Lactate availability is not the major factor limiting muscle glycogen repletion during recovery from an intense sprint in previously active fasted rats

Abstract

SUMMARY It is not clear whether the amount of accumulated lactate is the main factor limiting muscle glycogen accumulation during recovery from an intense sprint performed by previously active fasted laboratory rats. To address this question, groups of fasted rats swam at moderate intensity for 30 min, each animal with a lead weight equivalent to 0.5% body mass attached to its tail,followed by a 3 min high intensity swim with a 10% lead weight and a recovery period of up to 2 hours afterwards. Moderately intense exercise for 30 min caused a decrease in muscle glycogen levels in the mixed, white and red gastrocnemius and the mixed quadriceps muscles, and a further rapid fall occurred in response to the 3 min sprint effort. During recovery, glycogen increased to comparable or above pre-sprint levels across all muscles, and this occurred to a large extent at the expense of net carbon sources other than lactate, with these carbon sources accounting for at least 36–65%of the glycogen deposited. The sustained dephosphorylation-mediated activation of glycogen synthase, but not the changes in glucose 6-phosphate levels, most probably played an important role in enabling the replenishment of muscle glycogen stores. In conclusion, our findings suggest that the amount of glycogen deposited during recovery from high intensity exercise in fasted animals is not limited by the amount of accumulated lactate.

Fiber-specific responses of muscle glycogen repletion in fasted rats physically active during recovery from high-intensity physical exertion

“…It is possible, however, that the lack of difference in glycogen levels in the white gastrocnemius muscle as the post-active recovery period progresses is due to a lack of statistical power as suggested by the 60 min glycogen bars appearing to be higher than the 30-min bars. Overall, these findings are consistent with those of earlier studies from our laboratory on the importance of both enzymes in the regulation of glycogen synthesis without food after an intense bout of exercise (11,18,19,48). Arguably, given the many factors that could play a role in controlling the rate of glycogen synthesis, such as the many phosphorylation sites and kinases affecting the activity and intracellular localization of glycogen synthase (16,46,49), glucose transport activation post-intense exercise (18), and the possible increased blood flow to actively recovering muscle, further work is required to elucidate the relative importance of these factors in the regulation of glycogen synthesis after active recovery in FOG and FG muscle fibers.…”

“…Rats being natural swimmers, an exercise protocol based on swimming was adopted in this study, the intensity of the exercise being determined by the amount of lead weight attached to the base of the tail (11,18). Previous studies from this laboratory showed that swimming with a lead weight corresponding to 9 -10% body mass allows the animal to remain at the water surface, while causing rapid and marked reproducible changes in muscle glycogen and lactate levels (11,18,47,48), thus offering a model of choice to investigate the response of muscle glycogen levels to intense physical activity. Another advantage of this exercise protocol over one that uses a treadmill is that a prolonged training period is not required for rats to exercise at near maximal intensity (11,18,47,48).…”

“…The study examined different muscles selected on the basis of their fiber compositions. The white, red, and mixed gastrocnemius muscles were selected because they are actively recruited during high-intensity exercise (47,48) and because the white gastrocnemius is rich in FG fibers (ϳ90%) and poor in FOG and SO fibers (2,39), the red gastrocnemius is rich in FOG fibers and poor in FG fibers (ϳ9%), and the mixed gastrocnemius muscle is rich in FG and FOG, but poor in SO fibers (ϳ5%; 2, 39), thus reflecting the composition of the hind limb musculature of the rat as a whole (2,37). In contrast, the soleus muscle was chosen on the basis that it is one of the very few muscles rich in SO fibers and poor in FG and FOG in the rat (2,37,39).…”

“…Although this approach does not allow one to identify which of the many phosphorylation state of glycogen synthase is responsible for the change in its fractional velocity (16,43,49), particularly those without any kinetic effect on this enzyme, a low fractional velocity of glycogen synthase typically reflects a phosphorylation-mediated inactivation of the enzyme (35,43). The extraction of glycogen synthase and the determination of its fractional velocity were performed using a modification (35) of the filter paper method of Thomas and colleagues (50), as described in our earlier studies (18,48). Changes in the phosphorylation state of glycogen phosphorylase were estimated by measuring its activity ratio, which is defined as the ratio of the enzyme activity in the absence and presence of excess concentrations of its activator, AMP (23).…”

Restoration of the secretory activity of the digestive glands under high physical load

“…Within several minutes, one can see normalization of the pulse, blood pressure, stroke volume, cardiac output, circulation rate, i.e., indices providing the restoration of the lactate phase of oxygen debt. Indices of ventilation, glucose, and glyco gen are restored within several hours [28]. Metabolism, the peripheral blood, water-salt balance, enzymes, and hormones are restored within a few days [8,29].…”