Respiratory consequences of feeding in the snake Python molorus

Snakes can ingest large meals and exhibit marked increases in metabolic rate during digestion. Because postprandial oxygen consumption in some snakes may surpass that attained during exercise, studies of digestion offers an alternative avenue to understand the cardio-respiratory responses to elevated metabolic rate in reptiles. The effects of feeding on metabolic rate, arterial oxygen levels, and arterial acid-base status in the snake Python molorus are described. Four snakes (180-250 g) were cannulated in the dorsal aorta and blood samples were obtained during 72 h following ingestion of a meal (rat pups) exceeding 20% of body weight. Oxygen consumption increased from a fasting value of 1.71 +/- 0.08 to 5.54 +/- 0.42 ml kg-1 min-1 at 48 h following feeding, and the respiratory gas exchange ratio increased from 0.67 +/- 0.02 to a maximum of 0.92 +/- 0.03 at 32 h. Plasma lactate was always less than 0.5 mM, so the postprandial increase in metabolic rate was met by aerobic respiration. In fasting animals, arterial PO2 was 66 +/- 4 mmHg and haemoglobin-O2 saturation was 92 +/- 3%; similar values were recorded during digestion, but haematocrit decreased from 15.8 +/- 1.0 to 9.8 +/- 0.8 due to repeated blood sampling. Plasma HCO3-] increased from a fasting level of 19.3 +/- 0.8 to 25.8 +/- 1.0 mmol l-1 at 24 h after feeding. However, because arterial PCO2 increased from 21.1 +/- 0.5 to 27.9 +/- 1.4 mmHg, there was no significant change in arterial pH from the fasting value of 7.52 +/- 0.01. Acid-base status returned to pre-feeding levels at 72 h following feeding. The increased arterial PCO2 is most likely explained by a reduction in ventilation relative to metabolism, but we predict that lung PO2 does not decrease below 115 mmHg. Although ingestion of large meals is associated with large metabolic changes in pythons, the attendant changes in blood gases are relatively small. In particular, the small changes in plasma HCO3-] and stable pH show that pythons respond very differently to digestion than alligators where very large alkaline tides have been observed. It is unclear why pythons and alligators differ in the magnitude of their responses, but given these interspecific differences it seems worthwhile to describe arterial blood gases during digestion in other species of ectothermic vertebrates.