Noninvasive evaluation of liver repopulation by transplanted hepatocytes using 31P MRS imaging in mice

Hepatocyte transplantation (HT) is being explored as a substitute for liver transplantation for the treatment of liver diseases. For the clinical application of HT, a preparative regimen that allows preferential proliferation of transplanted cells in the host liver and a noninvasive method to monitor donor cell engraftment, proliferation, and immune rejection would be useful. We describe an imaging method that employs the creatine kinase (CK) gene as a marker of donor hepatocytes. Creatine kinase is unique among marker genes, because it is normally expressed in brain and muscle tissues and is therefore not immunogenic. Preferential proliferation of transplanted CK-expressing hepatocytes was induced by preparative hepatic irradiation and expression of hepatocyte growth factor using a recombinant adenoviral vector. CK is normally not expressed in mouse liver and its expression by the donor cells led to the production of phosphocreatine in the host liver, permitting (31)P magnetic resonance spectroscopic imaging of liver repopulation by engrafted hepatocytes. In conclusion, this study combined a noninvasive imaging technique to assess donor hepatocyte proliferation with a preparative regimen of partial liver irradiation that allowed regional repopulation of the host liver. Our results provide groundwork for future development of clinical protocols for HT.

SENSE imaging with a quadrature half-volume transverse electromagnetic (TEM) coil at 4T

PURPOSE

To demonstrate the feasibility of high-field SENSE imaging of large objects, such as the human head, using a semicircular (half-volume) coil for both transmission and multi-channel reception.

MATERIALS AND METHODS

As a proof of concept, we present experimental data obtained using a seven-element half-volume (180 degrees of arc) transmit/receive quadrature transverse electromagnetic (TEM) coil. SENSE images of the human brain were acquired with a reduction factor of R=2, using two degenerate linear modes of the same coil as independent receive channels at 4T. Since the need for additional hardware (i.e., a separate set of receive coils) is eliminated, the design can be substantially simplified.

RESULTS

The experimental data demonstrate that linear modes of the half-volume TEM coil have essentially no noise correlation, and their sensitivity profiles satisfy the requirement for small g-factors. Also, this type of coil provides efficient transmission with a relatively large uniform region and a reception profile that is more uniform than that of the surface coils.

CONCLUSION

We demonstrate the feasibility of SENSE imaging using a half-volume coil. Half-volume coils allow reduced total power deposition compared to full-volume coils, and may replace the latter in body imaging applications in which the target region of interest (ROI) is smaller than the entire torso.

Imaging β-galactosidase activity using 19F chemical shift imaging of LacZ gene-reporter molecule 2-fluoro-4-nitrophenol-β-d-galactopyranoside

2-Fluoro-4-nitrophenol-beta-D-galactopyranoside (OFPNPG) belongs to a novel class of NMR active molecules (fluoroaryl-beta-D-galactopyranosides), which are highly responsive to the action of beta-galactosidase (beta-gal). OFPNPG has a single 19F peak (-55 ppm relative to aqueous sodium trifluoroacetate). Upon cleavage by beta-gal, the pH sensitive aglycone 2-fluoro-4-nitrophenol (OFPNP) is observed at a chemical shift of -59 to -61 ppm. The chemical shift response is sufficient to observe beta-gal activity using chemical shift imaging (CSI). 19F CSI studies of enzyme activity and lacZ gene expression in 9L-glioma and MCF7 breast cancer cells are presented, providing further evidence for the utility of OFPNPG as a gene-reporter molecule for future in vivo studies.

