Effects of exercise and time elapsed after exercise on VO2, VCO2 and R responses to norepinephrine in rats

The effects of norepinephrine (NE) injection (300 microgram . kg-1 of body weight) on oxygen consumption (VO2), carbon dioxide production (VCO2) and respiratory exchange ratio (R) were investigated in female rats after 1 h of running on a treadmill (21.5 m . min-1) at 10% inclination. Six groups of animals were injected respectively at various times after the exercise (1, 3, 6, 9, 21, and 47 h), and were compared to six non-exercised groups injected at corresponding times. VO2 and VCO2 were monitored continuously during the 20 min preceding injection and for the 60 min following it. The increases in VO2 and VCO2, and the decrease in R were of similar magnitude in both exercised and non-exercised rats (about 30% and 20% for VO2 and VCO2, respectively, and -12% for R). Peak VO2 and R values attained after NE injection varied however with time of injection, specially in exercised animals 1 and 9 h after the run. Exercise significantly delayed time of response to NE for VO2 and VCO2 particularly 1 and 9 h after the running bout. It is concluded that time of day, exercise, and time elapsed after exercise are important factors to consider when studying metabolic responses to catecholamines. Furthermore, it is suggested that such experimental controls might be meaningful in human studies as well.