Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Pioglitazone treatment restores in vivo muscle oxidative capacity in a rat model of diabetes

AIM

To determine the effect of pioglitazone treatment on in vivo and ex vivo muscle mitochondrial function in a rat model of diabetes.

METHODS

Both the lean, healthy rats and the obese, diabetic rats are Zucker Diabetic Fatty (ZDF) rats. The homozygous fa/fa ZDF rats are obese and diabetic. The heterozygous fa/+ ZDF rats are lean and healthy. Diabetic Zucker Diabetic Fatty rats were treated with either pioglitazone (30 mg/kg/day) or water as a control (n = 6 per group), for 2 weeks. In vivo ¹H and ³¹P magnetic resonance spectroscopy was performed on skeletal muscle to assess intramyocellular lipid (IMCL) content and muscle oxidative capacity, respectively. Ex vivo muscle mitochondrial respiratory capacity was evaluated using high-resolution respirometry. In addition, several markers of mitochondrial content were determined.

RESULTS

IMCL content was 14-fold higher and in vivo muscle oxidative capacity was 26% lower in diabetic rats compared with lean rats, which was, however, not caused by impairments of ex vivo mitochondrial respiratory capacity or a lower mitochondrial content. Pioglitazone treatment restored in vivo muscle oxidative capacity in diabetic rats to the level of lean controls. This amelioration was not accompanied by an increase in mitochondrial content or ex vivo mitochondrial respiratory capacity, but rather was paralleled by an improvement in lipid homeostasis, that is lowering of plasma triglycerides and muscle lipid and long-chain acylcarnitine content.

CONCLUSION

Diminished in vivo muscle oxidative capacity in diabetic rats results from mitochondrial lipid overload and can be alleviated by redirecting the lipids from the muscle into adipose tissue using pioglitazone treatment.

In vivo postprandial lipid partitioning in liver and skeletal muscle in prediabetic and diabetic rats

AIMS/HYPOTHESIS

Insulin resistance and type 2 diabetes have been associated with ectopic lipid deposition. This study investigates the derangements in postprandial lipid handling in liver and skeletal muscle tissue at different stages during the pathogenesis of type 2 diabetes in a rat model.

METHODS

Four groups (n = 6) of male Zucker diabetic fatty rats were used for this study: prediabetic fa/fa rats and healthy fa/+ littermates at the age of 6 weeks, and diabetic fa/fa rats and healthy fa/+ littermates at the age of 12 weeks. In vivo (1)H-[(13)C] magnetic resonance spectroscopy measurements were performed in liver and tibialis anterior muscle at baseline and 4, 24 and 48 h after oral administration of 1.5 g [U-(13)C]algal lipid mixture per kilogram body weight. Total and (13)C-labelled intracellular lipid contents were determined from the magnetic resonance spectra.

RESULTS

In both prediabetic and diabetic rats, total lipid contents in muscle and liver were substantially higher than in healthy controls and this was accompanied by a 2.3-fold greater postprandial lipid uptake in the liver (p < 0.001). Interestingly, in prediabetic rats, skeletal muscle appeared to be protected from excess lipid uptake whereas after developing overt diabetes muscle lipid uptake was 3.4-fold higher than in controls (p < 0.05). Muscle lipid use was significantly lower in prediabetic and diabetic muscle, indicative of impairments in lipid oxidation.

CONCLUSIONS/INTERPRETATION

In vivo postprandial lipid handling is disturbed in both liver and skeletal muscle tissue in prediabetic and diabetic rats, but the uptake of dietary lipids in muscle is only increased after the development of overt diabetes.

Combined in vivo and in silico investigations of activation of glycolysis in contracting skeletal muscle

The hypothesis was tested that the variation of in vivo glycolytic flux with contraction frequency in skeletal muscle can be qualitatively and quantitatively explained by calcium-calmodulin activation of phosphofructokinase (PFK-1). Ischemic rat tibialis anterior muscle was electrically stimulated at frequencies between 0 and 80 Hz to covary the ATP turnover rate and calcium concentration in the tissue. Estimates of in vivo glycolytic rates and cellular free energetic states were derived from dynamic changes in intramuscular pH and phosphocreatine content, respectively, determined by phosphorus magnetic resonance spectroscopy ((31)P-MRS). Computational modeling was applied to relate these empirical observations to understanding of the biochemistry of muscle glycolysis. Hereto, the kinetic model of PFK activity in a previously reported mathematical model of the glycolytic pathway (Vinnakota KC, Rusk J, Palmer L, Shankland E, Kushmerick MJ. J Physiol 588: 1961-1983, 2010) was adapted to contain a calcium-calmodulin binding sensitivity. The two main results were introduction of regulation of PFK-1 activity by binding of a calcium-calmodulin complex in combination with activation by increased concentrations of AMP and ADP was essential to qualitatively and quantitatively explain the experimental observations. Secondly, the model predicted that shutdown of glycolytic ATP production flux in muscle postexercise may lag behind deactivation of PFK-1 (timescales: 5-10 s vs. 100-200 ms, respectively) as a result of accumulation of glycolytic intermediates downstream of PFK during contractions.

Physical activity is the key determinant of skeletal muscle mitochondrial function in type 2 diabetes

CONTEXT

Conflicting data exist on mitochondrial function and physical activity in type 2 diabetes mellitus (T2DM) development.

OBJECTIVE

The aim was to assess mitochondrial function at different stages during T2DM development in combination with physical exercise in longstanding T2DM patients.

DESIGN AND METHODS

We performed cross-sectional analysis of skeletal muscle from 12 prediabetic 11 longstanding T2DM male subjects and 12 male controls matched by age and body mass index.

INTERVENTION

One-year intrasubject controlled supervised exercise training intervention was done in longstanding T2DM patients.

MAIN OUTCOME MEASUREMENTS

Extensive ex vivo analyses of mitochondrial quality, quantity, and function were collected and combined with global gene expression analysis and in vivo ATP production capacity after 1 yr of training.

