Skeletal muscle vasodilation during systemic hypoxia in humans

Area of Science:

• Physiology
• Cardiovascular Science
• Exercise Physiology

Background:

• Systemic hypoxia in quiescent skeletal muscle typically causes vasodilation, increasing perfusion and oxygen delivery.
• The precise local vascular control mechanisms, particularly the roles of endothelial-derived substances like nitric oxide (NO) and prostaglandins (PGs), remain under investigation.
• Understanding these mechanisms is vital for maintaining tissue oxygenation during hypoxic conditions.

Purpose of the Study:

• To investigate the independent and combined roles of nitric oxide (NO) and prostaglandins (PGs) in regulating vascular tone during acute systemic hypoxia in human skeletal muscle.
• To determine the contribution of NO and PGs to hypoxic vasodilation under both resting and exercise conditions.
• To identify potential alternative local dilator mechanisms involved when NO and PGs are inhibited.

Main Methods:

• Utilized sympathoadrenal blockade to isolate local vascular control mechanisms.
• Administered inhibitors for nitric oxide (NO) and prostaglandins (PGs) pathways.
• Assessed skeletal muscle blood flow and vasodilation responses during systemic hypoxia at rest and during submaximal rhythmic exercise.
• Compared responses under normoxic and hypoxic conditions with and without pharmacological inhibition.

Main Results:

• Nitric oxide (NO) plays a significant role in regulating vascular tone during hypoxia, independent of the prostaglandin (PG) pathway.
• Prostaglandins (PGs) do not typically contribute to hypoxic vasodilation but become important when NO is inhibited.
• Combined inhibition of NO and PGs completely abolished vasodilation during resting hypoxia.
• During hypoxic exercise, combined NO and PG inhibition blunted augmented vasodilation and hyperemia by approximately 50%, suggesting the involvement of other local dilator substances.

Conclusions:

• Nitric oxide (NO) and prostaglandins (PGs) are essential, and their combined action is obligatory for hypoxic vasodilation in human skeletal muscle at rest.
• During hypoxic exercise, while NO and PGs contribute significantly, other local dilator mechanisms, potentially involving ATP release or nitrite reduction, are also involved in augmenting blood flow.
• These findings underscore the complex interplay of local factors in maintaining oxygen delivery to skeletal muscle under varying hypoxic stress.