Substrate utilisation during high Intensity exercise >85% VO 2ma

High intensity exercise utilises the ATP-CP system and anaerobic glycolysis to produce ATP, meeting the high energy cost of this exercise type (Randle 1969; Brouns and van der Vusse, 1998; Gastin 2001). Glycogen is the primary fuel source, contributing to 2/3 of energy production, with the other 1/3 from fat sources (Brooks and Mercier, 1994; Coyle

et al., 1997). During a 30 s single bout of maximal exercise, 25-30% of ATP synthesised comes from phosphocreatine (PCr) breakdown while the remaining 65-70% of ATP production comes from glycogen entering anaerobic glycolysis (Bogdanis et al., 1996;

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Brouns and van der Vusse, 1998; Dawson et al., 1998). PCr replenishment is an aerobic process and takes approximately 3 mins for complete recovery from cessation of exercise (Edwards et al., 1973; McCartney et al., 1986; Chasiotis et al., 1987; Bogdanis et al., 1995; Bogdanis et al., 1996; Dawson et al., 1997; Wadley and Rossignol, 1998; Tomlin et al., 2001; Tomlin et al., 2002; Dorado et al., 2004; Dupont et al., 2004; Thevenent et al., 2007; Forbes

et al., 2008). Interestingly one study observed that after 6 mins of recovery, there was only 85% PCr replenishment complete. This may be due to PCr content falling even lower in fast twitch muscle fibres due to poor capillary density, fewer mitochondria number and increased H+ _{ion concentration (Bogdanis et al., 1995), limiting the oxidative processes}

required for replenishment. Nonetheless, once ATP synthesis from PCr has diminished, glycogen is the dominant fuel source used for ATP generation. As muscle glycogen content declines (Broberg and Sahlin 1989; Bogdanis et al., 1995; Bogdanis et al., 1996; Bogdanis et al., 1998; Balsom et al., 1999), muscle pyruvate and lactate concentrations increase (Broberg and Sahlin 1989; Medbo and Tabata 1989; Bogdanis et al., 1995; Bogdanis et al.,

1996; Bogdanis et al., 1998; Balsom et al., 1999) as metabolic by-products of anaerobic glycolysis (see figure 2.7).

2.7.1 High intensity exercise stimulates anaerobic glycolysis

At high exercise intensities, oxygen cannot be extracted and utilised fast enough hence ATP is produced anaerobically. In the cytosol, glycogen is degraded to glucose 1 phospahte (G-1-P) which is then converted to G-6-P (see figure 2.6). Phosphofructokinase (PFK), the rate limiting enzyme of glycolysis converts downstream G-6-P to F1-6-P and ultimately the enzyme pyruvate kinase will make pyruvate. Pyruvate can be utilised aerobically discussed in section 2.6.1 or anaerobically where it is broken down further to

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lactate which subsequently effluxes the muscle (see figure 2.6). The rapid hydrolysis of ATP and PCr during high intensity exercise results in the build-up of free phosphate (Pi) and AMP, increasing the activity of glycolysis. Henceforth an increased glycolytic rate elevates pyruvate production, wielding an allosteric effect on PDHa (Parolin et al., 1999).

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Figure 2.6: Simplified pathway of glycolysis, displaying the aerobic and anaerobic fate of pyruvate (modified from Gropper, 2009). Anaerobically, pyruvate is converted to lactate and aerobically it shuttled to the mitochondria.

When glycolytic production of pyruvate overcomes the oxidation rate via PDH, accumulated pyruvate is converted to lactate via lactate dehydrogenase (LDH) (see figure 2.6). Lactate effluxes the muscle with the concentration of lactate in the blood the balance of production rate and removal (Olson 1963; Alpert, 1965; Schroder et al., 1969; Hubbard, 1973). Once in the blood stream, lactate can be taken up by exercising or non-exercising skeletal muscle, the myocardium, kidney or the liver (Hubbard 1973) where it is converted to pyruvate for gluconeogenesis (produce glycogen) and subsequent storage, this process called the Cori Cycle (Long and White 1938; Schroder et al., 1969; Katz and Tayek, 1998; Katz and Tayek, 1999). Blood lactate can also be filtered at the kidney and excreted in the urine, this will be discussed in detail in section 2.9.2.5 and chapter 6.

2.7.2 High intensity exercise decreases fat oxidation

As exercise intensity gradually increases from 25%, 65% to 85% VO2max, the

contribution of CHO to energy production increases (Romijn et al., 1993). At intensities >85% VO2max, CHO utilisation increases (Jeukendrup, 2003; Achten and Jeukendrup, 2004), causing lactate production (Krebs et al., 1964; Gastin, 2001). Contrastingly, fat oxidation decreases as exercise intensity increases, becoming depressed when exercise intensity reaches approximately 80-85% VO2max, as seen in figure 2.7 (Romijn et al., 1993; Thompson,

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Figure 2.7: The relationship between fat oxidation and lactate concentration as exercise intensity increases (Taken from Achten and Jeukendrup, 2004a). As lactate production increases, fat oxidation decreases.

Numerous systemic and local factors may cause depressed fat oxidation during high intensity exercise. Vasoconstriction to adipose tissue with the majority of blood flow redirected to exercising skeletal muscles for increased substrate supply and waste removal may result in re uptake and reesterification of FFA in adipose tissue, decreasing delivery uptake and oxidation in muscle mitochondria (Helge et al., 2007). However, the plasma FFA availability may not play a role in limited fat oxidation, as muscle blood flow is at its highest during high intensity exercise (van Loon et al., 2001). Indeed at exercise intensities greater than 85% VO2max, no difference in thigh plasma FFA oxidation has been observed, thereby

the use of other fat sources at this exercise intensity such as IMTG, may be decreased, explaining decreased fat oxidation at higher exercise intensities (Helge et al., 2007).

The metabolic by-products of high intensity exercise may also lead to declining fat utilisation. When glycolytic flux exceeds the utilisation of pyruvate, considerable muscle

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lactate concentrations are produced. Acetylcarnitine is also produced, reducing the available free carnitine which is needed for FFA uptake into the mitochondria. Therefore fatty acid oxidation rates may be limited to the amount of free carnitine availability (van Loon 2001; Jeukendrup, 2002), providing an avenue to the reduced action of CPT1 and thus inadequate entry across the mitochondria membrane. Furthermore, increased lactate production with the subsequent H+ ion release decreases muscle pH (Stisen et al., 2006), causing intramuscular acidity. An acidic skeletal muscle environment inhibits the activity of CPT1 by 40% when pH drops from 7.0 to 6.8 (Stisen et al., 2006; Sahlin and Harris 2008), limiting FFA transport into the mitochondria for oxidation (Sidossis et al., 1997; Jeukendrup, 2002; Helge et al., 2007). Lastly, an additional outcome of anaerobic glycolysis is the potential elevation of acetyl CoA, derived from pyruvate. High concentrations of acetyl CoA may lead to increased production of malonyl coenzyme A in the cytosol (Elayan and Winder 1991; Jeukendrup 2002; Sahlin and Harris 2008), an intermediate of fatty acid synthesis which can inhibit the action of CPT1 (Odland et al., 1998; Roepstorff et al., 2005). This occurs under acidic conditions; hence CPT1 activity is optimal during exercise modes that do not alter muscle pH (Odland et al., 1998). For this reason the optimal exercise conditions for promoting fat oxidation should be when there is sufficient FFA delivery to the exercising skeletal muscle at an exercise intensity that produces little lactate, thus not altering muscle pH.