RESULTS

Mitochondrial density, complex I activity, and the expression of Krebs cycle and oxidative phosphorylation system-related genes were lower in longstanding T2DM subjects but not in prediabetic subjects compared with controls. This indicated a reduced capacity to generate ATP in longstanding T2DM patients only. Gene expression analysis in prediabetic subjects suggested a switch from carbohydrate toward lipid as an energy source. One year of exercise training raised in vivo skeletal muscle ATP production capacity by 21 ± 2% with an increased trend in mitochondrial density and complex I activity. In addition, expression levels of β-oxidation, Krebs cycle, and oxidative phosphorylation system-related genes were higher after exercise training.

CONCLUSIONS

Mitochondrial dysfunction is apparent only in inactive longstanding T2DM patients, which suggests that mitochondrial function and insulin resistance do not depend on each other. Prolonged exercise training can, at least partly, reverse the mitochondrial impairments associated with the longstanding diabetic state.

The effects of deformation, ischemia, and reperfusion on the development of muscle damage during prolonged loading

Deep tissue injury (DTI) is a severe form of pressure ulcer where tissue damage starts in deep tissues underneath intact skin. In the present study, the contributions of deformation, ischemia, and reperfusion to skeletal muscle damage development were examined in a rat model during a 6-h period. Magnetic resonance imaging (MRI) was used to study perfusion (contrast-enhanced MRI) and tissue integrity (T2-weighted MRI). The levels of tissue deformation were estimated using finite element models. Complete ischemia caused a gradual homogeneous increase in T2 (∼20% during the 6-h period). The effect of reperfusion on T2 was highly variable, depending on the anatomical location. In experiments involving deformation, inevitably associated with partial ischemia, a variable T2 increase (17–66% during the 6-h period) was observed reflecting the significant variation in deformation (with two-dimensional strain energies of 0.60–1.51 J/mm) and ischemia (50.8–99.8% of the leg) between experiments. These results imply that deformation, ischemia, and reperfusion all contribute to the damage process during prolonged loading, although their importance varies with time. The critical deformation threshold and period of ischemia that cause muscle damage will certainly vary between individuals. These variations are related to intrinsic factors, such as pathological state, which partly explain the individual susceptibility to the development of DTI and highlight the need for regular assessments of individual subjects.

Unchanged muscle fiber conduction velocity relates to mild acidosis during exhaustive bicycling

Muscle fiber conduction velocity (MFCV) has often been shown to decrease during standardized fatiguing isometric contractions. However, several studies have indicated that the MFCV may remain constant during fatiguing dynamic exercise. It was investigated if these observations can be related to the absence of a large decrease in pH and if MFCV can be considered as a good indicator of acidosis, also during dynamic bicycle exercise. High-density surface electromyography (HDsEMG) was combined with read-outs of muscle energetics recorded by in vivo ³¹P magnetic resonance spectroscopy (MRS). Measurements were performed during serial exhausting bouts of bicycle exercise at three different workloads. The HDsEMG recordings revealed a small and incoherent variation of MFCV during all high-intensity exercise bouts. ³¹P MRS spectra revealed a moderate decrease in pH at the end of exercise (~0.3 units down to 6.8) and a rapid ancillary drop to pH 6.5 during recovery 30 s post-exercise. This additional degree of acidification caused a significant decrease in MFCV during cycling immediately after the rest period. From the data a significant correlation between MFCV and [H⁺] ([H⁺] = 10^−pH) was calculated (p < 0.001, Pearson’s R = −0.87). Our results confirmed the previous observations of MFCV remaining constant during fatiguing dynamic exercise. A constant MFCV is in line with a low degree of acidification, considering the presence of a correlation between pH and MFCV after further increasing acidification.

Ischemia-reperfusion injury in rat skeletal muscle assessed with T2-weighted and dynamic contrast-enhanced MRI

Pressure ulcers are localized areas of soft tissue breakdown due to mechanical loading. Susceptible individuals are subjected to pressure relief strategies to prevent long loading periods. Therefore, ischemia-reperfusion injury may play an important role in the etiology of pressure ulcers. To investigate the inter-relation between postischemic perfusion and changes in skeletal muscle integrity, the hindlimbs of Brown Norway rats were subjected to 4-h ischemia followed by 2-h reperfusion. Dynamic contrast-enhanced MRI was used to examine perfusion, and changes in skeletal muscle integrity were monitored with T2-weighted MRI. The dynamic contrast-enhanced MRI data showed a heterogeneous postischemic profile in the hindlimb, consisting of areas with increased contrast enhancement (14-76% of the hindlimb) and regions with no-reflow (5-77%). For T2, a gradual increase in the complete leg was observed during the 4-h ischemic period (from 34 to 41 msec). During the reperfusion phase, a heterogeneous distribution of T2 was observed. Areas with increased contrast enhancement were associated with a decrease in T2 (to 38 msec) toward preischemic levels, whereas no-reflow areas exhibited a further increase in T2 (to 42 msec). These results show that reperfusion after prolonged ischemia may not be complete, thereby continuing the ischemic condition and aggravating tissue damage.

Comparison of in vivo postexercise phosphocreatine recovery and resting ATP synthesis flux for the assessment of skeletal muscle mitochondrial function

(31)P magnetic resonance spectroscopy (MRS) has been used to assess skeletal muscle mitochondrial function in vivo by measuring 1) phosphocreatine (PCr) recovery after exercise or 2) resting ATP synthesis flux with saturation transfer (ST). In this study, we compared both parameters in a rat model of mitochondrial dysfunction with the aim of establishing the most appropriate method for the assessment of in vivo muscle mitochondrial function. Mitochondrial dysfunction was induced in adult Wistar rats by daily subcutaneous injections with the complex I inhibitor diphenyleneiodonium (DPI) for 2 wk. In vivo (31)P MRS measurements were supplemented by in vitro measurements of oxygen consumption in isolated mitochondria. Two weeks of DPI treatment induced mitochondrial dysfunction, as evidenced by a 20% lower maximal ADP-stimulated oxygen consumption rate in isolated mitochondria from DPI-treated rats oxidizing pyruvate plus malate. This was paralleled by a 46% decrease in in vivo oxidative capacity, determined from postexercise PCr recovery. Interestingly, no significant difference in resting, ST-based ATP synthesis flux was observed between DPI-treated rats and controls. These results show that PCr recovery after exercise has a more direct relationship with skeletal muscle mitochondrial function than the ATP synthesis flux measured with (31)P ST MRS in the resting state.

