Effect of decreased availability of substrates on intramuscular triglyceride utilization during exercise.

Intramuscular triglycerides mobilization during prolonged physical exercise was examined in rats fed ad libitum, in rats fasted for 24 h and in rats treated with nicotinic acid. It has been found that during exercise the intramuscular triglyceride level was markedly reduced only in the red muscle but not in the white and intermediate muscles. Fasting significantly augmented the utilization of triglycerides in the red muscle during exericse. The post-exercise triglyceride level in the red muscle of the rats treated with nicotinic acid was similar to that in the control group whereas blood FFA level, in the nicotinic acid-treated group was much lower than in the control group. Nicotinic acid increased glycogen utilization in the liver and in the skeletal muscles during exercise. It may be concluded that the major cause of the reduction of the triglyceride level in the red muscle during exercise is a developing shortage of available carbohydrates. The greatly elevated blood FFA level during exercise does not seem to have a sparing effect on the intramuscular triglyceride level during exercise. However, it does spare glycogen content in the liver and the skeletal muscles.