PL - 035 Effects of lactate infusion on the molecular response to resistance exercise in men and women
Authors: Moberg Marcus , Liegnell Rasmus , Apró William , Holmberg Hans-Christer , Van Hall Gerrit , Ekblom Björn 
Published: October 2018

Objective It is well known that a bout of resistance exercise stimulates mechanistic target of rapamycin complex 1 (mTORC1) signaling and the rate of protein synthesis in skeletal muscle, which after repeated stimulus ultimately leads to muscle hypertrophy. However, the precise molecular events that leads to the activation of mTORC1 and protein synthesis following resistance exercise remains to be elucidated. A number of previous studies have shown that resistance exercise protocols that cause a significant degree of metabolic stress, such as blood flow restriction or low-load repetitions to failure, activate mTORC1 and protein synthesis, as well as induce muscle growth, to a similar extent as conventional high-load muscle contractions. It is suggested that the accumulated metabolites are responsible for the anabolic response in those intense and stressful exercise protocols, either as direct signaling molecules or indirectly e.g. by enhancing fiber activation. Here, one metabolite is of particular interest, namely lactate. Interestingly, it has been shown that incubation of C2C12 myotubes with lactate increases mTORC1 signaling and reduces myostatin expression. Moreover, in vitro lactate incubation has been shown to induce the expression of peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC-1α), suggestively through increased reactive oxygen species production. The latter is also of interest since intense high volume resistance exercise can activate AMP-activated protein kinase (AMPK) and induce PGC-1α expression. Accordingly the purpose of the present study is to investigate the acute molecular response in human skeletal muscle to resistance exercise performed with our without a venous infusion of sodium lactate. The objective was to create a situation where muscle contractions were performed with low or high levels of lactate and where other factors such as load, volume, time under tension, hormonal levels and etc. would not differ between interventions. The primary outcomes of the study is analysis of intracellular signaling, rate of muscle protein synthesis (FSR), mRNA expression as well as blood and muscle levels of lactate and pH.
Methods Four female and four male, young and healthy, subjects who performed resistance exercise of the lower body on a regular basis participated in the study. Following pre-testing subjects performed two familiarization sessions and two experimental trials, with each session separated by seven to ten days. The experimental trials consisted of unilateral knee-extension resistance exercise performed under venous infusion of sodium lactate or saline (placebo), in a randomized counterbalanced fashion, where subjects were blinded to the treatment. On the days before the trials subjects consumed an oral dose of deuterium oxide to enable measurement of muscle protein synthesis rates during recovery. On the day of the trials subjects reported to the laboratory at 0700 hours in the fasted state, had their blood status cleared and donated a first muscle biopsy from the vastus lateralis. There after an infusion of sodium lactate or saline was initiated at rest, and following 20 min of infusion subjects performed a bout of unilateral knee extension exercise. Immediately after exercise completion the infusion was terminated and a second muscle biopsy was taken. This was followed by a three hour recovery period where additional biopsies were taken at 90 and 180 min post exercise. Blood samples were drawn repeatedly in 10-30 min intervals throughout the trial. After the three hours of recovery subjects ingested a standardized meal, left the laboratory to be free-living and then return to the lab in the overnight fasted state the next morning for a 24 h muscle biopsy which concluded the trial.Whole blood levels of lactate, glucose, sodium, potassium as well as base excess and pH were analyzed in each sample immediately after collection. Muscle levels of lactate were analyzed on lyophilized and dissected muscle samples. Immunoblotting was used to analyze mTORC1-, AMPK- and MAPK signaling. Analysis of 2H-alanine using mass spectrometry enabled calculation of muscle protein FSR during 3h and 24h post exercise. Muscle pH and mRNA expression remains to be determined.
Results Load, number of repetitions and time under tension during resistance exercise did not differ between trials. No subject was able to identify in which trial the received the sodium lactate infusion. In the placebo trial blood levels of lactate increased during exercise to 3.00 ± 0.28 mmol/l and remained higher than at baseline up until 45 min post exercise (P<0.05). In the lactate trial blood levels of lactate were increased above baseline after 10 min of infusion and reached a maximal level of 7.00 ± 0.50 mmol/l directly after exercise, and then remained higher than baseline and placebo levels until 60 min of recovery (P<0.05). Blood pH increased in response to the sodium lactate infusion with a peak of 7.44 ± 0.01 following 60 min of recovery (P<0.05). Muscle levels of lactate were higher in the lactate trial immediately as well as 90 min after exercise, 26.1 ± 2.6 and 10.2 ± 1.9 mmol/kg dry muscle compared to 21.7 ± 2.6 and 6.4 ± 1.1 mmol/kg dry muscle in the placebo trial, respectively (P<0.05). With regard to mTORC1-signaling, the phosphorylation of S6K1T389 increased 6-fold above rest directly after exercise in both trials, and remained elevated throughout the entire 180 min of recovery (P<0.05), with no differences between trials at any time point. The phosphorylation of AMPKT172 increased by 18 and 31% after exercise in the placebo and lactate trials (P<0.05), respectively, with no differences between trials. In both trials the phosphorylation of p44T202/Y204 increased 3- to 4-fold immediately after exercise, stayed elevated above rest at 90 min post exercise (P<0.05) and returned to baseline after 180 min, with no differences between trials. Although statistically underpowered at this point, potential sex differences with regards to the effect of lactate infusion on cell signaling have been observed. For S6K1T389, at 90 min post exercise, a 7-fold increase above baseline was observed in the placebo trial for both females and males, but in the lactate trial the females had merely a 2-fold increase while the males had an 11-fold increase. A similar observation was made for p44T202/Y204 at that same time point. In the placebo trials both females and males had a 3- to-4 fold increase in p44T202/Y204 phosphorylation from rest, while in the lactate trial females had a 12% reduction and the males a 170% increase. For AMPKT172 the men had a 14% and 21% increase in phosphorylation immediately after exercise in the placebo and lactate trials respectively, while the women displayed a 24% increase in the placebo trial and a 62% increase in the lactate trial at that time point. The mean plasma deuterium oxide enrichment was 0.36 ± 0.02% in the placebo trial and 0.33 ± 0.01% in the lactate trial. In our hands valid estimates of muscle protein FSR during 3 hours of recovery from exercise could not be made using deuterium oxide. The period for incorporation seems too short for valid increases in enrichment and resulted in calculated FSR values ranging from -0.075%/h to 0.347%/h. Muscle protein FSR during the 24 h post exercise period was 0.099 ± 0.010 %/h in the placebo trial and 0.094 ± 0.003 %/h in the lactate trial, with no differences between the two.
Conclusions Infusion of sodium lactate during unilateral resistance exercise induces a robust elevation of blood levels of lactate during and after exercise, as well as a small but significant increase in muscle levels of lactate. On a group level, infusion of sodium lactate does not alter resistance exercise induced activation of mTORC1-, AMPK- or MAPK signaling in human skeletal muscle. However, there seems to be sex differences in response to the lactate infusion, where males get an amplified response with regard to mTORC1- and MAPK signaling, whereas the females get a reduced response. Further work with additional subjects is required to draw more firm conclusions from these initial observations. Muscle protein FSR during 24 h post exercise is not influenced by a sodium lactate infusion directly before and during the resistance exercise session.