Alpha-Adrenergic receptor responsiveness is preserved during prolonged exercise

Area of Science:

• Physiology
• Exercise Science
• Cardiovascular Regulation

Background:

• Prolonged mild-intensity exercise may involve reduced sympathetic nervous system (SNS) restraint on skeletal muscle blood flow.
• A potential mechanism for this reduction is a decrease in alpha(1)- and alpha(2)-adrenergic receptor responsiveness over time.

Purpose of the Study:

• To investigate the impact of exercise duration on alpha(1)- and alpha(2)-adrenergic receptor responsiveness during prolonged, constant-load exercise.

Main Methods:

• Mongrel dogs were instrumented to measure hindlimb blood flow (HBF) and mean arterial pressure (MAP).
• Selective alpha(1)- (phenylephrine) and alpha(2)-adrenergic-receptor (clonidine) agonists, and tyramine (to stimulate norepinephrine release), were infused at 5, 30, and 50 minutes of exercise.
• Vascular conductance (VC) was calculated (HBF/MAP) to assess receptor responsiveness.

Main Results:

• Phenylephrine infusion caused similar decreases in VC across all time points (5, 30, 50 min).
• Clonidine infusion also resulted in comparable reductions in VC at 5, 30, and 50 min.
• Tyramine infusion demonstrated consistent decreases in VC throughout the exercise duration.

Conclusions:

• Alpha(1)- and alpha(2)-adrenergic receptor responsiveness to agonists and endogenous norepinephrine does not diminish during prolonged mild-intensity exercise.
• Reduced alpha-adrenergic receptor responsiveness is unlikely to be the cause of decreased SNS restraint on muscle blood flow during extended exercise.