The long road to steady state in gas exchange: metabolic and ventilatory responses to hypercapnia and hypoxia in Cuvier's dwarf caiman

摘要

Animals with intermittent lung ventilation and those exposed to hypoxia and hypercapnia will experience fluctuations in the bodily O-2 and CO2 stores, but the magnitude and duration of these changes are not well understood amongst ectotherms. Using the changes in the respiratory exchange ratio (RER; CO2 excretion divided by O-2 uptake) as a proxy for changes in bodily gas stores, we quantified time constants in response to hypoxia and hypercapnia in Cuvier's dwarf caiman. We confirm distinct and prolonged changes in RER during and after exposure to hypoxia or hypercapnia. Gas exchange transients were evaluated in reference to predictions from a two-compartment model of CO2 exchange to quantify the effects of the levels of hypoxia and hypercapnia, duration of hypercapnia (30-300 min) and body temperature (23 versus 33 degrees C). For hypercapnia, the transients could be adequately fitted by two-phase exponential functions, and slow time constants (after 300 min hypercapnia) concurred reasonably well with modelling predictions. The slow time constants for the decays after hypercapnia were not affected by the level of hypercapnia, but they increased (especially at 23 degrees C) with exposure time, possibly indicating a temporal and slow recruitment of tissues for CO2 storage. In contrast to modelling predictions, elevated body temperature did not reduce the time constants, probably reflecting similar ventilation rates in transients at 23 and 33 degrees C. Our study reveals that attainment of steady state for gas exchange requires considerable time and this has important implications for designing experimental protocols when studying ventilatory control and conducting respirometry.