Patterns of cardiovascular and ventilatory response to elevated metabolic states in the lizard Varanus exanthematicus

The principal function of the cardiopulmonary system is the precise matching of O(2) and CO(2) transport to the metabolic requirements of different tissues. In some ecothermic vertebrates (amphibians and reptiles), vdot (O2) increases dramatically following feeding. Factorial increments in vdot (O2) range from 1.7 to 44 times above resting rates, and in some cases vdot (O2) approaches or even exceeds values measured during physical activity. There is virtually no information on the cardiopulmonary response during the postprandial period in these animals or how the pattern of cardiopulmonary support compares with that during activity. In our experiments, pulmonary ventilation ( vdot e), heart rate (fh), systemic blood flow ( qdot (sys)), rate of oxygen consumption ( vdot (O2)) and rate of carbon dioxide production ( vdot (CO2)) were measured at 35 degrees C in the lizard Varanus exanthematicus for 24 h prior to the ingestion of meals of various sizes and measured continuously for up to 72 h during the postprandial period. The results of this study were compared with previously published values for treadmill exercise in the same experimental animals. The change in fh and stroke volume (V(S)) for a given increment in vdot (O2) did not differ during exercise and digestion. In contrast, the ventilatory response was very dependent on the nature of the elevated metabolic state. During digestion, an increase in vdot (O2) resulted in a relative hypoventilation in comparison with resting values, whereas hyperventilation characterized the response during activity. During exercise, breathing frequency (f) increased 10- to 40-fold above resting values accompanied by large reductions in tidal volume (V(T)). In contrast, postprandial increases in vdot (O2) resulted in relatively minor changes in f and V(T) almost doubled. These results indicate that, in these lizards, the cardiac response to elevated vdot (O2) is stereotyped, the response being predictable irrespective of the source of the metabolic increment. In contrast, the ventilatory response is flexible and state-dependent, not only in pattern but also in its frequency and volume components.