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Kerri-Jo Smithurst, Department of Human Biology, University of Guelph, September 2002
Track racing relies heavily on the horse’s anaerobic metabolism to produce the speeds necessary to win. One widely accepted theory is that a major limiting factor to high intensity exercise is the production of lactic acid in the muscles. This paper examines the way lactate is produced and utilised in the horse. It outlines how lactate affects the horse during exercise and describes metabolic changes occurring with high intensity exercise.
Traditionally it was thought that the accumulation of Lac- during high intensity exercise was a result of an oxygen limitation within the muscle cell. The term ‘anaerobic’ or ‘lactate’ threshold was given to indicate the point where exercise intensity reaches an abrupt increase in [Lac-]p (Wasserman et al. 1986). However, lactate is also created and utilized under fully aerobic conditions (Brooks 2001) and there seems to be more of a moderate rise in plasma lactate levels (Myers and Ashley 1997). The formation of lactate depends on several factors, only one of them being oxygen availability (Brooks 2001).
Lactate is a metabolic intermediate that can be oxidized or utilized as energy. The mandatory coupling of Lac- and H+ increases intramuscular acidification that may have many effects resulting in the limitation of high intensity exercise. During experiments where the acid-base variables were manipulated during exercise inconsistencies appeared in these relationships (Jones et al.1977). Problems also appeared in animal experiments where [Lac-] changes were not related to changes in [H+] (Jones and Heigenhauser 1992). Subsequent studies indicated that a variety of conditions were contributing to increased [H+] with exercise (Stainsby and Eitzman 1988, Heigenhauser et al.1990).
Using a multifactorial theory to explain force reduction during high intensity has [H+] affecting Lac- directly through the inhibition of key glycolytic enzymes as well as indirectly through inhibiting the contractile process (Bonen 2001). Heigenhauser and colleagues (1990) estimated the relative contributions of the different factors from studies with exercising humans. They found that increases up to 30% may be due to increases in PCO2, 40% or more may be due to reductions in [SID] and 30% due to changes in [Atot] and KA (Jones and Heigenhauser 1992).
A major problem with the Henderson-Hasselbalch approach is that in principle it is much more descriptive than mechanistic (Jones 1987, Stewart 1981) and the greatest advantage of the physicochemical approach over the traditional is in its quantitative assessment (Constable 1999). A limitation with the traditional approach is that the equation can only be accurately applied to ruminant plasma at approximately normal T, pH, protein concentration, and sodium concentration (Constable 1999). The physicochemical approach permits a better evaluation of a greater range of acid-base disorders as well as allowing greater understanding of acid-base physiology (Aguilera-Tejero 2000). It also brings us past the conventional descriptions of acid-base in terms of the CO2 system as the descriptions of base excess and deficit are limited do not work intramuscularly (Johnson et al. 1996). By acknowledging the numerous variables involved in acid-base balance, bicarbonate becomes only one of the many factors influencing lactate concentration (Johnson et al. 1996).
The existence of the Lac-/H+ cotransporter reduces the increasing [Lac-] during activity in the active muscles and facilitates its uptake in other fibres (Juel 2001). From their study Tonouchi and co-workers (2002) found that the contraction-induced increases in Lac- transport, occurring at high [Lac-], may be attributed to changes in the intrinsic activity of MCT transporters. The internal pH is regulated by the H+/Na+ exchanger, which serves as a safety system against any major changes (Juel 2001). It seems that the transport systems not involving Lac- regulate pH at rest, and contribute to its fine adjustment, and that the MCTs are the main transporter for the large Lac- productions with intense exercise (Juel 2001).