31P NMR study of phosphorus metabolites in fast and slow muscles

Noninvasive monitoring of training induced muscle adaptation with 31P-MRS: fibre type shifts correlate with metabolic changes

Purpose. To evaluate training induced metabolic changes noninvasively with ³¹P magnetic resonance spectroscopy (³¹P-MRS) for measuring muscle fibre type adaptation. Methods. Eleven volunteers underwent a 24-week training, consisting of speed-strength, endurance, and detraining (each 8 weeks). Prior to and following each training period, needle biopsies and ³¹P-MRS of the resting gastrocnemius muscle were performed. Fibre type distribution was analyzed histologically and tested for correlation with the ratios of high energy phosphates ([PCr]/[P_i], [PCr]/[βATP] and [PCr + P_i]/[βATP]). The correlation between the changes of the ³¹P-MRS parameters during training and the resulting changes in fibre composition were also analysed. Results. We observed an increased type-II-fibre proportion after speed-strength and detraining. After endurance training the percentage of fast-twitch fibres was reduced. The progression of the [PCr]/[P_i]-ratio was similar to that of the fast-twitch fibres during the training. We found a correlation between the type-II-fibre proportion and [PCr]/[P_i] (r = 0.70, P < 0.01) or [PCr]/[βATP] (r = 0.69, P < 0.01); the correlations between its changes (delta) and the fibre-shift were significant as well (delta[PCr]/[P_i] r = 0.66, delta[PCr]/[βATP] r = 0.55, P < 0.01). Conclusion. Shifts in fibre type composition and high energy phosphate metabolite content covary in human gastrocnemius muscle. Therefore ³¹P-MRS might be a feasible method for noninvasive monitoring of exercise-induced fibre type transformation.

Mechanisms of resistance to pathogenesis in muscular dystrophies

A mechanistic definition of the dystrophic process is proposed, and the effects of growth factors vs. down-regulation of growth are critically analyzed. A conceptual scheme is presented to illustrate the steps leading to pathology, and various compensatory systems which ameliorate the pathology are examined, particularly in regards to the mdv mouse which is resistant to the deficiency of dystrophin, the main protein product of the Duchenne and Becker muscular dystrophy (DMD/BMD) gene. These compensatory systems are analyzed in terms of the differential resistance of fiber types to pathogenesis. The generation of a stable population of maturationally arrested centronucleated fibers which express the mature adult myosin isoforms is proposed to be the main strategy of mdx muscle to minimize apoptosis. Physiological properties of these fibers, such as utrophin expression, and high mitochondrial and endoplasmic reticulum content, together with probable increased glycerophosphorylcholine concentrations and facile access to the vascular system, are hypothesized to be instrumental in their resistance to pathogenesis. It is proposed that the major element that determines the susceptibility of most human muscles to the dystrophic process is their inability to arrest the maturation of regenerated fibers at the centronucleated stage with a concomitant expression of the adult myosins.

Effect of training on fibre composition and phosphate metabolites in rest measured in vitro in muscles of young pigs

1. Fibre type composition and phosphate metabolites were studied in m. longissimus thoracis (MLT), m. rectus femoris (MRF) and m. triceps brachii (MTB) in trained (N = 6) and sedentary (N = 6) pigs. 2. Samples were analyzed histochemically and by means of in vitro 31P NMR spectroscopy. 3. Training (duration 11 weeks) consisted of treadmill running at a speed of 1.1 m/sec. The daily exercise time of trained animals gradually increased from 10 min during the very first days to 60 min at the end of the 4th week. 4. During the final 7 weeks exercise time remained unchanged. Sedentary animals were not subjected to training. 5. A higher proportion of type beta R fibres in MRF, MTB and MLT and a lower proportion of type alpha W fibres were found in the trained group of animals compared to the control group. 6. In MLT no significant differences in the proportion of type alpha W were observed between both groups. 7. No significant differences in average fibre diameter of muscle fibres were found between groups. 8. No differences in concentration of phosphate compounds were observed between trained and sedentary groups. 9. Muscles with a higher proportion of type IIb fibres in both groups of pigs contained higher amounts of phosphocreatine (PCr) and were also characterized by a higher ratio of PCr to inorganic phosphate (Pi).