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This paper examined the role of metabolites in causing muscle fatigue. Previous studies have shown that Pi (H2PO4-, HPO4-2) and H+ may be important factors in causing fatigue. A key question is the potential interaction between metabolic end-products and calcium related excitation-contraction coupling fatigue (ECC). An in vivo rat muscle model was used to measure tension development and metabolic end-products in response to electrical stimulation. Two stimulation protocols were used, high intensity stimulation followed by a medium intensity stimulation (High Group), and low intensity stimulation followed by a medium intensity stimulation (Low Group). Metabolic fatigue was based on concentrations of (H2PO4-measured with phosphorus magnetic resonance spectroscopy. ECC fatigue was measured as the fatigue in excess of metabolic fatigue, and as the relative decline of force at low compared to high stimulation frequencies. During the initial stimulation period, the High Group had greater metabolic fatigue (p < 0.001) and greater ECC fatigue (p = 0.007). During the second stimulation period and recovery, the High Group had no difference in metabolic fatigue (p = 0.07) and greater ECC fatigue (p = 0.015). These results present a method for determining the relative amounts of metabolic and ECC fatigue, and suggest that metabolites can increase the amount of ECC fatigue.

Clones of human embryonic alpha-like zeta-globin cDNA were isolated, by detection using cross-hybridization to human alpha-globin cDNA probes, from a cDNA library derived from the mRNA of the human erythroleukemia cell line K562. Nucleotide sequence analysis of these cDNA clones revealed a coding sequence that corresponds perfectly to the independently derived amino acid sequence of the human zeta-globin chain. Comparison of the nucleotide sequence of human zeta-globin cDNA with that of human alpha-globin cDNA confirmed previous estimates of very distant evolutionary divergence between the human zeta- and alpha-globin genes. Nevertheless, the human zeta-globin cDNA sequence shares a remarkable similarity to that of the alpha-globin gene in its codon usage, high G + C base composition, and lack of bias against usage of CG dinucleotides.

Perfluorinated blood substitutes can be detected by 19F NMR in vivo in small concentrations. Reactive hyperemia was induced in rat leg by the transient application of a tourniquet, causing a 50% increase in 19F NMR signal. These results show that this method can be used to monitor the kinetics of tissue perfusion.

Phosphorus-31 MR spectroscopy allows non-invasive evaluation of the energy state of a tissue and was used to study effects of acute ischemia in rat skeletal muscle. Male Wistar rats were anesthetized with pentobarbital (50 mg.kg-1) and the femoral artery ligated on leaving the abdominal cavity. Animals were then set inside a superconducting magnet (Bruker 2.35 T) and a circular coil (15 mm diameter) placed on leg facing gastrocnemius muscle. Pulse lengths were chosen in such a may that the sensitive zone would include mainly gastrocnemius muscle. Signals were accumulated over 2 or 18 minutes. One hour after insertion of ligature muscular exercise was provoked over 20 minutes by electrical stimulation of sciatic nerve (4 Hz, 2 to 5 V). Muscles were removed at different stages of the test for biochemical assays. Figure 1 includes spectra at 2 and 18 minutes showing the effects of ischemic when compared with normoxic muscle exercise. Figure 2, showing phosphocreatine (PC) and ATP levels, illustrates the accentuation under the effect of ischemia of degradation in PC during muscular exercise and its subsequent slow reconstitution. Table I lists more precise MR data obtained during 18 minutes and compares them with biochemical findings. Ischemic exercise appears also to induce significant degradation of ATP, accumulation of inorganic phosphate (Pi) and phosphomonoesters (PME) as well as persistent intracellular acidosis (pH 6.5 as against 7.1).(ABSTRACT TRUNCATED AT 250 WORDS)

This study uses 31P NMR as a tool for the study of the capacity of recovery of the rat skeletal muscle after an exercise performed during an acute state of ischaemia. The leg muscle of a rat submitted to a 20 minute exercise period one hour after irreversible femoral artery ligation, manifested a dramatic (75%) decrease in phosphocreatine (PC) content, a less pronounced (30%) decrease in ATP, an accumulation of inorganic phosphate (Pi) and an increase in the phosphomonoester (PME) resonances, in addition to acidosis to pH 6.4. An investigation over a 40 minute post-exercise period using 31P NMR and biochemical analysis led to the following observations: 1. The PC and Pi contents of the muscle experienced no further significant changes, remaining at the level reached by the end of the exercise. 2. The ATP content similarly remained at the level reached at the end of this period, the adenylate charge being 0.91 (controls 0.93). 3. The IMP accumulated during ischaemic exercise remained at its high level. It seems likely that this compound contributes in a large part to the resonances in the PME region of the spectra. 4. Intracellular acidosis persisted despite a decrease in lactate content. The most important finding from this study is that the situation created by ischaemic exercise--as revealed by the NMR spectra--is characterized by a blocking of the main biochemical processes (phosphorylations, purine nucleotide cycle, pH regulation). Such a condition, which does not seem to entail lethal cell injury, could thus be used as a basis for the study via 31P NMR of the therapeutic effect of various treatments.

Rat leg muscles, rendered ischaemic 1 hour previously by ligation of the femoral artery, were submitted to 20 minutes exercise by electrical stimulation of the sciatic nerve. 31P-NMR spectroscopy was used to monitor the changes in high-energy phosphate content of the muscles before, during, and after exercise. Fifteen of the 35 studied muscles evolved toward total necrosis, whereas the others showed signs of recovery over a 2-5-hour postexercise period. Those muscles which did not subsequently recover contained significantly more inorganic phosphate (Pi) at rest (before exercise) than those which recovered. It is suggested that under acute ischaemic conditions the Pi level at rest is correlated with the extent of blood flow restriction and can be used to predict the severity of the ischemia.