Phosphorus-31 nuclear magnetic resonance study on the effects of endurance training in rat skeletal muscle

Effect of long-term caloric restriction and exercise on muscle bioenergetics and force development in rats

We evaluated the hypothesis that long-term caloric restriction and exercise would have beneficial effects on muscle bioenergetics and performance in the rat. By themselves, each of these interventions is known to increase longevity, and bioenergetic improvements are thought to be important in this phenomenon. Accordingly, we investigated rats that underwent long-term caloric restriction and were sedentary, ad libitum-fed rats permitted to exercise by daily spontaneous wheel running (AE), and the combination of the dietary and exercise interventions (RE). Ad libitum-fed, sedentary rats comprised the control group. 31P NMR spectra of the gastrocnemius muscle (GM) were collected in vivo at rest and during two periods of electrical stimulation. Neither caloric restriction nor exercise affected the ratio of phosphocreatine to ATP or pH at rest. During the first stimulation and after recovery, the RE group had a significantly smaller decline in pH than did the other groups (P < 0.05). During the second period of stimulation, the decrease in pH was much smaller in all groups than during the first stimulation, with no differences observed among the groups. The combination of caloric restriction and exercise resulted in a significant attenuation in the decline in developed force during the second period of stimulation (P < 0.05). A biochemical correlate of this was a significantly higher concentration of citrate synthase in the GM samples from the RE rats (32.7 +/- 5.4 micromol. min-1. g-1) compared with the AE rats (17.6 +/- 5.7 micromol. min-1. g-1; P < 0.05). Our experiments thus demonstrated a synergistic effect of long-term caloric restriction and free exercise on muscle bioenergetics during electrical stimulation.

Optical isometric force transducer for measurement of rat skeletal muscle contraction in the NMR spectrometer

In order to measure force of contraction of the rat gastrocnemius muscle in the NMR spectrometer with a simultaneous observation of 31P NMR spectra, an optical force transducer was constructed using gratings and optical fibers. Muscle contraction was accurately recorded in the spectrometer without interfering with NMR measurements. The half height width of the phosphocreatine (PCr) peak in the presence or absence of the transducer was 5 Hz. Decrease in PCr and increase in inorganic phosphate (Pi) in the muscle were successfully followed during monitoring of the force developed during exercise. The change in the PCr:(PCr + Pi) ratio correlated well with the time-force integral from the optical force transducer. In general, there was no interference with the 31P NMR spectrum when the transducer was attached to rat leg, so this optical force transducer should prove valuable for studying muscle physiology in the NMR spectrometer.

31P nuclear magnetic resonance study on changes in phosphocreatine and the intracellular pH in rat skeletal muscle during exercise at various inspired oxygen contents

We measured ATP, phosphocreatine (PCr), inorganic phosphate (Pi), and the intracellular pH in rat hindlimb muscles during submaximal isometric exercise with various O2 deliveries using 31P nuclear magnetic resonance spectroscopy (31P NMR) to evaluate changes in energy metabolism in relation to O2 availability. Delivery of O2 to muscles was altered by controlling the fractional concentration of inspired oxygen (F1O2) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial pressure of oxygen and carbon dioxide, and bicarbonate at the femoral artery. The steady-state ratio of PCr: (PCr + Pi) during exercise decreased as a function of F1O2 even at 0.21. Significant acidification of the intracellular pH during exercise occurred at 0.08 F1O2. Change in the PCr: (PCr + Pi) ratio demonstrated that the oxidative capacity, i.e. the maximal rate of the oxidative phosphorylation reaction, in muscle was not limited by O2 delivery at 0.50 F1O2, but was significantly limited at 0.21 F1O2 or below. Change in the intracellular pH at 0.08 F1O2 could be interpreted as an increase in lactate, suggesting activation of glycolysis. Correlation between the PCr: (PCr + Pi) ratio and the intracellular pH revealed the existence of a critical PCr: (PCr + Pi) ratio and pH for glycolysis activation at around 0.4 and 6.7, respectively.

