Postprandial exercise: Prioritization or addivity of the metabolic responses?

Monitor lizards (Varanus exanthematicus) were used to examine the prioritiz ation or additivity of the metabolic responses associated with exercise and digestion, either of which can elevate metabolic rate independently. Rates of oxygen consumption ((V) over dot(O2)) and ventilation ((V) over dot(E)) were measured in lizards during fasting exercise, postprandial rest and po stprandial exercise. In fasting animals, (V) over dot(O2) increased with wa lking speed to a maximal value of 15.9 ml O-2 kg(-1) min(-1) at 1.25 km h(- 1). Postprandial resting metabolic rate was elevated significantly above fa sting levels (4.1 versus 2.0 ml O-2 kg(-1) min(-1)). During postprandial ex ercise, (V) over dot(O2) increased to a maximal value of 18.8 ml O-2 kg(-1) min(-1) at 1.25 km h(-1). At every level of exercise, (V) over dot(O2) was significantly higher in postprandial animals by a similar increment; the m aximal rate of oxygen consumption was significantly increased by 18 % in po stprandial individuals. Maximal lie did not differ in fasting and postprand ial animals and, therefore, the greater (V) over dot(O2max) of postprandial animals cannot be attributed to a higher ventilation rate. Air convection requirement ((V) over dot(E)/(V) over dot(O2)) is significantly lower in po stprandial animals at rest and at all levels of exercise, indicating a rela tive hypoventilation and increased pulmonary oxygen extraction efficiency. We suggest that this increased oxygen extraction may be due to decreased ca rdiopulmonary shunts and/or to lower mixed venous oxygen content. The data unequivocally support an additivity model rather than prioritization models for the allocation of elevated metabolic rate: the postprandial metabolic increment is not suspended during exercise, but rather is added onto the co st of exercise, It is clear that fasting exercise did not elicit truly maxi mal levels of cardiopulmonary oxygen transport in these animals, indicating problems for design models that make this assumption.