1.2 GESTIÓN DE RECURSOS MULTIMEDIA
1.2.2 CARACTERÍSTICAS DE LA GESTIÓN DE RECURSOS MULTIMEDIA
Plasma lactate concentrations were elevated from rest in the HIIE, RSA and CCT trials with the more intense protocols producing higher plasma lactates profiles irrespective of the total workload performed (see figure 6.1). HIIE resulted in higher plasma lactate levels compared with CON (p<0.05) further elevated with RSA and CCT protocols compared with CON (p<0.0001). The work performed in the CCT was higher than the RSA (p<0.01) however, they amounted to 20% and 10%, respectively, of the work performed in the HIIE and CON trials, which were workload matched (see figure 6.0). This highlights the importance of the mode of exercise, in particular, the influence of exercise intensity on plasma lactate concentrations. A similar pattern was observed with urinary lactate excretion profiles which were significantly greater after HIIE compared to CON (p<0.05) and further elevated after
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RSA and CCT (p<0.01, see figure 6.3). Importantly, these observations in CCT and RSA were made against a substantially reduced total work output. This suggests that exercise duration is less important than intensity for lactate production and subsequent efflux from the muscle. This is further supported by the two very high intensity, fatiguing exercise protocols (RSA and CCT), with a high glycolytic contribution of energy supply, further exacerbating plasma lactate accumulation and the urinary lactate excretion rate (see figures 6.1 and 6.3 respectively), despite performing substantially lower total work (kJ) compared with the HIIE and CON protocols (see figure 6.0) and over a much shorter duration.
Presence of lactate in the urine following physical exertion has been recognised for decades (Liljestrand and Wright-Wilson, 1925; Johnson and Edwards, 1937; Miller and Miller 1948; Pechilvanis et al., 2010). Unlike plasma lactate and on occasion muscle lactate, which has been used for monitoring and understanding metabolism of physical performance and training applications (Billat, 1996), little consideration has been given to the metabolic outcome of post exercise urinary lactate excretion. During vigorous exercise, an increased rate of glycogenolysis results in elevations in muscle pyruvate and lactate concentrations (refer to section 2.7.1). Muscle lactate production is influenced by exercise intensity and duration with concentrations of lactate accumulating in the bloodstream increasing when energy demand exceeds the metabolic oxidative capacity of the muscle, requiring a greater glycolytic contribution to sustain the exercise (Broberg and Sahlin 1989; Medbo and Tabata 1993; Bogdanis et al., 1995; Bogdanis et al., 1996; Bogdanis et al., 1998; Balsom et al.,
1999).
The excretion of lactate in the urine is influenced by the magnitude of the post exercise plasma lactate concentration and subsequent renal lactate handling. Pyruvate is
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also excreted in the urine, however excretion of pyruvate is relatively insignificant compared to lactate (Hubbard, 1937), and was not measured in this study. A closer look at urinary excretion patterns of lactate relative to plasma accumulation after exercise indicates that lactate excretion follows a similar pattern to that of glucose excretion (Miller and Miller, 1949). At low plasma concentrations, reuptake of lactate from the filtrate in the renal tubules occurs to a threshold (~6 mmol/L) before excess lactate molecules not taken up in the tubules are subsequently excreted in the Miller and Miller 1949). The urinary excretion patterns observed in this study support this trend and a greater magnitude of urinary lactate excretion is excreted with the more intense bouts (Pechilivanis et al., 2010).
Urinary lactate excretion results from this current study provides additional weight to studies where elevated urinary excretion of purine bases (Hx and uric acid) were significantly elevated after greater number of repeated sprints (Stathis et al., 1994; Hellsten et al., 1999; Stathis et al., 1999; Saiki et al., 2001; Stathis et al., 2006; Pechlivanis et al.,
2010) and in HIIE when compared to CON (Borg et al., 2008). Purine base excretion was twice the magnitude after an acute bout of 30mins HIIE than a workload matched CON exercise (Borg et al., 2008), thus more energy utilised to restore de novo ATP, a metabolically costly mechanism (Newsholme and Leech, 1983; Hellsten et al., 1998) demonstrated in section 2.9.2.4.
Interestingly, the degree of urinary lactate excretion and the relative molecular energy loss is greater by a factor of 3200 compared with urinary purine base excretion. This is taken from the assumed de novo replacement cost of 5 ATP for every Hx molecule that originated from the muscle (Newsholme and Leech, 1983), compared to 16 molecules of
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ATP that can be generated from a lactate molecule and the 1000 fold difference in relative excretion rates between purines and lactate.
Thus the metabolic impact of excreting lactate compared to purine bases is considerably greater in respect to energy expenditure. Losing lactate in the urine is a potential avenue for lost energy which is not accounted for in whole body energy balance equation, and may need to be considered more closely in estimations with exercise that requires significant glycolytic or adenine nucleotide replenishment or restoration, respectively. Nonetheless, together and over periods of time, these imbalances in energy balance accumulate and can explain the greater reduction in adiposity and expended energy associated with HIIE training compared with CON training (Tremblay et al., 1994; Trapp et al., 2008).
6.4.2 Conclusion
These results substantiate the hypothesis that urinary lactate excretion is influenced by the extent of plasma lactate accumulation and exercise intensity. The results also demonstrate that urinary lactate excretion is a major metabolic mechanism influencing energy balance during intense exercise due to exercise induced energy deficit.
Lactate lost from the body post metabolically stressing exercise results is subsequent energy loss which must be replaced. Thus these results offer an additional metabolic avenue for enhancing energy expenditure and reducing adiposity.
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