Ventilation and acid-base balance during graded activity in lizards

Oxygen transfer during aerobic exercise in a varanid lizardVaranus mertensiis limited by the circulation

Oxygen transfer during sustained maximal exercise while locomoting on a treadmill at 0.33 m s-1 was examined in a varanid lizard Varanus mertensi at 35 °C. The rate of oxygen consumption(V̇O2) increased with locomotion from 3.49±0.75 (mean ± S.D.) to 14.0±4.0 ml O2 kg-1 min-1. Ventilation(V̇E) increased, aided by increases in both tidal volume and frequency, in direct proportion to V̇O2. The air convection requirement(V̇E/V̇O2=27)was therefore maintained, together with arterial PaCO2 and PaO2. The alveolar—arterial PO2 difference(PAO2—PaO2)also remained unchanged during exercise from its value at rest, which was approximately 20 mmHg. Pulmonary diffusion for carbon monoxide(0.116±0.027 ml kg-1 min-1 mmHg-1) was double the value previously reported in V. exanthematicus and remained unchanged with exercise. Furthermore, exercise was associated with an increase in the arterial—venous O2 content difference(CaO2—CvO2),which was assisted by a marked Bohr shift in the hemoglobin saturation curve and further unloading of venous O2. During exercise the increase in cardiac output (Q̇tot) did not match the increase in V̇O2, such that the blood convection requirement(Q̇tot/V̇O2)decreased from the pre-exercise value of approximately 35 to 16 during exercise. Together, the results suggest that ventilation and O2transfer across the lung are adequate to meet the aerobic needs of V. mertensi during exercise, but the decrease in the blood convection requirement in the presence of a large arterial—venous O2content difference suggests that a limit in the transport of O2 is imposed by the circulation.

Postprandial exercise: prioritization or additivity of the metabolic responses?

Monitor lizards (Varanus exanthematicus) were used to examine the prioritization or additivity of the metabolic responses associated with exercise and digestion, either of which can elevate metabolic rate independently. Rates of oxygen consumption (V̇.O2) and ventilation (V̇.E) were measured in lizards during fasting exercise, postprandial rest and postprandial exercise. In fasting animals, V̇.O2 increased with walking speed to a maximal value of 15.9mlO2kg−1min−1 at 1.25kmh−1. Postprandial resting metabolic rate was elevated significantly above fasting levels (4.1 versus 2.0mlO2kg−1min−1). During postprandial exercise, V̇.O2 increased to a maximal value of 18.8mlO2kg−1min−1 at 1.25kmh−1. At every level of exercise, V̇.O2 was significantly higher in postprandial animals by a similar increment; the maximal rate of oxygen consumption was significantly increased by 18% in postprandial individuals. Maximal V̇.E did not differ in fasting and postprandial animals and, therefore, the greater V̇.O2max of postprandial animals cannot be attributed to a higher ventilation rate. Air convection requirement (V̇.E/V̇.O2) is significantly lower in postprandial animals at rest and at all levels of exercise, indicating a relative hypoventilation and increased pulmonary oxygen extraction efficiency. We suggest that this increased oxygen extraction may be due to decreased cardiopulmonary 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 cost of exercise. It is clear that fasting exercise did not elicit truly maximal levels of cardiopulmonary oxygen transport in these animals, indicating problems for design models that make this assumption.