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

Area of Science:

• Physiology
• Comparative Physiology
• Metabolic Regulation

Background:

• Vagal nerve stimulation can significantly impact cardiovascular function and oxygen delivery.
• Understanding metabolic responses to reduced oxygen is crucial for various physiological states.

Purpose of the Study:

• To investigate the effects of vagal stimulation-induced reductions in oxygen delivery on oxygen consumption in freshwater turtles.
• To test the hypothesis that decreased arterial oxygen partial pressure (PO2) and/or systemic oxygen transport (SOT) trigger metabolic downregulation.

Main Methods:

• Anesthetized freshwater turtles (Trachemys scripta) underwent electrical stimulation of the vagus nerve.
• Measurements included cardiac output, arterial PO2, systemic oxygen transport (SOT), and oxygen consumption (VO2).
• Metabolic uncoupler 2,4-dinitrophenol (DNP) was used to assess maximal metabolic capacity.

Main Results:

• Vagal stimulation reduced cardiac output (60-70%), systemic PO2 (approx. 30%), and SOT (60-70%).
• Oxygen consumption (VO2) decreased by approximately 35% during vagal stimulation.
• Even after vagal stimulation, VO2 remained elevated when metabolic rate was increased with DNP, suggesting sufficient oxygen availability.

Conclusions:

• Vagal stimulation induces metabolic downregulation in turtles, primarily mediated by alterations in systemic oxygen transport.
• While PO2 may play a role, changes in SOT appear more critical in triggering hypometabolism.
• The specific factors initiating this vagal-induced hypometabolism require further investigation.