| Summary: | Volume and composition of the body fluid compartments are kept within narrow limits. This is done by changes in intake and excretion of water and salt. I found that changes in food intake and salt excretion play an important role in body fluid regulation during water deprivation and investigated the mechanisms of these changes. Urine volume changed little in rats deprived of water for 10 hours. The main osmoregulatory response during water deprivation was a loss of solute from the body. Food intake fell by 43%, which reduced the load of solute to the tissues and allowed absorption of water already in the gut. Water deprived rats also excreted more sodium, potassium and chloride. Thus, although rats lost 8% of their body water during 10 hours of water deprivation, plasma tonicity rose by only 2%. On rehydration, when no food was present rats rapidly reduced excretion of sodium, potassium and chloride. Rats allowed water and food drank more and excreted more solute. These changes contribute to restoration of the body fluid compartments. Brain infusions caused changes in electrolyte excretion that were similar to those seen during water deprivation and rehydration. Electrolyte excretion increased during infusion (1 $\mu$L/min for 2 h) of cerebrospinal fluid (CSF) with 300 mM NaCl in the lateral ventricle. Infusion of low-sodium CSF reduced electrolyte excretion in water deprived rats, but had little effect in rats that were not water deprived. The time course of the changes after rehydration and brain infusions was the same, and the same solutes were involved. This suggests the mechanisms are similar. Low-sodium CSF made isotonic with mannitol had the same effect as hypotonic low-sodium CSF. The sensor probably monitors brain interstitial fluid, not CSF. Food intake fell within 1 hour of water deprivation. Meals were smaller, but meal frequency did not change. Dehydration anorexia is caused by a sensor located in the gut, portal circulation or liver, because infusion of water in the stomach, jejunum or cecum (10 mL/6 h) restored food intake in rats not allowed to drink, but intravenous infusions had no effect. Intravenous infusions did not alter urinary water loss and did not alter food intake in rats allowed to drink.
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