Computational modelling identifies the impact of subtle anatomical variations between amphibian and mammalian skeletal muscle on spatiotemporal calcium dynamics

The physical sites of calcium entry and exit in the skeletal muscle cell are distinct and highly organised in space. It was investigated whether the highly structured spatial organisation of sites of Ca(2+) release, uptake and action in skeletal muscle cells substantially impacts the dynamics of cytosolic Ca(2+) handling and thereby the physiology of the cell. Hereto, the spatiotemporal dynamics of the free calcium distribution in a fast-twitch (FT) muscle sarcomere was studied using a reaction-diffusion computational model for two genotypes with known anatomical differences. A computational model of a murine FT muscle sarcomere is developed, de novo including a closed calcium mass balance to simulate spatiotemporal high stimulation frequency calcium dynamics at 35 degrees C. Literature data on high-frequency calcium dye measurements were used as a first step towards model validation. The murine and amphibian sarcomere models were phenotypically distinct to capture known differences in positions of troponin C, actin-myosin overlap and calcium release within the sarcomere between frog and mouse. The models predicted large calcium gradients throughout the myoplasm as well as differences in calcium concentrations near the mitochondria of frog and mouse. Furthermore, the predicted Ca(2+) concentration was high at positions where Ca(2+) has a regulatory function, close to the mitochondria and troponin C.

Validation of a Numerical Model of Skeletal Muscle Compression With MR Tagging: A Contribution to Pressure Ulcer Research

Sustained tissue compression can lead to pressure ulcers, which can either start superficially or within deeper tissue layers. The latter type includes deep tissue injury, starting in skeletal muscle underneath an intact skin. Since the underlying damage mechanisms are poorly understood, prevention and early detection are difficult. Recent in vitro studies and in vivo animal studies have suggested that tissue deformation per se can lead to damage. In order to conclusively couple damage to deformation, experiments are required in which internal tissue deformation and damage are both known. Magnetic resonance (MR) tagging and T2-weighted MR imaging can be used to measure tissue deformation and damage, respectively, but they cannot be combined in a protocol for measuring damage after prolonged loading. Therefore, a dedicated finite element model was developed to calculate strains in damage experiments. In the present study, this model, which describes the compression of rat skeletal muscles, was validated with MR tagging. Displacements from both the tagging experiments and the model were interpolated on a grid and subsequently processed to obtain maximum shear strains. A correlation analysis revealed a linear correlation between experimental and numerical strains. It was further found that the accuracy of the numerical prediction decreased for increasing strains, but the positive predictive value remained reasonable. It was concluded that the model was suitable for calculating strains in skeletal muscle tissues in which damage is measured due to compression.

Early or advanced stage type 2 diabetes is not accompanied by in vivo skeletal muscle mitochondrial dysfunction

OBJECTIVE

Several lines of evidence support a potential role of skeletal muscle mitochondrial dysfunction in the pathogenesis of insulin resistance and/or type 2 diabetes. However, it remains to be established whether mitochondrial dysfunction represents either cause or consequence of the disease. We examined in vivo skeletal muscle mitochondrial function in early and advanced stages of type 2 diabetes, with the aim to gain insight in the proposed role of mitochondrial dysfunction in the aetiology of insulin resistance and/or type 2 diabetes.

METHODS

Ten long-standing, insulin-treated type 2 diabetes patients, 11 subjects with impaired fasting glucose, impaired glucose tolerance and/or recently diagnosed type 2 diabetes, and 12 healthy, normoglycaemic controls, matched for age and body composition and with low habitual physical activity levels were studied. In vivo mitochondrial function of the vastus lateralis muscle was evaluated from post-exercise phosphocreatine (PCr) recovery kinetics using (31)P magnetic resonance spectroscopy (MRS). Intramyocellular lipid (IMCL) content was assessed in the same muscle using single-voxel (1)H MRS.

RESULTS

IMCL content tended to be higher in the type 2 diabetes patients when compared with normoglycaemic controls (P=0.06). The(31)P MRS parameters for mitochondrial function, i.e. PCr and ADP recovery time constants and maximum aerobic capacity, did not differ between groups.

CONCLUSIONS

The finding that in vivo skeletal muscle oxidative capacity does not differ between long-standing, insulin-treated type 2 diabetes patients, subjects with early stage type 2 diabetes and sedentary, normoglycaemic controls suggests that mitochondrial dysfunction does not necessarily represent either cause or consequence of insulin resistance and/or type 2 diabetes.

31P MR spectroscopy and in vitro markers of oxidative capacity in type 2 diabetes patients

BACKGROUND

Skeletal muscle mitochondrial function in type 2 diabetes (T2D) is currently being studied intensively. In vivo (31)P magnetic resonance spectroscopy ((31)P MRS) is a noninvasive tool used to measure mitochondrial respiratory function (MIFU) in skeletal muscle tissue. However, microvascular co-morbidity in long-standing T2D can interfere with the (31)P MRS methodology.

AIM

To compare (31)P MRS-derived parameters describing in vivo MIFU with an in vitro assessment of muscle respiratory capacity and muscle fiber-type composition in T2D patients.

METHODS

(31)P MRS was applied in long-standing, insulin-treated T2D patients. (31)P MRS markers of MIFU were measured in the M. vastus lateralis. Muscle biopsy samples were collected from the same muscle and analyzed for succinate dehydrogenase activity (SDH) and fiber-type distribution.

RESULTS

Several (31)P MRS parameters of MIFU showed moderate to good correlations with the percentage of type I fibers and type I fiber-specific SDH activity (Pearson's R between 0.70 and 0.75). In vivo and in vitro parameters of local mitochondrial respiration also correlated well with whole-body fitness levels (VO (2peak)) in these patients (Pearson's R between 0.62 and 0.90).

CONCLUSION

Good correlations exist between in vivo and in vitro measurements of MIFU in long-standing insulin-treated T2D subjects, which are qualitatively and quantitatively consistent with previous results measured in healthy subjects. This justifies the use of (31)P MRS to measure MIFU in relation to T2D.

