Effect of temperature on oxygen stores during aerobic diving in the freshwater turtle Mauremys caspica leprosa

Oxygen stores available for aerobic diving were studied in the freshwater turtle (Mauremys caspica leprosa) at three constant body temperatures (15 degrees, 25 degrees, and 35 degrees C) and during the thermal transient (30 degrees-15 degrees C) induced by immersion in cold water. The term ''aerobic dive limit'' has been defined as the maximal duration of the dive before lactate increases. This increase occurs when a critical P-O2 value is reached, and it is well characterized at lung level by a sharp increase in the lung apnoeic respiratory quotient. Kinetic analysis of lung gas composition during forced dives at fixed body temperature shows that critical P-O2 values rise with temperature and that the postventilatory P-O2 at the beginning of a dive decreases, so that the two temperature-dependent factors lead to a significant decrease with temperature in the lung O-2, stores available for aerobic diving. During dives with transient body cooling, a natural condition in M. caspica leprosa, temperature equilibration occurs fast enough to expand aerobic scope by bearing the critical P-O2 to the same value obtained at a fixed temperature of 15 degrees C. These dives are characterized by reversed CO2 transport (from lung to tissues) and therefore by negative values of the lung respiratory quotient; a decrease in temperature increases CO2 capacitance of tissues, resulting in a fall in P-CO2, at constant CO2 content. Because this does not occur in the gas phase, P-CO2 difference can lead to diffusion in the direction opposite from normal. This pattern may favour lung-to-tissue O-2 transfer, through the Bohr effect. Therefore, the aerobic dive limit is reduced at high temperature not only through a metabolic rate effect but also through a marked decrease in the available O-2 stores; fast body cooling (30 degrees-15 degrees C) associated with immersion in cold water extends the O-2 stores available for aerobic diving to a level similar to that of immersions at constant body temperatures that are in equilibrium with water temperature.