Effects of feeding on metabolism, gas transport, and acid-base balance in the bullfrog Rana catesbeiana

Massive feeding in ectothermic vertebrates causes changes in metabolism and acid-base and respiratory parameters. Most investigations have focused on only one aspect of these complex changes, and different species have been used, making comparison among studies difficult. The purpose of the present study was, therefore, to provide an integrative study of the multiple physiological changes taking place after feeding. Bullfrogs (Rana catesbeiana) partly submerged in water were fed meals (mice or rats) amounting to similar to 1/10 of their body weight. Oxygen consumption increased and peaked at a value three times the predigestive level 72-96 h after feeding. Arterial PO2 decreased slightly during digestion, whereas hemoglobin-bound oxygen saturation was unaffected. Yet, arterial blood oxygen content was pronouncedly elevated because of a 60% increase in hematocrit, which appeared mediated via release of red blood cells from the spleen. Gastric acid secretion was associated with a 60% increase in plasma HCO3- concentration ([HCO3-]) 48 h after feeding. Arterial pH only increased from 7.86 to 7.94, because the metabolic alkalosis was countered by an increase in PCO2 from 10.8 to 13.7 mmHg. Feeding also induced a small intracellular alkalosis in the sartorius muscle. Arterial pH and HCO3- returned to control values 96-120 h after feeding. There was no sign of anaerobic energy production during digestion as plasma and tissue lactate levels remained low and intracellular ATP concentration stayed high. However, phosphocreatine was reduced in the sartorius muscle and ventricle 48 h after feeding.