Computational model of excitable cell indicates ATP free energy dynamics in response to calcium oscillations are undampened by cytosolic ATP buffers

Mitochondria in excitable cells are recurrently exposed to pulsatile calcium gradients that activate cell function. Rapid calcium uptake by the mitochondria has previously been shown to cause uncoupling of oxidative phosphorylation. To test (i) if periodic nerve firing may cause oscillation of the cytosolic thermodynamic potential of ATP hydrolysis and (ii) if cytosolic adenylate (AK) and creatine kinase (CK) ATP buffering reactions dampen such oscillations, a lumped kinetic model of an excitable cell capturing major aspects of the physiology has been developed. Activation of ATP metabolism by low-frequency calcium pulses caused large oscillation of the cytosolic, but not mitochondrial ATP/ADP, ratio. This outcome was independent of net ATP synthesis or hydrolysis during mitochondrial calcium uptake. The AK/CK ATP buffering reactions dampened the amplitude and rate of cytosolic ATP/ADP changes on a timescale of seconds, but not milliseconds. These model predictions suggest that alternative sources of capacitance in neurons and striated muscles should be considered to protect ATP-free energy-driven cell functions.

A new MR-compatible loading device to study in vivo muscle damage development in rats due to compressive loading

To study the aetiology of pressure ulcers an MR-compatible loading device was developed. Magnetic resonance imaging provides the possibility of non-invasive evaluation of muscle tissue after compressive loading. Pressure was applied to the tibialis anterior region of rats by means of an indenter. The developed MR-compatible loading device allowed high quality consecutive MR measurements for up to 6h. Tissue was evaluated both during and after loading. Two loading protocols were used; a large indentation of 4.5mm (mean pressure 150 kPa) was applied for 2h and a small indentation of 2.9 mm (mean pressure 50 kPa) was applied for 4h. T2-weighted MR images after the large indentation showed an immediate increase in signal intensity, associated with damage, following load removal. After 20 h the signal intensity remained higher in the affected regions. Afterwards the tissue was perfusion fixated for histological examination. Histological evaluation revealed an inflammatory response and severe muscle necrosis. No signal increase was observed after small indentation. With this new set-up, the different factors that may play a role in the onset of muscle damage can be studied, what we believe will lead to a better understanding of the contributing factors to pressure ulcer development.

Compression-induced deep tissue injury examined with magnetic resonance imaging and histology

The underlying mechanisms leading to deep tissue injury after sustained compressive loading are not well understood. It is hypothesized that initial damage to muscle fibers is induced mechanically by local excessive deformation. Therefore, in this study, an animal model was used to study early damage after compressive loading to elucidate on the damage mechanisms leading to deep pressure ulcers. The tibialis anterior of Brown-Norway rats was loaded for 2 h by means of an indenter. Experiments were performed in a magnetic resonance (MR)-compatible loading device. Muscle tissue was evaluated with transverse relaxation time (T2)-weighted MRI both during loading and up to 20 h after load removal. In addition, a detailed examination of the histopathology was performed at several time points (1, 4, and 20 h) after unloading. Results demonstrated that, immediately after unloading, T2-weighted MR images showed localized areas with increased signal intensity. Histological examination at 1 and 4 h after unloading showed large necrotic regions with complete disorganization of the internal structure of the muscle fibers. Hypercontraction zones were found bilateral to the necrotic zone. Twenty hours after unloading, an extensive inflammatory response was observed. The proposed relevance of large deformation was demonstrated by the location of damage indicated by T2-weighted MRI and the histological appearance of the compressed tissues. Differences in damage development distal and proximal to the indenter position suggested a contribution of perfusion status in the measured tissue changes that, however, appeared be to reversible.

Comparison of perfusion MRI by flow-sensitive alternating inversion recovery and dynamic susceptibility contrast in rats with permanent middle cerebral artery occlusion

We compared cerebral blood flow (CBF) parameters obtained by dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) with those obtained by flow-sensitive alternating inversion recovery (FAIR) in brain regions with different perfusion levels in rats with permanent middle cerebral artery (MCA) occlusion. MCA occlusion was performed in 19 rats. T2-weighted MRI, FAIR and DSC-MRI were performed within 48 h after occlusion. CBF parameters were analyzed in regions of interest with either prolonged or less prolonged mean transit time (MTT). Ratios of ipsi- vs contralateral CBF values were calculated and tested for correlation and differences between FAIR and DSC-MRI. FAIR-aCBF ratios correlated significantly with DSC-rCBF ratios. The mean FAIR-aCBF ratio was significantly lower than mean DSC-rCBF ratio in the area with prolonged MTT. In the area with less prolonged MTT, the mean FAIR-aCBF ratio and mean DSC-rCBF values did not differ significantly. We conclude that FAIR correlates with DSC-MRI if perfusion is preserved. FAIR provides lower CBF values than DSC-MRI if perfusion is reduced and MTT is prolonged. This probable underestimation of perfusion may be caused by transit delays. Care should be taken when quantifying CBF with FAIR and when comparing the results of FAIR- and DSC-MRI in areas with hypoperfusion.

Relaxivity of liposomal paramagnetic MRI contrast agents

Paramagnetic liposomes, spherical particles formed by a lipid bilayer, are able to accommodate a high payload of Gd-containing lipid and therefore can serve as a highly potent magnetic resonance imaging contrast agent. In this paper the relaxation properties of paramagnetic liposomes were studied as a function of composition, temperature and magnetic field strength. The pegylated liposomes with a diameter of approximately 100 nm were designed for favorable pharmacokinetic properties in vivo. The proton relaxivity, i.e. the T1 relaxation rate per mmol of Gd(III) ions, of liposomes with unsaturated DOPC phospholipids was higher than those with saturated DSPC lipids. Addition of cholesterol was essential to obtain monodisperse liposomes and led to a further, although smaller, increase of the relaxivity. Nuclear magnetic relaxation dispersion measurements showed that the relaxivity was limited by water exchange. These results show that these paramagnetic liposomes are very effective contrast agents, making them excellent candidates for many applications in magnetic resonance imaging.

