Metabolic heterogeneity in human calf muscle during maximal exercise.

Abstract

Human skeletal muscle is composed of various muscle fiber types. We hypothesized that differences in metabolism between fiber types could be detected noninvasively with 31P nuclear magnetic resonance spectroscopy during maximal exercise. This assumes that during maximal exercise all fiber types are recruited and all vary in the amount of acidosis. The calf muscles of seven subjects were studied. Two different coils were applied: an 11-cm-diameter surface coil and a five-segment meander coil. The meander coil was used to localize the 31p signal to either the medial or the lateral gastrocnemius. Maximal exercise, consisting of rapid plantar flexions, resulted in an 83.7% ± 7.8% decrease of the phosphocreatine pool and an 8-fold increase of the inorganic phosphate (Pi) pool. At rest the Pi pool was observed as a single resonance (pH 7.0). Toward the end of the first minute of exercise, three subjects showed three distinct Pi peaks. During the second minute of exercise the pH values stabilized at 7.12 ± 0.12, 6.63 ± 0.15, and 6.27 ± 0.23. The same pattern was seen when the signal was collected from the medial or lateral gastrocnemius. In four subjects only two distinct Pi peaks were observed. The Pi peaks had differing relative areas in different subjects, but they were reproducible in each individual. This method allowed us to study the appearance and disappearance of the different Pi peaks, together with the changes in pH.

Dipolar coupling and ordering effects observed in magnetic resonance spectra of skeletal muscle

“…In turn, macroscopic compartmentation is better described as partial volume effect and is a very common finding in MR spectroscopy and imaging. Different metabolite levels [55][56][57] and splitting of the resonance of inorganic phosphate 56,[58][59][60][61][62] may be the result of the inclusion of slow-and fast-twitch fibers in the sensitive volume of 31 P-MR spectra acquisition. An interpretation of this effect obviously can lead to physiological knowledge; however, since it does not contain specific information on cellular or subcellular order of tissue, it will not be discussed in detail here.…”

Fiber Types in Mammalian Skeletal Muscles

“…Later, 31 P NMR spectroscopy opened a new approach to study muscle metabolism in vivo at rest and during contraction making detectable the diversity related to fiber type composition (587,819). In the magnetic resonance spectra, the shift of the P i peak with change in pH gives a direct indications of the activation of three distinct fiber type populations (587,819).…”

“…The subsequent development of microchemical methods at the scale of single fibers allowed the determination of enzyme activities and the quantitative description of the metabolic properties of individual muscle fibers (209,210,493,644). Later, 31 P NMR spectroscopy opened a new approach to study muscle metabolism in vivo at rest and during contraction making detectable the diversity related to fiber type composition (587,819). In the magnetic resonance spectra, the shift of the P i peak with change in pH gives a direct indications of the activation of three distinct fiber type populations (587,819).…”

Skeletal muscle energetics with PNMR: personal views and historic perspectives

“…With higher intensity, type IIb and type IIa are recruited to meet the needs of multiple tasks (71). Thus, bioenergetic heterogeneities between anaerobic and oxidative ATP synthesis and consumption rates are expected during exercise, and are more between different fiber type cells than within the myocytes (72)(73)(74). These three types of cell intermingle in a muscle, so that lactate as a final product of type IIb fiber is transported to type I and type IIa myocytes and used as a fuel for oxidative phosphorylation.…”