. We describe a 36-year-old man who recovered completely from an episode of hypoxia related to drug overdose, and 2 weeks later progressed from a confusional state to deep coma. MRI showed diffuse white matter signal changes, and brain biopsy demonstrated a noninflammatory demyelinating process. Proton magnetic resonance spectroscopy revealed elevated choline and lactate and reduced N-acetyl aspartate signal in the affected white matter, consistent with demyelination and a shift to anaerobic metabolism. Arylsulfatase A activity from peripheral leukocytes was approximately 50% of normal, consistent with a "pseudodeficiency" phenotype. These findings confirm the hypothesis that relative arylsulfatase A deficiency predisposes susceptible individuals to delayed posthypoxic leukoencephalopathy and implicates lactic acidosis in the pathogenesis of this disorder.

Effects of blood flow modifiers on tumor metabolism observed in vivo by proton magnetic resonance spectroscopic imaging

Perfusion plays a key role in tumor proliferation and therapeutic response. Tumor heterogeneity necessitates use of the highest spatial resolution to monitor metabolic correlates of blood flow changes. This is best achieved with 1H NMR spectroscopy, which permits noninvasive acquisition of high resolution spectroscopic images (SI) of subcutaneous tumors in a relatively short scan time (e.g., 12-25 microliters voxels with signal-to-noise ratio 7:1 in 30 min at 4.7 T). This study seeks to identify 1H spectroscopic indices of tumor blood flow. Proton SI of subcutaneous murine RIF-1 tumors were recorded (a) before and after administration of nicotinamide (1 g/kg) to increase blood flow, and (b) before and after hydralazine (10 mg/kg) to decrease flow. Nicotinamide produced a significant decrease in the total choline peak amplitudes, which subsequent high resolution NMR spectroscopy of tumor extracts revealed to be due to decreases in phosphocholine and glycerophosphocholine. The deamidation of nicotinamide to nicotinic acid, which is known to have hypolipidemic effects and to stimulate the formation of prostaglandins, may have sufficiently altered lipid metabolism to affect the in vivo concentration of the NMR-visible choline-containing compounds. The main effect of hydralazine was a significant increase of lactate, which is consistent with a reduction of tumor blood flow.

Effect of aminophylline on high-energy phosphate metabolism and fatigue in the diaphragm

BACKGROUND

Diaphragmatic fatigue causes respiratory failure, for which aminophylline has been used as therapy. Because the mechanism of action of aminophylline in reversing diaphragmatic fatigue is unclear, we used in vivo 31P magnetic resonance spectroscopy (MRS) to determine the relation between diaphragmatic activation, force output, and aerobic metabolism.

METHODS

Bilateral phrenic stimulation was used to pace the diaphragm in pentobarbital-anesthetized piglets (6-10 weeks old; n = 44). Esophageal and abdominal pressures were measured to calculate transdiaphragmatic pressure (Pdi) (Pdi = abdominal pressure-esophageal pressure) as an index of force output. Activation was determined by the amplitude of the compound action potential of the diaphragmatic electromyogram. Aerobic metabolism was assessed with a 31P MRS surface coil on the right hemidiaphragm with the animal in a 4.7-T magnet. The animals were divided into four groups based on aminophylline loading dose: saline, aminophylline 10 mg/kg (A10), aminophylline 20 mg/kg (A20), and aminophylline 40 mg/kg (A40). After aminophylline loading the diaphragm was paced for 25 min followed by a 10-min recovery.

RESULTS

Aminophylline concentrations were 12.2 +/- 0.7, 21.9 +/- 2.4, and 44.9 +/- 3.6 mg/l in the A10, A20, and A40 groups, respectively. Compound action potential amplitude decreased in all groups by 30% after 25 min of pacing. Conversely, Pdi remained at 100 +/- 3% of the initial value after 5 min of pacing in the A40 group but decreased to 75 +/- 3% in the saline group. Pdi recovered completely (103 +/- 17%) in the A40 group but remained depressed (72 +/- 6%) in the saline group. Pdi values were intermediate in the A10 and A20 groups. MRS data revealed inadequate energy supply/demand ratio in the saline group such that the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) increased to 1.01 +/- 0.09 after 5 min of pacing. Pi/PCr remained unchanged in the A40 group and was intermediate in the A10 and A20 groups. beta-Adenosine triphosphate and intracellular pH did not differ among groups or as a function of pacing. Diaphragmatic blood flow increased from a resting value of 35-60 to 300-410 ml.min-1 x 100 g-1 during pacing in all groups and was not affected by aminophylline dose.

