Phosphorus metabolites in different muscles of the rat leg by 31P image-selected in vivo spectroscopy

Age-related structural and functional changes of low back muscles

During aging declining maximum force capacity with more or less unchanged fatigability is observed with the underlying mechanisms still not fully understood. Therefore, we compared morphology and function of skeletal muscles between different age groups. Changes in high-energy phosphate turnover (PCr, Pi and pH) and muscle functional MRI (mfMRI) parameters, including proton transverse relaxation time (T2), diffusion (D) and vascular volume fraction (f), were investigated in moderately exercised low back muscles of young and late-middle-aged healthy subjects with (31)P-MR spectroscopy, T2- and diffusion-weighted MRI at 3T. In addition, T1-weighted MRI data were acquired to determine muscle cross-sectional areas (CSA) and to assess fat infiltration into muscle tissue. Except for pH, both age groups showed similar load-induced MR changes and rates of perceived exertion (RPE), which indicates comparable behavior of muscle activation at moderate loads. Changes of mfMRI parameters were significantly associated with RPE in both cohorts. Age-related differences were observed, with lower pH and higher Pi/ATP ratios as well as lower D and f values in the late-middle-aged subjects. These findings are ascribed to age-related changes of fiber type composition, fiber size and vascularity. Interestingly, post exercise f was negatively associated with fat infiltration with the latter being significantly higher in late-middle-aged subjects. CSA of low back muscles remained unchanged, while CSA of inner back muscle as well as mean T2 at rest were associated with maximum force capacity. Overall, applying the proposed MR approach provides evidence of age-related changes in several muscle tissue characteristics and gives new insights into the physiological processes that take place during aging.

Integrating muscle cell biochemistry and whole-body physiology in humans:(31)P-MRS data from the InSight trial

We acquired ³¹P-MRS data from skeletal muscle of subjects of mixed gender and ethnicity, combined with a panel of physiological characteristics, and tested several long-standing hypotheses regarding relationships between muscle cell biochemistry and whole-body physiology with unusually high statistical power. We hypothesized that i) whole-body VO₂max would correlate with muscle respiratory capacity, ii) resting muscle phosphocreatine concentration ([PCr]) would negatively correlate with delta efficiency and iii) muscle mitochondrial function would positively correlate with both resting VO₂ and total daily energy expenditure (TDEE). Muscle respiratory capacity explained a quarter of the variation in VO₂max (r² = 26, p < .001, n = 87). There was an inverse correlation between muscle [PCr] and delta efficiency (r = −23, p = 046, n = 87). There was also a correlation between [PCr] recovery halftime and TDEE (r = −23, p = 035, n = 87). Our data not only provide insights into muscle cell chemistry and whole-body physiology but our mixed cohort means that our findings are broadly generalizable.

Quantitative hepatic phosphorus-31 magnetic resonance spectroscopy in compensated and decompensated cirrhosis

Few studies have examined the physiological/biochemical status of hepatocytes in patients with compensated and decompensated cirrhosis in situ. Phosphorus-31 magnetic resonance spectroscopy ((31)P MRS) is a noninvasive technique that permits direct assessments of tissue bioenergetics and phospholipid metabolism. Quantitative (31)P MRS was employed to document differences in the hepatic metabolite concentrations among patients with compensated and decompensated cirrhosis as well as healthy controls. All MRS examinations were performed on a 1.5-T General Electric Signa whole body scanner. The concentration of hepatic phosphorylated metabolites among patients with compensated cirrhosis (n = 7) was similar to that among healthy controls (n = 8). However, patients with decompensated cirrhosis (n = 6) had significantly lower levels of hepatic ATP compared with patients with compensated cirrhosis and healthy controls (P < 0.02 and P < 0.009, respectively) and a higher phosphomonoester/phosphodiester ratio than controls (P < 0.003). The results of this study indicate that metabolic disturbances in hepatic energy and phospholipid metabolism exist in patients with decompensated cirrhosis that are not present in patients with compensated cirrhosis or healthy controls. These findings provide new insights into the pathophysiology of hepatic decompensation.

Mapping fiber orientation in human muscle by proton MR spectroscopic imaging

Proton magnetic resonance spectroscopic imaging ((1)H-MRSI) was used to determine muscle fiber orientations in human calf muscles. The method is based on the fact that some resonances show orientation-dependent dipolar splitting, caused by incomplete motional averaging. This leads to proton spectra that depend strongly on the angle between muscle fibers and the magnetic field B(0). The orientation-dependent dipolar splittings were mapped using a fit with a basis set of predefined coupling patterns reflecting the fiber orientation. The fitted coupling patterns were displayed as images and assigned to different muscles based on segmented MR images. They showed gross differences in fiber orientation between some muscles, including m. soleus and m. tibialis anterior, for all subjects. In addition, smaller but significant differences between subjects were detected, which could be due to localization differences or real interindividual differences. Since dipolar splitting affects metabolite intensities, it is important to take this effect into account when calculating metabolite concentrations from MR spectra in muscle tissue. Spatial maps of the MR signals of trimethyl-ammonium groups and creatine/ phosphocreatine revealed significant differences in intensity between muscles.

