Substrate and O2 fluxes during rest and exercise in a high-altitude-adapted animal, the llama

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.