CONCLUSIONS

Aminophylline, in a dose-dependent fashion, delays the onset of fatigue and improves recovery from fatigue. Delayed fatigue is associated with improved aerobic metabolism as reflected in a low Pi/PCr ratio.

Single-scan in vivo lactate editing with complete lipid and water suppression by selective multiple-quantum-coherence transfer (Sel-MQC) with application to tumors

A novel single-scan selective homonuclear multiple-quantum coherence-transfer technique, Sel-MQC, is presented that achieves lactate editing with complete lipid and water suppression. The method is suitable for studying tissues with high fat content and those subject to substantial motion. Frequency-selective excitation is employed to selectively prepare lactate into its multiple-quantum states; lipid and water are left in the single-quantum modes and eliminated by the multiple-quantum selection gradients. The efficiency of lipid suppression is monitored by a 2D Sel-MQC experiment which separates lipid and lactate along the multiple-quantum-evolution dimension. The spatial distribution of lactate can be imaged by the spectroscopic imaging version of Sel-MQC. Sel-MQC sequences were demonstrated both in phantoms and in vivo, using subcutaneously implanted murine EMT6 tumors.

Application of the magnetization-inversion-transfer technique to the transport of 7Li+, 23Na+, and 39K+ ions through the gramicidin channel and the M2 delta transmembrane domain of the nicotinic acetylcholine receptor

The magnetization-inversion-transfer NMR technique has been used to determine the activation enthalpy (delta H not equal to) for the transport of the 7Li+, 23Na+, and 39K+ ions through the gramicidin channel incorporated into vesicle membranes. The activation enthalpy obtained for the overall transport of the monovalent cations was found to be in agreement with the selectivity order and single-channel ion conductance values. Furthermore, the magnetization-inversion-transfer technique was also found to be applicable to the channel formed from a synthetic peptide having the same amino acid sequence as the M2 delta transmembrane segment of the nicotinic acetylcholine receptor.

Band-selective spin echoes for in vivo localized 1H NMR spectroscopy

This study describes a new single spin-echo spatial localization sequence, BASSALE or BAnd-Selective Spin echo Acquisition for Localized Editing, that overcomes a number of the limitations of the STEAM and PRESS volume selection pulse sequences. It achieves conformal volume localization in a single shot by spatially tailored suppression of all magnetization outside a 2D region of interest followed by selection of a single orthogonal slice. This separation of spatial localization from the echo formation process has permitted use of a spectrally selective cosine-modulated sinc refocusing pulse to acquire localized 1H spectra with the water suppression efficiency of STEAM and the sensitivity of PRESS. Echoes formed by such spectrally selective pulses have been termed band-selective spin echoes. The BASSALE sequence attains shorter echo times than PRESS, inhibits scalar spin-spin interactions to permit localized editing and T2 relaxometry of metabolites with J-coupled spins (e.g., lactate), is insensitive to homonuclear multiple-quantum and polarization transfer effects, and can be made sensitive or insensitive to spin displacement effects. Applications are shown both with phantoms and in situ in the rat brain.

Glucose metabolism in RIF-1 tumors after reduction in blood flow: an in vivo 13C and 31P NMR study

Low pH appears to enhance the effectiveness of therapeutic hyperthermia. 13C and 31P NMR spectroscopy have been employed to examine the possibility that elevating glucose in a solid tumor while simultaneously reducing tumor blood flow would induce a more profound acidosis than either treatment alone. When blood flow in RIF-1 tumors was acutely reduced by administration of hydralazine and additional glucose was delivered locally by intratumoral injection, tumor acidosis (as determined by 31P NMR spectroscopy) during the period of reduced blood flow was not enhanced, relative to administration of hydralazine alone. Tumor NTP/P1 ratios decreased significantly within 20 min of hydralazine administration, whether or not glucose was injected, although NTP/P1 ratios were slightly higher in tumors that received extra glucose. Tumor lactate concentrations were not significantly different in glucose-supplemented tumors, despite glucose concentrations that were 4 to 5 times higher. When the added glucose was labeled with 13C, no correlation was detected between the pH in an individual tumor and the intensity of the 3-[13C]-lactate resonance in the same tumor.

