Magnetic resonance imaging of the proximal upper extremity musculature in boys with Duchenne muscular dystrophy.

There is a pressing need for biomarkers and outcomes that can be used across disease stages in Duchenne muscular dystrophy (DMD), to facilitate the inclusion of a wider range of participants in clinical trials and to improve our understanding of the natural history of DMD. Quantitative magnetic resonance imaging (qMRI) and spectroscopy (MRS) biomarkers show considerable promise in both the legs and forearms of individuals with DMD, but have not yet been examined in functionally important proximal upper extremity muscles such as the biceps brachii and deltoid. The primary objective of this study was to examine the feasibility of implementing qMRI and MRS biomarkers in the proximal upper extremity musculature, and the secondary objective was to examine the relationship between MR measures of arm muscle pathology and upper extremity functional endpoints. Biomarkers included MRS and MRI measures of fat fraction and transverse relaxation time (T 2). The MR exam was well tolerated in both ambulatory and non-ambulatory boys. qMR biomarkers differentiated affected and unaffected participants and correlated strongly with upper extremity function (r = 0.91 for biceps brachii T 2 versus performance of upper limb score). These qMR outcome measures could be highly beneficial to the neuromuscular disease community, allowing measurement of the quality of functionally important muscles across disease stages to understand the natural history of DMD and particularly to broaden the opportunity for clinical trial participation.

Determination of the optimal load setting for arm crank anaerobic testing in men and women.

This study compared different relative load factors for eliciting the highest peak 5 s and mean 30 s absolute power output (watts) during an arm crank 30 s Wingate anaerobic power test in 40 upper body trained and recreationally active men and women. The relative load factor of 0.075 kg · kg(- 1) BM elicited a higher peak 5 s power output than 0.070 and 0.080 kg · kg(- 1) for trained males, and 0.070 was higher than 0.055 and 0.080 kg · kg(- 1) for active males (P<0.05). In trained women, the peak 5 s power output was greatest at 0.065 kg · kg(- 1) and 0.060 kg · kg(- 1) for active women. Mean 30 s power output at a relative load factor of 0.060, 0.065 and 0.070 kg · kg(- 1) was higher than 0.080, 0.085 and 0.090 kg · kg(- 1) in trained men, and mean power output at 0.080 kg · kg(- 1) was lower than all other relative load factors in active men (P<0.05). Mean 30 s power was greatest at 0.050 kg · kg(- 1) for trained and active women. In conclusion, the optimal relative load factor was different for eliciting peak 5 s and mean 30 s power outputs during an arm crank Wingate anaerobic test and depends on training status and gender.

Gene transfer of arginine kinase to skeletal muscle using adeno-associated virus.

In this study, we tested the feasibility of non-invasively measuring phosphoarginine (PArg) after gene delivery of arginine kinase (AK) using an adeno-associated virus (AAV) to murine hindlimbs. This was achieved by evaluating the time course, regional distribution and metabolic flux of PArg using (31)phosphorus magnetic resonance spectroscopy ((31)P-MRS). AK gene was injected into the gastrocnemius of the left hindlimb of C57Bl10 mice (age 5 weeks, male) using self-complementary AAV, type 2/8 with desmin promoter. Non-localized (31)P-MRS data were acquired over 9 months after injection using 11.1-T and 17.6-T Bruker Avance spectrometers. In addition, (31)P two-dimensional chemical shift imaging and saturation transfer experiments were performed to examine the spatial distribution and metabolic flux of PArg, respectively. PArg was evident in each injected mouse hindlimb after gene delivery, increased until 28 weeks, and remained elevated for at least 9 months (P<0.05). Furthermore, PArg was primarily localized to the injected posterior hindimb region and the metabolite was in exchange with ATP. Overall, the results show the viability of AAV gene transfer of AK gene to skeletal muscle, and provide support of PArg as a reporter that can be used to non-invasively monitor the transduction of genes for therapeutic interventions.

Longitudinal measurements of MRI-T2 in boys with Duchenne muscular dystrophy: effects of age and disease progression.

Duchenne muscular dystrophy (DMD) is characterized by an increased muscle damage and progressive replacement of muscle by noncontractile tissue. Both of these pathological changes can lengthen the MRI transverse proton relaxation time (T2). The current study measured longitudinal changes in T2 and its distribution in the lower leg of 16 boys with DMD (5-13years, 15 ambulatory) and 15 healthy controls (5-13years). These muscles were chosen to allow extended longitudinal monitoring, due to their slow progression compared with proximal muscles in DMD. In the soleus muscle of boys with DMD, T2 and the percentage of pixels with an elevated T2 (⩾2SD above control mean T2) increased significantly over 1year and 2years, while the width of the T2 histogram increased over 2years. Changes in soleus T2 variables were significantly greater in 9-13years old compared with 5-8years old boys with DMD. Significant correlations between the change in all soleus T2 variables over 2years and the change in functional measures over 2years were found. MRI measurement of muscle T2 in boys with DMD is sensitive to disease progression and shows promise as a clinical outcome measure.

Effects of recovery time on phosphocreatine kinetics during repeated bouts of heavy-intensity exercise.