31P NMR quantitation of phosphorus metabolites in rat heart and skeletal muscle in vivo

The aim of this study was to examine two methods of 31P NMR quantitation of phosphocreatine (PCr), ATP, and P(i) in rat heart and skeletal muscle in vivo. The first method employed an external standard of phenylphosphonic acid (PPA; 10 mM), and the second method used an enzymatic measurement of tissue ATP equated to the area under the betaATP peak. With the use of the external standard, the concentrations of ATP, PCr, and P(i) in the rat heart were 4.48 +/- 0.33, 9.21 +/- 0.65, and 2.25 +/- 0.16 micromol/g wet wt, respectively. With the use of the internal ATP standard, measured on the same tissue, the contents (means +/- SE) were 4.78 +/- 0.19, 9.83 +/- 0.18, and 2.51 +/- 0.33 micromol/g wet wt, respectively (n = 7). In skeletal muscle, ATP, PCr, and P(i) were 6.09 +/- 0.19, 23.44 +/- 0.88, and 1.81 +/- 0.18 micromol/g wet wt using the PPA standard and 6.03 +/- 0.19, 23.30 +/- 1.30, and 1.82 +/- 0.19 micromol/g wet wt using the internal ATP standard (n = 6). There was no significant difference for each metabolite as measured by the two methods of quantification in heart or skeletal muscle. The results validate the use of an external reference positioned symmetrically above the coil and imply that each has similar NMR sensitivities (similar signal amplitude per mole of 31P between PPA and tissue phosphorus compounds). We conclude that PCr, ATP, and P(i) are nearly 100% visible in the normoxic heart and nonworking skeletal muscle given the errors of measurement.

Force reduction uncoupled from pH and H(2)PO(-)(4) in rat gastrocnemius in vivo with continuous 2-Hz stimulation

The aim of this study was to examine the effect of the products of ATP hydrolysis on the fatigue process in rat gastrocnemius in vivo. Adult male Sprague-Dawley rats (300-400 g) were anesthetized and ventilated in a custom-built cradle fitted with a force transducer that could be placed into a 7-T NMR magnet. The muscle was stimulated continuously at 2 Hz for 20 min (n = 7). Isometric twitch force increased in the first 4 min of stimulation accompanied by changes in twitch duration (20% increase in relaxation time). Prolonged relaxation was associated with changes in cytosolic pH (6.91 to 6.58), lactate (1.8 to 12.6 micromol/g wet wt), and H(2)PO (7.57 to 13.99 mM). After 4 min, relaxation time, pH, lactate, and H(2)PO returned toward control values as twitch force progressively decreased. No correlation was found between force decline (or twitch broadening) and total phosphate (3 to 23 mM), free [ADP] (18 to 95 microM), free [Mg(2+)] (0.58 to 0.96 mM), or free energy of ATP hydrolysis (-65 to -55 kJ/mol). We conclude that force decline is not due to increased pH and/or H(2)PO but to fatigue of the fast-twitch fibers, possibly linked to glycogen depletion and/or failure of nerve impulse transmission in these fibers.

Impairment of cardiac function and bioenergetics in adult transgenic mice overexpressing the bovine growth hormone gene

Cardiovascular abnormalities represent the major cause of death in patients with acromegaly. We evaluated cardiac structure, function, and energy status in adult transgenic mice overexpressing bovine GH (bGH) gene. Female transgenic mice expressing bGH gene (n = 11) 8 months old and aged matched controls (n = 11) were used. They were studied with two-dimensional guided M-mode and Doppler echocardiography. The animals (n = 6) for each group were examined with 31P magnetic resonance spectroscopy to determine the cardiac energy status. Transgenic mice had a significantly higher body weight (BW), 53.2+/-2.4 vs. 34.6+/-3.7 g (P < 0.0001) and hypertrophy of left ventricle (LV) compared with normal controls: LV mass/BW 5.6+/-1.6 vs. 2.7+/-0.2 mg/g, P < 0.01. Several indexes of systolic function were depressed in transgenic animals compared with controls mice such as shortening fraction 25+/-3.0% vs. 39.9+/-3.1%; ejection fraction, 57+/-9 vs. 77+/-5; mean velocity of circumferential shortening, 4.5+/-0.8 vs. 7.0+/-1.1 circ/sec, p < 0.01. Creatine phosphate-to-ATP ratio was significantly lower in bGH overexpressing mice (1.3+/-0.08 vs. 2.1+/-0.23 in controls, P < 0.05). Ultrastructural examination of the hearts from transgenic mice revealed substantial changes of mitochondria. This study provides new insight into possible mechanisms behind the deteriorating effects of long exposure to high level of GH on heart function.

In vivo metabolic imaging of cardiac bioenergetics in transgenic mice

Recent advances in transgenic technology have made the mouse a particularly interesting small animal in cardiovascular research. Increasingly sophisticated experimental methods and tools are needed for detailed characterization of cardiovascular physiology and biochemistry in the mice. The objective of this study was to develop a method for noninvasive evaluation of cardiac energy metabolism in the mouse. Cardiac gated (31)P magnetic resonance spectroscopy using Image Selected in Vivo Spectroscopy (ISIS) method was applied in old mice overexpressing bovine growth hormone (bGH) (n = 5) and control mice (n = 5). The localized volumes of interest were 128 and 112 microL, respectively. Phosphocreatine-to-ATP ratio was 1.5 +/- 0.13 in the bGH mice and 2.1 +/- 0.04 in the control group (P < 0.01). The study demonstrates the feasibility of application of volume-selective (31)P MRS for evaluation of cardiac energy metabolism in the mouse under maintained physiological conditions.