Reductions in systemic oxygen delivery induce a hypometabolic state in theturtle Trachemys scripta

We investigated the effects of vagal reductions in O-2 delivery on oxygen c onsumption ((V) over dot O-2) in the anesthetized freshwater turtle Trachem ys scripta. Specifically, these experiments tested the hypothesis that redu ctions in arterial oxygen partial pressure (PO2) and/or systemic oxygen tra nsport (SOT) trigger a metabolic downregulation. During electric stimulatio n of the efferent branch of the sectioned right vagus nerve (RVEF), systemi c cardiac output decreased 60-70%, systemic PO2 fell by similar to 30%, and SOT decreased by 60-70%. During RVEF simulation, (V) over dot O-2 dropped similar to 35%. During control conditions, injection of the metabolic uncou pler 2,4-dinitrophenol (DNP) more than doubled (V) over dot O-2, reflecting an increase in ATP turnover. RVEF stimulation after DNP injection produced similar cardiovascular and blood gas changes as before DNP, but (V) over d o O-2 was higher than the (V) over dot O-2 measured in untreated control an imals, indicating that oxygen availability during RVEF stimulation is still sufficient to support (V) over dot O-2 rates that are even higher than res ting rates. We conclude that vagal stimulation triggers metabolic downregul ation, primarily through the effects on oxygen transport, although the fact or(s) that trigger the hypometabolism remain unknown. The PO2 may still be an important messenger in metabolic control, but our results suggest that c hanges in SOT to the metabolically active tissues, rather than changes in P O2 per se, play an important role in triggering hypometabolism in the fresh water turtle.