Magnetic resonance imaging in experimental subarachnoid haemorrhage

BACKGROUND

We developed an MRI protocol to measure cerebrovascular diameter and blood flow velocity, and if we could detect cerebrovascular alterations after SAH and their impact on cerebral ischaemia.

METHOD

SAH was induced in 15 Wistar rats by means of the endovascular filament method; 6 other rats served as control. MRI measurements were performed on a 4.7T NMR spectrometer 1 and 48 hours after SAH and 9 days thereafter. Diffusion-weighted and T2-weighted images were acquired to detect cerebral ischaemia. The arterial spin labelling method was used to measure CBF. MR angiography was used to measure vessel diameter and blood flow velocity, from which the arterial blood flow was calculated.

FINDINGS

The ischemic lesion volume increased between 1 and 48 hours after SAH from 0.039 to 0.26 ml (P = 0.003). CBF decreased from 53.6 to 39.1 ml/100 g/min. The vessel diameter had narrowed, the blood flow velocity diminished as did the arterial blood flow in most vessels, but only the vasoconstriction in the right proximal ICA reached significance (0.49 mm to 0.43 mm, P = 0.016). Baseline values were restored at day 9.

CONCLUSIONS

We showed that it is feasible to detect alterations of in-vivo vessel diameter and blood flow velocities and their consequences for brain damage after experimental SAH in the rat. The growth of the infarct volume between day 0 and 2 after SAH and the parallel vasoconstriction suggest that delayed cerebral ischaemia after SAH occurs in rats and that this may be caused by vasoconstriction.

Ischemia-induced ADC changes are larger than osmotically-induced ADC changes in a neonatal rat hippocampus model

Diffusion-weighted imaging (DWI) is frequently used to diagnose stroke. However, the origin of the observed reduction in the apparent diffusion coefficient (ADC) in the acute phase following ischemia is not well understood. Although cell swelling is considered to play an important role, it is unclear whether this can completely explain the large ADC decrease. We developed a method to induce in neonatal rat hippocampal slices both osmotic perturbations, which lead to cell swelling, and oxygen/glucose deprivation (OGD), which simulates ischemia. A perfusion system was used to provide the hippocampal slices with nutrients and oxygen to maintain slice viability, which was verified with the use of fluorescent dyes (live/dead staining). Upon induction of OGD, the ADC decreased to approximately 57% of the initial value within 2 hr. The ADC reduction cannot fully be explained by changes due to cell swelling, since these led only to a maximum decrease of approximately 83%. Therefore, in addition to cell swelling, other changes must contribute significantly to the ADC reduction.

MRI of hip prostheses using single-point methods: in vitro studies towards the artifact-free imaging of individuals with metal implants

Use of magnetic resonance imaging (MRI) in individuals with orthopedic implants is limited because of the large distortions caused by metallic components. As a possible solution for this problem, we suggest the use of single-point imaging (SPI) methods, which are immune to the susceptibility artifacts observed with conventional MRI methods. A further advantage of SPI, based on the fact that signal encoding is achieved in ultra-short times (as short as tens of microseconds), is that they enable the direct visualization of the polymeric elements of the implants, allowing the detection of possible implant failures. We present in vitro SPI images of polymeric sockets of two hip prostheses together with artifact-free images of gelatin phantoms containing their respective metallic stems. These data underscore the great potential of the SPI technique for obtaining artifact-free images of individuals with large metal implants.

Magnetic resonance imaging of regional cardiac function in the mouse

In this paper we introduce an improved harmonic phase (HARP) analysis for complementary spatial modulation of magnetization (CSPAMM) tagging of the mouse left ventricular wall, which enables the determination of regional displacement fields with the same resolution as the corresponding CINE anatomical images. CINE MRI was used to measure global function, such as the ejection fraction. The method was tested on two healthy mouse hearts and two mouse hearts with a myocardial infarction, which was induced by a ligation of the left anterior descending coronary artery. We show that the regional displacement fields can be determined. The mean circumferential strain for the left ventricular wall of one of the healthy mice was -0.09 +/- 0.04 (mean +/- standard deviation), while for one of the infarcted mouse hearts strains of -0.02 +/- 0.02 and -0.10 +/- 0.03 were found in the infarcted and remote regions, respectively.

The time course of ischemic damage and cerebral perfusion in a rat model of space-occupying cerebral infarction

We aimed to establish a rat model of space-occupying hemispheric infarction to evaluate potential treatment strategies. For adequate timing of therapy in future experiments, we studied the development of tissue damage, edema formation, and perfusion over time with different MRI techniques. Permanent middle cerebral artery (MCA) occlusion was performed in 32 Fisher-344 rats. Forty-six MRI experiments including diffusion weighted (DW), T2-weighted (T2W), flow-sensitive alternating inversion recovery (FAIR) perfusion-weighted, and T1-weighted (T1W) imaging before and after gadolinium were performed at 1, 3, 8, 16, 24, and 48 h of ischemia. MCA occlusion consistently led to infarction of the complete MCA territory. Mortality was 75%. Lesion volumes as derived from apparent diffusion coefficient (ADC) and T2 maps increased to maximum values of 400+/-48 mm3 at 24 h and 420+/-54 mm3 at 48 h of ischemia, respectively. Midline shift peaked at 24 h. The area with diffusion-perfusion deficit decreased to a minimum at 24 h after onset of ischemia and perfusion of the contralateral hemisphere dropped at the same time point. Leakage of gadolinium through the blood-brain barrier in the entire infarct occurred within 3 h of ischemia. Permanent intraluminal MCA occlusion in Fisher-344 rats is an adequate model for space-occupying cerebral infarction. Rats may benefit from intervention aimed at reducing tissue shift and intracranial pressure (ICP), and at improving cerebral blood flow, if initiated before 24 h after MCA occlusion. The value of treatment modalities depending on an intact blood-brain barrier should be questioned.

Delayed decompressive surgery increases apparent diffusion coefficient and improves peri-infarct perfusion in rats with space-occupying cerebral infarction

BACKGROUND AND PURPOSE

There is no conclusive experimental support that decompressive surgery in late stages of space-occupying cerebral infarction will improve outcome. We studied the effects of delayed decompressive surgery on the development of tissue damage, edema formation, and cerebral perfusion with different MRI techniques in a rat model of space-occupying cerebral infarction.

