Palatability and body fluid homeostasis

The results show that rats drink large volumes of a palatable saccharine solution and rely on the kidneys to eliminate excess water. Rats in fluid balance at the start of the experiment overdrink sodium saccharine (0.005 M) and go significantly into positive fluid balance. If the kidneys are functioning normally the maximum change in fluid balance occurs one half hour after the start of drinking (plasma osmolality is decreased by 9 m osm/kg H₂O). The production of a hypotonic urine rapidly restores the plasma to more normal levels. If rats are injected with antidiuretic hormone so that a hypotonic urine cannot be produced, drinking persists in spite of significant plasma dilution. After 3 hr plasma osmolality is reduced by 22 m osm/kg H₂O (p<0.001), plasma sodium is reduced by 8 m equiv/L (p<0.001) and plasma protein is reduced by 0.4 g% (p<0.05). At this point saccharine consumption decreases but does not stop. The rats showed signs of ill health and subsequent transitory aversion to saccharine. Thus if renal function is impaired, palatability can lead to severe overhydration and hyponatremia.