Nitric oxide increases myocardial efficiency in the hypoxia-tolerant turtle Trachemys scripta

Area of Science:

• Cardiovascular Physiology
• Environmental Physiology
• Biochemistry

Background:

• Nitric oxide (NO) influences cardiac function by affecting respiration and excitation-contraction coupling.
• Hypoxia-tolerant species, like the turtle Trachemys scripta, may exhibit unique responses to NO under low oxygen conditions.

Purpose of the Study:

• To investigate the role of nitric oxide (NO) in modulating cardiac mechanical performance relative to oxygen consumption in the hypoxia-tolerant turtle, Trachemys scripta.
• To determine if NO affects cardiac efficiency and anaerobic metabolism during normoxia and hypoxia.

Main Methods:

• Isolated ventricle preparations from Trachemys scripta were used to measure twitch force and oxygen consumption.
• L-arginine (NO substrate) and asymmetric dimethylarginine (ADMA, NOS inhibitor) were administered.
• Lactate production was assessed during hypoxia.
• Histochemical analysis using DAF-2 DA was performed to detect NO production.

Main Results:

• The ratio of twitch force to O2 consumption increased significantly with L-arginine, especially during hypoxia (approx. 60%).
• This increase was primarily due to a reduction in O2 consumption, not increased force.
• Lactate production remained unchanged, indicating NO does not stimulate anaerobic metabolism.
• ADMA pre-treatment blocked L-arginine's effect, confirming NOS involvement.

Conclusions:

• Nitric oxide enhances cardiac efficiency by decreasing oxygen consumption in turtles, particularly under hypoxic stress.
• This NO-mediated effect contributes to preserving cardiac function and the turtle's exceptional hypoxic tolerance.
• NO augments the mechanical performance per unit of oxygen consumed without altering lactate production or developed force.