Metabolic, Ventilatory, and Acid-Base Responses Associated with Specific Dynamic Action in the ToadBufo marinus

Abstract

Specific dynamic action (SDA), the postprandial increase in metabolic rate that has been well documented in numerous vertebrates, intensifies demand for O₂ delivery by the ventilatory and cardiovascular systems. Yet the well-documented increase in plasma pH following feeding (the "alkaline tide") should, paradoxically, result in a decrease rather than an increase in ventilation. The intent of this study was to investigate in the marine toad Bufo marinus whether there is a change in the relationship between metabolism, lung ventilation, and blood pH associated specifically with SDA. We measured V̇o2, V̇co2; respiratory quotient (RE); lung ventilation volume and frequency; arterial pH, PO₂, and hematocrit; and heart rate before and after the induction of SDA bypeptone injection directly into the stomach. Levels of V̇o2 and V̇co2; approximately 40-60 mL · g⁻¹ h⁻¹, doubled 5-6 h afterpeptone injection and then declined within 24 h. Accompanying this profound peptone-stimulated SDA, which was dose-dependent, lung ventilation increased from about 3,750 mL · kg⁻¹ · h⁻¹ to 5, 750 mL · kg⁻¹ · h⁻¹ with a time course similar to that of V̇o2. Heart rate increased from 36 to 45 beats · min⁻¹. In spite of these changes, arterial PO₂ andpH (approximately 80 mmHg and 7 75, respectively) did not change during the SDA response. Although the anticipated alkaline tide did not develop in Bufo, the large increase in lung ventilation without a corresponding fall in blood pH nonetheless suggests an uncoupling of hyperventilation and an acid stimulus during SDA. Respiratory physiology during the postprandial period clearly deserves more investigation.