The purpose of this study was to examine the kinetics of phosphocreatine (PCr) breakdown in repeated bouts of heavy-intensity exercise separated by three different durations of resting recovery. Healthy young adult male subjects (n = 7) performed three protocols involving two identical bouts of heavy-intensity dynamic plantar flexion exercise separated by 3, 6, and 15 min of rest. Muscle high-energy phosphates and intracellular acid-base status were measured using phosphorus-31 magnetic resonance spectroscopy. In addition, the change in concentration of total haemoglobin (Delta[Hb(tot)]) and deoxy-haemoglobin (Delta[HHb]) were monitored using near-infrared spectroscopy. Prior exercise resulted in an elevated (P < 0.05) intracellular hydrogen ion ([H(+)](i)) after 3 min (182 +/- 72 (SD) nM; pH 6.73) and 6 min (112 +/- 19 nM; pH 6.95) but not after 15 min (93 +/- 8 nM; pH 7.03) compared to pre-exercise in Con (90 +/- 3 nM; pHi 7.05). The on-transient time constant (tau) of the PCr primary component was not different amongst the exercise bouts. However, in each of the subsequent bouts the amplitude of the PCr slow component, total PCr breakdown, and rise in [H(+)](i) were reduced (P < 0.05). At exercise onset, Delta[Hb(tot)] was increased (P < 0.05) and the Delta[HHb] kinetic response was slowed (P < 0.05) in the exercise after 3 min, consistent with improved muscle perfusion. In summary, neither the level of acidosis or muscle perfusion at the onset of exercise appeared to be directly related to the time course of the on-transient PCr primary component or the magnitude of the PCr slow component during subsequent bouts of exercise.

Effects of hyperventilation on phosphocreatine kinetics and muscle deoxygenation during moderate-intensity plantar flexion exercise.

The effects of controlled voluntary hyperventilation (Hyp) on phosphocreatine (PCr) kinetics and muscle deoxygenation were examined during moderate-intensity plantar flexion exercise. Male subjects (n = 7) performed trials consisting of 20-min rest, 6-min exercise, and 10-min recovery in control [Con; end-tidal Pco(2) (Pet(CO(2))) approximately 33 mmHg] and Hyp (Pet(CO(2)) approximately 17 mmHg) conditions. Phosphorus-31 magnetic resonance and near-infrared spectroscopy were used simultaneously to monitor intramuscular acid-base status, high-energy phosphates, and muscle oxygenation. Resting intracellular hydrogen ion concentration ([H(+)](i)) was lower (P < 0.05) in Hyp [90 nM (SD 3)] than Con [96 nM (SD 4)]; however, at end exercise, [H(+)](i) was greater (P < 0.05) in Hyp [128 nM (SD 19)] than Con [120 nM (SD 17)]. At rest, [PCr] was not different between Con [36 mM (SD 2)] and Hyp [36 mM (SD 1)]. The time constant (tau) of PCr breakdown during transition from rest to exercise was greater (P < 0.05) in Hyp [39 s (SD 22)] than Con [32 s (SD 22)], and the PCr amplitude was greater (P < 0.05) in Hyp [26% (SD 4)] than Con [22% (SD 6)]. The deoxyhemoglobin and/or deoxymyoglobin (HHb) tau was similar between Hyp [13 s (SD 8)] and Con [10 s (SD 3)]; however, the amplitude was increased (P < 0.05) in Hyp [40 arbitrary units (au) (SD 23)] compared with Con [26 au (SD 17)]. In conclusion, our results indicate that Hyp-induced hypocapnia enhanced substrate-level phosphorylation during moderate-intensity exercise. In addition, the increased amplitude of the HHb response suggests a reduced local muscle perfusion in Hyp compared with Con.

NaHCO3-induced alkalosis reduces the phosphocreatine slow component during heavy-intensity forearm exercise.

During heavy-intensity exercise, the mechanisms responsible for the continued slow decline in phosphocreatine concentration ([PCr]) (PCr slow component) have not been established. In this study, we tested the hypothesis that a reduced intracellular acidosis would result in a greater oxidative flux and, consequently, a reduced magnitude of the PCr slow component. Subjects (n = 10) performed isotonic wrist flexion in a control trial and in an induced alkalosis (Alk) trial (0.3g/kg oral dose of NaHCO3, 90 min before testing). Wrist flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; intracellular pH threshold) and heavy-intensity (125% intracellular pH threshold) exercise. 31P-magnetic resonance spectroscopy was used to measure intracellular [H+], [PCr], [Pi], and [ATP]. The initial recovery data were used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials, venous blood sampling was used to measure plasma [H+], [HCO3-], and [Lac-]. Throughout rest and exercise, plasma [H+] was lower (P < 0.05) and [HCO3-] was elevated (P < 0.05) in Alk compared with control. During the final 3 min of heavy-intensity exercise, Alk caused a lower (P < 0.05) intracellular [H+] [246 (SD 117) vs. 291 nmol/l (SD 129)], a greater (P < 0.05) [PCr] [12.7 (SD 7.0) vs. 9.9 mmol/l (SD 6.0)], and a reduced accumulation of [ADP] [0.065 (SD 0.031) vs. 0.098 mmol/l (SD 0.059)]. Oxidative flux was similar (P > 0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis, causing a decrease in the magnitude of the PCr slow component. The decreased PCr slow component in Alk did not appear to be due to an elevated oxidative flux.