Sensitivity and localization enhancement in multinuclear in vivo NMR spectroscopy by outer volume presaturation

Sampling of the entire brain tissue in 1H NMR metabolic studies is complicated by the presence of intense pericranial lipid and tissue water resonances, which can obscure metabolite resonances. This study extends the concept of localization by outer volume suppression (OVS) to achieve fully conformal in vivo localization, sampling at least 95% of rat brain with complete elimination of pericranial lipids. This has permitted acquisition of lipid-free short echo time (7.5 ms), high spatial resolution 2D spectroscopic images exhibiting high sensitivity and information content in relatively short measurement time (68 min). Incorporation of spatially tailored OVS into existing methods (e.g., DRESS, STEAM, PRESS) extends their localization efficiency and performance. Two single spin-echo localization sequences have been described that attain much shorter echo times or achieve better water suppression than PRESS. One of these sequences allows editing and relaxometric studies of J-coupled spins. The methods described are suitable for in vivo localization of most tissues and can be applied with any biologically important nucleus.

Diaphragmatic fatigue assessed by 31P-magnetic resonance spectroscopy in vivo

We tested whether fatigue of the piglet diaphragm is associated with inadequate oxidative metabolism as measured by magnetic resonance spectroscopy (MRS). An MRS measured ratio of inorganic phosphate to phosphocreatine (Pi/PCr) > or = 1 was taken as evidence of inadequate oxidative metabolism. Piglets (n = 10) underwent phrenic nerve pacing for 90 min with stimulation frequency of 30 Hz and duty cycle of 0.33. In a separate group of six piglets PCr, Pi, ATP, and intracellular pH were measured by in vivo MRS, and diaphragmatic blood flow was measured with radioactive microspheres at control, 2, 10, 45, 60, and 90 min of pacing. Transdiaphragmatic pressure fell from 25 +/- 3 to 15 +/- 2 mmHg (61 +/- 5%) at 2 min and remained depressed in a separate group of four piglets (P < 0.05). Conversely, compound action potential amplitude remained constant for the first 10 min of pacing and fell to 68 +/- 5% of control at 45 min (P < 0.05). Pi/PCr rose from a control value of 0.32 +/- 0.06 to 0.92 +/- 0.23 at 2 min and 0.79 +/- 0.03 at 10 min (P < 0.05) before returning toward control at 45-90 min. O2 delivery increased from 4.6 +/- 1.2 to 24.7 +/- 4.8 ml.min-1.100 g-1 at 2 min and 18.4 +/- 2.2 ml.min-1.100 g-1 at 10 min (P < 0.05) but then fell to lower levels at 45-90 min. ATP and intracellular pH remained constant except for a decline in pH to 6.98 +/- 0.09 at 45 min (P < 0.05) from the control value of 7.26 +/- 0.06.(ABSTRACT TRUNCATED AT 250 WORDS)

23Na-NMR study of ion transport across vesicle membranes facilitated by phenylalanine analogs of gramicidin

The transport of Na+ ions across phosphatidylcholine/phosphatidylglycerol large unilamellar vesicle membranes facilitated by phenylalanine analogs of gramicidin A has been studied using 23Na-NMR spectroscopy. The four analogs studied were Phe9-, Phe11-, Phe13-and Phe15-gramicidin A. These analogs were found to transport Na+ ions in the following order Phe15 > Phe13 > Phe11 > Phe9. The entropy and enthalpy of activation for the transport of Na+ ions were determined for each analog. A correlation is made between the activation enthalpies and the single channel conductance values of the analogs.