METHODS

Permanent middle cerebral artery (MCA) occlusion was performed in 6 Fisher 344 rats. Decompressive surgery was performed 17 hours after the occlusion. Each animal was assessed before surgery and 2 and 4 hours after surgery by means, of diffusion-weighted T2-weighted, and flow-sensitive alternating inversion recovery perfusion-weighted MRI. Ischemic damage was also evaluated in hematoxylin-eosin-stained brain sections.

RESULTS

Lesion volume as derived from apparent diffusion coefficient (ADC) maps decreased from 522+/-98 mm3 before to 405+/-100 mm3 (P=0.016) 4 hours after decompressive surgery, whereas lesion volume from T2 maps increased from 420+/-66 mm3 before to 510+/-92 mm3 (P=0.048) 4 hours after decompressive surgery. Midline shift decreased from 1.4+/-0.1 mm to 0.5+/-0.2 mm (P=0.001). Blood flow in the noninfarcted area of the ipsilateral hemisphere improved from 25+/-9 mL/min/100 g of tissue to 38+/-9 mL/min/100 g of tissue (P=0.035). Despite the pseudonormalization of ADC, irreversible damage was found in the entire MCA territory on histological evaluation.

CONCLUSIONS

In rats with space-occupying cerebral infarction, delayed decompressive surgery leads to a decrease in lesion volume derived from ADC maps, which is probably because of an increase of extracellular water formation. There are no signs that this reflects rescue of ischemic tissue.

Blood-brain barrier permeability and monocyte infiltration in experimental allergic encephalomyelitis: a quantitative MRI study

Enhanced cerebrovascular permeability and cellular infiltration mark the onset of early multiple sclerosis lesions. So far, the precise sequence of these events and their role in lesion formation and disease progression remain unknown. Here we provide quantitative evidence that blood-brain barrier leakage is an early event and precedes massive cellular infiltration in the development of acute experimental allergic encephalomyelitis (EAE), the animal correlate of multiple sclerosis. Cerebrovascular leakage and monocytes infiltrates were separately monitored by quantitative in vivo MRI during the course of the disease. Magnetic resonance enhancement of the contrast agent gadolinium diethylenetriaminepentaacetate (Gd-DTPA), reflecting vascular leakage, occurred concomitantly with the onset of neurological signs and was already at a maximal level at this stage of the disease. Immunohistochemical analysis also confirmed the presence of the serum-derived proteins such as fibrinogen around the brain vessels early in the disease, whereas no cellular infiltrates could be detected. MRI further demonstrated that Gd-DTPA leakage clearly preceded monocyte infiltration as imaged by the contrast agent based on ultra small particles of iron oxide (USPIO), which was maximal only during full-blown EAE. Ultrastructural and immunohistochemical investigation revealed that USPIOs were present in newly infiltrated macrophages within the inflammatory lesions. To validate the use of USPIOs as a non-invasive tool to evaluate therapeutic strategies, EAE animals were treated with the immunomodulator 3-hydroxy-3-methylglutaryl Coenzyme A reductase inhibitor, lovastatin, which ameliorated clinical scores. MRI showed that the USPIO load in the brain was significantly diminished in lovastatin-treated animals. Data indicate that cerebrovascular leakage and monocytic trafficking into the brain are two distinct processes in the development of inflammatory lesions during multiple sclerosis, which can be monitored on-line with MRI using USPIOs and Gd-DTPA as contrast agents. These studies also implicate that USPIOs are a valuable tool to visualize monocyte infiltration in vivo and quantitatively assess the efficacy of new therapeutics like lovastatin.

Intramyocellular lipid content is increased after exercise in nonexercising human skeletal muscle

Intramyocellular lipid (IMCL) content has been reported to decrease after prolonged submaximal exercise in active muscle and, therefore, seems to form an important local substrate source. Because exercise leads to a substantial increase in plasma free fatty acid (FFA) availability with a concomitant increase in FFA uptake by muscle tissue, we aimed to investigate potential differences in the net changes in IMCL content between contracting and noncontracting skeletal muscle after prolonged endurance exercise. IMCL content was quantified by magnetic resonance spectroscopy in eight trained cyclists before and after a 3-h cycling protocol (55% maximal energy output) in the exercising vastus lateralis and the nonexercising biceps brachii muscle. Blood samples were taken before and after exercise to determine plasma FFA, glycerol, and triglyceride concentrations, and substrate oxidation was measured with indirect calorimetry. Prolonged endurance exercise resulted in a 20.4 +/- 2.8% (P < 0.001) decrease in IMCL content in the vastus lateralis muscle. In contrast, we observed a substantial (37.9 +/- 9.7%; P < 0.01) increase in IMCL content in the less active biceps brachii muscle. Plasma FFA and glycerol concentrations were substantially increased after exercise (from 85 +/- 6 to 1450 +/- 55 and 57 +/- 11 to 474 +/- 54 microM, respectively; P < 0.001), whereas plasma triglyceride concentrations were decreased (from 1498 +/- 39 to 703 +/- 7 microM; P < 0.001). IMCL is an important substrate source during prolonged moderate-intensity exercise and is substantially decreased in the active vastus lateralis muscle. However, prolonged endurance exercise with its concomitant increase in plasma FFA concentration results in a net increase in IMCL content in less active muscle.

Quantifying pressure sore-related muscle damage using high-resolution MRI

To obtain insight into the etiology of deep pressure sores, understanding of the relationship between prolonged transverse loading and local muscle damage is required. To date, the amount and location of muscle damage have been determined by histological examination. In the present study, we determined whether T2-weighted high-resolution magnetic resonance imaging (MRI) can also be applied to evaluate muscle tissue after prolonged transverse loading. The tibialis anterior muscle and overlying skin in the right hindlimbs of five rats were compressed between an indenter and the tibia. The in vivo magnetic resonance images of the loaded and contralateral hindlimbs were obtained 24 h after load application. The tibialis anterior muscles were then processed for histological examination. In the magnetic resonance images of all five loaded hindlimbs, signal intensity appeared higher in the loaded regions of the muscle compared with the unloaded regions. The location of the higher signal intensity coincided with the location of damage assessed from histology. Also the amount of damage determined with MRI was in good agreement with the amount of damage assessed from histological examination. Because MRI is nondestructive, it is a promising alternative for histology in research on pressure sore etiology, especially in follow-up studies to evaluate the development of muscle damage in time and in clinical studies.

