The long road to steady state in gas exchange: metabolic and ventilatory responses to hypercapnia and hypoxia in Cuvier's dwarf caiman.
Animals with intermittent lung ventilation and those exposed to hypoxia and hypercapnia will experience fluctuations in the bodily O2and CO2stores, but the magnitude and duration of these changes are not well understood amongst ectotherms. Using the changes in the respiratory exchange ratio (RER; CO2excretion divided by O2uptake) 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 CO2exchange to quantify the effects of the levels of hypoxia and hypercapnia, duration of hypercapnia (30-300 min) and body temperature (23 versus 33°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°C) with exposure time, possibly indicating a temporal and slow recruitment of tissues for CO2storage. 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°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.
Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C 8000, Denmark christian.malte@bios.au.dk.
Christian Lind
0000-0002-7003-6370
CL
Malte
Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C 8000, Denmark.
Hans
H
Malte
Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C 8000, Denmark.
Tobias
T
Wang
D000489:Alligators and Crocodiles / Q000378:metabolism* / Q000502:physiology*; D000818:Animals; D001831:Body Temperature; D002245:Carbon Dioxide / Q000378:metabolism; D015687:Cell Hypoxia / Q000502:physiology*; D006935:Hypercapnia / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D012119:Respiration; D056152:Respiratory Rate
D002245:Carbon Dioxide; D010100:Oxygen
Gas exchange; Hypercapnia; Hypoxia; Reptile; Steady state; Time constants