Determination of absolute phosphate metabolite concentrations in RIF-1 tumors in vivo by 31P-1H-2H NMR spectroscopy using water as an internal intensity reference

The absolute metabolite quantification method of Thulborn and Ackerman [J. Magn. Reson. 55, 357 (1983)] in which the tissue water proton signal is used as an internal intensity standard and its more recent variation in which NMR peak intensities are referenced to that of the natural abundance deuterium signal of water [Li et al., SMRM Abstr. 2, 825 (1988); Song et al., Magn. Reson. Med. 25, 45 (1992) have been implemented to obtain absolute phosphate metabolite concentrations in subcutaneous RIF-1 tumors during untreated growth and following treatment with 5-fluorouracil. The equivalence of these two hydrogen isotopes as intensity standards and the validity of their use in the determination of absolute metabolite concentrations in vivo by NMR has been demonstrated. On matched in vivo and extract tumor samples (n = 5), excellent agreement has been obtained between nucleoside triphosphate concentrations determined by NMR and those derived by HPLC analysis for the control tumors. Following 3 days of untreated growth, absolute concentrations of phosphate metabolites in RIF-1 tumors (n = 10) decreased significantly, except for the Pi concentration which did not vary. For the treated tumors (n = 10) there were no changes in metabolite concentrations except for a decrease in the PCr and, possibly, Pi concentrations. The PCr/Pi ratio in the latter tumors did not change. These observations suggest that changes in absolute metabolite concentrations may be more sensitive indices of response to therapy than changes in metabolite peak amplitude ratios, a parameter commonly used to express in vivo NMR data.

Thermodynamic parameters for the binding of divalent cations to gramicidin A incorporated into a lipid environment by Tl-205 nuclear magnetic resonance

Thermodynamic parameters, enthalpy and entropy, for the binding of the divalent cations, Mg+2, Ca+2, Sr+2, Ba+2, and Cd+2, to gramicidin A, incorporated into lysophosphatidylcholine, have been determined using a combination of Tl-205 nuclear magnetic resonance spectroscopy and competition binding. The binding process is thermodynamically driven by the enthalpy and not the entropy. The enthalpy values are related to the process involving the transfer of cations from an aqueous environment to an amide environment. A comparison is made between the thermodynamic parameters for the binding of monovalent and divalent cations to gramicidin A to illustrate the channel blocking ability of the divalent cations with respect to monovalent cation transport.

TI-205 nuclear magnetic resonance determination of the thermodynamic parameters for the binding of monovalent cations to gramicidins A and C

Thermodynamic parameters for the binding of the monovalent cations, Li+, Na+, K+, Rb+, Cs+, NH4+, TI+, and Ag+, to gramicidin A and for the binding of TI+ to gramicidin C, incorporated into lysophosphatidylcholine, have been determined using a combination of TI-205 nuclear magnetic resonance spectroscopy and competition binding. The thermodynamic parameters, enthalpy and entropy, are discussed in terms of a process involving the transfer of cations from an aqueous to amide environment.

23Na-nuclear magnetic resonance investigation of gramicidin-induced ion transport through membranes under equilibrium conditions

A technique for investigating the gramicidin-facilitated transport of Na+ ions across lipid bilayers of large unilamellar vesicles under the condition of ionic equilibrium has been developed using a combination of heat incubation of the gramicidin with the vesicles and 23Na-nuclear magnetic resonance (NMR) spectroscopy. Isolation of the two 23Na-NMR signals from the intra- and extravesicular Na+ with the shift reagent, dysprosium (III) tripolyphosphate, allows the equilibrium flux of Na+ through the gramicidin channels to be detected and treated as a two-site exchange process. This study indicates that the transport of Na+ through gramicidin channels is second order with respect to the gramicidin concentration.

Investigation of the interaction between thallous ions and gramicidin A in dimyristoylphosphatidylcholine vesicles: a thallium-205 NMR equilibrium study

This study reports the first direct observation of multiple occupancy of the gramicidin A channel by Tl+ ions. 205Tl NMR has been used to study the equilibrium binding of Tl+ by gramicidin A incorporated in sonicated dimyristoylphosphatidylcholine vesicles. It is shown that only multiple-channel occupancy can account for the 205Tl chemical shifts measured. The data are analyzed to yield the equilibrium association constants of 450-600 and 5-20 M-1 for the binding of the first and the second ions at 34 degrees C, respectively.