Plasma ammonia responses during heavy-intensity constant-load cycling in young and older individuals

S. Sabapathy, D.A. Schneider and N.R. Morris, School of Physiotherapy and Exercise Science, and Heart Foundation Research Centre, Gold Coast campus, Griffith University, Southport, QLD 4215, Australia.

A delayed and slowly increasing component of O₂ uptake kinetics may be observed when performing high intensity constant-load exercise (Barstow, 1994). Additionally, the increase in plasma ammonia concentration ([NH₃]) during high intensity cycling has been associated with the recruitment of type II fibres (Dudley et al., 1983). This study sought to examine the relationship between the slow component of O₂ uptake kinetics and plasma [NH₃] during constant-load cycling in healthy young and older individuals.

Seven young (mean age ± SD: 21.4 ± 2.8 yr) and 8 older healthy male adults (71.7 ± 2.7 yr) performed 7 min of heavy constant-load exercise. The power output for the constant-load tests was quantified as 50% of the difference between the power output attained at the gas exchange threshold and that achieved at peak O₂ uptake. The kinetics of O₂ uptake measured during constant-load exercise (including the slow component amplitude) were characterised using established non-linear regression modelling techniques (Sabapathy et al., 2004), as illustrated in the Figure. Plasma [NH₃] was measured at rest, following 3 min of unloaded cycling, and at 3 and 7 min of constant-load exercise.

The amplitude of the slow component was 406 ± 65 mL/min in the young and 217 ± 59 mL/min in the older subjects. Plasma [NH₃] values measured after 3 min of unloaded cycling and at 3 min of constant-load exercise were not significantly different from resting values, but increased significantly (P<0.01) between 3 and 7 min of exercise in both groups and correlated significantly (P<0.05) with the slow component (Young: r = 0.79; Older: r = 0.75).

While these findings do not indicate a causal link between the two variables, they could be related to a common physiological mechanism. The increase in [NH₃] observed is consistent with a progressive recruitment of type II muscle fibres during the slow component phase of exercise in both young and older individuals. The measurement of plasma [NH₃] during high-intensity exercise could provide a relatively non-invasive index of muscle fiber recruitment patterns.

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Dudley G.A., Staron R.S., Murray T.F., Hagerman F.C. & Luginbuhl, A. (1983) Journal of Applied Physiology, 54: 582-586.

Sabapathy S., Schneider D.A., Comadira G., Johnston I. & Morris N.R. (2004) Respiratory Physiology and Neurobiology, 139: 203-213.