Endothelin in a murine model of cerebral malaria

Cerebral malaria (CM) remains a deadly complication of Plasmodium falciparum infection, and children are at high risk of developing encephalopathy as a result of CM. This is probably a consequence of the activation of many of the inflammatory cytokines as well as the glial cells and the vascular endothelium in the brain. We have previously demonstrated that there is a striking reduction in cerebral blood flow by magnetic resonance imaging when mice are infected with Plasmodium berghei ANKA (PbA), and we now demonstrate a possible role for endothelin (ET-1) in the pathogenesis of CM. The brains of female C57BL/6 mice with PbA infection were examined at Day 5 for the expression of ET-1, endothelin converting enzyme (ECE), and the endothelin receptors A and B (ET(A) and ET(B)) by both reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time PCR. ET-1 and ECE mRNA expression was markedly increased by RT-PCR in PbA-infected mice. Real-time quantitative PCR demonstrated a 3-fold increase in ET-1 (P < 0.05) and a significant increase in ET(A) and ET(B) expression (P < 0.05) in PbA-infected mice. Histopathology bof PbA-infected mice demonstrated a transformation in the morphology of microglial cells and clustering of these cells consistent with activation. Though the full impact of ET-1 on CM remains to be elucidated, these findings demonstrate that in the murine model, there is a significant increase in ET-1 and its components, which is associated with the vasculopathy and immunopathology of CM.

Fast spectroscopic imaging using online optimal sparsek-space acquisition and projections onto convex sets reconstruction

Long acquisition times, low resolution, and voxel contamination are major difficulties in the application of magnetic resonance spectroscopic imaging (MRSI). To overcome these difficulties, an online-optimized acquisition of k-space, termed sequential forward array selection (SFAS), was developed to reduce acquisition time without sacrificing spatial resolution. A 2D proton MRSI region of interest (ROI) was defined from a scout image and used to create a region of support (ROS) image. The ROS was then used to optimize and obtain a subset of k-space (i.e., a subset of nonuniform phase encodings) and hence reduce the acquisition time for MRSI. Reconstruction and processing software was developed in-house to process and reconstruct MRSI using the projections onto convex sets method. Phantom and in vivo studies showed that good-quality MRS images are obtainable with an approximately 80% reduction of data acquisition time. The reduction of the acquisition time depends on the area ratio of ROS to FOV (i.e., the smaller the ratio, the greater the time reduction). It is also possible to obtain higher-resolution MRS images within a reasonable time using this approach. MRSI with a resolution of 64 x 64 is possible with the acquisition time of the same as 24 x 24 using the traditional full k-space method.

Regional energetic dysfunction in hippocampal epilepsy

OBJECTIVES

There is increasing evidence for a dysfunctional metabolic network in human mesial temporal lobe epilepsy (MTLE). To further describe this, we evaluated the bioenergetic status in unilateral MTLE inter-regionally and in relation to neuropathology.

MATERIALS AND METHODS

We used whole brain high field (4 T) 31P MR spectroscopic imaging to determine in vivo PCr and ATP, studying n=22 patients (all candidates for hippocampal resection) and n=14 control volunteers. The degree of bioenergetic impairment was assessed by calculating the ratio of PCr to ATP.

RESULTS

Compared to controls, patients demonstrated significant decreases in PCr/ATP from the ipsilateral amygdala and pes (0.84 +/- 0.14, 0.87 +/- 0.10, respectively, patients vs 0.97 +/- 0.15, 0.98 +/- 0.16, controls). In patients, the ipsilateral thalamic energetics positively correlated with contralateral hippocampal energetics. In addition, the ipsilateral thalamic and striatal energetics negatively correlated with hippocampal total glial counts.

CONCLUSIONS

These data are consistent with a view that in MTLE, the bilateral hippocampi, ipsilateral thalamus and striatum are linked in their energetic depression, possibly reflecting the propagation of seizures throughout the brain.

4 T actively detunable transmit/receive transverse electromagnetic coil and 4-channel receive-only phased array for (1)H human brain studies

The design and construction of a 4 T transverse electromagnetic (TEM) transmit/receive head coil and a four-channel phased array receive-only RF system are described. To enable both high-resolution imaging of the entire brain and high-resolution spectroscopic imaging, active PIN diode decoupling was used in both the TEM resonator and each surface coil in the array. This configuration allows for both transmission and reception from the volume coil as well as reception from the phased array. The surface coils were decoupled by overlapping the coils and using preamplifier decoupling. Since at high frequencies construction of a lumped element matching quarter wavelength transformer, an important component of the preamplifier decoupling, becomes difficult, a transmission line approach was used. The system was tested and compared to a TEM volume transmit/receive head coil. A four- to sixfold improvement in signal-to-noise ratio from the sensitive volume of the array was achieved.

