Metabolic responses to head-down suspension in hypophysectomized rats

Area of Science:

• Physiology
• Endocrinology
• Aerospace Medicine

Background:

• Head-down suspension (HDS) in rats causes reduced maximal oxygen consumption (VO2max) and hindlimb muscle atrophy.
• The role of endocrine factors in mediating these HDS-induced physiological changes is not fully understood.

Purpose of the Study:

• To test the hypothesis that endocrine deficiency would prevent HDS-induced reductions in VO2max and hindlimb muscle mass.
• To investigate the physiological and hormonal responses to HDS in endocrine-deficient rats.

Main Methods:

• Hypophysectomized (HYPX) and sham-operated (SHAM) rats were subjected to 28 days of HDS or cage control (CC).
• VO2max was measured before and after the experimental period.
• Hindlimb muscle mass, citrate synthase activity, thermoregulation, and plasma hormone levels (epinephrine, norepinephrine, thyroxine, triiodothyronine, testosterone, growth hormone) were assessed.

Main Results:

• HYPX rats showed no significant reduction in VO2max or hindlimb muscle mass.
• SHAM-HDS rats exhibited significant reductions in VO2max and soleus muscle atrophy.
• HYPX rats displayed atrophy in soleus and plantaris muscles, but preserved soleus citrate synthase activity compared to controls.
• HDS impaired thermoregulation and altered plasma catecholamine levels, with HYPX rats having lower pretreatment epinephrine.

Conclusions:

• Endocrine deficiency, particularly hypophysectomy, protects against HDS-induced decreases in aerobic capacity and hindlimb muscle atrophy.
• HDS affects thermoregulation and catecholamine levels, independent of pituitary hormones.
• Pituitary hormones like growth hormone and testosterone may play a role in mediating HDS-induced muscle atrophy.