Model for antiorthostatic hypokinesia: head-down tilt effects on water and salt excretion.

Water and electrolyte excretion was investigated in antiorthostatic hypokinetic and orthostatic hypokinetic and control rats in metabolic cages. Significant (t test, P < 0.05), diuresis, natriuresis, and kaliuresis occurred in the antiorthostatic hypokinetic subjects but did not occur in either the orthostatic hypokinetic or controls. Recovery from antiorthostatic hypokinesia was characterized by retention of water, sodium, and potassium. Patterns of changes in body weight and food and water orthostatic hypokinetic rats and thus could not account for the differences in renal handling of water and electrolytes. Also, differences in ingestion of food and water in controls could not account for differences in excretion of water and electrolytes between these and antiorthostatic hypokinetic rats. It was concluded that the antiorthostatic position was responsible for the diuresis and natriuresis and that the antiorthostatic hypokinetic rat appears to be a good model for the study of water and electrolyte excretion during conditions such as bed rest, water immersion, and exposure to weightlessness.