Fully automated shim mapping method for spectroscopic imaging of the mouse brain at 9.4 T

For spectroscopic imaging studies of the mouse brain, it is critical to obtain optimal B(0) homogeneity over a large region of interest (ROI). In this paper, a fully automated shimming method for mouse brain at 9.4 T, based on B(0) mapping, is described. B(0) maps were obtained using a multislice gradient echo sequence with multiple phase evolution time delays with a novel unwrapping scheme. The unwrapping method allows phase maps with large bandwidths (+/-1 kHz) but with high resolution (0.3 Hz/ degrees ) to be acquired in a single acquisition, thereby minimizing the number of iterations required. The SD of the B(0) over the ROI (8 x 5 x 1 mm) was less than 10 Hz after shimming. Application of this method to the in vivo mouse brain allowed reproducible, high-quality spectroscopic data to be collected with 1-microl voxels.

1H and 31P Spectroscopic Imaging of Epilepsy: Spectroscopic and Histologic Correlations

Although MRS measurements are useful in assessing the biochemical alterations underlying human epilepsy, to date their use has been limited primarily by three factors: (a) the lack of widespread methods and appropriate hardware for acquiring high-resolution spectroscopic imaging data, (b) difficulties in spectral interpretation associated with metabolic heterogeneity, and (c) difficulties in biological interpretation due to a lack of correlative histologic studies. In this work, we (a) describe approaches to overcome these hurdles, and (b) discuss the biological interpretation of the spectroscopic findings in TLE.

Modified perturbation method for transverse electromagnetic (TEM) coil tuning and evaluation

A modified perturbation method for tuning and testing volume head coils was developed. The common perturbation method, utilizing the change in resonance frequency of a resonator in response to the presence of a small dielectric or magnetic probe, was modified to modulate the frequency shift due to rotation of a probe. This modification enabled the RF magnetic and electric fields as well as the angular distribution of current in the longitudinal elements of the coil to be mapped. The latter serves as a quick test of magnetic field homogeneity by comparing the measured distribution with the sinusoidal function required for the field to be homogenous. The method was experimentally applied to tune and test a transverse electromagnetic (TEM) head coil.

1H and 31P spectroscopy and bioenergetics in the lateralization of seizures in temporal lobe epilepsy

Over the past decade, (1)H and (31)P spectroscopy measurements have demonstrated that significant metabolic alterations occur in temporal lobe epilepsy. However, to most accurately interpret these changes, metabolic heterogeneity and differences between gray and white matter must be accounted for. These alterations, decreased NAA and the ratio of phosphocreatine/inorganic phosphate, can be reversed with successful treatment of seizures. The reversibility of these two measures is consistent with the localization of NAA synthesis to neuronal mitochondria and the important role for bioenergetics in the pathophysiology of temporal lobe epilepsy.

[2,4-13 C2 ]-beta-Hydroxybutyrate metabolism in human brain

Infusions of [2,4-13C2]-beta-hydroxybutyrate and 1H-13C polarization transfer spectroscopy were used in normal human subjects to detect the entry and metabolism of beta-hydroxybutyrate in the brain. During the 2-hour infusion study, 13C label was detectable in the beta-hydroxybutyrate resonance positions and in the amino acid pools of glutamate, glutamine, and aspartate. With a plasma concentration of 2.25 +/- 0.24 mmol/L (four volunteers), the apparent tissue beta-hydroxybutyrate concentration reached 0.18 +/- 0.06 mmol/L during the last 20 minutes of the study. The relative fractional enrichment of 13C-4-glutamate labeling was 6.78 +/- 1.71%, whereas 13C-4-glutamine was 5.68 +/- 1.84%. Steady-state modeling of the 13C label distribution in glutamate and glutamine suggests that, under these conditions, the consumption of the beta-hydroxybutyrate is predominantly neuronal, used at a rate of 0.032 +/- 0.009 mmol. kg-1. min-1, and accounts for 6.4 +/- 1.6% of total acetyl coenzyme A oxidation. These results are consistent with minimal accumulation of cerebral ketones with rapid utilization, implying blood-brain barrier control of ketone oxidation in the nonfasted adult human brain.

