Effects of feeding on arterial blood gases in the American alligator Alligator mississippiensis

Reptiles habitually ingest large meals at infrequent intervals, leading to changes in acid-base status as the net secretion of acid to the stomach cau ses a metabolic alkalosis (the alkaline tide). In chronically cannulated an d undisturbed amphibians and reptiles, the pH changes in arterial blood are , nevertheless, reduced by a concomitant respiratory acidosis (increased P- CO2 caused by a relative hypoventilation), Alligators (Alligator mississipp iensis) have been reported to exhibit exceptionally large increases in plas ma [HCO3-] following feeding, but these studies were based on blood samples obtained by cardiac puncture, so stress and disturbance may have affected the blood gas levels. Furthermore, crocodilian haemoglobin is characterised by a unique binding of HCO3- that act to reduce blood oxygen-affinity, and it has been proposed that this feature safeguards oxygen offloading by cou nteracting pH effects on blood oxygen-affinity. Therefore, to study acid-ba se regulation and the interaction between the alkaline tide and oxygen tran sport in more detail, we describe the arterial blood gas composition of chr onically cannulated and undisturbed alligators before and after voluntary f eeding (meal size 7.5+/-1 % of body mass). Digestion was associated with an approximately fourfold increase in metabol ic rate (from 0.63+/-0.04 to 2.32+/-0.24 ml O-2 min(-1) kg(-1)) and was acc ompanied by a small increase in the respiratory gas exchange ratio. The art erial Po, of fasting alligators was 60.3+/-6.8 mmHg (1 mmHg=0.133 kPa) and reached a maximum of 81.3+/-2.7 mmHg at 96 h following feeding; there was o nly a small increase in lactate levels, so the increased metabolic rate see ms to be entirely aerobic, Plasma [HCO3-] increased from 24.4+/-1.1 to 36.9 +/-1.7 mmol l(-1) (at 24 h), but since arterial P-CO2 increased from 29.0+/ -1.1 to 36.8+/-1.3 mmHg, arterial pH remained virtually unaffected (changin g from 7.51+/-0.01 to 7.58+/-0.01 at 24 h), The changes in plasma [HCO3-] w ere mirrored by equimolar reductions in plasma [Cl-]. The in vitro blood ox ygen-affinity was reduced during the post-prandial period, whereas the esti mated in vivo blood oxygen-affinity remained virtually constant. This suppo rts the view that the specific HCO3- effect prevents an increased blood oxy gen-affinity during digestion in alligators.