Validity of lactate minimum test for anaerobic threshold determination in rats during swimming exercise

Abstract

The break point of the curve blood lactate vs exercise load has been called Anaerobic Threshold (AT) and is considered a good indicator of exercise capacity in human subjects. There are few studies of AT determination in rats. The present study aimed the determination of the AT in adult rats by the lactate minimum test (LMT) and to verify: a) the effects of muscle glycogen stores on AT determined by LMT; b) the sensibility of the LMT to the alterations of the aerobic conditioning imposed by physical training; c) the applicability of the LMT to rats with altered metabolic condition and d) the effects of the swimming exercise performed at intensities below, equivalent and above AT, on the protein metabolism of rats. The LMT is based on the premise that during an incremental exercise test performed after a bout of maximal exercise, blood lactate decreases to a minimum value and then increases again. Thus, the minimum value indicates the intensity of the AT. The initial state of lactic acidosis was obtained by making the animals jump into the water while carrying a load equivalent of 50% of body weight (bw) for 6 min (30-s exercise interrupted by a 30-s rest). After a 9-min rest, blood was collected and the incremental test was started. The incremental test consisted of swimming while supporting increasing (0,5% of bw) loads from 4,0% of bw until exhaustion. Each exercise load lasted 5 min and was followed by a short time rest, during which blood samples were collected. LMT was applied to rats 113 in different conditions: sedentarism, exercise-training, muscle glycogen depletion (12 or 48h fast) and metabolic disturbance (monosodium glutamate induced obesity). The effects of swimming exercise performed at intensities below, equivalent and above to AT on protein metabolism was also evaluated. According to the results: a) physical training increased the work load corresponding to the AT and decrease the blood lactate concentration in which appeared, indicating the sensibility of the test to the alterations of the aerobic conditioning caused by training; b) the depletion of the muscle glycogen stores imposed by fasting did not modify the work load at AT, but reduced the blood lactate concentration; c) the reduced body density associated to obesity did not change the workload equivalent to AT but reduced the blood lactate concentration at this load, showing the viability of LMT to animals submitted to metabolic disturbance; d) the LMT seems to be sensitive to metabolic alterations (protein metabolism) imposed by exercise and e ) taken together these findings indicate the feasibility of the LMT test application in studies with rats.