Regional cerebral blood flow and magnetic resonance spectroscopic imaging findings in diaschisis from stroke

BACKGROUND AND PURPOSE

This study evaluated blood flow and metabolite changes in cerebral diaschisis from internal capsule region infarction using regional cerebral blood flow (rCBF) single-photon emission computed tomography (SPECT) and 1H magnetic resonance spectroscopic imaging (MRSI). We hypothesized that complementary measures of diaschisis effects in white matter (characterized by 1H MRSI) and gray matter (characterized by changes in rCBF) can be measured and exhibit parallel changes.

METHODS

Five stroke patients and 16 normal controls underwent Tc-99m hexamethylpropyleneamine-oxime brain SPECT and 1H MRSI at 4.1 T. The metabolites N-acetyl aspartate (NAA) and creatine (Cr) were measured using 1H MRSI. The tissue content was expressed as the percent of gray or white matter in each MRSI voxel to allow comparison of the differential effects of diaschisis in gray and white matter tissue types. The blood flow and metabolite changes were evaluated at superior cerebral regions distant from the stroke to allow a measure of diaschisis relatively unconfounded by their expected changes in the infarction region.

RESULTS

The rCBF SPECT data in stroke patients showed a perfusion defect, with size ranging from 1.23 cc to 10.23 cc, in the region of cortical diaschisis. 1H MRSI showed increased Cr/NAA ratios in regions of white matter diaschisis. There was a tendency for larger rCBF defect size to be associated with greater increases in Cr/NAA values in the same diaschitic cerebral hemisphere, ipsilateral to the infarction.

CONCLUSION

Diaschisis ipsilateral to stroke in white matter can be characterized by 1H MRSI, and diaschisis ipsilateral to stroke in cortical gray matter regions can be characterized by changes in rCBF. The tendency for greater reductions in cortical rCBF values to be associated with increased Cr/NAA values in the same diaschitic cerebral hemisphere implies that a relationship exists between rCBF reductions in gray matter and abnormal changes in white matter subservient to it.

Metabolic differences between multiple sclerosis subtypes measured by quantitative MR spectroscopy

We used quantitative magnetic resonance (MR) spectroscopic imaging with T1-based image segmentation to evaluate the subtypes of multiple sclerosis (MS) (eight patients each group of relapsing-remitting [RR], secondary progressive [SP] and primary progressive [PP]). There was no significant difference in age between the PP group with the RP, SP or control group. We found that the metabolite ratio of choline/NA from the periventricular white matter region was not significantly different between the RR and SP groups. Using an ANOVA, the ratios of periventricular choline/NA or creatine/NA of these combined groups were significantly higher than the PP and control groups. Quantification of these data suggest that the major cause of the elevation of these parameters is due to an increase in choline and creatine in the RR group while NA is decreased in the SP group. Thus, early PP disease appears to be relatively intact with respect to neuronal loss.

Measurement of beta-hydroxybutyrate in acute hyperketonemia in human brain

We report the measurement of D-beta-hydroxybutyrate (BHB) in the brains of six normal adult subjects during acute infusions of BHB. We used high field in vivo (1)H magnetic resonance (MR) spectroscopy in the occipital lobe in conjunction with an acute infusion protocol to elevate plasma BHB levels from overnight fasted levels (0.20 +/- 0.10 mM) to a steady state value of 2.12 +/- 0.30 mM. At this level of hyperketonemia, we determined a tissue BHB level of 0.24 +/- 0.04 mM. No increases in brain lactate levels were seen in these data. The concentrations of BHB and lactate were both considerably lower in comparison with previous data acquired in fasted adult subjects. This suggests that up-regulation of the monocarboxylic acid transporter occurs with fasting.

