Setting Treadmill Intensity for Rat Aerobic Training Using Lactate and Gas Exchange Thresholds.

Purpose: To open up new horizons of translational research, we studied the lactate threshold (LT)-dependent physiological responses and adaptations to exercise in rats, highlighting the importance of intensity-specific studies for optimizing exercise prescriptions. LT is physiologically related to the non-invasive gas exchange threshold (GET), and both thresholds are moderate-heavy-intensity boundary indices in determining an effective intensity of aerobic training in humans. While their practical utility is presumed to extend to rats, the actual existence of GET, the thresholds' relations to maximal oxygen consumption (VO2max), and whether aerobic adaptations by training differ around the LT intensity remain uncertain.

Methods: This study sought to identify the GET using our previously established rat LT-model by combining the use of a metabolic chamber and the V-slope method and to confirm the thresholds' relations to VO2max. We investigated changes in the thresholds and VO2max following 6 weeks of endurance training at below or above LT intensity.

Results: GET and LT were significantly correlated, and agreed with high precision, although with a fixed bias. Untrained rats exhibited GET and LT at 56% and 52% of their VO2max, respectively. Endurance training at supra-, but not below-, the LT intensity significantly improved VO2max and both thresholds; however, their %VO2max remained unaltered.

Conclusions: GET in rats is identifiable as a threshold associated with LT using the V-slope method. Further, both thresholds can serve as moderate-heavy-intensity boundary indices for the aerobic training of rats. This study advances our understanding of exercise intensity regulation in rats, thereby contributing to the development of a more nuanced and effective model for exercise prescription, with implications for human health and fitness.