A method for calculating lactate production rate using blood lactate concentration during exercise in mice

Introduction Lactate acts as an energy fuel (Brooks, 2018) and a signaling molecule for muscle metabolic adaptation such as mitochondrial biogenesis (Hashimoto et al., 2007). Lactate production rate is an important parameter for muscle metabolic adaptation but the measurement requires muscle biopsy in general. Blood lactate concentration is lower invasive and widely measured in sports. It is meaningful to establish a method to calculate lactate production rate from blood lactate concentration. The purpose of this study was to establish a method to determine the amount of lactate production from blood lactate concentration in mice. Methods C57BL6/L mice divided into two groups: a control group (CON) and an endurance treadmill training (EX) for 6 weeks (15~25 m/min; 30 min/day; 5 days/week). A lactate tolerance test was performed following i.p. injection of sodium lactate (1 g/kg of body weight). Blood lactate concentration was measured for 120 min after lactate injection, and lactate uptake capacity “k” was calculated from the following equation d[lactate]/dt = k[lactate]n Next, animals were subjected to an incremental exercise test (increasing from 10 m/min to 40 m/min) of 2 min run and 2 min rest on the treadmill, and blood lactate concentration was measured at each intensity. Lactate production at each intensity was calculated by combining the blood lactate concentration with the calculated lactate uptake capacity. We measured monocarboxylate transporter (MCT) 1 and 4 protein levels and citrate synthase (CS) activity. These are involved in lactate metabolism in skeletal muscle. To verify calculated lactate production, we measured muscle glycogen concentration after 10 bouts of 2 min run at each speed (0, 10, 20, 30 and 40 m/min) with 2 min rest using other control mice. Results We calculated “k” using time series data of lactate concentration after lactate injection. When the lactate concentration dynamics were computationally simulated with various “k”, it was found that lactate reduction rate was determined by k. However, “k” was not altered by 6 wk-endurance training. In the incremental exercise test, the blood lactate concentration in the EX group was significantly lower at 25-40 m/min than in the CON group (p<0.05). The lactate production rate in the EX group was also significantly lower at 35 and 40 m/min than in the CON group (p<0.05). These results suggest that lower lactate concentration during higher intensity exercise is attributable by lower lactate production rate in the EX group. Endurance training also decreased MCT4 protein (p<0.05), which acts release of lactate from muscle, and increased CS activity (p<0.05). These results support the lower lactate production rate in the EX group. Finally, we assessed whether the lactate production rate was valid by the measurement of muscle glycogen concentration after acute exercise at each exercise intensity. We confirmed there was a strong negative correlation between the muscle glycogen concentrations immediately after exercises and the calculated lactate production rates (r=0.99, p<0.01). [Conclusion] We established a reasonable method for calculating lactate production rate from blood lactate concentration during exercise.