Differentiation of glucose transport in human brain gray and white matter

Localized 1H nuclear magnetic resonance spectroscopy has been applied to determine human brain gray matter and white matter glucose transport kinetics by measuring the steady-state glucose concentration under normoglycemia and two levels of hyperglycemia. Nuclear magnetic resonance spectroscopic measurements were simultaneously performed on three 12-mL volumes, containing predominantly gray or white matter. The exact volume compositions were determined from quantitative T1 relaxation magnetic resonance images. The absolute brain glucose concentration as a function of the plasma glucose level was fitted with two kinetic transport models, based on standard (irreversible) or reversible Michaelis-Menten kinetics. The steady-state brain glucose levels were similar for cerebral gray and white matter, although the white matter levels were consistently 15% to 20% higher. The ratio of the maximum glucose transport rate, V(max), to the cerebral metabolic utilization rate of glucose, CMR(Glc), was 3.2 +/- 0.10 and 3.9 +/- 0.15 for gray matter and white matter using the standard transport model and 1.8 +/- 0.10 and 2.2 +/- 0.12 for gray matter and white matter using the reversible transport model. The Michaelis-Menten constant K(m) was 6.2 +/- 0.85 and 7.3 +/- 1.1 mmol/L for gray matter and white matter in the standard model and 1.1 +/- 0.66 and 1.7 +/- 0.88 mmol/L in the reversible model. Taking into account the threefold lower rate of CMR(Glc) in white matter, this finding suggests that blood--brain barrier glucose transport activity is lower by a similar amount in white matter. The regulation of glucose transport activity at the blood--brain barrier may be an important mechanism for maintaining glucose homeostasis throughout the cerebral cortex.

Regional differences in intramyocellular lipids in humans observed by in vivo 1H-MR spectroscopic imaging

Regional differences in the content of intramyocellular lipids (IMCL), extramyocellular lipids, and total creatine (TCr) were quantified in soleus (S), tibialis posterior (TP), and tibialis anterior (TA) muscles in humans using in vivo 1H proton spectroscopic imaging at 4 T. Improved spatial resolution (0.25-ml nominal voxel resolution) made it feasible to measure IMCL in S, TP, and TA simultaneously in vivo. The most significant regional difference was found in the content of IMCL compared with extramyocellular lipids or TCr. The concentrations of TCr were found to be 29-32 mmol/kg, with little regional variation. IMCL content was measured to be 4.8 +/- 1.6 mmol/kg tissue wt in S, 2.8 +/- 1.3 mmol/kg tissue wt in TP, and 1.6 +/- 0.9 mmol/kg tissue wt in TA in the order of S > TP > TA (P < 0.05). It is likely that these IMCL values are consistent with the known fiber types of these muscles, with S having the greatest fraction of type I (slow-twitch, oxidative) fibers and TA having a large fraction of type IIb (fast-twitch, glycolytic) fibers.

Alkaline pH changes in the cerebellum of asymptomatic HIV-infected individuals

Human immunodeficiency virus (HIV) infection of the brain causes a complex cascade of cellular events involving several different cell types that eventually leads to neuronal cell death and the manifestation of the AIDS-associated dementia complex (ADC). Upon autopsy HIV-infected individuals show lesions within subcortical regions of the brain, including the cerebellum. Previously we have demonstrated, in primary and cell culture models of rat and human astrocytes, a change in intracellular pH (pH(i)) due to increased Na(+)/H(+) exchange following exposure to inactivated virus or gp120, the major HIV envelope glycoprotein. To further investigate whether any such in vivo pH(i) changes occur in human brains subsequent to HIV infection, we measured the pH(i) of the cerebellum in eight HIV-positive individuals and nine healthy volunteers using (31)P magnetic resonance spectroscopy imaging (MRSI) at high field strength (4.1 T). The results showed a significant difference between the age-adjusted mean pH(i) in the cerebellum in control group and patient groups (7.11 +/- 0.03 vs 7.16 +/- 0.04), and further HIV-infected individuals displayed a significant increase in the number of cerebellar volume elements that were alkaline. We hypothesize that this propensity towards alterations in cerebellar pH(i) may portend later neurological involvement resulting from HIV infection.

