Continuous arterial PO2 profiles in unrestrained, undisturbed aquatic turtles during routine behaviors

Mammals and birds maintain high arterial partial pressure of oxygen (P_O ) values in order to preserve near-complete hemoglobin (Hb) oxygen (O₂) saturation. In diving mammals and birds, arterial O₂ follows a primarily monotonic decline and then recovers quickly after dives. In laboratory studies of submerged freshwater turtles, arterial O₂ depletion typically follows a similar pattern. However, in these studies, turtles were disturbed, frequently tethered to external equipment and confined either to small tanks or breathing holes. Aquatic turtles can alter cardiac shunting patterns, which will affect arterial P_O  values. Consequently, little is known about arterial O₂ regulation and use in undisturbed turtles. We conducted the first study to continuously measure arterial P_O  using implanted microelectrodes and a backpack logger in undisturbed red-eared sliders during routine activities. Arterial P_O  profiles during submergences varied dramatically, with no consistent patterns. Arterial P_O  was also lower than previously reported during all activities, with values rarely above 50 mmHg (85% Hb saturation). There was no difference in mean P_O  between five different activities: submerged resting, swimming, basking, resting at the surface and when a person was present. These results suggest significant cardiac shunting occurs during routine activities as well as submergences. However, the lack of relationship between P_O  and any activity suggests that cardiac shunts are not regulated to maintain high arterial P_O  values. These data support the idea that cardiac shunting is the passive by-product of regulation of vascular resistances by the autonomic nervous system.