Neuroprotection by the endogenous cannabinoid anandamide and arvanil against in vivo excitotoxicity in the rat: role of vanilloid receptors and lipoxygenases

Type 1 vanilloid receptors (VR1) have been identified recently in the brain, in which they serve as yet primarily undetermined purposes. The endocannabinoid anandamide (AEA) and some of its oxidative metabolites are ligands for VR1, and AEA has been shown to afford protection against ouabain-induced in vivo excitotoxicity, in a manner that is only in part dependent on the type 1 cannabinoid (CB1) receptor. In the present study, we assessed whether VR1 is involved in neuroprotection by AEA and by arvanil, a hydrolysis-stable AEA analog that is a ligand for both VR1 and CB1. Furthermore, we assessed the putative involvement of lipoxygenase metabolites of AEA in conveying neuroprotection. Using HPLC and gas chromatography/mass spectroscopy, we demonstrated that rat brain and blood cells converted AEA into 12-hydroxy-N-arachidoylethanolamine (12-HAEA) and 15-hydroxy-N-arachidonoylethanolamine (15-HAEA) and that this conversion was blocked by addition of the lipoxygenase inhibitor nordihydroguaiaretic acid. Using magnetic resonance imaging we show the following: (1) pretreatment with the reduced 12-lipoxygenase metabolite of AEA, 12-HAEA, attenuated cytotoxic edema formation in a CB1 receptor-independent manner in the acute phase after intracranial injection of the Na+/K+-ATPase inhibitor ouabain; (2) the reduced 15-lipoxygenase metabolite, 15-HAEA, enhanced the neuroprotective effect of AEA in the acute phase; (3) modulation of VR1, as tested using arvanil, the VR1 agonist capsaicin, and the antagonist capsazepine, leads to neuroprotective effects in this model, and arvanil is a potent neuroprotectant, acting at both CB1 and VR1; and (4) the in vivo neuroprotective effects of AEA are mediated by CB1 but not by lipoxygenase metabolites or VR1.

The increase in intramyocellular lipid content is a very early response to training

The present study investigated the influences of a 2-wk training program on intramyocellular lipid (IMCL) content, IMCL decrease during exercise, fat oxidation, and insulin sensitivity. Nine untrained men (age, 23.3 +/- 3.2 yr; body mass index, 22.6 +/- 2.6 kg/m(2); maximal power output, 3.8 +/- 0.6 W/kg body weight) trained for 2 wk. Before and after training, subjects cycled for 3 h while substrate oxidation was measured. IMCL content in the vastus lateralis muscle was determined before and after cycling by proton magnetic resonance spectroscopy. Before and after training, insulin sensitivity was assessed by an insulin tolerance test. The training period resulted in a significant increase in IMCL content by 42 +/- 14%. IMCL content decreased significantly during cycling. However, 2 wk of training were not sufficient to achieve increases in fat oxidation and/or use of IMCL during exercise. All markers used to test insulin sensitivity point toward improved insulin sensitivity, albeit not significant. We conclude that the increase in IMCL content is a very early response to training, preceding significant changes in insulin sensitivity. The results suggest that the presence of triglycerides alone does not necessarily have detrimental effects on insulin sensitivity. We confirm earlier reports that IMCL contributes to the energy used during prolonged submaximal exercise.

Water ADC, extracellular space volume, and tortuosity in the rat cortex after traumatic injury

The diffusion parameters in rat cortex were studied 3-35 days following a cortical stab wound, using diffusion-weighted MR to determine the apparent diffusion coefficient of water (ADC(W)) in the tissue, and the real-time iontophoretic tetramethylammonium (TMA) method to measure the extracellular space (ECS) diffusion parameters: ECS volume fraction alpha and the ADC of TMA(+) (ADC(TMA)). Severe astrogliosis was found close to the wound, and mild astrogliosis was found in the ipsilateral but not the contralateral cortex. Chondroitin sulfate proteoglycan (CSPG) expression was increased throughout the ipsilateral cortex. In the hemisphere contralateral to the wound, alpha, ADC(TMA), and ADC(W) were not significantly different from control values. ECS volume fraction was increased only in the vicinity of the wound, in the region of cell death and severe astrogliosis, at 3 and 7 days after injury. However, both ADC(TMA) and ADC(W) were significantly decreased after lesion in the vicinity of the wound as well as in the rest of the ipsilateral hemisphere distant from the wound. Thus, both ADC(W) and ADC(TMA) decreased in regions wherein alpha did not change but CSPG increased. An increase in extracellular matrix expression may therefore impose diffusion barriers for water as well as for TMA molecules.

Characterization of the normal cardiac myofiber field in goat measured with MR-diffusion tensor imaging

Cardiac myofiber orientation is a crucial determinant of the distribution of myocardial wall stress. Myofiber orientation is commonly quantified by helix and transverse angles. Accuracy of reported helix angles is limited. Reported transverse angle data are incomplete. We measured cardiac myofiber orientation postmortem in five healthy goat hearts using magnetic resonance-diffusion tensor imaging. A novel local wall-bound coordinate system was derived from the characteristics of the fiber field. The transmural course of the helix angle corresponded to data reported in literature. The mean midwall transverse angle ranged from -12 +/- 4 degrees near the apex to +9.0 +/- 4 degrees near the base of the left ventricle, which is in agreement with the course predicted by Rijcken et al. (18) using a uniform load hypothesis. The divergence of the myofiber field was computed, which is a measure for the extent to which wall stress is transmitted through the myofiber alone. It appeared to be <0.07 mm(-1) throughout the myocardial walls except for the fusion sites between the left and right ventricles and the insertion sites of the papillary muscles.