Quantitative (31)P spectroscopic imaging of human brain at 4 Tesla: assessment of gray and white matter differences of phosphocreatine and ATP

This report describes the implementation and application of a multicompartment analysis of (31)P spectroscopic imaging data to determine the tissue-specific heterogeneities in metabolite content in the human brain and surrounding tissue. Using this information and a multicompartment regression analysis the phosphocreatine and ATP content of "pure" cerebral gray and white matter, the cerebellum, and skeletal muscle was determined in a group of 10 healthy volunteers. The data were converted to mM units using previously reported values for the T(1)s of phosphocreatine and ATP at 4 T, the water content of human brain, and an external reference for absolute quantification. The phosphocreatine concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 3.53 +/- 0.33, 3.33 +/- 0.37, 3.75 +/- 0.66, and 25.8 +/- 2.3 mM, respectively. The ATP concentration in cerebral gray and white matter, the cerebellum, and skeletal muscle was 2.19 +/- 0.33, 3.41 +/- 0.33, 1.75 +/- 0.58, and 8.5 +/- 1.9 mM, respectively. Magn Reson Med 45:46-52, 2001.

Spectroscopic imaging of glutamate C4 turnover in human brain

One-dimensional spectroscopic imaging of (13)C-4-glutamate turnover is performed in the human brain with a 6 cc nominal voxel resolution at 4T. Data were acquired with an indirect detection approach using a short spin echo single quantum (1)H-(13)C heteronuclear editing method and a 7 cm surface coil with quadrature (13)C decoupling coils. To analyze the data as a function of tissue type, T(1)-based image segmentation through the surface coil was performed to determine the gray and white matter contributions to each voxel. The tricarboxylic acid (TCA) cycle rate in gray and white matter was then determined using a two-compartment model with the tissue fractionation derived from the image segmentation. The mean values for the TCA cycle rate for occipital gray and white matter from three volunteers was 0.88 +/- 0.12 and 0.28 +/- 0.13 respectively, in agreement with literature data.

Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis

Ketones are known to constitute an important fraction of fuel for consumption by the brain, with brain ketone content generally thought to be low. However, the recent observation of 1-mmol/L levels of brain beta-hydroxybutyrate (BHB) in children on the ketogenic diet suggests otherwise. The authors report the measurement of brain BHB and lactate in the occipital lobe of healthy adults using high field (4-T) magnetic resonance spectroscopy, measured in the nonfasted state and after 2- and 3-day fasting-induced ketosis. A 9-mL voxel located in the calcarine fissure was studied, detecting the BHB and lactate upfield resonances using a 1H homonuclear editing sequence. Plasma BHB levels also were measured. The mean brain BHB concentration increased from a nonfasted level of 0.05 +/- 0.05 to 0.60 +/- 0.26 mmol/L (after second day of fasting), increasing further to 0.98 +/- 0.16 mmol/L (after the third day of fasting). The mean nonfasted brain lactate was 0.69 +/- 0.17 mmol/L, increasing to 1.47 +/- 0.22 mmol/L after the third day. The plasma and brain BHB levels correlated well (r = 0.86) with a brain-plasma slope of 0.26. These data show that brain BHB rises significantly with 2- and 3-day fasting-induced ketosis. The lactate increase likely results from ketones displacing lactate oxidation without altering glucose phosphorylation and glycolysis.

Measurements of human brain ethanol T(2) by spectroscopic imaging at 4 T

Previous MRS measurements of ethanol in human brain have yielded a range of transverse relaxation times for ethanol methyl resonance at 1.5 T (200-380 ms). To determine the T(2) of the methyl proton resonance of ethanol in human brain, 8 x 8 spectroscopic images were acquired at 16 different TE values. A frequency-selective refocusing pulse was used to suppress J-modulation of the ethanol triplet, permitting nonintegral multiples of 1/J to be used for TE values. The measured T(2) values for the methyl resonances of ethanol, creatine, and N-acetyl aspartate in mixed tissues were 82 +/- 12, 148 +/- 20, and 227 +/- 25 ms, respectively. Regression analysis of the measured T(2) as a function of gray matter content indicates a shorter T(2) value for ethanol in pure white matter compared to that in pure gray matter. Magn Reson Med 44:35-40, 2000.

