Effects of exposure to hypobaric-hypoxia on body weight, muscular and hematological characteristics, and work performance in rats.

The metabolic activities of skeletal muscles were studied in male rats exposed to hypobaric-hypoxia at about 550 Torr for 8 h per day for 2 weeks. Rats were divided into three groups; control (normoxic control), diurnal hypoxic (DH) exposure, and nocturnal hypoxic (NH) exposure groups. The changes in body weight and daily diet intake of the NH group were lower than the other groups (p < 0.01). The weights of fat in the abdominal cavity of both NH and DH groups were less than that of the control group. The red blood cell count, hemoglobin concentration, and hematocrit values were significantly increased in the hypoxic groups. The plasma glucose level in the NH group was significantly less than the control group (p < 0.05). The lactate dehydrogenase/citrate synthase (LDH/CS) activity ratios in the skeletal muscle tended to be lower in both hypoxic groups than in the control group. The swimming times to exhaustion at mild and high intensities that were measured after 2 weeks, loaded with a weight equivalent to 2.5% of the body weight, improved in the DH group. There were insignificant differences in the metabolic activity of skeletal muscles and blood characteristics between the NH and DH groups, but endurance swimming times in the DH group tended to be improved as compared to the NH group. We conclude that the DH group became competent in endurance work, which is believed to be driven from the combined effects of increased O2 transport capacity of the blood and enhanced O2 utilization capability by mitochondrial enzyme activity.