Renal adjustments to microgravity.

Area of Science:

• Space Physiology
• Renal Physiology
• Cardiovascular Physiology

Background:

• Microgravity exposure alters fluid and electrolyte balance.
• Previous observations indicated attenuated renal responses to fluid loads in space.

Purpose of the Study:

• To investigate the mechanisms behind attenuated renal fluid excretion during spaceflight.
• To compare renal responses to fluid loads in microgravity with ground-based simulations.

Main Methods:

• Analysis of diuretic and natriuretic responses to intravenous saline load during a 10-day shuttle mission.
• Observation of renal fluid excretion following oral water load in astronauts on space station Mir.
• Comparison with a ground-based study using head-down bed rest simulating microgravity effects.

Main Results:

• Diuretic and natriuretic responses were attenuated in microgravity, accompanied by increased plasma noradrenaline.
• Renal fluid excretion was reduced in astronauts after water load compared to ground conditions.
• Head-down bed rest did not replicate the attenuated urinary flow rate observed in space.

Conclusions:

• Spaceflight-induced activation of antinatriuretic mechanisms, possibly due to reduced blood volume, explains attenuated renal responses.
• Increased renal sympathetic nervous activity and the renin-angiotensin-aldosterone axis may contribute to these changes.
• Further research is planned on the International Space Station to elucidate these mechanisms.