Improvement of muscular oxidative capacity by training is associated with slight acidosis and ATP depletion in exercising muscles

Metabolic and mechanical properties of female rat skeletal muscles, submitted to endurance training on a treadmill, were studied by a 60-min in vivo multistep fatigue test. 31P-NMR was used to follow energy metabolism and pH. Mechanical performance was greatly improved in trained muscles. The oxidative capacity of the skeletal muscles was evaluated from the relationship between ADP calculated from the creatine kinase equilibrium and work and from the measure of the rate of phosphocreatine (PCr) resynthesis following exercise. In trained muscles, ADP production was lower per unit of mechanical performance, showing an improvement of oxidative metabolism. However, the PCr resynthesis rate was not modified. Slight acidosis and ATP depletion were observed from the beginning of the fatigue test. These modifications suggest changes of the creatine kinase equilibrium favoring mitochondrial ATP production. Our results indicate that muscle status improvement could be accompanied by ATP depletion and minimal acidosis during contraction; this would be of particular importance for objective evaluation of muscle regeneration processes and of gene therapy in muscle diseases.

In vivo human myocardial metabolism during aerobic exercise by phosphorus-31 nuclear magnetic resonance spectroscopy

A few studies have been made in vivo on human myocardial energy metabolism. Hence, no discussion has taken place on metabolism during exercise or of training effects on metabolism. We examined human myocardial energy metabolism at rest and during exercise, and also training effects on the metabolism by phosphorus-31 nuclear magnetic resonance (31P NMR)-spectroscopy. Six sedentary male students (Cont) and six male long distance runners (Tr) were the subjects. Energy metabolism data were obtained from myocardium during rest and exercise by the region selection method using 31P NMR. Rotation of the legs while riding a bicycle, which was fitted with an ergometer we had made ourselves for NMR, imposed given exercise intensities. The heart rate was measured in a stationary phase during exercise. Although the heart rate at rest in the Tr group was significantly lower [Tr, 52.5 (SD 3.1) beat.min-1; Cont, 67.1 (SD 2.9) beat.min-1], no significant difference was observed in myocardial energy metabolism using the 31P NMR method [Tr, phosphocreatine/beta-adenosine 5'-triphosphate (PCr/beta-ATP); 1.51 (SD 0.02); Cont, 1.51 (SD 0.01)]. When NMR measurements were investigated at two different intensities of exercise, heart rates in the Cont group were significantly higher by about 20 beat.min-1 than those in the Tr group at both exercise intensities, while no difference in energy metabolism was observed between the groups or between rest and exercise [Tr, 75.9 (SD 3.6), 88.3 (SD 3.7) beat.min-1; PCr/beta-ATP 1.51 (SD 0.03), 1.51 (SD 0.03); Cont, 95.9 (SD 2.4), 115.1 (SD 3.5) beat.min-1, PCr/beta-ATP 1.51 (SD 0.01), 1.51 (SD 0.04)].(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle energetics in short-term training during hypoxia in elite combination skiers

Four well-trained combination skiers were studied through pre- and post-training for the effects of short-term intermittent training during hypoxia on muscle energetics during submaximal exercise as measured by Phosphorus-31 nuclear magnetic resonance and maximal aerobic power (VO2max). The hypoxia and training in the cold was conducted in a hypobaric chamber and comprised 60-min aerobic exercise (at an intensity equivalent to the blood lactate threshold), using a cycle ergometer or a treadmill twice a day for 4, consecutive days at 5 degrees C, in conditions equivalent to an altitude of 2000 m (593 mm Hg). No change in VO2max was observed over the training period, while in the muscle energetics during submaximal exercise, the values of phosphocreatine/(phosphocreatine+inorganic phosphate) and intracellular pH were found to be significantly increased by training during hypoxia. During recovery, the time constant of phosphocreatine was found to have been significantly reduced [pre, 27.9 (SD 6.7) s; post, 22.5 (SD 4.7) s, P < 0.01]. The observed inhibition of phosphocreatine as well as that of intracellular pH changes after training during hypoxia and quicker recovery of phosphocreatine in submaximal exercise tests, may indicate improved oxidative capacity (i.e. a high adenosine 5'-triphosphate formation rate) despite the short-term hypoxia training.