Effect of endurance training on oxidation of lactate in mice after supramaximal exercise

Physical performance and soleus muscle fiber composition in wild-derived and laboratory inbred mouse strains

We compared four inbred mouse strains in their physical performance, measured as a maximal treadmill running time, characteristics of soleus muscle, anatomic character, and growth. The strains used were Mus musculus domesticus [C57BL/6 (B6) and BALB/c], Mus musculus molossinus (MSM/Ms), and Mus spretus. Maximal running time was significantly different among these four mouse strains. Running time until exhaustion was highest in MSM/Ms and lowest in M. spretus. Maximal times for the laboratory mouse strains were nearly identical. Soleus muscle fiber type and cross-sectional area also differed significantly among the species. In particular, M. spretus was significantly different from the other inbred mouse strains. Growth in the wild-derived inbred mice appeared to be complete earlier than in the laboratory mice, and the body size of the wild strains was about half that of the laboratory strains. From these results, we propose that wild-derived inbred mouse strains are useful models for enhancing phenotypic variation in physical performance and adaptability.

The effects of intensity on the energetics of brief locomotor activity

The energetic costs associated with locomotion are often estimated only from the energy expended during activity and do not include the costs incurred during recovery. For some types of locomotion, this method overlooks important aspects of the metabolic costs incurred as a result of the activity. These estimates for energetic cost have also been predicted from long-duration, low-intensity activities that do not necessarily reflect all the behavior patterns utilized by animals in nature. We have investigated the effects of different activity intensities on the metabolic expenditure (per unit distance traveled) associated with brief exercise, and offer a more inclusive analysis of how the energetics of short-duration activities might be analyzed to estimate the costs to the animal. Mice ran on a treadmill for 15 or 60 s at 25 %, 50 % or 100 % of maximum aerobic speed (MAS) while enclosed in an open-flow respirometry system. Following the run, each mouse was allowed to recover while remaining enclosed in the respirometry chamber. Excess exercise oxygen consumption (EEOC), the excess volume of oxygen consumed during the exercise period, increased with the duration and increased linearly with the intensity of exercise. In contrast, the volume of oxygen consumed during the recovery period, or excess post-exercise oxygen consumption (EPOC), was independent of exercise intensity and duration and accounted for more than 90 % of the total metabolic cost. The net cost of activity (C(act)), calculated by summing EEOC and EPOC and then dividing by the distance run, increased as both activity duration and intensity decreased. The values for C(act) ranged from 553 ml O(2)g(−)(1)km(−)(1) for a 15 s run at 25 % MAS to 43 ml O(2)g(−)(1)km(−)(1) for a 60 s run at 100 % MAS. Combining these data with data from a companion paper, we conclude (1) that EPOC is independent of both the duration and intensity of activity when exercise duration is brief in mice, (2) that EPOC accounts for a majority of the oxygen consumed as a result of the activity when exercise durations are short, and (3) that animals can minimize their energy expenditure per unit distance by running faster for a longer period.

Decreased physical performance of congenic mice with mismatch between the nuclear and the mitochondrial genome

Maternal transmission of mitochondrial DNA (mtDNA) allows us to generate mtDNA congenic strain by repeating backcrosses of female mice to male mice of an inbred strain, which carries different mtDNA haplotype from that of the female progenitor. Since genetic backgrounds of inbred strains commonly used (e.g., C57BL/6J [B6] and BALB/c) are mainly derived from an European subspecies of Mus musculus domesticus, congenic strains, in which mtDNA originated from an Asian subspecies M. musculus musculus or an European species M. spretus, give in vivo condition that mismatch occurs between the mitochondrial and the nuclear genome. So far, little has been known how the mismatch condition affects the physiological phenotype of the mice. To address this question, we established two mtDNA congenic strains, C57BL/6J(B6)-mtSPR and BALB/c-mtSHH, which carry M. spretus- and M. m. musculus-derived mtDNAs, representing the conditions of interspecific and intersubspecific mitochondrial-nuclear genome mismatch, respectively. Using these congenic strains, we examined their physical performance by measuring their running time on a treadmill belt until exhaustion. The result clearly showed that the mtDNA congenic strains manifested a significant decrease in the level of physical performance, when compared with their progenitor strains. It also appeared that the congenic mice manifested growth rate. Thus, all results indicated that mismatch between the mitochondrial and the nuclear genome causes phenotypic changes in individuals of mice.

An adjustable-current swimming pool for the evaluation of endurance capacity of mice

A new forced-swimming apparatus for determining maximum swimming time in mice was devised for use in the evaluation of the endurance capacity of Std and ddY and CDF1 mice after various diet and drug treatments. With the apparatus, a water current is generated by circulating water with a pump in a swimming pool. A spout and suction slit were contrived to generate a constant current while the strength of the current is regulated by a valve. The decrease in the leg-kicking intervals of mice accompanying the increase in the current speed confirmed that the workload is adjustable by regulation of the current speed. Compared with the number of forelimb strokes, that of the hindlimb kicks was greater. The swimming time until fatigue was observed to decrease with increasing current speed in the two strains of mice. As biochemical indexes, the blood lactate and muscle glycogen levels corroborated the correlation between current speed and increase in workload. These results indicate that the apparatus employed in the present study is suitable for the evaluation of the endurance capacity of mice and that is useful for detecting the effects of dietary differences and drug pretreatments on this capacity.

Lactate kinetics during passive and partially active recovery in endurance and sprint athletes

We investigated the effects of passive and partially active recovery on lactate removal after exhausting cycle ergometer exercise in endurance and sprint athletes. A group of 14 men, 7 endurance-trained (ET) and 7 sprint-trained (ST), performed two maximal incremental exercise tests followed by either passive recovery (20 min seated on cycle ergometer followed by 40 min more of seated rest) or partially active recovery [20 min of pedalling at 40% maximal oxygen uptake (VO2max) followed by 40 min of seated rest]. Venous blood samples were drawn at 5 min and 1 min prior to exercise, at the end of exercise, and during recovery at 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30, 40, 50, 60 min post-exercise. The time course of changes in lactate concentration during the recovery phases were fitted by a bi-exponential time function to assess the velocity constant of the slowly decreasing component (tau 2) expressing the rate of blood lactate removal. The results showed that at the end of maximal exercise and during the 1st min of recovery, ET showed higher blood lactate concentrations than ST. Furthermore, ET reached significantly higher maximal exercise intensities [5.1 (SEM 0.5) W.kg-1 vs 4.0 (SEM 0.3) W.kg-1, P < 0.05] and VO2max [68.4 (SEM 1.1) ml.kg-1.min-1 vs 55.5 (SEM 5.1) ml.kg-1.min-1, P < 0.01]. There was no significant difference between the two groups during passive recovery for tau 2. During partially active recovery, tau 2 was higher than during passive recovery for both groups (P < 0.001), but ET recovered faster and sooner than ST (P < 0.05). Compared to passive recovery, the tau 2 measured during partially active recovery was increased threefold in ET and only 1.5-fold in ST. We concluded that partially active recovery potentiates the enhanced ability to remove blood lactate induced by endurance training.