Reductions in systemic oxygen delivery induce a hypometabolic state in the turtle Trachemys scripta.

We investigated the effects of vagal reductions in O(2) delivery on oxygen consumption (VO(2)) in the anesthetized freshwater turtle Trachemys scripta. Specifically, these experiments tested the hypothesis that reductions in arterial oxygen partial pressure (PO(2)) and/or systemic oxygen transport (SOT) trigger a metabolic downregulation. During electric stimulation of the efferent branch of the sectioned right vagus nerve (RVEF), systemic cardiac output decreased 60-70%, systemic PO(2) fell by approximately 30%, and SOT decreased by 60-70%. During RVEF simulation, VO(2) dropped approximately 35%. During control conditions, injection of the metabolic uncoupler 2,4-dinitrophenol (DNP) more than doubled VO(2), reflecting an increase in ATP turnover. RVEF stimulation after DNP injection produced similar cardiovascular and blood gas changes as before DNP, but VO(2) was higher than the VO(2) measured in untreated control animals, indicating that oxygen availability during RVEF stimulation is still sufficient to support VO(2) rates that are even higher than resting rates. We conclude that vagal stimulation triggers metabolic downregulation, primarily through the effects on oxygen transport, although the factor(s) that trigger the hypometabolism remain unknown. The PO(2) may still be an important messenger in metabolic control, but our results suggest that changes in SOT to the metabolically active tissues, rather than changes in PO(2) per se, play an important role in triggering hypometabolism in the freshwater turtle.