ABSTRACT
West et al. (2004) recalled that, in intact frog fibres, the rates of energy release and MgATP hydrolysis are high at the beginning of an isometric contraction, declining thereafter as contraction continues. These declines in energy release and MgATP splitting may be related to the rapid onset of fatigue, as suggested and discussed in Section 3.4.3.2 and quantified in Sections 7.2 and 7.3. In isolated intact single fibres, whole muscles and muscles within the whole body, it is generally thought that an accumulation of lactic acid, resulting in significant acidification of the internal medium and toxicity, causes fatigue (e.g. decline in isometric tetanic tension, demonstrated in this chapter to be mostly coupled with decrease in MgATPase activity with increasing duration of contraction for very long-lasting tetani; see Sections 7.2 and 7.3 for precise details in this area; see also below). As recalled by Dawson et al. (1978), the ‘lactic acid hypothesis’, valid for muscles within the body, was put forward as early as 1807 by the Swedish chemist Berzelius. This assumption has gradually become a dogma and is still considered irrefutable by non-specialists and even by many specialists. Dawson et al. (1978, 1980), using phosphorus nuclear magnetic resonance (31P-NMR), studied the problem of fatigue on whole skeletal muscles
from frog stimulated electrically and under strict anoxic conditions. They briefly stimulated the muscles and then waited for several seconds: ‘six experiments were done, two using each the following patterns of stimulation: 1 s every 20 s, 1 s every 60 s, or 5 s every 500 s; the experiments lasted 18-92 min’. Using these traditional conditions for inducing fatigue, the authors found that fatigue and some of its main features were correlated, to various degrees, with metabolite levels, including increases in Pi (see also Potma and Stienen 1996) and lactic acid, and with a decrease in pH (acidification) from 7.0 to 6.5. Dawson et al. (1978) claimed that, in fatigued muscles, force development is proportional to the rate at which MgATP is hydrolysed, apparently at odds with what is demonstrated on pp. 78-80 in Section 3.4.5 and pp. 83-84 in Section 3.4.6, concerning very loose coupling between MgATPase activity and isometric tetanic tension, in intact fibres and, almost certainly, also in whole muscles. However, very loose coupling is observed only in short tetani and may not be valid in fatigued fibres. Nonetheless, given the approximations introduced into their equations and the wide scattering of their experimental results, I wonder whether the conclusions drawn by Dawson et al. (1978), particularly for the tight (one-to-one) coupling between isometric force and MgATPase activity, are well founded and of general interest.
