AJP - Regulatory, Integrative and Comparative Physiology, Vol 253, Issue 2 298-R305, Copyright © 1987 by American Physiological Society
Substrate and O2 fluxes during rest and exercise in a high-altitude-adapted animal, the llama
P. W. Hochachka, T. P. Mommsen, J. H. Jones and C. R. Taylor
Llamas were trained to exercise on a treadmill at graded speeds. O2 uptake
values at rest, at modest exercise (ME, 0.58 m/s), and at heavy exercise
(HE, 1.84 m/s) were 0.069, 0.297, and 0.594 ml O2 X kg-1 X s-1.
Corresponding values for cardiac output were 1.64, 4.39, and 6.85 ml X kg-1
X s-1. Palmitate, glucose, and lactate replacement rates (Ra values) were
determined at these varying work rates using a single-bolus injection of
14C- or 3H-labeled metabolites, two at a time. Ra values for palmitate,
glucose, and lactate at rest were 6.2, 61.4, and 36.1 mumol X kg-1 X min-1,
respectively. During ME, the Ra values for glucose and palmitate increased
about twofold but showed no further increase when exercise intensity was
increased about twofold. In contrast to these results, Ra values for
lactate increased much more with increasing work rate. A combination of
increased perfusion (increased cardiac output) and increased lactate
availability (increased plasma concentration) could account for the lactate
Ra data. During prolonged ME, the Ra for lactate decreased because lactate
availability decreased; concurrently, both the plasma free fatty acid
availability and the Ra values for palmitate greatly increased. Because O2
carrying capacity is low in the llama (as in all high-altitude-adapted
mammals), a given level of exercise required exaggerated increases in
cardiac output (and hence in perfusion rates of working muscles). As a
result, the flux rates of palmitate, glucose, and lactate through the
plasma compartment could readily account for O2 flux rates even at
relatively high sustainable rates of exercise.
