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 matchin g of O-2 and CO2 transport to the metabolic requirements of different tissu es. In some ecothermic vertebrates (amphibians and reptiles), (V) over dot O-2 increases dramatically following feeding. Factorial increments in (V) o ver dot O-2 range from 1.7 to 44 times above resting rates, and in some cas es (V) over dot O-2 approaches or even exceeds values measured during physi cal activity. There is virtually no information on the cardiopulmonary resp onse during the postprandial period in these animals or how the pattern of cardiopulmonary support compares with that during activity, In our experime nts, pulmonary ventilation ((V) over dot(E)), heart rate (f(H)), systemic b lood flow ((Q) over dot(sys)), rate of oxygen consumption ((V) over dotO(2) ) and rate of carbon dioxide production ((V) over dot(CO2)) were measured a t 35 degrees C in the lizard Varanus exanthematicus for 24 h prior to the i ngestion of meals of various sizes and measured continuously for up to 72 h during the postprandial period. The results of this study were compared wi th previously published values for treadmill exercise in the same experimen tal animals. The change in f(H) and stroke volume (Vs) for a given incremen t in (V) over dot(O2) did not differ during exercise and digestion. In cont rast, the ventilatory response was very dependent on the nature of the elev ated metabolic state. During digestion, an increase in Tie, resulted in a r elative hypoventilation in comparison with resting values, whereas hyperven tilation characterized the response during activity. During exercise, breat hing frequency (f) increased 10- to 40-fold above resting values accompanie d by large reductions in tidal volume (VT) In contrast, postprandial increa ses in (V) over dot(O2) resulted in relatively minor changes in f and VT al most doubled. These results indicate that, in these lizards, the cardiac re sponse to elevated (V) over dot(O2) is stereotyped, the response being pred ictable irrespective of the source of the metabolic increment. In contrast, the ventilatory response is flexible and state-dependent, not only in patt ern but also in its frequency and volume components.