Thirst and sodium appetite after colloid treatment in rats

Subcutaneous injection of polyethylene glycol (PEG) solution produced a progressive sequestration of extracellular fluid at the injection site. Appropriately, PEG-treated rats showed both thirst and sodium appetite. However, water intake began within 1 to 2 hr after the injections, whereas consumption of NaCl solution did not start until 3-4 hr later. Then rats ingested both fluids alternately until plasma volumes were restored, whereupon saline intake became even more prominent while water was consumed due to induced osmoregulatory needs. These three phases were seen regardless of the dose of PEG that was administered or the concentration of saline that was available. In contrast, after maintenance on a sodium-deficient diet for 2-4 days, or after bilateral adrenalectomy, rats increased their intake of saline immediately after PEG treatment. These and other findings suggest that the delayed onset of sodium appetite after PEG treatment that occurs when rats are maintained on standard sodium-rich chow results from the buffer provided by surplus extracellular fluid in those animals. They further suggest that sodium appetite is not directly associated with decreases in plasma volume or sodium concentration but instead may be stimulated by decreased availability of sodium in the brain.