Effect of hindlimb unloading on salt and water intake and output in male and female rats.

INTRODUCTION/PURPOSE: Although fluid intake decreases during spaceflight, there is no increase in urine output, despite the microgravity-induced cephalic fluid shifts. We proposed that fluid and electrolyte balance is controlled not only by cardiac receptors but also by splanchnic baroreceptors.

METHODS

Male and female rats bearing indwelling intravenous cannulae, were subjected to hindlimb unloading (HU) for 24 h at 30 degrees . Water and salt (3% saline) intake, urine output, and plasma osmolality were measured. Right atrial pressure (RAP) and splenic blood pressure/flow were measured during tilt.

RESULTS

Whereas HU reduced water intake in both males (100.9 +/- 9.0 to 71.8 +/- 6.1 mL.kg) and females (118.9 +/- 8.9 to 83.1 +/- 6.5 mL x kg), there was a sustained increase in urine volume only in females (males: 30.9 +/- 3.8 to 34.0 +/- 2.8 mL.kg; females: 26.3 +/- 3.8 to 41.1 +/- 5.8 mL.kg). HU did not change sodium intake/output in either males or females. Plasma osmolality (321.8 +/- 24.1 to 347.5 +/- 22.7 mOsm x L) increased in females but not in males (349.2 +/- 3.3 to 354.4 +/- 7.7 mOsm x L). HU was associated not only with increased RAP but also with reduced splenic perfusion: splenic venous pressure/flow decreased by 0.90 +/- 0.16 mm Hg and 0.19 +/- 0.01 mL.min, respectively; splenic arterial pressure/flow decreased by 3.97 +/- 1.68 mm Hg and 0.17 +/- 0.06 mL x min, respectively.

CONCLUSION

We propose that it is the balance between discordant signals from cardiac and splenic baroreceptors that controls ingestion and excretion. The ultimate reflex response thus depends on the differential effects of changes in gravity on systemic hemodynamics.