Bud abortion in tulip bulbs studied by magnetic resonance imaging

After storage and subsequent planting of flower bulbs, the flower bud frequently appears to be aborted. This physiological aberration is probably caused by a change in the water status of the bulb and may be initiated during storage. The development of bud abortion in tulip bulbs was studied during long-term dry storage of the bulbs at 5 degrees C. The anatomy of individual tulip bulbs was followed non-invasively with T2-weighted NMR imaging, which allowed the monitoring of the growth of the shoot and daughter bulbs. Quantitative maps of T1 and T2 relaxation times of individual bulbs were used to assess regional changes in the water status of different tissues. Parallel to the NMR measurements, bulbs were planted to assess the ultimate flower quality. Moreover, water content, osmolality of tissue sap and ion leakage of excised shoot and scale tissues were determined to obtain information about the water status and viability of the bulbs. Significant decreases during long-term storage were found in T1 and T2 relaxation times in the shoot and particularly in the stamens. An increase in the osmolality of tissue sap and the decrease in relaxation times in the shoot below a certain threshold value attained after 24 weeks of storage, could be indicative for the emergence of bud abortion in tulips.

Exogenous anandamide protects rat brain against acute neuronal injury in vivo

The endocannabinoid anandamide [N-arachidonoylethanolamine (AEA)] is thought to function as an endogenous protective factor of the brain against acute neuronal damage. However, this has never been tested in an in vivo model of acute brain injury. Here, we show in a longitudinal pharmacological magnetic resonance imaging study that exogenously administered AEA dose-dependently reduced neuronal damage in neonatal rats injected intracerebrally with the Na(+)/K(+)-ATPase inhibitor ouabain. At 15 min after injury, AEA (10 mg/kg) administered 30 min before ouabain injection reduced the volume of cytotoxic edema by 43 +/- 15% in a manner insensitive to the cannabinoid CB(1) receptor antagonist SR141716A. At 7 d after ouabain treatment, 64 +/- 24% less neuronal damage was observed in AEA-treated (10 mg/kg) rats compared with control animals. Coadministration of SR141716A prevented the neuroprotective actions of AEA at this end point. In addition, (1) no increase in AEA and 2-arachidonoylglycerol levels was detected at 2, 8, or 24 hr after ouabain injection; (2) application of SR141716A alone did not increase the lesion volume at days 0 and 7; and (3) the AEA-uptake inhibitor, VDM11, did not affect the lesion volume. These data indicate that there was no endogenous endocannabinoid tone controlling the acute neuronal damage induced by ouabain. Although our data seem to question a possible role of the endogenous cannabinoid system in establishing a brain defense system in our model, AEA may be used as a structural template to develop neuroprotective agents.

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

Neuroprotection by Delta9-tetrahydrocannabinol, the main active compound in marijuana, against ouabain-induced in vivo excitotoxicity

Excitotoxicity is a paradigm used to explain the biochemical events in both acute neuronal damage and in slowly progressive, neurodegenerative diseases. Here, we show in a longitudinal magnetic resonance imaging study that Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the main active compound in marijuana, reduces neuronal injury in neonatal rats injected intracerebrally with the Na(+)/K(+)-ATPase inhibitor ouabain to elicit excitotoxicity. In the acute phase Delta(9)-THC reduced the volume of cytotoxic edema by 22%. After 7 d, 36% less neuronal damage was observed in treated rats compared with control animals. Coadministration of the CB(1) cannabinoid receptor antagonist SR141716 prevented the neuroprotective actions of Delta(9)-THC, indicating that Delta(9)-THC afforded protection to neurons via the CB(1) receptor. In Delta(9)-THC-treated rats the volume of astrogliotic tissue was 36% smaller. The CB(1) receptor antagonist did not block this effect. These results provide evidence that the cannabinoid system can serve to protect the brain against neurodegeneration.

Single-shot diffusion trace (1)H NMR spectroscopy

Ignoring diffusion anisotropy can severely hamper the quantitative determination of water and metabolite diffusion in complex tissues. The measurement of the trace of the diffusion tensor provides unambiguous and rotationally invariant ADC values, but usually requires three separate experiments. A single-shot technique developed earlier, originally designed for diffusion trace MR imaging (Mori and van Zijl, Magn Reson Med 1995;33:41-52), was improved and adapted for diffusion trace MR spectroscopy. A double spin-echo pulse sequence was incorporated with four pairs of bipolar gradients with specific predetermined relative signs in each of the three orthogonal directions. The combination of gradient directions leads to cancellation of all off-diagonal tensor elements while constructively adding the diagonal elements. Furthermore, the pulse scheme provides complete compensation for cross-terms between static magnetic field gradients and the applied diffusion gradients, while simultaneously avoiding cross-terms with localization gradients. The sequence was tested at 4.7 T in vivo on rat brain for MRI and on rat skeletal muscle and brain for MRS. It is shown that the average ADC as determined from the measurement of the ADCs in the three orthogonal directions is in close agreement with the ADC obtained along the trace of the diffusion tensor in a single acquisition, for both water and metabolite diffusion. The large differences in water and metabolite diffusion coefficients as measured in the individual orthogonal directions illustrate the need for diffusion trace measurements when accurate and rotationally invariant diffusion quantitation is required. The pulse scheme presented here may be applied for such purposes in MRS and MRI studies.

Diffusion NMR spectroscopy

MR offers unique tools for measuring molecular diffusion. This review focuses on the use of diffusion-weighted MR spectroscopy (DW-MRS) to non-invasively quantitate the translational displacement of endogenous metabolites in intact mammalian tissues. Most of the metabolites that are observed by in vivo MRS are predominantly located in the intracellular compartment. DW-MRS is of fundamental interest because it enables one to probe the in situ status of the intracellular space from the diffusion characteristics of the metabolites, while at the same time providing information on the intrinsic diffusion properties of the metabolites themselves. Alternative techniques require the introduction of exogenous probe molecules, which involves invasive procedures, and are also unable to measure molecular diffusion in and throughout intact tissues. The length scale of the process(es) probed by MR is in the micrometer range which is of the same order as the dimensions of many intracellular entities. DW-MRS has been used to estimate the dimensions of the cellular elements that restrict intracellular metabolite diffusion in muscle and nerve tissue. In addition, it has been shown that DW-MRS can provide novel information on the cellular response to pathophysiological changes in relation to a range of disorders, including ischemia and excitotoxicity of the brain and cancer.