Influence of simulated microgravity on the sympathetic response to exercise

Area of Science:

• Physiology
• Space Medicine
• Exercise Science

Background:

• Microgravity exposure reduces aerobic exercise capacity.
• This may stem from impaired sympathetic nervous system (SNS) function.
• Specifically, reduced cardiac output and blood flow redistribution are suspected.

Purpose of the Study:

• To quantify the SNS response to exercise in rats after simulated microgravity.
• To compare SNS activity between head-down suspension (HDS) and control conditions.
• To elucidate the physiological mechanisms behind reduced aerobic capacity.

Main Methods:

• Rats underwent 14 days of head-down suspension (HDS) or cage control (CC).
• Norepinephrine (NE) synthesis was blocked using alpha-methyl-p-tyrosine on day 14.
• NE depletion rate post-blockade estimated SNS activity during exercise at 75% maximal oxygen uptake.

Main Results:

• HDS rats showed significantly lower NE depletion rates in the left ventricle (-82%) and spleen (-42%) compared to CC rats.
• Conversely, NE depletion in the soleus muscle was 47% higher in HDS rats.
• These altered NE depletion patterns indicate a blunted sympathetic response.

Conclusions:

• Simulated microgravity impairs the sympathetic nervous system's ability to increase cardiac output during exercise.
• Reduced blood flow redistribution to active skeletal muscles may occur.
• These SNS alterations likely contribute to the decreased aerobic capacity observed in HDS rats.