Metabolic and Blood Gas Dependence on Digestive State in the Savannah Monitor Lizard Varanus exanthematicus: An Assessment of the Alkaline Tide

A large alkaline tide (up to 20 mmol l ^–1 increase in bicarbonate concentration [HCO ₃ ^– ] with an accompanied increase in blood pH) has previously been reported for some carnivorous reptiles within 24 h after ingesting a large meal. This phenomenon has been attributed to the secretion of large amounts of H ⁺ ions into the stomach, which is required for digestion of large prey items. To test the generality of this phenomenon in carnivorous reptiles, this study quantified the metabolic and acid–base status of the Savannah monitor lizard, Varanus exanthematicus , during digestion at 35°C. Following a meal of approximately 10% of body mass, V̇ _O2 and V̇ _CO2 were measured continuously and arterial pH, blood gases and strong ions were measured every 8 h for 5 days. During peak digestion (24 h post feeding), V̇ _O2 and V̇ _CO2 increased to approximately threefold fasting values ( V̇ _O2 , 0.95–2.57 ml min ^–1 kg ^–1 ; V̇ _CO2 0.53–1.63 ml min ^–1 kg ^–1 ) while respiratory exchange ratio (R) remained constant (0.62–0.73). During digestion, arterial P _CO2 increased (from 4.6 kPa to 5.8 kPa), and [HCO ₃ ^– ] also increased (from 24.1 mmol l ^–1 to 40.3 mmol l ^–1 ). In contrast to early studies on crocodilians, arterial pH in V. exanthematicus remained relatively stable during digestion (7.43–7.56). Strong ions contributed little to the acid–base compensation during the alkalosis. Collectively the data indicate that the metabolic alkalosis associated with H ⁺ secretion (as indicated by increased plasma bicarbonate) is partially compensated by a respiratory acidosis.