Statistically driven identification of focal metabolic abnormalities in temporal lobe epilepsy with corrections for tissue heterogeneity using 1H spectroscopic imaging

1H spectroscopic imaging of N-acetyl-aspartate, creatine, and choline has proven to be a sensitive indicator for the lateralization of seizure foci in temporal lobe epilepsy. Previous studies have used right-left comparisons to identify the epileptogenic tissue assuming that alterations due to the disease process outweigh the effects of tissue heterogeneity. To evaluate the effectiveness of tissue heterogeneity corrected analyses, we evaluated three criteria for lateralization of the seizure focus: 1) a statistically driven method adjusted for tissue composition, 2) a single valued threshold, and 3) a single global index of the hippocampus. The statistically driven analysis lateralized all eight patients correctly, whereas the single threshold method incorrectly lateralized one case and the global index failed to identify a significant difference in two cases. These findings indicate that increased accuracy and sensitivity can be obtained by correcting for tissue heterogeneity when analyzing spectroscopy studies of temporal lobe epilepsy.

31P MRS measurement of mitochondrial function in skeletal muscle: reliability, force-level sensitivity and relation to whole body maximal oxygen uptake

The reliability, relation to whole-body maximal oxygen uptake (VO(2max)), and force-level sensitivity of (31)P MRS markers of mitochondrial function were studied in 39 normal-weight women. Following 90 s isometric plantar-flexion exercises at 45, 70 and 100% of maximum voluntary contraction, skeletal muscle mitochondrial function was determined from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). VO(2max) was measured on a treadmill. Test-retest measurements were obtained in a subset of seven women. Overall, TC(PCr), TC(ADP) and OxPhos were reproducible for all exercises (coefficients of variation = 2.3-19.3%). With increasing force-level, TC(PCr) was prolonged (29.0 +/- 8.2, 31.9 +/- 9.0 and 35.4 +/- 9.5 s), OxPhos was increased (0.159 +/- 0.081, 0.247 +/- 0.090 and 0.310 +/- 0.114), and TC(ADP) was shortened (22.4 +/- 7.9, 21.3 +/- 6.2, and 19.5 +/- 6.7; p < 0.01). All MRS markers of mitochondrial function were correlated with VO(2max) (r = 0.41-0.72; p < 0.05). These results suggest that measurements of TC(PCr), TC(ADP) and OxPhos yield reproducible results that correlate with whole-body VO(2max) and that vary in force-level sensitivity.

Effect of weight reduction, obesity predisposition, and aerobic fitness on skeletal muscle mitochondrial function

We used (31)P magnetic resonance spectroscopy to measure maximal mitochondrial function in 12 obesity-prone women before and after diet-induced weight reduction and in 12 matched, never-obese, and 7 endurance-trained controls. Mitochondrial function was modeled after maximum-effort plantar flexion from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). Weight reduction was not associated with a significant change in mitochondrial function by TC(PCr), TC(ADP), or OxPhos. Mitochondrial function was not different between postobese and never-obese controls by TC(PCr) [35.1 +/- 2.5 (SE) vs. 34.6 +/- 2.5 s], TC(ADP) (22.9 +/- 1.8 vs. 21.2 +/- 1.8 s), or OxPhos (0.26 +/- 0. 03 vs. 0.25 +/- 0.03 mM ATP/s), postobese vs. never-obese, respectively. However, TC(ADP) was significantly faster (14.5 +/- 2. 3 s), and OxPhos was significantly higher (0.38 +/- 0.04 mM ATP/s) in the endurance-trained group. These results suggest that maximal mitochondrial function is not impaired in normal-weight obesity-prone women relative to their never-obese counterparts but is increased in endurance-trained women.